Gas Surface Brightness Research Articles

THE UNUSUALLY HIGH HALO CONCENTRATION OF THE FOSSIL GROUP NGC 6482: EVIDENCE FOR WEAK ADIABATIC CONTRACTION

ABSTRACT Although fossil galaxy systems are thought to be very old, and thus should possess above-average halo concentrations, typically fossils have unexceptional concentrations of their masses. We revisit the massive isolated elliptical galaxy/fossil group NGC 6482 for which previous X-ray studies of a modest Chandra observation obtained a very uncertain, but also possibly very high, halo concentration. We present new measurements of the hot gas surface brightness, temperature, and iron abundance using the modest Chandra observation and a previously unpublished Suzaku observation, the latter of which allows the measurements of the gas properties to be extended out to . By constructing hydrostatic equilibrium models of the gas with separate components for the gas, the stellar mass of the large central galaxy (BCG), and the dark matter (DM), we measure and using a Navarro-Frenk-White (NFW) DM profile. For a halo of this mass, the measured concentration c 200 exceeds the mean value (7.1) expected for relaxed ΛCDM halos by in terms of the observational error, and by considering the intrinsic scatter in the ΛCDM c–M relation, which situates NGC 6482 as the most extreme outlier known for a fossil system. We explored several variants of adiabatic contraction (AC) models and, while the AC models provide fits of the same quality as the uncontracted models, they do have the following advantages: (1) lower c 200 that is less of an outlier in the ΛCDM c–M relation, and (2) baryon fractions that agree better with the mean cosmic value. While the standard AC prescriptions yield a BCG stellar mass that is uncomfortably low compared to results from stellar population synthesis (SPS) models, a weaker AC variant that artificially shuts off cooling and star formation at z = 2 yields the same stellar mass as the uncontracted models. These are probably the reasons why our X-ray analysis prefers this weaker AC variant applied to either an NFW or Einasto DM halo. Finally, the stellar mass we measure for the BCG from the hydrostatic analysis strongly favors results from SPS models with a Chabrier or Kroupa initial mass function (IMF) over a Salpeter IMF.

A detailed study of the radio-FIR correlation in NGC 6946 withHerschel-PACS/SPIRE from KINGFISH

We derive the distribution of the synchrotron spectral index across NGC6946 and investigate the correlation between the radio continuum (synchrotron) and far-infrared (FIR) emission using the KINGFISH Herschel PACS and SPIRE data. The radio--FIR correlation is studied as a function of star formation rate, magnetic field strength, radiation field strength, and the total gas surface brightness. The synchrotron emission follows both star-forming regions and the so-called magnetic arms present in the inter-arm regions. The synchrotron spectral index is steepest along the magnetic arms ($\alpha_n \sim 1$), while it is flat in places of giant H{\sc ii} regions and in the center of the galaxy ($\alpha_n \sim 0.6-0.7$). The map of $\alpha_n$ provides an observational evidence for aging and energy loss of cosmic ray electrons propagating in the disk of the galaxy. Variations in the synchrotron--FIR correlation across the galaxy are shown to be a function of both star formation and magnetic fields. We find that the synchrotron emission correlates better with cold rather than with warm dust emission, when the interstellar radiation field is the main heating source of dust. The synchrotron--FIR correlation suggests a coupling between the magnetic field and the gas density. NGC6946 shows a power-law behavior between the total (turbulent) magnetic field strength B and the star formation rate surface density $\Sigma_{\rm SFR}$ with an index of 0.14\,(0.16)$\pm$0.01. This indicates an efficient production of the turbulent magnetic field with the increasing gas turbulence expected in actively star forming regions. The scale-by-scale analysis of the synchrotron--FIR correlation indicates that the ISM affects the propagation of old/diffused cosmic ray electrons, resulting in a diffusion coefficient of $D_0=4.6\times 10^{28}$\,cm$^2$\,s$^{-1}$ for 2.2\,GeV CREs.

