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Register a new userThe mass and angular momentum distribution of simulated massive early-type galaxies to large radii
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We study the dark and luminous mass distributions, circular velocity curves (CVC), line-of-sight kinematics, and angular momenta for a sample of 42 cosmological zoom simulations of massive galaxies. Using a temporal smoothing technique, we are able to reach large radii. We find that: (i)The dark matter halo density profiles outside a few kpc follow simple power-law models, with flat dark matter CVCs for lower-mass systems, and rising CVCs for high-mass haloes. The projected stellar density distributions at large radii can be fitted by Sersic functions with n>10, larger than for typical ETGs. (ii)The massive systems have nearly flat total CVCs at large radii, while the less massive systems have mildly decreasing CVCs. The slope of the CVC at large radii correlates with v_circ itself. (iii)The dark matter fractions within Re are in the range 15-30% and increase to 40-65% at 5Re. Larger and more massive galaxies have higher dark matter fractions. (iv)The short axes of simulated galaxies and their host dark matter haloes are well aligned and their short-to-long axis ratios are correlated. (v)The stellar vrms(R) profiles are slowly declining, in agreement with planetary nebulae observations in the outer haloes of most ETGs. (vi)The line-of-sight velocity fields v show that rotation properties at small and large radii are correlated. Most radial profiles for the cumulative specific angular momentum parameter lambda(R) are nearly flat or slightly rising, with values in [0.06,0.75] from 2Re to 5Re. (vii)Stellar mass, ellipticity at 5Re, and lambda(5Re) are correlated: the more massive systems have less angular momentum and are rounder, as for observed ETGs. (viii)More massive galaxies with a large fraction of accreted stars have radially anisotropic velocity distributions outside Re. Tangential anisotropy is seen only for galaxies with high fraction of in-situ stars. (Full abstract in PDF)
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We studied the stellar populations, distribution of dark matter, and dynamical structure of a sample of 25 early-type galaxies in the Coma and Abell 262 clusters. We derived dynamical mass-to-light ratios and dark matter densities from orbit-based dynamical models, complemented by the ages, metallicities, and alpha-elements abundances of the galaxies from single stellar population models. Most of the galaxies have a significant detection of dark matter and their halos are about 10 times denser than in spirals of the same stellar mass. Calibrating dark matter densities to cosmological simulations we find assembly redshifts z_{DM} approx 1-3. The dynamical mass that follows the light is larger than expected for a Kroupa stellar initial mass function, especially in galaxies with high velocity dispersion sigma_{eff} inside the effective radius r_{eff}. We now have 5 of 25 galaxies where mass follows light to 1-3 r_{eff}, the dynamical mass-to-light ratio of all the mass that follows the light is large (approx 8-10 in the Kron-Cousins R band), the dark matter fraction is negligible to 1-3 r_{eff}. This could indicate a massive initial mass function in massive early-type galaxies. Alternatively, some of the dark matter in massive galaxies could follow the light very closely suggesting a significant degeneracy between luminous and dark matter.
We study mass distributions within and beyond 5~effective radii ($R_{rm e}$) in 23 early-type galaxies from the SLUGGS survey, using their globular cluster (GC) kinematic data. The data are obtained with Keck/DEIMOS spectrograph, and consist of line-of-sight velocities for ~$3500$ GCs, measured with a high precision of ~15 $rm km s^{-1}$ per GC and extending out to $~13 R_{rm e}$. We obtain the mass distribution in each galaxy using the tracer mass estimator of Watkins et al. and account for kinematic substructures, rotation of the GC systems and galaxy flattening in our mass estimates. The observed scatter between our mass estimates and results from the literature is less than 0.2 dex. The dark matter fraction within $5R_{rm e}$ ($f_{rm DM}$) increases from ~$0.6$ to ~$0.8$ for low- and high-mass galaxies, respectively, with some intermediate-mass galaxies ($M_*{sim}10^{11}M_odot$) having low $f_{rm DM}sim0.3$, which appears at odds with predictions from simple galaxy models. We show that these results are independent of the adopted orbital anisotropy, stellar mass-to-light ratio, and the assumed slope of the gravitational potential. However, the low $f_{rm DM}$ in the ~$10^{11}M_odot$ galaxies agrees with the cosmological simulations of Wu et al. where the pristine dark matter distribution has been modified by baryons during the galaxy assembly process. We find hints that these $M_*sim10^{11}M_odot$ galaxies with low $f_{rm DM}$ have very diffuse dark matter haloes, implying that they assembled late. Beyond $5R_{rm e}$, the $M/L$ gradients are steeper in the more massive galaxies and shallower in both low and intermediate mass galaxies.
