HUBBLE 2018 Space TeleScope - ESO.org
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
2018 HUBBLE Space Telescope Confirmation (2019–2020) Mission Extension (2021–2022) Proposal prepared by: Antonella Nota Paule Sonnentrucker Linda Smith
Lagoon Nebula WFC3/IR
3 Contents Executive Summary . . . . . . . . . . . . . . . . . . . . . . . 7 A.1 The Science Instruments . . . . . . . . . . . . . . . . . . 9 A.2 Hubble Science . . . . . . . . . . . . . . . . . . . . . . . 10 A.2.1 The Exploration of the Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 A.2.2 Exploring Planets outside our Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 A.2.3 The distant Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 A.2.5 High Precision Astrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 A.3 Synergy with Other Missions . . . . . . . . . . . . . . . 21 A.3.1 Hubble is a key player in multi-messenger astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 A.3.2 Synergy with GAIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 A.4 New Initiatives Offered . . . . . . . . . . . . . . . . . . 26 A.4.1 New Hubble Observing Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 While the instruments on Hubble are mature, and relatively well understood, new initiatives are continuously implemented to augment the scientific productivity and efficiency of the observatory. Innovative new observing modes are being explored. . 26 A.4.1.1 New COS/FUV Central Wavelengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 A.4.1.2 The STIS Spatial Scanning Capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 A.4.2 New Science Initiatives . . . . . . . . . . . . . . . . . . 26 A.4.2.1 The Gap Filler Initiative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 A.4.2.2 Fundamental Physics with HST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 A.4.2.3 Ultraviolet Initiative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 A.4.2.4 Midterm Proposals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 A.4.2.5 Preparatory Science for JWST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 B.1. Impact of Hubble on Science: Metrics . . . . . . . . . . . 33 B.1.1. Publications and Citations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 B.1.2 Proposal Pressure and European Success . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 B.2 Hubble in the Media: Impact and Benefits . . . . . . . . . 37 B.2.1 Press Releases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 B.2.1.1. Science and Photo Releases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 B.2.1.2 Announcements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 B.2.1.3 Pictures of the Week . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 B 2.1.4 Youtube: Hubblecasts and other videos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4
B.2.2 Social Media . . . . . . . . . . . . . . . . . . . . . . . 43
B.2.2.1 Facebook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
B.2.2.2 Twitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
B.2.3 Science NewsLetter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
B.2.4 Other Initiatives . . . . . . . . . . . . . . . . . . . . . 46
B. 2.4.1 Art & Science – Our Place In Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
B.2.4.2 Universe Awareness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
C.1 Observatory/Operations . . . . . . . . . . . . . . . . . . 51
C.2 Observatory Status and Future Projections . . . . . . . 51
C.1.2 Science Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
C.2.2 Fine Guidance Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
C.2.3 Gyros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
C.2.4.Reaction Wheels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
C.2.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SI
C&DH Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
C.2.6 Batteries and Solar Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
C.2.7 Subsystem Lifetime Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
C.2.8 Orbit Decay and Debris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
C.3 Mission Operations and Sustaining Engineering . . . . . 57
C.3.1 Anomaly Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
C.3.3.1 The COS2025 Initiative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
C.3.2 Engineering and Flight Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
C.3.3 Mission Life Extension Initiatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
C.3.4 Instruments Legacy Initiatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
C.3.4.1 STIS Blaze function shift correction tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
C.3.4.2 LINEAR: A Novel Way of Reconstructing WFC3/IR Slitless Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
D.1 The Hubble Data Archive at STScI . . . . . . . . . . . . . 63
D.1.1 The Hubble Legacy Archive (HLA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
D.1.2 The Hubble Spectroscopic Legacy Archive (HSLA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
UV spectra publicly available from the STIS spectrograph will be made available in future releases of the HSLA. . . . . . . . . 64
E.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 67
5
6 NGC 4302 and NGC 4298 WFC3/UVIS
7 Executive Summary In over 28 years in orbit, the Hubble Space Telescope has revolutionized our understanding of the Universe, educated both astronomers and the public at large, and inspired new generations of students to become interested in astronomy, space science and engineering. More powerful and innovative than ever, Hubble continues its relentless investigation of a broad range of astrophysical objects and phenomena, ranging from our own solar system to the most distant galaxies, from the collapse of ordinary matter into black holes to the distribution of dark matter in galaxies and clusters, from the characterization of the atmospheres of newly discovered exoplanets, to the precise studies of relative motions of nearby stars, to tracing the expansion of the Universe. Working in synergy with other observatories, from the ground and space, Hubble continues to be a prominent presence on the astronomical discovery scene (see Annex A) and its observing time is in high community demand. At every yearly cycle, approximately 1000 proposals are received requesting new and compel- ling Hubble observations, with an oversubscription of ~6. This is testament to the strong interest of the community worldwide in Hubble observations. ESA member state astronomers especially play a key role in submitting clever and ambitious ideas, and are very successful in obtaining Hubble time, cycle after cycle (see B.1.2.2). Hubble has witnessed rare astronomical events, such as observing for the first time the source of gravitational waves, and continues making discoveries that are beyond the dreams of its designers and builders. For so many years, Hubble has defined our collective knowledge of the Universe, and has shaped and adapted to the evolution of space astronomy. Hubble is a dynamic observatory: new and innovative observing techniques are implemented to expand on Hubble’s current observing capabilities and new initiatives are continuously designed to exploit Hubble’s unique characteristics (see A.4), and enhance the scientific return of the mission. Very high in priority is the optimization of the synergy with other missions such as XMM, Spitzer and the James Webb Space Telescope, slated for launch in 2021 (see A.4.2.5). For example, the UV initiative encourages astronomers to take advantage of the unique UV capability that Hubble offers. Mid-term proposals have been introduced to allow astronomers to follow up on their discoveries on shorter than traditional timescales (see A.4.2.4). Treasury programs are allowed for compelling scientific investigations that require a significant investment of Hubble’s observing time. New to the next cycle, astronomers are encouraged to submit proposals that address questions in fundamental physics (see A.4.2.2). All of this translates into more than 900 refereed papers published in 2017 based on Hubble data, of which 39% have first authors from ESA member states. In parallel, new efforts are invested in expanding high level science data products and dis- covery tools to accelerate the pace of scientific discovery from archival usage, which generates half of the Hubble published papers. ESA maintains a dedicated Hubble mirror archive at ESAC (eHST, see D.2) which showcases an easier search mechanism, a new powerful user interface, and new visualization tools. In a collaboration with the National Observatory of Athens, ESA is developing a Hubble Catalogue of Variables (HCV) out of the Hubble Source Catalogue. The HCV is being installed at ESAC and STScI (see D.3). In summary, Hubble is an aging, but healthy, observatory. All subsystems are currently operat- ing at nominal status, and extrapolations of current trends predict a very viable observatory well into 2025 (see Annex C). Even given the recently announced delay in the launch date for JWST, it is very likely that Hubble will scientifically overlap with JWST for two-three years, enabling new scientific opportunities for the community to exploit. Hubble will continue making ground-break- ing results for years to come. The European community is having much success exploiting this powerful observatory and ESA’s continued support to the Hubble mission is crucial to even greater success in the years to come.