The ATLAS3D Project – XIV. The extent and kinematics of the molecular gas in early-type galaxies

We use interferometric 12CO(1–0) observations to compare and contrast the extent, surface brightness profiles and kinematics of the molecular gas in CO-rich ATLAS3D early-type galaxies (ETGs) and spiral galaxies. We find that the molecular gas extent is smaller in absolute terms in ETGs than in late-type galaxies, but that the size distributions are similar once scaled by the galaxies optical/stellar characteristic scalelengths. Amongst ETGs, we find that the extent of the gas is independent of its kinematic misalignment (with respect to the stars), but does depend on the environment, with Virgo cluster ETGs having less extended molecular gas reservoirs, further emphasizing that cluster ETGs follow different evolutionary pathways from those in the field. Approximately half of ETGs have molecular gas surface brightness profiles that follow the stellar light profile. These systems often have relaxed gas out to large radii, suggesting they are unlikely to have had recent merger/accretion events. A third of the sample galaxies show molecular gas surface brightness profiles that fall off slower than the light, and sometimes show a truncation. These galaxies often have a low mass, and either have disturbed molecular gas or are in the Virgo cluster, suggesting that recent mergers, ram pressure stripping and/or the presence of hot gas can compress/truncate the gas. The remaining galaxies have rings, or composite profiles, that we argue can be caused by the effects of bars. We investigated the kinematics of the molecular gas using position–velocity diagrams, and compared the observed kinematics with dynamical model predictions, and the observed stellar and ionized gas velocities. We confirm that the molecular gas reaches beyond the turnover of the circular velocity curve in ≈70 per cent of our CO-rich ATLAS3D ETGs, validating previous work on the CO Tully–Fisher relation. In general we find that in most galaxies the molecular gas is dynamically cold, and the observed CO rotation matches well model predictions of the circular velocity. In the galaxies with the largest molecular masses, dust obscuration and/or population gradients can cause model predictions of the circular velocity to disagree with observations of the molecular gas rotation; however, these effects are confined to the most star forming systems. Bars and non-equilibrium conditions can also make the gas deviate from circular orbits. In both these cases, one expects the model circular velocity to be higher than the observed CO velocity, in agreement with our observations. Molecular gas is a better direct tracer of the circular velocity than the ionized gas, justifying its use as a kinematic tracer for Tully–Fisher and similar analyses.

IONIZED NITROGEN AT HIGH REDSHIFT

We present secure [NII[ detections in two mm-bright, strongly lensed objects at high redshift, APM08279+5255 (z=3.911) and MM18423+5938 (z=3.930), using the IRAM Plateau de Bure Interferometer. Due to its ionization energy [NII] is a good tracer of the ionized gas phase in the interstellar medium. The measured fluxes are S([NII])=(4.8+/-0.8) Jy km/s and (7.4+/-0.5) Jy km/s respectively, yielding line luminosities of L([NII]) =(1.8+/-0.3) x 10^9 \mu^{-1} Lsun for APM08279+5255 and L([NII]) =(2.8+/-0.2) x 10^9 \mu^{-1} Lsun for MM18423+5938. Our high-resolution map of the [NII] and 1 mm continuum emission in MM18423+5938 clearly resolves an Einstein ring in this source, and reveals a velocity gradient in the dynamics of the ionized gas. A comparison of these maps with high-resolution EVLA CO observations enables us to perform the first spatially-resolved study of the dust continuum-to-molecular gas surface brightness (Sigma_{FIR} Sigma_{CO}^N, which can be interpreted as the star formation law) in a high-redshift object. We find a steep relation (N=1.4+/-0.2), consistent with a starbursting environment. We measure a [NII]/FIR luminosity ratio in APM0828+5255 and MM18423+5938 of 9.0 x 10^{-6} and 5.8 x 10^{-6}, respectively. This is in agreement with the decrease of the [NII]/FIR ratio at high FIR luminosities observed in local galaxies.

The Radial Distribution of the Interstellar Medium in Disk Galaxies: Evidence for Secular Evolution