We investigate the origin, the shape, the scatter, and the cosmic evolution in the observed relationship between specific angular momentum $j_star$ and the stellar mass $M_star$ in early-type (ETGs) and late-type galaxies (LTGs). Specifically, we exploit the observed star-formation efficiency and chemical abundance to infer the fraction $f_{rm inf}$ of baryons that infall toward the central regions of galaxies where star formation can occur. We find $f_{rm inf}approx 1$ for LTGs and $approx 0.4$ for ETGs with an uncertainty of about $0.25$ dex, consistent with a biased collapse. By comparing with the locally observed $j_star$ vs. $M_star$ relations for LTGs and ETGs we estimate the fraction $f_j$ of the initial specific angular momentum associated to the infalling gas that is retained in the stellar component: for LTGs we find $f_japprox 1.11^{+0.75}_{-0.44}$, in line with the classic disc formation picture; for ETGs we infer $f_japprox 0.64^{+0.20}_{-0.16}$, that can be traced back to a $z<1$ evolution via dry mergers. We also show that the observed scatter in the $j_{star}$ vs. $M_{star}$ relation for both galaxy types is mainly contributed by the intrinsic dispersion in the spin parameters of the host dark matter halo. The biased collapse plus mergers scenario implies that the specific angular momentum in the stellar components of ETG progenitors at $zsim 2$ is already close to the local values, in pleasing agreement with observations. All in all, we argue such a behavior to be imprinted by nature and not nurtured substantially by the environment.
We measure the average mass properties of a sample of 41 strong gravitational lenses at moderate redshift (z ~ 0.4 - 0.9), and present the lens redshift for 6 of these galaxies for the first time. Using the techniques of strong and weak gravitational lensing on archival data obtained from the Hubble Space Telescope, we determine that the average mass overdensity profile of the lenses can be fit with a power-law profile (Delta_Sigma prop. to R^{-0.86 +/- 0.16}) that is within 1-sigma of an isothermal profile (Delta_Sigma prop. to R^{-1}) with velocity dispersion sigma_v = 260 +/- 20 km/s. Additionally, we use a two-component de Vaucouleurs+NFW model to disentangle the total mass profile into separate luminous and dark matter components, and determine the relative fraction of each component. We measure the average rest frame V-band stellar mass-to-light ratio (Upsilon_V = 4.0 +/- 0.6 h M_sol/L_sol) and virial mass-to-light ratio (tau_V = 300 +/- 90 h M_sol/L_sol) for our sample, resulting in a virial-to-stellar mass ratio of M_vir/M_* = 75 +/- 25. Finally, we compare our results to a previous study using low redshift lenses, to understand how galaxy mass profiles evolve over time. We investigate the evolution of M_vir/M_*(z) = alpha(1+z)^{beta}, and find best fit parameters of alpha = 51 +/- 36 and beta = 0.9 +/- 1.8, constraining the growth of virial to stellar mass ratio over the last ~7 Gigayears. We note that, by using a sample of strong lenses, we are able to constrain the growth of M_vir/M_*(z) without making any assumptions about the IMF of the stellar population.
Stellar metallicity gradients in the outer regions of galaxies are a critical tool for disentangling the contributions of in-situ and ex-situ formed stars. In the two-phase galaxy formation scenario, the initial gas collapse creates steep metallicity gradients, while the accretion of stars formed in satellites tends to flatten these gradients in the outskirts, particularly for massive galaxies. This work presents the first compilation of extended metallicity profiles over a wide range of galaxy mass. We use the DEIMOS spectrograph on the Keck telescope in multi-slit mode to obtain radial stellar metallicity profiles for 22 nearby early-type galaxies. From the calcium triplet lines in the near-infrared we measure the metallicity of the starlight up to 3 effective radii. We find a relation between the outer metallicity gradient and galaxy mass, in the sense that lower mass systems show steeper metallicity gradients than more massive galaxies. This result is consistent with a picture in which the ratio of ex-situ to in-situ formed stars is lower in less massive galaxies as a consequence of the smaller contribution by accretion. In addition, we infer a correlation between the strength of the calcium triplet feature in the near-infrared and the stellar initial mass function slope that is consistent with recent models in the literature.
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