8
Galaxy
NGC 4258
WFC3/UVIS, ACS/WFC
9
Annex A
Hubble Science
A.1 The Science Instruments
With its unique capability upgraded in orbit by astronauts, Hubble has been at the forefront
of observational astronomy since its launch. The current suite of six science instruments - two of
which were installed during the last servicing mission – are in excellent condition and performing
well (see Table A.1.1).
Of the current complement of science instruments, NICMOS is presently offline due to low
demand, and to impurities in the NICMOS Cooling System (NCS) that prevent NICMOS from
reaching its nominal operating temperature of 77 K. Procedures for purging the NCS and
restarting NICMOS are available if the scientific need arises.
The five operating science instruments offer a rich set of observing capabilities at UV, optical
and near IR wavelengths. Hubble has two cameras, which are often operated together. Wide
Field Camera 3 (WFC3) was installed in Serciving Mission 4(SM4) and offers panchromatic
(200–1700nm) imaging capabilities. The Advanced Camera for Surveys (ACS) provides imaging
in the far UV (115–170nm) and optical (370–1100nm). The Cosmic Origins Spectrograph (COS)
operates in the far UV (90–200nm) and near-UV (170–320nm). The Space Telescope Imaging
Spectrograph (STIS) was repaired in SM4 and has FUV (115–175nm), NUV (170–320nm) and
optical (200–1030 nm) capabilities.
Overall there are thousands of instrument modes resulting from possible combinations of
optics, filters, gratings, slits, and detectors. There are five distinct types of detectors in the
instruments – three ultraviolet-sensitive multi-anode micro-channel arrays (MAMAs) and one
cross-delay-line miro-crochannel plate (MCP) detector, three Si-based CCD arrays used at optical
and near-UV wavelengths, one near-infrared HgCdTe array, and three white-light photomul-
tipliers. All of these detectors exhibit time-dependent characteristics that are calibrated and
constantly monitored.
The initial decrease in COS sensitivity at far-UV wavelengths seen in the first three years of
operation (~5% per year) has arrested, and the overall sensitivity remains very high. Gain sag
Table A.1.1 HST Science Instruments
Channel λ (nm) Details FOV
ACS SBC 115-170 6 filters; 2 prisms (R=λ/Δλ~100; 115-180 nm) 35''x31''
WFC 370-1100 36 filters; 1 grism (R~100; 550-1050 nm) 202''x202''
COS FUV 90-200 R~3k; R~18k 2.5'' circle
NUV 170-320 R~3k; R~20k; full-band imaging 2.5'' circle
FGS-1R ------ 400-700 4 filters; photometry; astrometry ~69 sq.arcmin
NICMOS* NIC1 800-1800 19 filters; polarimetry 11''x11''
NIC2 800-2450 19 filters; polarimetry; chronography 19''x19''
NIC3 800-2300 16 filters; 3 grisms (R~200; 800-2500 nm) 51''x51''
STIS FUV 115-175 3 filters; R~1k; R~15k; R~45k; R~114k 25''x25''
NUV 170-320 6 filters; R~500; R~20k, R~30k; R~114k; prism 25’’x25’’
CCD 200-1030 3 filters; R~1k; R~8k; coronagraphic fingers 52''x52''
WFC3 UVIS 200-1000 62 filters; 1 grism (R~70; 190-450 nm) 162''x162''
IR 800-1700 15 filters; 2 grisms (R~210; 800-1150 nm and R~130; 1075-1700 nm) 123''x136''
*NICMOS is currently dormant and not being used for science observations.10
resulting from charge extraction on the far-UV MCP is closely monitored and managed with peri-
odic adjustments to the detector high voltage settings and position of the science spectrum on
the detector. WFC3 sensitivity, stability, and photometric accuracy continue to meet or exceed
pre-launch expectations at both visible and IR wavelengths. ACS and STIS instrument properties
(dark count, readnoise, sensitivity, stability, etc.) continue to follow their well-established trends.
See Section C for addition details.
A.2 Hubble Science
Hubble’s capabilities, and its long baseline of 28 years and counting, have advanced as-
trophysics and planetary science across a broad front. As we celebrate its splendid past, the
telescope is looking forward to one of its most ambitious observing programs ever. Hubble
continues to stretch the boundaries of our knowledge, enhance the value of data from other
assets in space and on the ground, and build its archive to enable discoveries far into the future.
As in previous cycles, this past year and current observing program is notable for its incredible
breadth, from comets to the most distant galaxies, including the following highlights:
• Hubble solves the mystery on the nature of the interstellar visitor to our solar system,
‘Oumuamua.
• Hubble observes the Jupiter Giant Red Spot shrinking.
• Hubble detects Helium in the atmosphere of an exoplanet.
• Hubble characterizes the atmospheres of earth-size planets in the Trappist-1 system.
• Teaming up with GAIA, Hubble measures 3D stellar motions in the solar neighbourhood.
• Hubble finds the most distant star.
• Hubble provides an accurate test of General Relativity outside our Solar System
• Hubble is a key player in multi-messenger astronomy.
• Hubble increases the accuracy of the Hubble constant.
