DESI Legacy Imaging Surveys - On behalf of the imaging teams Yichang 2019.10.18 Hu Zou ( @ NAOC) - lamost
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DESI Legacy Imaging Surveys
Hu Zou ( @ NAOC)
On behalf of the imaging teams
Yichang
2019.10.18
zouhu@nao.cas.cn
1
The DESI project q Dark Energy Spectroscopic Instrument (DESI) is a stage- IV cosmological spectroscopic survey. q obtain optical spectra of 35 million galaxies and quasars q measure the expansion history of the Universe q measure the structure growth in the Universe q FoV: 3.2 degrees q 5000 fibers q Three-channel spectrograph q Wavelength range: 360-980nm q Resolution: ~3000 @550 nm
Spectroscopic Targets
Galaxy targets
Spectroscopic targets: MWS
qMWS: Milky Way Survey
q Share bright time with BGS, 1/3 of a month
q targeting about 10M objects;
q Composition (thin, thick and halo), structure dynamics and origins of the
outer Galaxy, spectroscopic complement to Gaia.
q Select targets using the DESI imaging surveys, together with others,
such as JPLUS and Gaia
q Standard stars
q Rare stellar types
q White dwarfs
q Main-sequence, turnoff stars,
giants
Simple selection: complete
and no bias
16
DESI schedule
Milestones Date
Start of Installation Feb. 2018
Commissioning Instrument Install Oct. 2018
End of imaging surveys Mar. 2019
Start of Commissioning Oct. 2019
End of Commissioning (ends project) Feb. 2020
Survey Validation Survey Begins Feb. 2020
Science Survey Begins Jun. 2020
Data Assembly (DA1-DA5) Apr. 2021-2025
Science Survey End Jun. 2025
Final Data Assembly Mar. 2026
(
DESI imaging surveys
q Target selections based on three optical surveys and one
near-infrared survey
q Requirements: g=24, r=23.4, and z=22.5
q Baseline survey area: 14000 deg2
Survey Institute Telescope Filter Area Depth (AB mag)
BASS NAOC Bok 2.3m gr 5100 g=24.0, r=23.5
MzLS NOAO Mayall 4m z 5100 z=22.9
g=24.5, r=23.9,
DECaLS LBNL Blanco 4m grz 9000
z=22.9
unWISE DESI WISE W1,W2 Full sky W1=20.7, W2=20.0
q DECaLS = Dark Energy Camera Legacy Survey (Blanco/DECam)
q MzLS = MOSAIC z-band Legacy Survey (Mayall/MOSAIC-z)
q BASS = Beijing-Arizona Sky Survey (Bok/90Prime)
Footprints
BASS MzLS Blanco
Bok 90Prime Mayall MOSAIC-3
DECaLS
DECam
DESI Imaging Footprint
7
7
unWISE q New coadds from AllWISE and latest observations (NEOWISE) of WISE W1 and W2 qForced photometry based on shape parameters from optical bands q low resolution of about 6”; 5-yr deep stacks q Deeper than official AllWISE: 1-1.5 mag deeper q More complete than the official catalog q Consistent photometry with optical bands q WISE W1 and W2 are key for the target selections of LRGs and quasars
Time-resolved WISE coadds
Credit: Aaron Meisner
q A given sky region is imaged once every ~6 months, for a
time span of ~1 day, during which ~12 exposures are acquired.
