galaxies, with the high redshift phase atz>2 dominated by cold gas accretion, gas-rich merger events and in-situ star formation, while the later phase is dominated by more or less gas-poor merger events as the main driver of mass growth. This is in agreement with the results presented by Hilz et al. (2012, 2013), who showed that the observed evolution of the mass-size relation with redshift can be well explained through the accretion of multiple dry minor merger events.
Another aspect of this evolution scenario is the increase of the central dark matter fraction of spheroidal galaxies with decreasing redshift. We find this increase for all galaxies in our sample, independent of the selection method and simulation, in good agreement with recent observational results by Tortora et al. (2014b). While there is only a weak correlation with the mass of the galaxies, we find a strong anti-correlation between the in-situ fraction and the central dark matter fraction of galaxies, with galaxies with high central dark matter fractions having only very little stars formed in situ. This is a natural consequence of the late growth through (dry) merger events.
We had already shown in Remus et al. (2013) that there exists a dark-halo–spheroid conspiracy for the CETGs, that is that, atz=0, their total (stellar plus dark matter) radial density profiles can be well described by an isothermal power-law with a slope ofγtot ≈ −2. In this work, we demonstrate that this is also true for the spheroidals from the Magneticum simulations, and that both simulations show a similar scatter inγtot, with a tendency towards steeper slopes found at higher redshifts. This trend for steeper slopes at high redshifts is more pronounced for the CETGs than for the METGs, but the overall trend is the same in both simulations, indicating that this is a real evolution trend and not a numerical feature. However, observations from strong lensing (Treu & Koopmans, 2004; Auger et al., 2010; Ruffet al., 2011; Sonnenfeld et al., 2013b) find no changes in the total density slopes with redshift, or, if any, a tendency towards flatter slopes at higher redshifts.
Using mock observations of our simulated spheroidal samples and applying the same analysis tool as for the observed samples, following Sonnenfeld et al. (2013b), we demonstrate that the applied assumptions for the modeling of the density profiles provide good fits for spheroidals with nearly isothermal density profiles, but for those spheroidals with steeper slopes, the slopes obtained from the mocked observations are much flatter than the intrinsic profiles. Using the slopes obtained from the mock observations leads to an excellent match between our simulations and the observations, however, to successfully measure the total density slopes even for the more compact spheroidals with the steeper slopes, a better refinement of the modeling is needed.
Simulations and observations agree in that compact spheroidal galaxies tend to have steeper total density slopes than their more extended counterparts, see Sonnenfeld et al. (2013b), but also Tortora et al. (2014a). Spheroidals with flatter slopes also have larger central dark matter fractions, in agree-ment with the fact that there are more compact spheroidal galaxies at higher redshifts, where we also find more galaxies with steeper density slopes and smaller dark matter fractions.
In summary, we find clear indications from our new set of simulations for the two-phase evo-lution scenario for central spheroidal galaxies: At high redshift, gas dominates the mass growth of (spheroidal) galaxies, thus many stars are formed in situ and only few are accreted. The gas dissipates its energy and sinks to the center of the potential well where it forms the stars in a compact central structure, thus the dark matter fractions are small and the total density slopes are steeper. At lower redshifts, (dry) merger events of all mass ratios start to dominate the mass growth of the galaxies, leading to an enhanced growth in size compared to the growth in mass, as mass is mostly added to the outskirts (apart from the rare major merger events which actually mix the whole galaxy). This leads to a growth of the central dark matter fraction, a flattening of the total density slopes and a decrease of the in-situ fraction of stars. Therefore, we conclude that the central dark matter fractions and the
4.6. SUMMARY AND DISCUSSION 109
slopes of the total radial density profiles of spheroidal galaxies are good indicators for the amount of dry merging events a galaxy has undergone.
Chapter 5
A “Universal” Density Profile for the Stellar Halos of Galaxies
5.1 Introduction
In addition to the clearly visible content of a galaxy, every galaxy is surrounded by a diffuse global stellar halo, which usually is assumed to be spherical. This outer halo consists of old and thus not very massive stars, since there is basically no in-situ star formation in the outskirts of a galaxy be-cause the gas density is much too low to form stars. (However, recent studies of extended Hidisks indicate that in those disk galaxies there is a constant albeit low star formation rate per unit of gas even at large radii in the metal poor, low density gas disks, see for example Espada et al. (2011).) Therefore, those old populations must (mostly) be accreted through merging events and stripped from the main or the accreted galaxies during the encounters, as suggested for example by observations from Mart´ınez-Delgado et al. (2010), or by the wealth of substructures observed around the Milky Way or Andromeda. The outer stellar halo contains vital information about the formation history of a galaxy, as in these outer regions the stars still remember the violent mass accretion the galaxy has gone through in its lifetime, since the mixing times in the outer halo are much larger than in the inner regions of the galaxy. Measuring densities and velocities of the outer stellar halo could therefore pro-vide a multitude of unique information about the mass accretion history and morphological changes of a galaxy.