A.2.1 The Exploration of the Solar System
Hubble continues to produce exciting new science results about objects within our Solar
System, generating some of the most popular press releases and producing new beautiful
images which capture the imagination of the public.
Figure A.2.1.1: Artist’s impression of
the first interstellar object discovered
in the Solar System, `Oumuamua.
Observations made with Hubble and
other telescopes show that the object
is moving faster than predicted while
leaving the Solar System.11
In October 2017, the object now known as 1I/2017 U1 ‘Oumuamua was discovered by the
Pan-STARRS1 survey (Figure A.2.1.1). Within a few days, additional observations collected with
ESA’s Optical Ground Station (OGS) telescope, together with pre-discovery data from Pan-
STARRS1, led to the determination of a preliminary orbit that was highly hyperbolic (eccentricity
of 1.2), identifying the object as originating from outside the Solar System and approaching
from the direction of the constellation Lyra (Figure A.2.1.2), with an asymptotic inbound velocity
of V∞~26 km s -1 (Micheli et al. 2018).
The extreme eccentricity of ‘Oumuamua’s orbit led the Minor Planet Centre to initially classify
the object as a comet. However, this classification was later withdrawn when imaging obtained
immediately after discovery using the Canada-France-Hawaii Telescope (CFHT) and, in the
following weeks, the ESO VLT and the Gemini South Telescope, found no sign of coma despite
optimal seeing conditions. Spectroscopic data obtained at around the same time showed no
evidence of identifiable gas emission in the visible wavelength region of the spectrum. Although
the object has a surface reflectivity similar to comets, all other observational evidence available
at the time suggested that ‘Oumuamua was likely inactive and of asteroidal nature, contrary to
the expectation that most interstellar objects are cometary. Additional data was subsequently
obtained with CFHT, VLT, and Hubble. A final set of images obtained with Hubble in early 2018
allowed the extract high-precision astrometry. The resulting dataset provided dense coverage
from the moment of discovery to January 2018, when the object became fainter than V~27 at a
heliocentric distance of 2.9 AU. The data analysis showed that the observed orbital arc could not
be fit in its entirety by a trajectory governed solely by gravitational forces due to the Sun, the
eight planets, the Moon, Pluto, the 16 biggest bodies in the asteroid main belt, and relativistic
effects.
The motion of celestial bodies is mostly governed by gravity, with non-gravitational effects
having been observed only for a limited number of Solar System objects. The detection, at 30σ
significance, of non-gravitational acceleration in the motion of ‘Oumuamua, made this the first
Figure A.2.1.2: This diagram shows the orbit of the interstellar object ‘Oumuamua as it passes through the Solar System. It shows the predicted
path of ‘Oumuamua and the new course, taking the new measured velocity of the object into account.
‘Oumuamua passed the distance of Jupiter’s orbit in early May 2018 and will pass Saturn’s orbit in January 2019. It will reach a distance
corresponding to Uranus’ orbit in August 2020 and of Neptune in late June 2024. In late 2025 ‘Oumuamua will reach the outer edge of the
Kuiper Belt, and then the heliopause — the edge of the Solar System — in November 2038.12
and only known object of interstellar origin to have entered the Solar System. The most plausible
physical model of the observed non-gravitational acceleration is likely that ‘Oumuamua behaves
like a comet of miniature size. By establishing the object as an icy body (albeit one with possibly
unusual dust properties), this scenario resolves the puzzle of the object’s apparent asteroidal
nature and reconciles ‘Oumuamua’s properties with predictions that only a small fraction of
interstellar objects are asteroidal (rocky-to-icy ratio in the 0.01% to 0.5% range). The lack of
observed dust lifted from the object the hypothesized cometary activity can then be explained
by an atypical dust grain size distribution that is devoid of small grains, smaller-than-usual pores
in the nucleus, a low dust-to-ice ratio or surface evolution from its long journey.
In addition to solving interstellar mysteries, Hubble has continued surveying the familiar ob-
jects in the Solar System, observing planets in special phases, such as opposition, and providing
long term monitoring of evolving phenomena, such as the Giant Red Spot (GRS) on Jupiter. In
Figure A.2.1.3 we show a recent image of Mars at opposition, taken in 2018.
Figure A.2.1.3: This annotated image of Mars shows features of the planet that were visible in summer 2018 despite
a global dust storm. During the time of observation it was spring in Mars’ southern hemisphere, where a dust storm
erupted and ballooned into a global event that is blanketing the entire planet. Even so, several distinctive features can
be identified.
The large oval area at the lower right is the bright Hellas Basin. About 2200 kilometres across and nearly eight
kilometres deep, it was formed about four billion years ago by an asteroid impact. Many global dust storms originate
in this region. The orange area in the upper centre of the image is Arabia Terra, a vast upland region in northern
Mars. The landscape is densely cratered and heavily eroded, indicating that it could be among the oldest terrains on
the planet.
South of Arabia Terra, running east to west along the equator, are the long dark features known as Sinus Sabaeus
and Sinus Meridiani. These regions are covered by dark bedrock and fine-grained sand deposits ground down from
ancient lava flows and other volcanic features. These sand grains are coarser and less reflective than the fine dust
that gives the brighter regions of Mars their rusty appearance. Because it is autumn in the northern hemisphere, a
bright blanket of clouds covers the north polar region. Clouds also can be seen over the southern polar cap. The two
small moons of Mars, Phobos and Deimos, appear in the lower half of the image.13
The advantage of having a flexible and powerful observatory such as Hubble in orbit for 28
years is that it can monitor evolving astronomical phenomena on timescales of decades, such
as the Giant Red Spot (GRS) on Jupiter. Through their monitoring program, the Outer Planet
Atmospheres Legacy (OPAL), Amy Simon and collaborators (2018) have observed the GRS with
Hubble since 1994 to present, complementing previous observations by Voyager, Galileo and
Cassini (Figure A.2.1.4). This wealth of data allowed Simon and collaborators to analyse trends
in the recent behaviour and characteristics of the GRS, further the mechanisms powering and
sustaining this long-lived storm. Simon and collaborators found that:
• As measured by colour and cloud patterns in optical images, the longitudinal length has
continued to decrease, as has the latitudinal width, with the GRS becoming smaller and
rounder over time.
