Galaxies grow primarily by the cooling of the intergalactic gas accreted by dark matter halos. Numerical simulations have shown that the gas accreting in dark matter halos <~ 10^12 Msun is not heated to the virial temperature, but instead is accreted via the `cold' mode (see http://arxiv.org/abs/astro-ph/0407095). For halos less massive than this critical mass, the cooling time of the gas at the virial radius is shorter than the dynamical time, and thus a hot hydrostatic atmosphere is not formed. This recent paper (http://arxiv.org/abs/1104.5046) argues that, in addition to the halo mass, the environment also plays an important role in galaxy formation. They have simulated two large (~ 20^3 Mp^3) cosmological volumes, one centered on a void (underdensity) and one on a cluster (overdensity), and compared galaxy formation in these different environments. As expected, stellar/light density is much smaller in the void simulation than in the cluster simulation. Their simulations reproduce galaxy downsizing, i.e., the peak star formation occurs in lower mass halos with decreasing redshift, for both the void and cluster galaxies.
They argue that the ratio of tcool (cooling time at the virial radius) and the Hubble time (halo age), which is roughly a function of entropy and is relatively independent of redshift, determines the specific star-formation rate (sSFR). If tcool/tH is small, sSFR is large and the galactic mass can double in the Hubble time. At high z, because of a large cosmic density, tcool is shorter than tH and sSFR is large for all galaxies, both in void and cluster environments. However, at z=0, not only the massive halos (as predicted by the 'cold mode' scenario), but also the lower mass halos in high temperature/entropy cluster environment, have low sSFR such that they can't double their mass in a Hubble time. Thus, in addition to the halo mass, environment also plays an important role in galaxy formation. But fundamentally it is still tcool/tH which determines sSFR; temperature/entropy in cluster environment is higher and tcool/tH>1, and hence sSFR is suppressed.
Some studies compare tcool/tff and some compare tcool/tH. If there is feedback heating which roughly balances radiative cooling, as in the intracluster medium, the relevant ratio should be tcool/tff. In absence of heating, tcool/tH seems more appropriate. It will be useful to study these important timescales in more detail. Clarification: tcool<tH is absolute minimum requirement (necessary but not sufficient) for efficient cooling; since tcool is minimum at fewx1e5 K, that tcool is an absolute upper limit for efficient star formation.
They argue that the ratio of tcool (cooling time at the virial radius) and the Hubble time (halo age), which is roughly a function of entropy and is relatively independent of redshift, determines the specific star-formation rate (sSFR). If tcool/tH is small, sSFR is large and the galactic mass can double in the Hubble time. At high z, because of a large cosmic density, tcool is shorter than tH and sSFR is large for all galaxies, both in void and cluster environments. However, at z=0, not only the massive halos (as predicted by the 'cold mode' scenario), but also the lower mass halos in high temperature/entropy cluster environment, have low sSFR such that they can't double their mass in a Hubble time. Thus, in addition to the halo mass, environment also plays an important role in galaxy formation. But fundamentally it is still tcool/tH which determines sSFR; temperature/entropy in cluster environment is higher and tcool/tH>1, and hence sSFR is suppressed.
Some studies compare tcool/tff and some compare tcool/tH. If there is feedback heating which roughly balances radiative cooling, as in the intracluster medium, the relevant ratio should be tcool/tff. In absence of heating, tcool/tH seems more appropriate. It will be useful to study these important timescales in more detail. Clarification: tcool<tH is absolute minimum requirement (necessary but not sufficient) for efficient cooling; since tcool is minimum at fewx1e5 K, that tcool is an absolute upper limit for efficient star formation.
No comments:
Post a Comment