However, since these halos are usually made up of very old and thus faint stars, those outer stel-lar halos are difficult to observe as their surface brightness is very low. The usual method to gain information about the stellar outer halos is to use their brightest objects (or objects that are espe-cially dominant in a certain narrow waveband) as tracers, such as globular clusters (GCs) or planetary nebulae (PNe), or, in nearby galaxies, the brighter classes of stars within those low mass stellar popu-lations. Using stellar population models, it is then possible to calculate the actual total density profiles from the number densities of the tracer populations. However, this approach includes a multitude of assumptions in the models for the different tracers as well as the additional problem of substructures and merger remnants that cause local overdensities and therefore could imply a higher global stellar halo density than the actual one. Thus, it is very important to use as many different tracer populations as possible and carefully subtract substructures from the analysis to ensure that we do not measure the stellar populations of the substructures.
While PNe and GCs are bright enough to be used as tracer populations for galaxies outside the local group (see Chap. 6.1), the resolution of present-day telescopes is not high enough to detect indi-vidual stars in massive galaxies other than the Milky Way and Andromeda. Hence, those two galaxies are perfect laboratories to gather information about their stellar halos which might provide insight into the properties of stellar halos of massive galaxies in general, and learn about the significance of the different stellar tracer populations. This has been done for the Milky Way in a multitude of studies, for example Preston et al. (1991) and Kinman et al. (1994) used blue horizontal branch stars in the nearby stellar halo regions, Miceli et al. (2008) used RR Lyrae stars out to 30 kpc from the galactic center, and Carollo et al. (2007) used 10123 stars within 4 kpc around the sun from SDSS Data Release 5 to measure density and metallicity gradients in the stellar halo. They report a dichotomy in the Milky Way halo density, metallicity and net rotation, indicating that the Milky Way has indeed two distinct stellar halo components. Bovy & Rix (2013) obtained the mass of the Galactic stellar disk and the radial profile of the dark halo at small radii from a sample of G-type dwarf stars from the SEGUE survey at radii between 5 kpc < RGalactic Plane < 12 kpc and 0.3 kpc < Rz < 3 kpc. More recently, Kafle et al. (2014) used K giant stars from SEGUE to measure the density of the stellar halo out to
≈160 kpc, reporting a power-law break in the density at≈17 kpc and an exponential cutoffat radii larger than 97.7 kpc, while Deason et al. (2014) found a drop in the density profile of the stellar halo at radii larger than 50 kpc, changing from a power-lawρ ∝ rγout with a slope of−2 > γout > −3 at radii smaller than≈25 kpc to a steeper slope of −5 > γout > −6 at ≈50 kpc, with indications for an even steeper slope at larger radii. They interpret that this behaviour indicates that the Milky Way had a quiet mass accretion history, with the last accretion into the stellar outer halo about 6 Gyr ago.
This interpretation is supported by Bullock & Johnston (2005) who used dark matter merger trees from simulations with semi-analytic stellar components to study Milky Way mass halos and found a steepening in the stellar halo profiles at large radii, with the smooth component accreted much earlier than the structures which are still visible as satellites in the stellar halo. Additionally, Rashkov et al.
(2013) also reported a steepening of the density profile at radii larger than 60. . .70 kpc for the ERIS simulation of a Milky Way type spiral galaxy (Guedes et al., 2011).
For our neighbouring galaxy, Andromeda, the density profile of the stellar outer halo was mea-sured in several studies as well. For example, Tanaka et al. (2010) reported a density profile for mhe smooth halo component which can be described by a power-lawρ∝rγout with a slope ofγout ≈ −2.17 out toR ≈ 100 kpc. More recently, Ibata et al. (2014) measured the stellar halo density profile from the RGB stars from the PANDAS-survey and found a power-law fit to the smooth, metal poor halo component with a slope ofγout=−3.08 that stays nearly constant from 30 kpc to 300 kpc.
Both studies find a much flatter density profile for the outer stellar halo of Andromeda than what has been reported for the Milky Way stellar halo, and they find no indication of a steepening in the density profile. Thus, it was concluded in many papers (e.g., Deason et al. (2014), Pillepich et al.
(2014)) that the Milky Way and Andromeda, although both are large spiral galaxies of similar mass, have very different formation histories, in that Andromeda had a much more violent accretion history than the Milky Way, with multiple dwarf galaxy mergers compared to the relatively quiet accretion history assumed for the Milky Way due to its steep outer stellar slope and its low number of satellite galaxies.
In this work we use the Magneticum Pathfinder simulations to address the question of the steep-ness of the density slopes of the outer halos of Milky Way mass galaxies and its implications for the accretion history of those galaxies. This simulation set provides a statistically relevant sample of galaxies in the Milky Way mass range to address those questions. Additionally, it allows us to broaden