Early-type galaxies are known to be mostly “red and dead”, meaning their cold gas content is relatively low compared to disk galaxies, and most of their gas mass is inside their surrounding hot halo. Some early-type galaxies have a small fraction of cold, even star forming gas, but the star formation rates are usually low (e.g., Knapp, 1999). Recently, new detailed observations in the context of the Atlas3D survey (Cappellari et al., 2011a) have show that between 22% (Young et al., 2011) and 40% (Young et al., 2013) of all early-type galaxies in the survey host a cold gas disk. These cold gas disks usually containMCold gas = 107. . .109M, (for comparison, the Milky Way has MCold gasMW ' 109M) and the presence is independent of other internal dynamical properties tested in the survey (Krajnovi´c et al.
2011; Emsellem et al. 2011). Usually, the early-type galaxies that host the largest cold gas masses live in environments with low densities (Young et al. 2011, 2013). The angular momentum vectors of the stellar and cold-gas components of galaxies in dense environments (for example in the Virgo cluster) generally are aligned, while 50% of all field early-type galaxies show a misalignment between those angular momentum vectors (Davis et al., 2011), similar to the result found for the alignment of the angular momentum vectors of stars and cold gas for the Magneticum spheroidals (see Sec. A.4).
This indicates that the cold gas in early-type galaxies in dense environments could have an internal origin, e.g., from cooling from the halo as seen for example in the Phoenix cluster BCG (McDonald et al. 2012, 2014) or the Perseus Cluster galaxy NGC 1275 (e.g., Lim et al., 2008), probably enhanced due to dusty ejections from the stars (Mathews & Brighenti, 2003). In contrast, the misalignment of the angular momentum vectors of the stars and the gas suggests an external origin through merging and/or smooth accretion (e.g., cold streams, Dekel et al. (2009)). With more advanced observational instruments and methods, even the mapping of the individual clouds inside the gas disks of spheroidals becomes possible (Utomo et al., 2015), and therefore the question of the origin of the cold gas in spheroidal galaxies becomes important.
To investigate the origin of the cold gas in field galaxies, we selected four isolated massive galax-ies (Mtot ≈ 1×1013M) in a void environment from the dark-matter-only cosmological simulation Dianoga (Borgani & Viel, 2009) atz=0 and performed full baryonic zoom-simulations (see Sec. 2.3) with very high resolutions (MpartDM=4×106h−1MandMGaspart=6.2×105h−1M) using a modified ver-sion of GADGET-3. We adapted aΛCDM cosmology withh=0.72,ΩΛ=0.76 andΩm=0.24 with a baryon fraction of fbar = 0.04. The selection was made such that the galaxies hosted inside these halos have to be spheroidals due to the mass range, and due to the isolation criterion the computational time for the zoom-simulations was manageable.
All four simulated galaxies are spheroidals atz = 0, but have very different formation histories.
One is formed in a classical major merger scenario (merger atz = 0.5), another one is formed only through continuous minor merging until present day. The third galaxy has accreted most of its mass already at a redshift of aboutz = 1.5 with a quiet, nearly merger-less history afterwards, while the last galaxy has just recently finished a massive merger event. One example of our galaxies atz = 0 is shown in the left panel of Fig. A.2 in an edge-on projection as density map, colored according to the stellar density. In all four galaxies we find gas disks, ranging fromMCold Gas = 0.4×109Mto MCold Gas=1.2×109M, independent of the individual formation scenarios. The stellar-mass to gas-mass fractions agree well with the values for gas-massive red-sequence galaxies presented in Wei et al.
(2010). In three of the four cases, we see a misalignment between the orientation of the stellar structure
† The results of this Master’s Thesis by Schlachtberger (2014) will be submitted asThe Origin of the Cold Gas in Isolated Elliptical GalaxiesbyR.-S. Remus, D. P. Schlachtberger & K. Dolag
A.2. THE ORIGIN OF THE COLD GAS IN ISOLATED ELLIPTICAL GALAXIES 167
-10 -5 0 5 10
x [kpc]
-10 -5 0 5 10
y [kpc]
0 1 2 3
Stars log density
Figure A.2:Left panel: Projection of the mass weighted stellar density distribution of one example from our four zoom-simulations. The cold gas particles are shown as blue dots, and the dashed circle marks the halfmass radius.Right panels: Infall time of the cold gas that is present in the gas disk of the spheroidal galaxy shown in the left panel into the virial radius rvir(lookback time). In the upper panel, the color indicates if the gas is cold (TGas <5×105K, blue) or hot (TGas>5×105K, red) at the time of infall. In the lower panel, the color indicates if the gas was accreted bound to a substructure (yellow) or smoothly (green).
and the gas disk, as shown in the left panel of Fig. A.2 (gas particles are shown as blue points), in agreement with the observations. This supports the idea of an external origin of the (misaligned) cold gas in field early-type galaxies and is in agreement with the results from semi-analytic models presented in Lagos et al. 2014, 2015.
To investigate the origin of the cold gas we traced all cold gas particles inside the galaxies atz=0 back in time to find the time of their accretion into the virial radiusrvir. If the temperature of the gas particle isTGas < 5×105K then we call it cold, if TGas > 5×105K we call it hot. As can be seen in the upper right panel of Fig. A.2 all of the gas that is in the cold disk at z = 0 was already cold at its time of accretion. In the lower panel we show the results from the test if the gas was accreted while bound to an infalling substructure (yellow) or as part of a smooth cold stream (green). We find that both feeding mechanisms are present in our galaxy. No gas which is inside the gas disk atz= 0 has fallen into the halo less than 2 Gyr ago, which is simply due to the fact that the gas that fell into the halo more recently did not have enough time to reach the center (the free-fall time in our galaxies is approx. tff ≈ 2.6 Gyr, Binney & Tremaine 2008, p.268). Generally, most of this cold gas was accreted relatively late, namely around the free-fall time, and only a small fraction is inside the halos longer than 2tff. Those gas particles, that have been inside the halo for the longest time where usually accreted in the vicinity of a substructure. Tracing the temperature of those gas particles during their lifetime inside the halo reveals that they never got heated up to high temperatures, proving that, at least for our four examples of isolated spheroidals, the hot mode accretion and cooling from the halo
do not play a role.
In summary, we find no cooling from the halo, but only accretion of cold gas from both streams and merging satellites with similar likelihood. In case of cold gas accretion through merging substructures, the main sources interestingly are not the most massive satellites accreted during the late formation history of the galaxy but small substructures which do not contribute strongly to the mass growth.
In case of cold gas accreted from streams, we find, in disagreement with the work presented by Dekel & Birnboim (2006), that the cold streams can still penetrate the hot halo and reach the central galaxy, although the halo masses of all our galaxies are well aboveMtot > 1012M. However, the amount of cold gas accreted through those streams is much smaller than for less massive galaxies. We conclude that the gas in isolated galaxies is most likely of external origin, and that this explains the random orientations of the gas disks with respect to the main stellar body. The environment seems to be the most important driver of the feeding mechanisms, while the formation scenario does not have an impact on the existence of a cold gas disk. Nevertheless, these simulations were performed without black holes and AGN feedback, metal cooling or stellar feedback and winds, and thus the behaviour might change if those physics are included. We therefore are currently performing the same simulations including these models, and that analysis will be included in the forthcoming paper.
A.2. THE ORIGIN OF THE COLD GAS IN ISOLATED ELLIPTICAL GALAXIES 169