Testing a simple recipe for estimating galaxy masses from minimal observational data

The accuracy and robustness of a simple method to estimate the total mass profile of a galaxy is tested using a sample of 65 cosmological zoom-simulations of individual galaxies. The method only requires information on the optical surface brightness and the projected velocity dispersion profiles and therefore can be applied even in case of poor observational data. In the simulated sample massive galaxies ($\sigma \simeq 200-400$ \kms) at redshift $z=0$ have almost isothermal rotation curves for broad range of radii (RMS $\simeq 5$ for the circular speed deviations from a constant value over $0.5R_{\rm eff} < r < 3R_{\rm eff}$). For such galaxies the method recovers the unbiased value of the circular speed. The sample averaged deviation from the true circular speed is less than $\sim 1$ with the scatter of $\simeq 5-8$ (RMS) up to $R \simeq 5R_{\rm eff}$. Circular speed estimates of massive non-rotating simulated galaxies at higher redshifts ($z=1$ and $z=2$) are also almost unbiased and with the same scatter. For the least massive galaxies in the sample ($\sigma < 150$ \kms) at $z=0$ the RMS deviation is $\simeq 7-9$ and the mean deviation is biased low by about $1-2$. We also derive the circular velocity profile from the hydrostatic equilibrium (HE) equation for hot gas in the simulated galaxies. The accuracy of this estimate is about RMS $\simeq 4-5$ for massive objects ($M > 6.5\times 10^{12} M_\odot$) and the HE estimate is biased low by $\simeq 3-4$, which can be traced to the presence of gas motions. This implies that the simple mass estimate can be used to determine the mass of observed massive elliptical galaxies to an accuracy of $5-8$ and can be very useful for galaxy surveys.Comment: 15 pages, 14 figures, 1 tabl

Non perturbative chiral approach to s-wave \bar{K}N interactions

The s-wave meson-nucleon interaction in the $S = -1$ sector is studied by means of coupled-channel Lippmann Schwinger equations, using the lowest order chiral Lagrangian and a cut off to regularize the loop integrals. The method reproduces succesfully the $\Lambda (1405)$ resonance and the $K^- p \to K^- p, \bar{K}^0 n, \pi^0 \Lambda, \pi^0 \Sigma, \pi^+ \Sigma^-, \pi^- \Sigma^+$ cross sections at low energies. The inclusion of the $\eta \Lambda, \eta \Sigma^0$ channels in the coupled system is found very important and allows a solution in terms of only the lowest order Lagrangian.Comment: 34 pages, 11 figures, uses epsf.sty, submitted to Nucl.Phys.

Dark matter halos around isolated ellipticals

We investigate the distribution of the luminous and the dark matter components in the isolated ellipticals NGC 7052 and NGC 7785, embedded in an emitting hot gas halo, by means of relevant X-ray and photometric data. In order to calculate the dark matter distribution in these rare objects, we performed an improved X-ray analysis of the XMM-Newton data of NGC 7785, and we used former results based on Chandra data of NGC 7052. For each object we also derived the stellar spheroid length scale from the surface photometry and the spheroid stellar mass from an analysis of the galaxy spectral energy distribution. We find that a dark matter component is present in these objects. It is subdominant and mixed with the luminous matter inside the optical region half-light radius wide, while it dominates the gravitational potential at outer radii. On the whole, the dark halo structure is very similar to that found around spirals of comparable luminosity and it is well reproduced by a Burkert halo, while a Sersic spheroid accounts well for the baryonic component.Comment: 9 pages, 5 figures, matching the version published by Astronomy & Astrophysic

The non-evolving internal structure of early-type galaxies: the case study SDSS J0728+3835 at z = 0.206

We study the internal dynamical structure of the early-type lens galaxy SDSS J0728+3835 at z = 0.206. The analysis is based on two-dimensional kinematic maps extending out to 1.7 effective radii obtained from Keck spectroscopy, on lensing geometry and on stellar mass estimates obtained from multiband Hubble Space Telescope imaging. The data are modelled under the assumptions of axial symmetry supported by a two-integral distribution function (DF), by applying the combined gravitational lensing and stellar dynamics code CAULDRON, and yielding high-quality constraints for an early-type galaxy at cosmological redshifts. Modelling the total density profile as a power-law of the form rho_tot ~ 1/r^{gamma}, we find that it is nearly isothermal (logarithmic slope gamma = 2.08^{+0.04}_{-0.02}), and quite flattened (axial ratio q = 0.60^{+0.08}_{-0.03}). The galaxy is mildly anisotropic (delta = 0.08 +/- 0.02) and shows a fair amount of rotational support, in particular towards the outer regions. We determine a dark matter fraction lower limit of 28 per cent within the effective radius. The stellar contribution to the total mass distribution is close to maximal for a Chabrier initial mass function (IMF), whereas for a Salpeter IMF the stellar mass exceeds the total mass within the galaxy inner regions. We find that the combination of a NFW dark matter halo with the maximally rescaled luminous profile provides a remarkably good fit to the total mass distribution over a broad radial range. Our results confirm and expand the findings of the SLACS survey for early-type galaxies of comparable velocity dispersion (sigma_SDSS = 214 +/- 11 km/s). The internal structure of J0728 is consistent with that of local early-type galaxies of comparable velocity dispersion as measured by the SAURON project, suggesting lack of evolution in the past two billion years.Comment: 13 pages, 10 figures. MNRAS in press. Revised to match accepted versio