q 11-epoch light curve per band per object from the coadds
Optical imaging surveys q Beijing-Arizona Sky Survey (BASS): q Collaboration between NAOC and UA q Co-PIs: Xu Zhou (NAOC), Xiaohui Fan (UA) q Duty PI: Hu Zou (NAOC) q Data and science teams from collaborators in China and DESI q 2.3m Bok telescope q g and r bands q 5100 deg2 q 387 nights q 2015.1-2019.3 q Common survey strategy with DECaLS and MzLS: three dithered passes q Pass 1: photometric and seeing < 1.7” (1.3”) q Pass 2: photometric or seeing < 1.7” (1.3”) q Pass 3: other weather conditions q Sky brightness conditions: q g dark nights q r dark/grey nights q z bright nights q Dynamic exposure time calculator ensures the imaging depths for different passes
q All optical observations were completed (Mar. 2019): 2015-2019 q BASS 397 nights, Jan 2015 — Mar. 2019 q MzLS 401 nights, Dec 2015 — Feb. 2018 q DECaLS 204 nights, Aug 2014 — Mar. 2019 q 5-yr new WISE coadds using WISE data taken as of 2018
Data Releases
qBASS releases:
q 2015 Dec. Eearly Data Release (EDR)
q 2017 Jan. First Data Release (DR1)
q 2017 Dec. Second Data Release (DR2)
More observations
q 2019 Aug. Third Data Releae (DR3)
q http://batc.bao.ac.cn/BASS/
Updated pipelines
qDESI releases
q DR1-DR3: DECaLS+unWISE
q DR4: BASS+MzLS+unWISE Better data quality
q DR5: DECaLS+unWISE
q DR6: BASS+MzLS+unWISE
q DR7: DECaLS+unWISE
q DR8: BASS+MzLS+DECaLS+unWISE (2019. Jun)
q http://legacysurvey.org/Legacy survey DR8
qBoth north and south sky
q About 20,000 deg2
q 5 filters: g,r,z,w1 and w2
q ~2 mag deeper than SDSS
q 1-1.5 mag deeper than
AllWISE
q Include 5000 deg2 from
DES
q Best seeing at z band for
best shape measurementsWhat’s in DR8? q Combined footprint for the first time! q DR8: DR8-north & DR8-south q Overlap region around dec=32 degrees (32.275) q Coverage reaching >100% (!) of 14k goal in individual bands, including additional 5000 deg2 from DES q Larger than 1.6 billion unique sources q Photometric catalogs from Tractor with object types: q PSF: point source (stars & QSOs) q REX: round exponential q EXP: exponential (w/ ellipticity) q DEV: Devaucouleurs profile q COMP: composite with EXP+DEV components q Forced photometry for optical grz and WISE W1-W4 q Light curve of 11 epoches for WISE W1 and W2 (from 2010-2018)
Sky viewer qhttp://legacysurvey.org/viewer
Some work based on the imaging data
p Low surface brightness galaxies and Ha emitters: Shi et
al. 2017, Zaritsky et al. 2019, Omkar Bait et al. 2019
p High-redshift quasars: Wang et al. 2017,2018, Yang, et
al. 2017,2018
p Scaling relation of star forming galaxies: Gao et al.
2019, Huang et al. 2019
p Strong lensing: Shu et al. 2018, Wang et al.2019
p Photo-z and galactic clusters: Hu et al. 2019
p Light variability: Garling et al. 2018, Li et al, 2018
)Substructures/stellar streams
p About 2 mag deeper and wider than the SDSS:
© /1 5 5 1 1G 5 5 1 51 E5 5 5
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Credit: V. Belokurov and the Sloan Digital Sky Survey.Proper motion p relative to SDSS: >10-year time lag between SDSS and DESI; pStars with high proper motions pCatalog of star proper motion (r down to ~22 mag), used for the MW dynamics.
Variable objects
p Comparing SDSS photometry or using WISE light curves
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WISE light
variabilityInteresting objects from unWISE
p Finding the Coldest, Nearest Brown Dwarfs
prior WISE motion searches
A possible high-velocity
brown dwarf
q µ = 1.9”/yr
q color-based spectral type
estimate (~T5) implies
distance of ~49 pc
q => extremely high Vtan ~
450 km/s
Co-mover to primary with
Gaia DR2 parallax
q primary is known L1
brown dwarf at 24 pc.
~1.4 mags deeper q Absolute W2 mag of co-
~6.5 x larger volume mover: T9 spectral type;
q projected separation of
326 AUSummary
p DESI imaging surveys provide very wide and deep legacy imaging
data, which can be used to explore the Milky Way:
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p Combining both deep imaging and LAMOST/DESI spectra is good for
studying the MW, including the substructure of the halo, satellite
dwarf galaxies, different stellar populations, interesting variable
objects, proper motion, moving objects, etc.
p Encourage people to dive into this unprecedented data set,Survey Institute Telescope Filter Area Depth (AB mag)
BASS NAOC Bok 2.3m gr 5100 g=24.0, r=23.5
MzLS NOAO Mayall 4m z 5100 z=22.9
g=24.5, r=23.9,
DECaLS LBNL Blanco 4m grz 9000
z=22.9
unWISE DESI WISE W1,W2 Full sky W1=20.7, W2=20.0Spectrograph
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