Digging for Formational Clues in the Halos of Early-type Galaxies

Many of the fundamental properties of early-type galaxies (ellipticals and lenticulars) can only be accessed by venturing beyond their oft-studied centers into their large-radius halo regions. Advances in observations of kinematical tracers allow early-type halos to be increasingly well probed. This review focuses on recent findings on angular momentum and dark matter content, and discusses some possible implications for galaxy structure and formation.Comment: 4 pages, 2 figures, to appear in "Hunting for the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista & C.C. Popescu, AIP Conf. Ser., in pres

Kinematics and Angular Momentum in Early Type Galaxy Halos

We use the kinematics of discrete tracers, primarily globular clusters (GCs) and planetary nebulae (PNe), along with measurements of the integrated starlight to explore the assembly histories of early type galaxies. Data for GCs and stars are taken from the SLUGGS wide field, 2-dimensional, chemo-dynamical survey (Brodie et al. 2014). Data for PNe are from the PN.S survey (see contributions by Gerhard and by Arnaboldi, this volume). We find widespread evidence for 2-phase galaxy assembly and intriguing constraints on hierarchical merging under a lambda CDM cosmology.Comment: proceedings IAU Symp. 317, The General Assembly of Galaxy Halos: Structure, Origin and Evolution, eds. A. Bragaglia, M. Arnaboldi, M. Rejkuba, O. Gerhard, in press, 7 pp, 7 figs; references update

Structural and dynamical uncertainties in modeling axisymmetric elliptical galaxies

Quantitative dynamical models of galaxies require deprojecting the observed surface brightness to determine the luminosity density of the galaxy. Existing deprojection methods for axisymmetric galaxies assume that a unique deprojection exists for any given inclination, even though the projected density is known to be degenerate to the addition of "konus densities" that are invisible in projection. We develop a deprojection method based on linear regularization that can explore the range of luminosity densities statistically consistent with an observed surface brightness distribution. The luminosity density is poorly constrained at modest inclinations (i > ~30 deg), even in the limit of vanishing observational errors. In constant mass-to-light ratio, axisymmetric, two-integral dynamical models, the uncertainties in the luminosity density result in large uncertainties in the meridional plane velocities. However, the projected line-of-sight velocities show variations comparable to current typical observational uncertainties.Comment: 20 pages, 8 Postscript figures, LaTeX, aaspp4.sty, submitted to MNRAS; paper w/figs (600 kb) also available at http://cfa-www.harvard.edu/~romanow/ell.mn.ps.gz GIF-format figures replaced by PostScrip

Central dark matter content of early-type galaxies: scaling relations and connections with star formation histories

We examine correlations between masses, sizes and star formation histories for a large sample of low-redshift early-type galaxies, using a simple suite of dynamical and stellar population models. We confirm an anticorrelation between the size and stellar age and go on to survey for trends with the central content of dark matter (DM). An average relation between the central DM density and galaxy size of 〈ρDM〉∝R−2eff provides the first clear indication of cuspy DM haloes in these galaxies – akin to standard Λ cold dark matter haloes that have undergone adiabatic contraction. The DM density scales with galaxy mass as expected, deviating from suggestions of a universal halo profile for dwarf and late-type galaxies. We introduce a new fundamental constraint on galaxy formation by finding that the central DM fraction decreases with stellar age. This result is only partially explained by the size–age dependencies, and the residual trend is in the opposite direction to basic DM halo expectations. Therefore, we suggest that there may be a connection between age and halo contraction and that galaxies forming earlier had stronger baryonic feedback, which expanded their haloes, or lumpier baryonic accretion, which avoided halo contraction. An alternative explanation is a lighter initial mass function for older stellar populations

Dynamical Models of Elliptical Galaxies -- II. M87 and its Globular Clusters

We study the Globular Cluster (GC) system of the nearby elliptical galaxy M87 using the newly available dataset with accurate kinematics (Strader et al.2011). We find evidence for three distinct sub-populations of GCs in terms of colours, kinematics and radial profiles. We show that a decomposition into three populations (blue, intermediate and red GCs) is statistically preferred to one with two or four populations, and relate them to the stellar profile. We exploit the sub-populations to derive dynamical constraints on the mass and Dark Matter (DM) content of M87 out to $\sim100$ kpc. We use a class of global mass-estimators (from Paper I), obtaining mass measurements at different locations. M87's DM fraction changes from $\approx$0.2 at the starlight's effective radius (6 kpc) to $\approx$0.95 at the distance probed by the most extended, blue GCs (135 kpc). We supplement this with \textit{virial decompositions}, exploiting the dynamical model to produce a separation into multiple components. These yield the luminous mass as $5.5^{+1.5}_{-2.0}\times 10^{11}M_\odot$ and the DM within 135 kpc as $8.0^{+1.0}_{-4.0}\times 10^{12}M_\odot.$ The inner DM density behaves as $\rho \sim r^{-\gamma}$ with $\gamma\approx 1.6$. This is steeper than the cosmologically preferred cusp $\rho \sim r^{-1},$ providing evidence of DM contraction. Finally, we combine the GC separation into three sub-populations and the Jeans equations, obtaining information on the orbits of the GC system. The centrally concentrated red GCs exhibit tangential anisotropy, consistent with radial-orbit depletion by tidal shredding. The most extended blue GCs have an isotropic velocity dispersion tensor in the central parts, which becomes more tangential moving outwards, consistent with adiabatic contraction of the DM halo.Comment: MNRAS (submitted), 16 pages, 10 figure

