DISEÑO EXPERIMENTAL DE MUESTREO

In this section, we treat satellite galaxies within and beyond Rvir of the main halo sepa- rately. The satellite galaxy in the center of a subhalo beyond Rvir is treated in the same manner as the central galaxy of a main halo, while satellite galaxies in subhalos withinRvir lose their gas due to environmental effects.

5.3.5.1 Gas Stripping

In most semi-analytic models, hot gas associated with a halo is assumed to be stripped immediately after accretion on to a larger system, leading to a rapid decline in star forma- tion and a reddening in color. In the real Universe (Sun et al. 2007; Jeltema et al. 2008) and in hydrodynamic simulations, however, the hot atmosphere of massive satellite galaxies may survive for a considerable while after accretion. McCarthy et al. (2008) found that for satellite galaxies with typical structural and orbital parameters, around 30% of the initial hot galactic halo gas can remain in place for more than 10 Gyr. In the following we introduce a model for the ram pressure and tidal effects in galaxy clusters, which gradually strip the hot atmospheres from satellite galaxies.

As for the main halo, we assume the hot gas relaxes to a distribution that exactly parallels that of its host dark matter subhalo. The specific tidal force on the hot atmosphere is then the same as on the dark matter in its subhalo. We calculate the remaining hot gas mass after tidal stripping assuming it is reduced in exactly the same way as the dark matter mass which is followed explicitly in the original simulations, i.e.

Mhot(Rtidal)

Mhot,inf all

= MDM

MDM,inf all

(5.25) where MDM,inf all and Mhot,inf all are the masses of the subhalo and of the associated hot gas just before accretion, andMDM and Mhot(r) are the current mass of the subhalo and the hot gas, respectively. Recall that we assume an isothermal profile for the hot gas distribution, thus Mhot(r) = r/Rtidal×Mhot(Rtidal). The tidal radius beyond which the hot gas is stripped can be expressed as

Rtidal = (

MDM

MDM,inf all

)RDM,inf all (5.26) whereRDM,inf all is the virial radius of the subhalo just before accretion.

In addition to the tidal force, the hot gas of satellite galaxies also feels pressure from the intracluster medium (ICM) due to their relative movement (ram pressure). At a certain radius,Rr.p., the self-gravity is balanced by this ram pressure:

ρsat(Rr.p.)V_sat2 =ρpar(R)V_orbit2 (5.27) whereρsat(Rr.p.) is the hot gas density of the satellite at the radiusRr.p.,Vsat is the virial velocity of the subhalo at infall (which we assume to be constant as the subhalo obits

around the main halo), ρpar(R) is the hot gas density of the parent dark matter halo at distanceRfrom the center of its potential well, andVorbitis the orbital velocity of satellite, which we assume to be the virial velocity of the main halo. The pressure from surroundings wins over the gravity beyond Rr.p. and the hot gas at these radii is stripped.

We compare the two radii Rtidal and Rr.p. and define the minimum of the two as the stripping radius

Rstrip=min(Rtidal, Rr.p.) (5.28) BeyondRstrip we assume all the hot gas is stripped away. The relative importance of these two mechanisms in stripping hot gas will be discussed elsewhere.

Besides the stripping, there are at least two other processes that may change the hot gas reservoir of satellites. One is cooling. The hot gas in satellite galaxies can cool and replenish the central cold star-forming disk. Here we assume that the temperature of hot gas within Rstrip does not change after infall. The cooling rate is calculated in the same way as in sec. 5.3.2, which ensures continuity in the treatment of cooling as central galaxies turn into satellite galaxies. Another process which changes the hot atmosphere around the satellite galaxies is SN feedback. As happened in central galaxies, stars formed in satellite galaxies evolve into supernovae, releasing a huge amount of energy and reheating the cold gas in the disk. Font et al. (2008) found that satellite galaxy properties are very sensitive to the way in which the reheated gas is distributed between components: if the reheated gas is stripped to the same degree as the hot gas at infall, satellite galaxies will lose their gas and become red very rapidly. In fact, in their work, they adopted a stripping efficiency for the reheated gas which is only _∼ 10% that for the hot gas. In contrast, we treat the reheated gas in satellite galaxies in the same way as in the central galaxies. We assume the reheated gas extends to a radius equal to the virial radius of the subhalo at infall. Taking into account the stripping mechanism discussed above, only reheated gas within

Rstripremains in the subhalo and the rest is added to the hot atmosphere of the main halo.

5.3.5.2 Disruption

The stellar component in subhalos can also be stripped in the presence of very strong tidal forces. Usually, the galaxy is harder to disrupt than the dark matter halo because it is more compact. We thus assume the stellar component of a satellite galaxy will only be affected by the tidal force after its host subhalo is entirely disrupted. The position of the satellite galaxy is then assigned to the most bound particle of its subhalo at the last time when this could be identified. To estimate when stripping of stars is important we assume the satellite orbits in an isothermal potential field:

φ(R) =V_vir2 lnR (5.29)

Assuming conservation of the orbital energy and angular momentum, the distance of the pericenter from the potential center can be estimated from:

( R Rperi )2 = ln R Rperi+ 1 2Vvir 1 2Vvir,t (5.30)

5.3 Galaxy Formation Models

where R is the current distant of the satellite galaxy from the center and Vvir,t is the tangential part of the orbital velocity of the satellite galaxy.

We compare the main halo density at pericenter with the average stellar mass density of satellite within its half light radius. If

MDM(Rperi) R3 peri ≡ ρDM > ρsat≡ Msat,stellar R3 sat,half