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Register a new userOn the Mass-to-Light Ratio of Large Scale Structure
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Jeremy L. Tinker
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We examine the dependence of the mass-to-light (M/L) ratio of large-scale structure on cosmological parameters, in models that are constrained to match observations of the projected galaxy correlation function w(rp). For a sequence of cosmological models with a fixed P(k) shape and increasing normalization sig8, we find parameters of the galaxy halo occupation distribution (HOD) that reproduce SDSS measurements of w(rp) as a function of luminosity. Using these HOD models we calculate mean M/L ratios as a function of halo mass and populate halos of N-body simulations to compute M/L in larger scale environments, including cluster infall regions. For all cosmological models, the M/L ratio in high mass halos or high density regions is approximately independent of halo mass or smoothing scale. However, the plateau value of M/L depends on sig8 as well as Omega_m, and it represents the universal mass-to-light ratio <M/L> only for models in which the galaxy correlation function is approximately unbiased, i.e., with sig8 ~ sig8_gal. Our results for cluster mass halos follow the trend M/L = 577(Omega_m/0.3)(sig8/0.9)^{1.7} h Msun/Lsun. Combined with Carlberg et al.s (1996) mean M/L ratio of CNOC galaxy clusters, this relation implies (sig8/0.9)(Omega_m/0.3)^{0.6} = 0.75 +/- 0.06. M/L ratios of clusters from the SDSS and CAIRNS surveys yield similar results. This constraint is inconsistent with parameter values Omega_m ~ 0.3, sig8 ~ 0.9 favored by recent joint analyses of CMB measurements and other large-scale structure data. We discuss possible resolutions, none of which seems entirely satisfactory. Appendices present an improved formula for halo bias factors and an improved analytic technique for calculating the galaxy correlation function from a given cosmological model and HOD. (Abridged)
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The dark matter content of early,- type galaxies (ETGs) is a hotly debated topic with contrasting results arguing in favour or against the presence of significant dark mass within the effective radius and the change with luminosity and mass. In order to address this question, we investigate here the global mass - to - light ratio $Upsilon(r) = M(r)/L(r)$ of a sample of 21 lenses observed within the Sloan Lens ACS (SLACS) survey. We follow the usual approach of modeling the galaxy as a two component systems, but we use a phenomenological ansatz for $Upsilon(r)$, proposed by some of us in Tortora et al. (2007), able to smoothly interpolate between constant $M/L$ models and a wide class of dark matter haloes. The resulting galaxy model is then fitted to the data on the Einstein radius and velocity dispersion. Our phenomenological model turns out to be in well agreement with the data suggesting the presence of massive dark matter haloes in order to explain the lensing and dynamics properties of the SLACS lenses. According to the values of the dark matter mass fraction, we argue that the halo may play a significant role in the inner regions probed by the data, but such a conclusion strongly depends on the adopted initial mass function of the stellar population. Finally, we find that the dark matter mass fraction within $R_{eff}$ scales with both the total luminosity and stellar mass in such a way that more luminous (and hence more massive) galaxies have a larger dark matter content.