Optical and near-infrared velocity dispersions of early-type galaxies

We have carried out a systematic, homogeneous comparison of optical and near-infrared dispersions. Our magnitude-limited sample of early-type galaxies in the Fornax cluster comprises 11 elliptical and 11 lenticular galaxies more luminous than MB = -17. We were able to determine the central dispersions based on the near-infrared CO absorption band head for 19 of those galaxies. The velocity dispersions range from less than 70 km/s to over 400 km/s. We compare our near-infrared velocity dispersions to the optical dispersions measured by Kuntschner (2000). Contrary to previous studies, we find a one-to-one correspondence with a median fractional difference of 6.4%. We examine the correlation between the relative dust mass and the fractional difference of the velocity dispersions, but find no significant trend. Our results suggest that early-type galaxies are largely optically thin, which is consistent with recent Herschel observations.Comment: 10 pages, 9 figures, accepted for publication in MNRA

Simulating multiple merger pathways to the central kinematics of early-type galaxies

Two-dimensional integral field surveys such as ATLAS^3D are producing rich observational data sets yielding insights into galaxy formation. These new kinematic observations have highlighted the need to understand the evolutionary mechanisms leading to a spectrum of fast-rotators and slow-rotators in early-type galaxies. We address the formation of slow and fast rotators through a series of controlled, comprehensive hydrodynamical simulations sampling idealized galaxy merger scenarios constructed from model spiral galaxies. Idealized and controlled simulations of this sort complement the more 'realistic' cosmological simulations by isolating and analyzing the effects of specific parameters, as we do in this paper. We recreate minor and major binary mergers, binary merger trees with multiple progenitors, and multiple sequential mergers. Within each of these categories of formation history, we correlate progenitor gas fraction, mass ratio, orbital pericenter, orbital ellipticity, and spin with remnant kinematic properties. We create kinematic profiles of these 95 simulations comparable to ATLAS^3D data. By constructing remnant profiles of the projected specific angular momentum (lambda_R = / , triaxiality, and measuring the incidences of kinematic twists and kinematically decoupled cores, we distinguish between varying formation scenarios. We find that binary mergers nearly always form fast rotators. Slow rotators can be formed from zero initial angular momentum configurations and gas-poor mergers, but are not as round as the ATLAS^3D galaxies. Remnants of binary merger trees are triaxial slow rotators. Sequential mergers form round slow rotators that most resemble the ATLAS^3D rotators.Comment: MNRAS, in press, 12 pages, 15 figure

Evolution of central dark matter of early-type galaxies up to z ~ 0.8

We investigate the evolution of dark and luminous matter in the central regions of early-type galaxies (ETGs) up to z ~ 0.8. We use a spectroscopically selected sample of 154 cluster and field galaxies from the EDisCS survey, covering a wide range in redshifts (z ~ 0.4-0.8), stellar masses ($\log M_{\star}/ M_{\odot}$ ~ 10.5-11.5 dex) and velocity dispersions ($\sigma_{\star}$ ~ 100-300 \, km/s). We obtain central dark matter (DM) fractions by determining the dynamical masses from Jeans modelling of galaxy aperture velocity dispersions and the $M_{\star}$ from galaxy colours, and compare the results with local samples. We discuss how the correlations of central DM with galaxy size (i.e. the effective radius, $R_{\rm e}$), $M_{\star}$ and $\sigma_{\star}$ evolve as a function of redshift, finding clear indications that local galaxies are, on average, more DM dominated than their counterparts at larger redshift. This DM fraction evolution with $z$ can be only partially interpreted as a consequence of the size-redshift evolution. We discuss our results within galaxy formation scenarios, and conclude that the growth in size and DM content which we measure within the last 7 Gyr is incompatible with passive evolution, while it is well reproduced in the multiple minor merger scenario. We also discuss the impact of the IMF on our DM inferences and argue that this can be non-universal with the lookback time. In particular, we find the Salpeter IMF can be better accommodated by low redshift systems, while producing stellar masses at high-$z$ which are unphysically larger than the estimated dynamical masses (particularly for lower-$\sigma_{\star}$ systems).Comment: 14 pages, 6 figures, 3 tables, MNRAS in pres