1. LAMOST 补充星系样本和LAMOST-SDSS星系对样本
沈世银
上海天文台

2. LAMOST 星系巡天计划 North Galactic Cap South Galactic Cap
Complementary galaxy sample
South Galactic Cap
Main galaxy sample
Emission line galaxy
Two complete fields(Wu Hong)

3. Complementary galaxy sample
SDSS main sample galaxies
r<17.77
Spectroscopic completeness >90%
~7% missed due to fiber collision effect
Input catalog of LAMOST
68,722 objects
Biased towards (small scale) high density environment, e.g. galaxy pair, triplet/quartlet, compact group

4. Galaxy pair sample in SDSS are strongly biased
towards large separation and low redshift

5. Current status Complementary galaxy with LAMOST spectra
DR2 ( ): 3456
Why very small fraction?
Lower priority than QSO targets
Few Medium plates
Low S/N
Y3 ( ): 2620

6. Redshift measurements
In DR2, 3456 complementary galaxies have LAMOST spectra
Only 1951 have redshifts measured by LAMOST 1-D pipeline
We developed a new PCA-redshift measurement code
2796 with redshift measured, increased about 25%

7. Early science: new galaxy pairs
Galaxy pairs are one of the key ingredients to study the galaxy merging and galaxy interaction.
How to define a galaxy pair?
Non-trivial
P<100kpc dV<500km/s

8. Galaxy pair sample (Shen et al., 2015)
1,102 galaxy pairs (P<100kpc, dV<500km/s)
Visual inspection

9. Multiple flag Pair definition: dV<500km/s, Rp<100kpc
Physical pair ?
interaction
Multiple system: triplet, group,cluster
One of the member within another pair
Not complete
Redshift dependent
A triplet system with M =-20,-21,-22 mag would be identified as a triplet, a pair and a single galaxy in the SDSS MG sample (r<17.77) at redshift z=0.05,0.1,0.15 respectively.

10. Overlap flag Visual inspection Interesting objects
Very close pair (image overlap each other)
Photometry not accurate
Color bias
Interesting objects
Example: an isolated triplet (Feng et al. 2015)
See

11. Discovery of an isolated galaxy triplet (Feng et al. 2015)
Chain-like structure
Formed in early time
Early type galaxies with Halpha emission
recent star formation

13. Other applications and future
Supply the galaxy pair sample
Supply redshifts to the galaxy group catalog of Yang et al.
Small-scale galaxy cross-correlation function (Guo Hong)
Future
How much can we finish the spectral survey of the complementary galaxies?
After three years (6000 of 6,8000), only two years left.
LAMOST spectral survey 2? (after 2017)

14. Two complete fields Ra = 37.8, Dec = 3.4 Ra = 21.5, Dec = −2.20
r<18.1 mag
~5000 galaxies, point sources
28 plates observed from
Yang et al. in preparation

15. South Galactic Cap Main goal of the galaxy survey in design Footprint
Winter season, good weather
Footprint
Dec>-10 & b<-30, 3700 sq deg
Input catalog
Main sample galaxies (r<18) ~660 k
Blue galaxies (18<r<18.8) ~200 k
Science motivation
Large galaxy spectroscopic sample in SGC, deeper than SDSS main sample galaxies
Probe the faint (higher z) star forming galaxies

16. Current status
Only 35,000 (<5%) objects observed in past four years

17.  We can not finish the galaxy survey in SGC as we designed in 5 years (2012-2017)

18. How is the spectra quality?

19. Reasons for the small number of galaxies being observed
Fiber assignment
Galaxy PRI < QSO
QSO candidates outnumbers galaxy
Will be changed
Small number of M plates in EG area
Could not be greatly improved because of requirement of successful rate of spectroscopy
Weather condition
Other survey (Galactic Anti Center , M31)

20. Possible Collaboration: TAIPAN
Measure redshifts for 500,000 galaxies(r~17) and peculiar velocities for 50,000 galaxies in the nearby universe
on the UKST over a 3.5-year period ( )
Scientific goal
Hubble constant
Bulk flow
Velocity power spectrum
Redshift space distortions
Galaxy evolution
LAMOST: extend the Taipan survey into the non-SDSS areas of the northern hemisphere?
Spectra quality?

21. summary
There are only about 50,000 galaxies that had been observed by LAMOST in its first three years
~6000 SDSS missed main sample galaxies
Useful for new galaxy pair identification
~5000 galaxies in two small complete fields
~35000 galaxies in South Galactic Cap
Other ~4000 bright galaxies (replicated observation)
The spectra quality is barely acceptable(S/N ~10) for galaxies brighter than r<17 mag
Redshift measurement
Stellar population?