• The westward drift rate, relative to the planetary rotation, has increased steadily since
~2005.
Figu re A . 2 .1.4: Images of
Jupiter’s Great Red Spot taken
by the Hubble Space Telescope
over a span of 20 years show that
the Great Red Spot is shrinking.14
• Since 2014, the GRS is darker at wavelengths shorter than 650 nm and shows less N/S
asymmetry over time. High-altitude structure may have also changed over that time,
causing it to be bright at methane-absorption wavelengths and darker in the UV.
• Changing size and internal wind speeds from 1979 to 2017 resulted in a decreased circula-
tion within the spot, even as its dynamical areas shrinks.
Finally, they found that the size and drift rate of the GRS are tightly correlated with its location
in the wind field and possibly with changes in background relative vorticity, as it deflects the
surrounding zonal winds. The most recent (2014–2017) changes in internal cloud morphology
and colour may be due to changes in divergence, internal vorticity, and vortex stretching rather
than being correlated to its drift rate.
A.2.2 Exploring Planets outside our Solar System
In recent years, exoplanet science has become a dominant subject in Hubble observations and
results, indicating how this versatile telescope has adapted to, and at some level, influenced,
the evolution of science. When Hubble was designed and built, the presence of planets around
nearby stars was suspected, but no detection had yet been made. In fact, one of the science
cases for the Faint Object Camera, the first generation Hubble instrument provided by ESA, was
indeed the search for sub-stellar companions to nearby stars. Fast forward to 28 years later, we
know of 3774 exoplanets, and Hubble has played a major role in the characterization of many of
their atmospheres. Here are some of the latest results:
Hubble finds Helium in the eroding atmosphere of an exoplanet
Helium is the second most abundant element in the universe after hydrogen and is a major
constituent of gas-giant planets in our Solar System. Early theoretical models predicted helium
to be among the most readily-detectable species in the atmospheres of exoplanets, especially
in extended and escaping atmospheres. However, searches for helium have been unsuccessful,
until the first detection was made, at a confidence level of 4.5σ in the atmosphere of WASP-107b,
one of the lowest density planets known (Spake et al. 2018). The near-infrared transmission
spectrum of the warm gas giant WASP-107b, taken with Hubble WFC3 during a primary transit,
Figure A.2.2.1: Combined near-infrared
a transmission spectrum for WASP-107b with
helium absorption feature. (a) Data plotted
on a linear scale. Points with 1σ error bars
are from a previous study and this work,
both corrected for stellar activity. The solid
purple line is the best fit lower atmosphere
retrieval model from, and the shaded pink
areas encompass 68%, 95% and 99.7% of
the samples. The gold line is the best-fit
helium 10,830 Å absorption profile from their
1-D escaping atmosphere model. (b) Same
as (a), on a log scale. The dashed blue line
shows the Roche radius (Spake et al. 2018).
b15 showed the narrow absorption feature of excited, metastable helium at 10,833 Å (Figure A.2.2.1). The amplitude of the feature, in transit depth, is 0.049±0.011% in a bandpass of 98 Å, which is more than 5 times greater than that which could be caused by nominal stellar chromospheric activity. The large absorption signal suggests that WASP-107b has an extended atmosphere that is eroding at a total rate of 1010 - 3 x 1011 g s-1 (0.1-4% of its total mass per Gyr), and may have a comet-like tail of gas shaped by radiation pressure. The Trappist-1 System Seven Earth-sized planets orbit the ultracool dwarf star Trappist-1 (Figure A.2.2.2). This makes Trappist-1 the planetary system with the largest number of Earth-sized planets discovered so far. In a wonderful collaboration between ground based and space observatories, these planets were discovered during their transit in front of the central star, by the ground-based TRAPPIST- South at ESO’s La Silla Observatory in Chile, TRAPPIST-North in Morocco, the orbiting NASA Spitzer Space Telescope, ESO’s HAWK-I instrument on the VLT at the Paranal Observatory in Chile, the 3.8-metre UKIRT in Hawaii, the 2-metre Liverpool and 4-metre William Herschel telescopes on La Palma in the Canary Islands; and the 1-metre SAAO telescope in South Africa (Gillon et al. 2017). The density measurements suggest that at least the innermost six planets are probably rocky in composition. The planetary orbits are not much larger than that of the Jupiter Galilean moon system and much smaller than the orbit of Mercury in the Solar System. However, TRAPPIST-1’s small size and low temperature mean that the energy input to its planets is similar to that received by the inner planets in our Solar System; TRAPPIST-1c, d and f receive similar amounts of energy as Venus, Earth and Mars, respectively. It was speculated that all seven planets discovered in the system could potentially have liquid water on their surfaces, though their orbital distances make some of them more likely candidates than others. Climate models suggested the innermost planets, TRAPPIST-1b, c and d, are probably too hot to support liquid water, except maybe on a small fraction of their surfaces. The orbital distance of the system’s outermost planet, TRAPPIST-1h, is unconfirmed, though it is likely to be too distant and cold to harbour liquid water — assuming no alternative heating processes are occurring. TRAPPIST-1e, f, and g, however, represent the holy grail for planet-hunting astron- omers, as they orbit in the star’s habitable zone and could host oceans of surface water. Hubble observed transits of four planets ( d, e, f, g) in the Trappist-1 system in 2017 (de Wit et al. 2018), using the ‘forward’ scanning mode with the near-infrared (1.1-1.7μm) G141 grism on Figure A.2.2.2: A size comparison of the planets of the TRAPPIST-1 system, lined up in order of increasing distance from their host star. The planetary surfaces are portrayed with an artist’s impression of their potential surface features, including water, ice, and atmospheres.
16
the WFC3, looking for presence of a hydrogen dominated atmosphere. Hydrogen is a powerful
greenhouse gas that might prevent habitability. An atmosphere largely dominated by hydrogen
should yield prominent signatures in the near-IR, detectable during transit. Observations of the
innermost planets ruled out such a signature. The conclusion therefore was that three of the
four planets - d e, and f - do not harbour a hydrogen dominated atmosphere and are potentially
habitable. Such an atmosphere cannot be excluded for planet g.