The initial mass function of early-type galaxies

We determine an absolute calibration of the initial mass function (IMF) of early-type galaxies, by studying a sample of 56 gravitational lenses identified by the SLACS Survey. Under the assumption of standard Navarro, Frenk & White dark matter halos, a combination of lensing, dynamical, and stellar population synthesis models is used to disentangle the stellar and dark matter contribution for each lens. We define an "IMF mismatch" parameter \alpha=M*(L+D)/M*(SPS) as the ratio of stellar mass inferred by a joint lensing and dynamical models (M*(L+D)) to the current stellar mass inferred from stellar populations synthesis models (M*(SPS)). We find that a Salpeter IMF provides stellar masses in agreement with those inferred by lensing and dynamical models (=0.00+-0.03+-0.02), while a Chabrier IMF underestimates them (=0.25+-0.03+-0.02). A tentative trend is found, in the sense that \alpha appears to increase with galaxy velocity dispersion. Taken at face value, this result would imply a non universal IMF, perhaps dependent on metallicity, age, or abundance ratios of the stellar populations. Alternatively, the observed trend may imply non-universal dark matter halos with inner density slope increasing with velocity dispersion. While the degeneracy between the two interpretations cannot be broken without additional information, the data imply that massive early-type galaxies cannot have both a universal IMF and universal dark matter halos.Comment: 10 pages 4 figures. Resubmitted to ApJ taking into account referee's comment

Oxygen Metallicity Determinations from Optical Emission Lines in Early-type Galaxies

We measured the oxygen abundances of the warm (T$\sim 10^{4}K$) phase of gas in seven early-type galaxies through long-slit observations. A template spectra was constructed from galaxies void of warm gas and subtracted from the emission-line galaxies, allowing for a clean measurement of the nebular lines. The ratios of the emission lines are consistent with photoionization, which likely originates from the UV flux of post-asymototic giant branch (PAGB) stars. We employ H II region photoionization models to determine a mean oxygen metallicity of $1.01\pm0.50$ solar for the warm interstellar medium (ISM) in this sample. This warm ISM 0.5 to 1.5 solar metallicity is consistent with modern determinations of the metallicity in the hot (T$\sim 10^{6}-10^{7}K$) ISM and the upper range of this warm ISM metallicity is consistent with stellar population metallicity determinations. A solar metallicity of the warm ISM favors an internal origin for the warm ISM such as AGB mass loss within the galaxy.Comment: Accepted Astrophysical Journa

Comparison of an approximately isothermal gravitational potentials of elliptical galaxies based on X-ray and optical data

We analyze six X-ray bright elliptical galaxies, observed with Chandra and XMM-Newton, and approximate their gravitational potentials by isothermal spheres phi(r)=v_c^2 ln(r) over a range of radii from ~0.5 to ~25 kpc. We then compare the circular speed v_c derived from X-ray data with the estimators available from optical data. In particular we discuss two simple and robust procedures for evaluating the circular speed of the galaxy using the observed optical surface brightness and the line-of-sight velocity dispersion profiles. The best fitting relation between the circular speeds derived from optical observations of stars and X-ray observations of hot gas is v_{c,opt}~ \eta * v_{c,X}, where \eta=1.10-1.15 (depending on the method), suggesting, albeit with large statistical and systematic uncertainties, that non-thermal pressure on average contributes ~20-30% of the gas thermal pressure.Comment: 24 pages, 15 figures; Accepted for publication in MNRA

The outer halos of elliptical galaxies

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.Comment: 8 pages. Invited review to appear in the proceedings of "Galaxies and their Masks" eds. Block, D.L., Freeman, K.C. & Puerari, I., 2010, Springer (New York

Anisotropic coarse-grained statistical potentials improve the ability to identify native-like protein structures

We present a new method to extract distance and orientation dependent potentials between amino acid side chains using a database of protein structures and the standard Boltzmann device. The importance of orientation dependent interactions is first established by computing orientational order parameters for proteins with alpha-helical and beta-sheet architecture. Extraction of the anisotropic interactions requires defining local reference frames for each amino acid that uniquely determine the coordinates of the neighboring residues. Using the local reference frames and histograms of the radial and angular correlation functions for a standard set of non-homologue protein structures, we construct the anisotropic pair potentials. The performance of the orientation dependent potentials was studied using a large database of decoy proteins. The results demonstrate that the new distance and orientation dependent residue-residue potentials present a significantly improved ability to recognize native folds from a set of native and decoy protein structures.Comment: Submitted to "The Journal of Chemical Physics