Galactic Astronomy銀河物理学特論 I

Lecture 1-5: Dependence of Galaxy Properties on Environment

Seminar: Park et al. 2007, ApJ, 658, 898

Lecture:

2011/11/07

Binggeli et al. 1988, ARAA, 26, 509

Before SDSS:

The composition of the galaxy luminosity function vary from field to clusters of galaxies. E/S0 is more common in the cluster region. (Need to note that Virgo cluster is relatively spiral-rich cluster).

Environmental Effects ?

Nature ( 生まれ )?

In high-density region, galaxies are formed earlier than low-density region due to the higher background density level. In such region, elliptical galaxies are preferentially formed ?

Nurture ( 育ち ) ?

Galaxies are formed as disk galaxies at first, afterthat, in high-density region, galaxies are transformed to elliptical galaxies ?

• Interaction between galaxies ?

• Merging or interaction ? High-speed interaction (Harassment) ?

• Ram-pressure stripping by hot intra-cluster gas

• S0 galaxies as anemic ( 貧血 ) spiral ?

Effect of “local” density field to “global” density field

Early SDSS:

Using the large database of SDSS, morphological composition of galaxies can be examined as a function of galaxy density. Morphological classification of galaxies are done with concentration parameter (C) etc. Abrupt change of the composition found only in the very high density region = core of clusters.

Goto et al. 2003, MNRAS, 346, 601

Galaxy “harassment” in a simulated cluster of galaxies ?

Moore et al. 1999, MNRAS, 304, 465

In the high density region, the relative movement of the galaxies have large velocity difference and the dynamical effect of one interaction between galaxies may be small. But the number of such encounter is large in such region and cumulative effect can make a big impact on the galaxy properties. Such effect is called galaxy “harassment”.

The green clouds in the figure represent stars from a “galaxy” spread by many encounters.

Examples in Virgo cluster : Ram pressure stripping ?

Chung et al. 2007, ApJ, 659, L115

HI gas observation of galaxies in the Virgo cluster region revealed gas structure with long tail. The contour of the figure shows the X-ray flux representing the column density of the hot intra cluster gas. The direction of the tailes are against the core of the cluster.

Virgo cluster: Interaction with cold gas of galaxy and hot gas of cluster ?

Pfrommer et al. 2010, Nature, 6, 520

Clustering of galaxies: Auto-correlation function

Auto-correlation function of galaxies determined from the SDSS database. The correlation function is explained with two components, 1) correlation between dark matter halos and 2) correlation between galaxies inside one dark halo.

Zehavi et al. 2004, ApJ, 608, 16

One-halo term:

Correlation between galaxies inside same halo

Two-halo term:

Correlation between dark-matter halos

“Classical” evolution of galaxies in clusters : Butcher-Oemler effect

The fraction of blue galaxies in clusters increasing with the redshift. The scatter of the fraction is large and it is difficult to define the same “population” of clusters.

Buther & Oemler , 1984, ApJ, 285, 426 Van Dokkum, 2000, ApJ, 541, 95

“Classical” evolution of galaxies in clusters : Butcher-Oemler effect

For example, the fraction of blue galaxies depends on the shape of the clusters of galaxies. The shape is though to reflect the “relaxation” level of the structure.

Smail et al. 1998, MNRAS, 293, 124

“Classical” evolution of galaxies in clusters : Butcher-Oemler effect

The morphological mixture of galaxies also vary with redshift. The fraction of early-type galaxies decreases with redshift.

Van Dokkum, 2000, ApJ, 541, 95