A.2.3 High precision astrometry to study stars, clusters and galaxies
HST has revolutionized the field of high-precision astrometry because of its long time-baseline
combined with the exquisite calibrations and well-characterised point-spread functions of the
ACS and WFC3 detectors. A point source can be measured in a single image to a precision
of 1/100th of a pixel or 0.4-0.5 milli-arcseconds. This precision has opened up many areas of
discovery from the motions of minor bodies in the Solar system, to the determination of stellar
parallaxes, to detecting the proper motions of jets in distant galaxies.
Einstein’s general theory of relativity was famously confirmed by observations of the grav-
itational deflection of starlight around the Sun during the 1919 total solar eclipse. Sahu and
collaborators (2017) used HST to measure the analogous process of astrometric microlensing
caused by a nearby star, the white dwarf Stein 2051 B. As Stein 2051 B passed closely in front of
a background star (Figure A.2.3.1), the background star’s position was deflected. Measurement
of a deflection of 2 milli-arcsec at eight epochs over 2 years provided a mass for Stein 2051 B
(the sixth-nearest white dwarf to the Sun) of 0.675 ± 0.051 solar masses. This mass determination
provides confirmation of the physics of degenerate matter and lends support to white dwarf
evolutionary theory.
The technique of spatial scanning with the WFC3/UVIS detector enables up to thousands
of more times sampling of a source and increases the ability to measure changes in a source’s
position to a precision of better than 20-40 micro-arcseconds. For comparison, the GAIA mission
is expected to measure parallaxes to a precision of 5-10 micro-arcsec by the end of the mission.
In two recent papers, Riess and collaborators (2018) used spatial scanning to measure parallaxes
Figure A.2.3.1: The close passage of the white dwarf Stein 2051 B (labelled source) in front of a distant star in shown.
Stein 2051 B is moving across the field in the direction of its proper motion towards the lower left-hand corner (cyan
line with wavy effects of the motion of the Earth around the Sun). The small blue squares mark the observing epochs.17 for seven long-period Cepheid variables and, in combination with GAIA DR2 data (see A.3.2), confirm the tension between the value for the Hubble constant measured locally and by the Planck mission from the cosmic microwave background (see also A.3.2). Precision astrometry providing the internal motions of stars in Globular Clusters has deep- ened our understanding of the components and evolution of these stellar systems. Tight limits have been placed on the mass of any intermediate-mass black hole in Omega Centauri. The existence of multiple populations in GCs was a major HST discovery 2 decades ago and is still unexplained. Stellar velocity dispersions of the multiple populations have been measured in several GCs. Clear differences are seen in the stellar motions in the outer regions, where the lower stellar density ensures that the kinematics have not been altered by two-body interactions. This fossil signature implies that the kinematics at birth of the multiple populations was different. Measurements of the absolute proper motions of GCs in the MW halo have provided the total MW mass, the survival expectancy based on the number of disk crossing times and the exact positions of likely halo streams based on the direction of motion of the GCs. Our understanding of the dynamics of our neighbouring galaxies has changed dramatically through HST precision astrometry. The measurement of the proper motions of the Magellanic Clouds, our two closest satellite galaxies, revealed that their 3D velocities are much higher than previously thought and approach the escape velocity of the Milky Way. This discovery upended the view that the Magellanic Clouds have been steadily orbiting the MW for a Hubble time. Instead, the HST astrometric results show that they are likely on their first infall about the MW or on an eccentric, long period (> 6 Gyr) orbit. The measurement of the proper motion of the nearest giant spiral M31 has also yielded surprises. By using a 5 year baseline and measuring the positions of thousands of M31 stars relative to stationary background galaxies, the proper mo- tion of M31 was measured to an accuracy of 12 micro-arcsec/year. The resulting velocity vector of M31 shows that it is heading directly towards the MW and will collide in 6 Gyr. Modelling of the collision shows that this will be the next major cosmic event to affect the environment of the Sun because it will be kicked out to orbit the merger remnant at a much larger radius. A.2.4 The distant Universe Hubble discovers the most distant star Kelly and collaborators (2018) recently discovered an individual star at redshift z=1.5 extremely magnified by a galaxy cluster. Galaxy-cluster gravitational lenses can magnify background galaxies by a total factor of up to ~50. This star, MACS J1149 Lensed Star 1 (LS1), was magnified by a factor >2000. A separate image, detected briefly 0.26” from LS1 is likely a counterimage of the first star demagnified for multiple years by a >3 Mo object in the cluster. Microlensing fluc- tuations in the stars’ light curves yield evidence about the mass function of intracluster stars and compact objects, including binary fractions and specific stellar evolution and supernova models. Dark-matter subhalos or massive compact objects may help to account for the two images’ long- term brightness ratio (Figure A.2.4.1). Hubble provides a precise extragalactic test of General Relativity General Relativity (GR) postulates that mass deforms space-time, such that light passing near to a massive object is deflected. If two galaxies are almost perfectly aligned, the deformation of space-time near the centre of the foreground galaxy can be large enough that multiple images of the background galaxy are observed. Such alignments are called strong gravitational lenses. In the case of a spherical foreground lens and a perfect alignment of lens and source, the background galaxy is distorted into an Einstein ring. The radius of this ring, the Einstein radius, is a function of the mass of the lens, the amount of spatial curvature produced per unit mass and a ratio of three angular diameter distances between the observer, lens and source. Angular diameter distances are calculated from the redshifts of the lens, and source, and the cosmological parameters of our Universe. Therefore, the combination of a non-lensing measure- ment of the mass of a strong lensing galaxy and a measurement of the Einstein radius constrains the amount of spatial curvature produced per unit mass and tests if GR is the correct theory of gravity.
18
Figure A.2.4.1: This image composite shows the discovery of the most distant known star using Hubble. The image to
the left shows a part of the deep-field observation of the galaxy cluster MACS J1149.5+2223 from the Frontier Fields
programme gathered in 2014. The square indicates the position where the star appeared in May 2016 — its image
magnified by gravitational microlensing. The upper right image pinpoints the position of the star, observed in 2011. The
lower right image shows where the star was undergoing the microlensing event in late May 2016.
