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The environment of galaxies

The environment of galaxies. Shen Shiyin 6/12/2006. Halo assemble history. In hierarchical cold-dark-matter cosmology, N-body simulation: More massive systems populate in denser regions (Mo & White 1996)

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The environment of galaxies

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  1. The environment of galaxies Shen Shiyin 6/12/2006

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

  3. 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

  4. 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 (?)

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

  6. 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

  7. 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(>1Gyr-1) • The shutdown of star formation in high density environment can not take place in 1 Gyr-1

  8. 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

  9. 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

  10. My practice • Density estimation for all SDSS galaxies (r>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.

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

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