The magnetization $|Omega_{mathrm e}|/omega_{mathrm{e}}$ is an important parameter in plasma astrophysics, where $Omega_{mathrm e}$ and $omega_{mathrm{e}}$ are the electron gyro-frequency and electron plasma frequency, respectively. It only depends on the mass ratio $m_{mathrm i}/m_{mathrm e}$ and the light-to-Alfven speed ratio $c/v_{mathrm{Ai}}$, where $m_{mathrm i}$ ($m_{mathrm e}$) is the ion (electron) mass, $c$ is the speed of light, and $v_{mathrm{Ai}}$ is the ion Alfven speed. Nonlinear numerical plasma models such as particle-in-cell simulations must often assume unrealistic values for $m_{mathrm i}/m_{mathrm e}$ and for $c/v_{mathrm{Ai}}$. Because linear theory yields exact results for parametric scalings of wave properties at small amplitudes, we use linear theory to investigate the dispersion relations of Alfven/ion-cyclotron and fast-magnetosonic/whistler waves as prime examples for collective plasma behaviour depending on $m_{mathrm i}/m_{mathrm e}$ and $c/v_{mathrm{Ai}}$. We analyse their dependence on $m_{mathrm i}/m_{mathrm e}$ and $c/v_{mathrm{Ai}}$ in quasi-parallel and quasi-perpendicular directions of propagation with respect to the background magnetic field for a plasma with $beta_jsim1$, where $beta_j$ is the ratio of the thermal to magnetic pressure for species $j$. Although their dispersion relations are largely independent of $c/v_{mathrm{Ai}}$ for $c/v_{mathrm{Ai}}gtrsim 10$, the mass ratio $m_{mathrm i}/m_{mathrm e}$ has a strong effect at scales smaller than the ion inertial length. Moreover, we study the impact of relativistic electron effects on the dispersion relations. Based on our results, we recommend aiming for a more realistic value of $m_{mathrm i}/m_{mathrm e}$ than for a more realistic value of $c/v_{mathrm{Ai}}$ in non-relativistic plasma simulations if such a choice is necessary, although $dots$
We have tested the effect of spatial gradients in stellar mass-to-light ratio (Y) on measurements of black hole masses (MBH) derived from stellar orbit superposition models. Such models construct a static gravitational potential for a galaxy and its central black hole, but typically assume spatially uniform Y. We have modeled three giant elliptical galaxies with gradients alpha = d(log Y)/d(log r) from -0.2 to +0.1. Color and line strength gradients suggest mildly negative alpha in these galaxies. Introducing a negative (positive) gradient in Y increases (decreases) the enclosed stellar mass near the center of the galaxy and leads to systematically smaller (larger) MBH measurements. For models with alpha = -0.2, the best-fit values of MBH are 28%, 27%, and 17% lower than the constant-Y case, in NGC 3842, NGC 6086, and NGC 7768, respectively. For alpha = +0.1, MBH are 14%, 22%, and 17% higher than the constant-Y case for the three respective galaxies. For NGC 3842 and NGC 6086, this bias is comparable to the statistical errors from individual modeling trials. At larger radii, negative (positive) gradients in Y cause the total stellar mass to decrease (increase) and the dark matter fraction within one effective radius to increase (decrease).
We analyze the stellar mass-to-light ratio (M/L) gradients in a large sample of local galaxies taken from the Sloan Digital Sky Survey, spanning a wide range of stellar masses and morphological types. As suggested by the well known relationship between M/L ratios and colors, we show that M/L gradients are strongly correlated with colour gradients, which we trace to the effects of age variations. Stellar M/L gradients generally follow patterns of variation with stellar mass and galaxy type that were previous found for colour and metallicty gradients. In late-type galaxies M/L gradients are negative, steepening with increasing mass. In early-type galaxies M/L gradients are shallower while presenting a two-fold trend: they decrease with mass up to a characteristic mass of M* sim 10^10.3 M_sun and increase at larger masses. We compare our findings with other analyses and discuss some implications for galaxy formation and for dark matter estimates.
We combine Spitzer $3.6mu$ observations of a sample of disk galaxies spanning over 10 magnitudes in luminosity with optical luminosities and colors to test population synthesis prescriptions for computing stellar mass. Many commonly employed models fail to provide self-consistent results: the stellar mass estimated from the luminosity in one band can differ grossly from that of another band for the same galaxy. Independent models agree closely in the optical ($V$-band), but diverge at longer wavelengths. This effect is particularly pronounced in recent models with substantial contributions from TP-AGB stars. We provide revised color--mass-to-light ratio relations that yield self-consistent stellar masses when applied to real galaxies. The $B-V$ color is a good indicator of the mass-to-light ratio. Some additional information is provided by $V-I$, but neither it nor $J-K_s$ are particularly useful for constraining the mass-to-light ratio on their own. In the near-infrared, the mass-to-light ratio depends weakly on color, with typical values of $0.6; mathrm{M}_{odot}/mathrm{L}_{odot}$ in the $K_s$-band and $0.47; mathrm{M}_{odot}/mathrm{L}_{odot}$ at $3.6mu$.
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