Einstein’s theory of gravity, General Relativity, has been precisely tested on Solar System
scales, but the long-range nature of gravity is still poorly constrained. The nearby strong gravita-
tional lens, ESO 325-G004, has provided a laboratory to probe the weak-field regime of gravity
and measure the spatial curvature generated per unit mass, γ (Figure A.2.4.2). By reconstructing
the observed light profile of the lensed arcs and the observed spatially resolved stellar kinemat-
ics with a single self-consistent model, Collett and collaborators (2018) concluded that γ = 0.97 ±
0.09 at 68% confidence. Their result is consistent with the prediction of 1 from General Relativity
and provides a strong extragalactic constraint on the weak-field metric of gravity.
Hubble gives the most precise measurement of the Universe expansion rate.
Using new high precision astrometric measurements of the parallax of 7 long-period (≥ 10
days) Milky Way Cepheid variables (SS CMa, XY Car, VY Car, VX Per, WZ Sgr, X Pup and S Vul),
taken with the spatial scanning technique of WFC3 over three years, Riess et al. (2018) have been
able to address two outstanding systematic uncertainties affecting their prior comparisons of
Milky Way and extragalactic Cepheids: their dissimilarity of periods and photometric systems.
Comparing the new parallaxes to their predicted values derived from reversing the distance
ladder (Figure A.2.4.3) gives a ratio (or independent scale for Ho) of 1.034 ± 0.036, consistent
with no change and inconsistent at the 3.3σ level with a ratio of 0.91 needed to match the value
predicted by Planck CMB data in concert with ΛCDM. Using these data instead to augment Riess
and collaborators (2016) measurements of Ho improves the precision to 2.3%, yielding 73.45
+- 1.66 km s-1 Mpc-1, and the tension with Planck + ΛCDM increases to 3.7σ. The future combi-
nation of GAIA parallaxes and Hubble spatial scanning photometry of 50 Milky Way Cepheids
can support a < 1% calibration of Ho.19
Figure A.2.4.2: An image of the nearby galaxy
ESO 325-G004, created using data collected
by Hubble and the MUSE instrument on the
ESO’ VLT. MUSE measured the velocity of
stars in ESO 325-G004 to produce the velocity
dispersion map that is overlaid on top of the
Hubble Space Telescope image. Knowledge of
the velocities of the stars allowed the astrono-
mers to infer the mass of ESO 325-G004. The
inset shows the Einstein ring resulting from
the distortion of light from a more distant
source by intervening lens ESO 325-004,
which becomes visible after subtraction of
the foreground lens light.
Figure A.2.4.3: This illustration shows the three steps astronomers use to measure the Universe’s expansion rate to an
unprecedented accuracy, reducing the total uncertainty to 2.3 percent.
Beginning at left, astronomers use Hubble to measure the distances to Cepheid variables. Astronomers then
move beyond the Milky Way to nearby galaxies (shown at centre). They look for Cepheid stars in galaxies that
recently hosted another reliable yardstick, Type Ia supernovae. The astronomers use the Cepheids to measure the
luminosity of the supernovae in each host galaxy.
They then look for supernovae in galaxies located even farther away from Earth. Unlike Cepheids, Type Ia
supernovae are brilliant enough to be seen from relatively longer distances. The astronomers compare the luminosity
and apparent brightness of distant supernovae to measure out to the distance where the expansion of the universe
can be seen. They use these two values to calculate the Hubble constant.20
A.3 Synergy with Other Missions
Astronomers use Hubble to obtain UV, optical, and infrared observations of objects detected
at other wavelengths, to survey fields and objects for follow-up by other facilities in space and on
the ground, and to push the limits of observations pioneered by smaller observatories or pilot
studies on the ground. Hubble’s location above the atmosphere, unique access to the UV, low
sky backgrounds in the near-IR, diffraction-limited imaging in the optical, well-known and stable
point spread function, and pointing stability make it a natural complement for astrophysical stud-
ies conducted by other observatories. Opportunities to propose joint HST observing programs
with Chandra or Spitzer have been possible for many years. Beginning in Cycle 20, a similar
arrangement was established for XMM-Newton, and starting in Cycle 22 a reciprocal agreement
with NRAO has taken effect. Such agreements foster cross-observatory cooperation and remove
the double jeopardy that proposals seeking coordinated observing time would otherwise face in
two independent, oversubscribed peer reviews.
Hubble also adds its unique capabilities to coordinated science programs and spacecraft
trajectory updates for NASA and ESA planetary missions, such as New Horizons or Juno. In the
past, the most compelling multi-observatory efforts anchored by Hubble were the deep-field
surveys. Following the initial Hubble Deep Field observations, the UDF, GOODS, and COSMOS
fields were also studied extensively by XMM- Newton and Herschel, and deep spectroscopic
follow-ups have been obtained with the VLT. This tradition carried over into the Multi Cycle
Treasury (MCT) programs and continued with the new Frontier Fields. These programs have
exemplified how astronomers have become adept at designing multi-wavelength observing
campaigns around Hubble’s unique capabilities.
The recent discovery of gravitational waves from astronomical phenomena has just opened a
new bright chapter in this decade long multi mission, multi telescope collaboration.
A.3.1 Hubble is a key player in multi-messenger astronomy
Multi-messenger astronomy is based on the coordinated observations and interpretation of
different messenger signals, with the four extrasolar messengers being electromagnetic radia-
tion, gravitational waves, neutrinos, and cosmic rays. They are created by different astrophysical
processes, and thus reveal different information about their sources. The main multi-messenger
sources outside the heliosphere are expected to be compact binary pairs (black holes and
neutron stars), supernovae, irregular neutron stars, gamma-ray bursts, active galactic nuclei, and
relativistic jets.
On 17 August 2017 the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the
Virgo Interferometer both alerted astronomical observers all over the globe about the detection
of a gravitational wave event named GW170817. About two seconds after the detection of the
gravitational wave, ESA’s Integral and NASA’s Fermi Gamma-ray Space Telescope observed a
short gamma-ray burst in the same direction. In the night following the initial discovery, a fleet
of telescopes started their hunt to locate the source of the event. Astronomers found it in the
lenticular galaxy NGC 4993. A point of light was visible where nothing was seen before and this
set off one of the largest multi-telescope observing campaigns ever — among these telescopes
was Hubble.
