Gal Env

The environment of galaxies : 

The environment of galaxies Shen Shiyin 6/12/2006

Halo assemble history: 

Halo assemble history In hierarchical cold-dark-matter cosmology, N-body simulation: More massive systems populate in denser regions (Mo andamp; White 1996) No dependence of clustering properties on structure parameter, e.g. concentration (Lemson andamp; Kauffmann 1999) At given mass, halos formed at higher redshift are more clustered, especially for low mass haloes (Gao et al. 2005).

Galaxy formation inside haloes : 

Galaxy formation inside haloes Cooling: dependent on mass of haloes Low masses, never shock-heated, collapse directly High masses, shock-heated to virial temperature, pressure-supported, cools by radiation. Mergers, encounters Most efficient in galaxy groups Clusters: cumulative effect of weaker encounters Tidal interactions Clusters: destroy disks, convert to E and S0. Ram pressure Interaction with dense hot intra-cluster medium, strip away interstellar medium, strong reduction of star formation rate

Observations: 

Observations Morphology-density relation (Oemler 1974, Dressler 198) Star-forming, disk dominated galaxies reside in lower density regions than inactive elliptical galaxies Dependence of luminosity on density (Hogg et al. 2003) Brighter galaxies located in denser region Dependence of color on density Redder galaxies located in denser region At constant color, over-density is independent of luminosity for blue branch galaxies; for red branch galaxy, over-density does depend on luminosity Spiral galaxies, Implication: the SFH(SFR) is more correlated with environment than stellar mass; Red galaxies: very bright and dwarf galaxies are more clustered than intermediate objects (?)

Slide5: 

Hogg et al. 2003, based on SDSS data of 105 galaxies

Observations (kauffmann et al. 2004): 

Observations (kauffmann et al. 2004) At fixed stellar mass, star formation and nuclear activity depend strongly on local density, while size and structure parameters (e.g. concentration) are almost independent of it. The galaxy property most sensitive to environment is sepcific star formation rate, SFR/M* . For galaxies with stellar masses in the range 1010-3£1010M¯, the median SFR/M* decreases by more than a factor of 10 as the population shifts from star-forming at low densities to inactive at high densities. Low mass galaxies have lower SFR/M* than high mass galaxies At fixed stellar mass, AGN host galaxies twice as many in low density as in high density AGN typically hosted in high mass galaxies AGN probably located in the center of dark matter halos (Li et al. 2006) Galaxies in low density environment contains more dust

Slide7: 

The recent star formation history is related to the over-density on small scales (e.g 1h-1Mpc ) rather than large scales (e.g. 6h-1Mpc) (Blanton et al. 2004) although a larger density on one scales typically indicates a larger density on other scales The relation between galaxy’s SFH and density act on long time scales(andgt;1Gyr-1) The shutdown of star formation in high density environment can not take place in 1 Gyr-1

Slide8: 

density estimator : 

density estimator Over-density inside given volume (Hogg et al. 2003, 2006) =N/Na-1 Number of neighbors inside given volume (Kauffmann et al. 2004) volume limit sample avoid survey boundary k-th Nearest neighbors distance (Clemens et al. 2006) Volume limit, survey boudary Distance to known cluster center (Bernardi et al. 2006, Mateus et al. 2006) e.g. C4 cluster catalog (Miller et al. 2005), group catalog (Yang et al. 2005, Berlind et al. 2006) cluster catalog completeness

Pros and Cons: 

Pros and Cons Volume/magnitude limit sample Volume complete sample: limited to bright galaxies Magnitude complete sample: correction for incompleteness Boundary effect Remove galaxies near the survey boundary bias: remove galaxies in low density at low redshift Very complicate boundary of SDSS survey (bright star masks). Fiber collision in SDSS Bias: the galaxies missed due to the fiber collision are more likely located in denser regions. assume the redshift of the missed galaxies to be the same as the counterpart in collision group. Redshift distortion Projected density

My practice: 

My practice Density estimation for all SDSS galaxies (randgt;17.6) Estimator: over-density Magnitude limit sample Boundary effect Fiber collision N: number of galaxies in the volume of given co-moving distance Na: predicted number of galaxies in this volume if they are randomly distributed. Random positions inside the survey area, correction for boundary effect, provided by Blanton et al. in VGAC LSS sample.

Slide12: 

Correction for missing spectroscopic observation In each sector, the fraction of objects with spectroscopic observation is known.