One possible way for spiral galaxies to internally evolve would be for gas to flow to the center and form stars in a central disk (pseudo-bulge). If the inflow rate is faster than the rate of star formation, a central concentration of gas will form. In this paper we present radial profiles of stellar and 8 μm emission from polycyclic aromatic hydrocarbons (PAHs) for 11 spiral galaxies to investigate whether the interstellar medium in these galaxies contains a central concentration above that expected from the exponential disk. In general, we find that the two-dimensional CO and PAH emission morphologies are similar, and that they exhibit similar radial profiles. We find that in 6 of the 11 galaxies there is a central excess in the 8 μm and CO emission above the inward extrapolation of an exponential disk. In particular, all four barred galaxies in the sample have strong central excesses in both 8 μm and CO emission. These correlations suggest that the excess seen in the CO profiles is, in general, not simply due to a radial increase in the CO emissivity. In the inner disk, the ratio of the stellar to the 8 μm radial surface brightness is similar for 9 of the 11 galaxies, suggesting a physical connection between the average stellar surface brightness and the average gas surface brightness at a given radius. We also find that the ratio of the CO to 8 μm PAH surface brightness is consistent over the sample, implying that the 8 μm PAH surface brightness can be used as an approximate tracer of the interstellar medium.

A Fundamental Plane Relation for the X-Ray Gas in Normal Elliptical Galaxies

We report on the discovery of a new correlation between global parameters of the hot interstellar gas in elliptical galaxies. We reanalyze archival Chandra data for 30 normal early-type systems, removing the contributions of resolved and unresolved point sources to reveal the X-ray morphology of the hot gas. We determine the half-light radius, R_X, and the mean surface brightness, I_X, from the gas surface brightness profiles. A spectral analysis determines the temperature, T_X, of the gas within 3 optical effective radii. We find that the galaxies lie on an X-ray Gas Fundamental Plane (XGFP) of the form T_X ~ R_X^{0.28} I_X^{0.22}. This is close to, but distinct from, a simple luminosity-temperature relation. The intrinsic width of the XGFP is only 0.07dex, nearly identical to that of the stellar (optical) fundamental plane (SFP). This is surprising since X-ray gas masses are typically ~10^{-2} of the stellar masses. We show that the XGFP is not a simple consequence of the virial theorem or hydrostatic equilibrium, and that it is essentially independent of the SFP. The XGFP thus represents a genuinely new constraint on the hydrodynamical evolution of elliptical galaxies.

Reconstructing the Three‐dimensional Structure of Underlying Triaxial Dark Halos from X‐Ray and Sunyaev‐Zel’dovich Effect Observations of Galaxy Clusters

While the use of galaxy clusters as tools to probe cosmology is established, their conventional description still relies on spherical and/or isothermal models that were proposed more than 20 years ago. We present, instead, a deprojection method for extracting their intrinsic properties from X-ray and Sunyaev-Zel'dovich effect observations in order to improve our understanding of cluster physics. First, we develop a theoretical model for the intracluster gas in hydrostatic equilibrium in a triaxial dark matter halo with a constant axis ratio. In this theoretical model, the gas density profiles are expressed in terms of the intrinsic properties of the dark matter halos. Then, we incorporate the projection effect into the gas profiles and show that the gas surface brightness profiles are expressed in terms of the eccentricities and the orientation angles of the dark halos. For the practical purpose of our theoretical model, we provide several empirical fitting formulae for the gas density and temperature profiles and also for the surface brightness profiles relevant to X-ray and Sunyaev-Zel'dovich effect observations. Finally, we construct a numerical algorithm to determine the halo eccentricities and orientation angles using our model and demonstrate that it is possible in principle to reconstruct the three-dimensional structures of the dark halos from the X-ray and/or Sunyaev-Zel'dovich effect cluster data alone, without requiring priors, such as weak-lensing information, and without relying on such restrictive assumptions as halo axial symmetry about the line of sight.

ITAL]Chandra[/ITAL] Limits on X-Ray Emission Associated with the Supermassive Black Holes in Three Giant Elliptical Galaxies

Elliptical galaxy nuclei are the sites of the largest black holes known but typically show little or no nuclear activity. We investigate this extreme quiescence using Chandra X-Ray Observatory observations of the giant elliptical galaxies NGC 1399, NGC 4472, and NGC 4636. The unique Chandra imaging power enables us to place upper limits of 7.3, 15, and 28 × 10-9LEdd for the ~108-109 M☉ black holes in NGC 1399, NGC 4472, and NGC 4636, respectively. The corresponding radiative efficiencies in this band are 4.1, 24, and 620 × 10-6 using Bondi accretion rates derived from the Chandra hot interstellar gas surface brightness profiles. These limits are inconsistent with basic advection-dominated accretion flow models for NGC 1399 and NGC 4472, indicating accretion onto the black hole at 10% of the Bondi rate.