Several teams of scientists (Tanvir et al. 2018; Levan et al. 2018) used Hubble over the two
weeks following the gravitational wave event alert to observe NGC 4993. Using Hubble’s
high-resolution imaging capabilities, they managed to get the first observational proof for a
kilonova, the visible counterpart of the merging of two extremely dense objects — most likely
two neutron stars. Such kilonova would be powered by the radioactive decay of massive neu-
tron- rich nuclides created via r-process nucleosynthesis in the neutron-star ejecta.
Such mergers were first suggested more than 30 years ago but this was the first firm observa-
tion of such an event. The distance to the merger makes the source both the closest gravitational
wave event detected so far and also one of the closest gamma-ray burst sources ever seen.
Hubble captured images of the galaxy in visible and infrared light, witnessing a new bright
object within NGC 4993 that was brighter than a nova but fainter than a supernova. The images21
Figure A.3.1.1: On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo
Interferometer both detected gravitational waves from the collision between two neutron stars. Within 12 hours ob-
servatories had identified the source of the event within the lenticular galaxy NGC 4993, shown in this image obtained
by Hubble. The associated stellar flare, a kilonova, is clearly visible in the Hubble observations. This is the first time the
optical counterpart of a gravitational wave event was observed.
Hubble observed the kilonova gradually fading over the course of six days, as shown in these observations taken
in between 22 and 28 August (insets).
showed that the object faded noticeably over the six days of the Hubble observations (Figure
A.3.1.1). Using Hubble’s spectroscopic capabilities the teams also found indications of material
being ejected by the kilonova as fast as one-fifth of the speed of light.
Connecting kilonovae and short gamma-ray bursts to neutron star mergers has so far been
difficult, but the multitude of detailed observations following the detection of the gravitational
wave event GW170817 has now finally verified these connections. The infrared spectra taken
with Hubble also showed several broad features that signal the formation of some of the heav-
iest elements in nature. These observations may help solve another long-standing question in
astronomy: the origin of heavy chemical elements, like gold and platinum. In the merger of two
neutron stars, the conditions appear just right for their production.22
A.3.2 Synergy with GAIA
The astrometric accuracy of pipeline-calibrated HST observations is limited by the quality of
the positions of the guide stars used, which have uncertainties of 0.05 to 0.15 arcseconds from
the current operational Guide Star Catalogue 2 (GSC2). The GSC2 has been in use for over a
decade and is gradually starting to ‘age’ due to the combination of inherent positional errors (up
to 0.15-0.2 arcsec) and 20+ years of proper motions. With the availability of the GAIA astrometric
catalogue, a project has been initiated to both update the GSC2 (version 2.3.3) to the GAIA
reference frame, and update the absolute astrometry of the HST observations in the archive. The
GSC2 has been cross-matched to the GAIA DR1 catalogue and the improved astrometry for all
matched objects has been transferred into the new GSC Version 2.4.1. Not only are the astro-
metric errors reduced to23
3D Motions in the Sculptor dwarf galaxy
The dwarf galaxy Sculptor is one of the best studied systems and inferred to be strongly dark
matter dominated. However, there are conflicting reports on its mean motion around the Milky
Way and the 3D internal motions of its stars have never been measured. Based on data from the
GAIA space mission and the Hubble, Massari and collaborators (2018) obtained a new precise
measurement of Sculptor’s mean proper motion. From this new measurement, they deduced
that Sculptor is currently at its closest approach to the Milky Way and moving on an elongated
high-inclination orbit that takes it much farther away than previously thought. For the first time,
they were also able to measure the internal motions of stars in Sculptor (Figure A.3.2.2). They
find σR=11.5±4.3 km/s and σT=8.5±3.2 km/s along the projected radial and tangential directions,
implying that the stars in their sample move preferentially on radial orbits as quantified by the
anisotropy parameter, which they find to be β∼0.86+0.12−0.83 at a location beyond the core
radius. Taken at face value such a high radial anisotropy requires abandoning conventional
models for the mass distribution in Sculptor. Their sample is dominated by metal-rich stars and
for these they find βMR∼0.95+0.04−0.27, a value consistent with multi-component models where
Sculptor is embedded in a cuspy dark halo as expected for cold dark matter.
Figure A.3.2.2: This image shows a small
part of the Sculptor Dwarf Galaxy, a satel-
lite galaxy of the Milky Way. This is one of
two different pointings of the telescope
that were used in a study combining data
from Hubble and ESA’s Gaia satellite to
measure the 3D motion of stars in this
galaxy.24
Hubble and GAIA pushing the precision on the measurements on the expansion of the
Universe
In their quest of determining the most accurate value for H0, Riess and collaborators (2018)
have combined Hubble and GAIA DR2 data. They started from the photometry of a selected
sample of 50 long-period, low-extinction Milky Way Cepheids measured on the same WFC3
F555W-, F814W-, and F160W-band photometric system as extragalactic Cepheids in Type Ia
supernova host galaxies. These bright Cepheids were observed with the WFC3 spatial scanning
mode in the optical and near-infrared to mitigate saturation and reduce pixel-to-pixel calibration
errors to reach a mean photometric error of 5 mmag per observation. Riess and collaborators
then used the new GAIA DR2 parallaxes and HST photometry to simultaneously constrain
the cosmic distance scale and to measure the DR2 parallax zeropoint offset appropriate for
Cepheids. They find the latter to be −46 ± 13 μas or ±6 μas for a fixed distance scale, higher
than found from quasars, as expected for these brighter and redder sources. The precision
of the distance scale from DR2 has been reduced by a factor of 2.5 because of the need to
independently determine the parallax offset. The best fit distance scale is now 1.006 +-0.0033,
relative to the scale from Riess and collaborators with H0 = 73.24 km s-1 Mpc-1 used to predict
the parallaxes photometrically, and is inconsistent with the scale needed to match the Planck
2016 cosmic microwave background data combined with ΛCDM at the 2.9σ confidence level
(99.6%). Including the DR2 parallaxes with all prior distance-ladder data raises the current tension
between the late and early universe route to the Hubble constant to 3.8σ (99.99%). With the
final expected precision from GAIA, the sample of 50 Cepheids with HST photometry will limit
to 0.5% the contribution of the first rung of the distance ladder to the uncertainty in H0 (Figure
A.3.2.3).
