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Cosmological simulations predict that early-type galaxies (ETGs) are the results of extended mass accretion histories. The latter are characterized by different numbers of mergers, mergers mass ratios and gas fractions, and timing. Depending on the sequence and nature of these mergers that follow the first phase of the in-situ star formation, these accretion histories may lead to ETGs that have low or high mass halos, and that rotate fast or slow. Since the stellar halos maintain the fossil records of the events that led to their formation, a discontinuity may be in place between the inner regions of ETGs and their outer halos, because the time required for the halos stars to exchange their energies and momenta is very long compared with the age of these systems. Exquisite deep photometry and extended spectroscopy for significant samples of ETGs are then used to quantify the occurrence and significance of such a transition in the galaxies structural and kinematical parameters. Once this transition radius is measured, its dependency with the effective radius of the galaxies light distribution and total stellar masses can be investigated. Such correlations can then be compared with the predictions of accreted, i.e. ex-situ vs. in-situ components from cosmological simulations to validate such models.
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Recent progress is summarized on the determination of the density distributions of stars and dark matter, stellar kinematics, and stellar population properties, in the extended, low surface brightness halo regions of elliptical galaxies. With integral field absorption spectroscopy and with planetary nebulae as tracers, velocity dispersion and rotation profiles have been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the outer edge at ~150 kpc. The results are generally consistent with the known dichotomy of elliptical galaxy types, but some galaxies show more complex rotation profiles in their halos and there is a higher incidence of misalignments, indicating triaxiality. Dynamical models have shown a range of slopes for the total mass profiles, and that the inner dark matter densities in ellipticals are higher than in spiral galaxies, indicating earlier assembly redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals and comparison with dynamical mass determinations indicates that non-thermal components to the pressure may be important in the inner ~10 kpc, and that the properties of these systems are closely related to their group environments. First results on the outer halo stellar population properties do not yet give a clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances indicate longer star formation histories pointing towards late accretion of the halo. This is consistent with independent evidence for on-going accretion, and suggests a connection to the observed size evolution of elliptical galaxies with redshift.
Without the interference of a number of events, galaxies may suffer in crowded environments (e.g., stripping, harassment, strangulation); isolated elliptical galaxies provide a control sample for the study of galaxy formation. We present the study of a sample of isolated ellipticals using imaging from a variety of telescopes, focusing on their globular cluster systems as tracers of their stellar halos. Our main findings are: (a) GC color bimodality is common even in the most isolated systems; (b) the specific frequency of GCs is fairly constant with galaxy mass, without showing an increase towards high-mass systems like in the case of cluster ellipticals; (c) on the other hand, the red fraction of GCs follows the same inverted V shape trend with mass as seen in cluster ellipticals; and (d) the stellar halos show low Sersic indices which are consistent with a major merger origin.
The kinematics of stars and planetary nebulae in early type galaxies provide vital clues to the enigmatic physics of their dark matter halos. We fit published data for fourteen such galaxies using a spherical, self-gravitating model with two components: (1) a Sersic stellar profile fixed according to photometric parameters, and (2) a polytropic dark matter halo that conforms consistently to the shared gravitational potential. The polytropic equation of state can describe extended theories of dark matter involving self-interaction, non-extensive thermostatistics, or boson condensation (in a classical limit). In such models, the flat-cored mass profiles widely observed in disc galaxies are due to innate dark physics, regardless of any baryonic agitation. One of the natural parameters of this scenario is the number of effective thermal degrees of freedom of dark matter (F_d) which is proportional to the dark heat capacity. By default we assume a cosmic ratio of baryonic and dark mass. Non-Sersic kinematic ideosyncrasies and possible non-sphericity thwart fitting in some cases. In all fourteen galaxies the fit with a polytropic dark halo improves or at least gives similar fits to the velocity dispersion profile, compared to a stars-only model. The good halo fits usually prefer F_d values from six to eight. This range complements the recently inferred limit of 7<F_d<10 (Saxton & Wu), derived from constraints on galaxy cluster core radii and black hole masses. However a degeneracy remains: radial orbital anisotropy or a depleted dark mass fraction could shift our models preference towards lower F_d; whereas a loss of baryons would favour higher F_d.
In our modern understanding of galaxy formation, every galaxy forms within a dark matter halo. The formation and growth of galaxies over time is connected to the growth of the halos in which they form. The advent of large galaxy surveys as well as high-resolution cosmological simulations has provided a new window into the statistical relationship between galaxies and halos and its evolution. Here we define this galaxy-halo connection as the multi-variate distribution of galaxy and halo properties that can be derived from observations and simulations. This connection provides a key test of physical galaxy formation models; it also plays an essential role in constraints of cosmological models using galaxy surveys and in elucidating the properties of dark matter using galaxies. We review techniques for inferring the galaxy-halo connection and the insights that have arisen from these approaches. Some things we have learned are that galaxy formation efficiency is a strong function of halo mass; at its peak in halos around a pivot halo mass of 10^12 Msun, less than 20% of the available baryons have turned into stars by the present day; the intrinsic scatter in galaxy stellar mass is small, less than 0.2 dex at a given halo mass above this pivot mass; below this pivot mass galaxy stellar mass is a strong function of halo mass; the majority of stars over cosmic time were formed in a narrow region around this pivot mass. We also highlight key open questions about how galaxies and halos are connected, including understanding the correlations with secondary properties and the connection of these properties to galaxy clustering.
There is mounting evidence that compact elliptical galaxies (CEGs) are local analogs of the high-redshift red nuggets thought to represent progenitors of todays early-type galaxies (ETGs). We report the discovery of extended X-ray emission from a hot interstellar / intragroup medium in two CEGs, Mrk 1216 and PGC 032873, using shallow archival Chandra observations. We find that PGC 032873 has an average gas temperature $k_BT=0.67pm 0.06$ keV within a radius of 15 kpc, and a luminosity $L_{rm x} = (1.8pm 0.2)times 10^{41}$ erg s$^{-1}$ within a radius of 100kpc. For Mrk 1216, which is closer and more luminous $[L_{rm x}(rm <100~kpc) = (12.1pm 1.9)times 10^{41}$ erg s$^{-1}]$, we performed a spatially resolved spectral analysis in 7 annuli out to a radius of 73 kpc. Using an entropy-based hydrostatic equilibrium (HE) procedure, we obtain a good constraint on the $H$-band stellar mass-to-light ratio, $M_{rm stars}/L_H=1.33pm 0.21$ solar, in good agreement with stellar dynamical (SD) studies, which supports the HE approximation. We obtain a density slope $2.22pm 0.08$ within $R_e$ consistent with other CEGs and normal local ETGs, while the dark matter (DM) fraction within $R_e$, $f_{rm DM}=0.20pm 0.07$, is similar to local ETGs. We place a constraint on the SMBH mass, $M_{rm BH} = (5pm 4)times 10^{9}, M_{odot}$, with a 90% upper limit of $M_{rm BH} = 1.4times 10^{10}, M_{odot}$, consistent with a recent SD measurement. We obtain a halo concentration $(c_{200}=17.5pm 6.7)$ and mass [$M_{200} = (9.6pm 3.7)times 10^{12}, M_{odot}$], where $c_{200}$ exceeds the mean $Lambda$CDM value ($approx 7$), consistent with a system that formed earlier than the general halo population. We suggest that these galaxies, which reside in group-scale halos, should be classified as fossil groups. (Abridged)
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