Two-halo Galactic Conformity as a Test of Assembly Bias

Transcription

1 Two-halo Galactic Conformity as a Test of Assembly Bias Angela Berti UC San Diego GalFRESCA

2 Galactic Conformity Correlation between star formation rates (SFRs) of central galaxies and their satellites/neighbors One-halo vs. two-halo scales How do galaxies know about central SFRs in other halos? one-halo scale { two-halo scale { 2

3 Galactic Conformity One-halo term in SDSS using group catalogs (Weinmann et al. 2006) Two-halo term in SDSS with isolation criteria (Kauffmann et al. 2013) Star-forming Satellite Fraction Weinmann et al. (2006) Kauffmann et al. (2013) 10.0 < log M * < 10.5 Projected Radius (Mpc) Halo Mass Standard halo model of galaxy evolution does not explain two-halo conformity. log SFR/M * (fib) 3

4 Halo assembly bias Assembly Bias Older halos less clustered than younger halos at a given halo mass Predicted by cosmological simulations Galaxy assembly bias? What besides its mass affects a halo s galaxy population? (and how?) Youngest 20% of Halos Oldest 20% of Halos Millenium Simulation 500 x 500 x 30 Mpc 3 4 Gao et al. (2005)

5 Assembly Bias Large-scale structure Large-scale tidal fields Halo accretion conformity (dark matter accretion rate correlated with local density of halos) + Correlated gas accretion Two-halo galactic conformity 5 Hearin et al. (2015)

6 Halo Assembly Bias Simulations predict halo accretion conformity (Hearin et al. 2016) Stronger for lower halo masses Weaker with increasing redshift and mostly gone by z~1 Testable to z~1 with current surveys? Hearin et al. (2015) 6

7 PRIsm MUlti-object Survey (PRIMUS) Low-resolution spectroscopic galaxy redshift survey 120,000 galaxies at 0.2 < z < deg 2 to depth of i ~ 23.5 Specific Star Formation Rate Relative Distance (Mpc) Stellar Mass 7 LOS Distance (Mpc)

8 Conformity in PRIMUS 5% one-halo conformity signal (> 3-sigma) Marginal 1% detection at two-halo scales Berti, Coil et al. (2017) Star-forming satellite fraction 8 Projected Radius (Mpc)

9 Conformity in PRIMUS SYSTEMATIC ERROR Small effect with large systematic errors. Differences in stellar mass and redshift can mimic a conformity signal. Solution: match quiescent and star-forming galaxy M * and redshift distributions (at expense of sample size). 9

10 Conformity in PRIMUS COSMIC VARIANCE Bootstrap Resampling Randomly select with replacement from full sample Doesn t account for variation among spatially distinct regions Jackknife Resampling Divide full sample into spatially distinct jackknife samples In our case, errors often greater than bootstrap by factor of ~2 10

11 Galactic Conformity Two-halo signal in SDSS due to small number of centrals with many satellites. Sin et al. (2017) 11

12 Completeness & Contamination VARIABLE NEIGHBOR MASS VARIABLE CYLINDER RADIUS 12

13 Completeness & Contamination Quantifiable with mock galaxy catalogs from simulations Two-halo conformity in PRIMUS an effect of contamination by satellite galaxies? 13

14 Summary Two-halo galactic conformity is a small statistical effect with large systematic errors, so Can be a test of assembly bias if methods are robust, but Larger, deeper spectroscopic redshift surveys also required for conformity measurements that account for cosmic variance Caution: isolation criteria are not necessarily a reliable proxy for central galaxies The silver lining: completeness and contamination can and should be quantified with mock galaxy catalogs We don t have all the data we want, but we can be smarter about using the data we have 14