Figure A.3.2.3: Using two of the world’s
most powerful space telescopes — Hubble
and GAIA — astronomers have made the
most precise measurements to date of the
universe’s expansion rate. This is calculated
by gauging the distances between nearby
galaxies using special types of stars called
Cepheid variables as cosmic yardsticks.
By comparing their intrinsic brightness as
measured by Hubble, with their apparent
brightness as seen from Earth, scientists
can calculate their distances. GAIA further
refines this yardstick by geometrically
measuring the distances to Cepheid var-
iables within our Milky Way galaxy. This
allowed astronomers to more precisely
calibrate the distances to Cepheids that
are seen in outside galaxies.25 A.4 New Initiatives Offered A.4.1 New Hubble Observing Modes While the instruments on Hubble are mature, and relatively well understood, new initiatives are continuously implemented to augment the scientific productivity and efficiency of the observato- ry. Innovative new observing modes are being explored. A.4.1.1 New COS/FUV Central Wavelengths Two new central wavelengths (cenwaves) are being offered in Cycle 26 to obtain COS/FUV observations: the 800Å cenwave for use with the G140L grating and the 1533Å cenwave for use with the G160M grating (Figure A.4.1.1.1). The G140L/800 configuration allows for contiguous coverage of the entire FUV spectral region from 800 to 1950 Å on a single detector segment (FUVA). The low spectral height below 1150 Å allows to achieve higher S/N which is critical for observations of faint astronomical object that are limited by the instrument background. This setting will be particularly useful for background-lim- ited observations of faint targets at the bluest wavelengths of the spectral range covered with this new setting. The G160M/1533 configuration extends coverage at the short-wavelength end of the G160M grating by 44 Å. This allows for an overlap with the longest wavelengths covered by G130M/1222 configuration, and is otherwise expected to have similar spectral resolution and sensitivity to the existing G160M/1577 configuration. With this new setting, a broad range of FUV wavelengths can now be covered at good spectral resolution with only two cenwave settings (G130M/1222+G160M/1533) allowing the full COS FUV range to be covered with an efficient use of Hubble orbits. Most importantly, with these two configurations the COS FUV detectors are not exposed to the damaging effects of Lyman-alpha photons, the major cause of local gain sag for the COS/FUV channel. The coverage of these two new modes compared with the existing COS modes is shown at: http://www.stsci.edu/~COS/waveranges.html. The calibration of these new modes is ongoing for usage with data obtained in Cycle 26. A.4.1.2 The STIS Spatial Scanning Capability Spatial scanning with the STIS CCD is a recently enabled, available-but-unsupported mode for obtaining high S/N ratio spectra of relatively bright targets by trailing the target in the spatial direction within one of the long STIS apertures (Figures A.4.1.2.1 and A.4.1.2.2). Scientific applications include the reliable detection of weak stellar and interstellar absorption features (particularly in the red and near-IR where ground-based observations can be severely compromised by strong telluric absorption) and the accurate monitoring of stellar fluxes (both broad-band and in narrower spectral intervals) for characterizing transiting exoplanets. Additionally, spatial scanning offers the potential advantages of improved flat fielding, improved fringe removal (beyond about 7000 Å), and better correction for cosmic rays, compared to the previously used method of deliberately saturating the CCD detector at a fixed pointing. The calibration of this new STIS CCD mode is ongoing for usage with data obtained in Cycle 26. A.4.2 New Science Initiatives A.4.2.1 The Gap Filler Initiative Each HST cycle, around 2-3% of orbits are unused because of slew time constraints. In Cycle 24, a pilot programme using ACS/WFC was initiated to fill these observing gaps and covered 500 unobserved galaxies in the NGC catalogue evenly distributed over the sky. This programme, with no proprietary period, was highly successful with typically 10 observations per month using a single filter (F606W) and two short exposures. The host galaxy of the multi-messenger event (see A.3.1) NGC 4493 was fortuitously observed by this programme a few months before the
26
A.4.1.1.1: Schematic view of the wavelength coverage offered by the COS FUV detectors (segments A and B) onboard Hubble. The two newly
commissioned central wavelengths are labelled in green.
merger of two neutron stars, and provided a much-used pre-discovery image. The success of
the ACS pilot programme led to a community call for “gap filler” proposals in Summer 2017. A
total of 53 proposals were submitted and 3 were chosen to execute on the telescope. These
programmes are based on large catalogues of galaxies previously unobserved by HST, and cover
nearby active galactic nuclei, unusual galaxies from the Galaxy Zoo project, and interacting Arp
galaxies. It is expected that these programmes will be active for 2-3 years.
A.4.2.2 Fundamental Physics with HST
Over the last three decades the Hubble Space Telescope has played a crucial role in probing
key parameters relevant to fundamental physics and cosmology. The H0 key project figured
prominently in the early years, and subsequent programs have reduced measurement uncertain-
ties to less than 3%. More recently, Hubble has investigated other parameters, including testing
the nature of dark matter through observations of merging galaxy clusters and using white dwarf
spectra to constrain the gravity dependence of the fine structure constant.
Recently, the STScI Director convened a working group drawn from the physics and cosmology
communities to provide advice on how Hubble might contribute to future investigations in
fundamental physics. The committee was chartered to:
• Explore how HST observations in the coming years can be applied directly to refining
measurements of key parameters;
• Determine how HST observations can complement and supplement observations by other
facilities; Identify key legacy observations that HST should obtain to lay the foundation for
future experiments in fundamental physics;
• Consider and recommend implementation strategies that could be applied to enabling
more effective use of HST in all of these areas.
The committee consulted with members of the community and submitted a final report in
November 2017. Their report describes a number of ways in which Hubble observations can
provide new insights in fundamental physics, including improving the determination of the
Hubble constant, improving the calibration of Cepheids at the distance of SNIae, calibrating the
distance scale with tip of the Red Giant branch (TRGB) stars and strong lens time delays. Their
recommendation is that the Director allocate observing time over the next three cycles to anYou can also read
