Central Surface Brightness Profiles Research Articles

The connection between radio loudness and central surface brightness profiles in optically selected low-luminosity active galaxies

Recent results indicate a correlation between nuclear radio-loudness of active galaxies and their central stellar surface-brightness profiles, in that `core' galaxies (with inner logarithmic slope {\gamma}<0.3) are significantly more radio loud than `power-law' galaxies ({\gamma}>0.5). This connection, which indicates possible links between radio-loudness and galaxy formation history (e.g. through black hole spin) has so far only been confirmed for a radio-selected sample of galaxies. Furthermore, it has since been shown that the Nuker law, which was used to parameterise the brightness profiles in these studies, gives a poor description of the brightness profile. Here, we present an analysis of the central surface brightness profiles of the active galaxies of Hubble Type T<3, that were identified by the optically-selected Palomar spectroscopic survey of nearby galaxies. We fit the brightness profiles using Sersic, Core-Sersic and, where necessary, Double-Sersic models, which we fit to the semi-major axis brightness profiles extracted from high resolution images of the galaxies from the Hubble Space Telescope (HST). We use these fits to classify the galaxies as `Core', `Sersic' or `Double-Sersic' and compare this classification with the properties of the Active Galactic Nuclei (AGNs). We find that AGN hosted in Core galaxies are generally more radio-loud than those hosted in Sersic galaxies, although there is a large overlap between the two subsamples. The correlation between radio-loudness and brightness profile can partly be explained by a correlation between radio-loudness and black hole mass. Additionally, there is a significant (99 per cent confidence) partial correlation between radio-loudness and the Core/Sersic classification of the host galaxy, which lends support to the previous results based on the radio-selected sample.

THE ACS FORNAX CLUSTER SURVEY. IV. DEPROJECTION OF THE SURFACE BRIGHTNESS PROFILES OF EARLY-TYPE GALAXIES IN THE VIRGO AND FORNAX CLUSTERS: INVESTIGATING THE “CORE/POWER-LAW DICHOTOMY”

Although early observations with the Hubble Space Telescope (HST) pointed to a sharp dichotomy among early-type galaxies in terms of the logarithmic slope gamma' of their central surface brightness profiles, several studies in the past few years have called this finding into question. In particular, recent imaging surveys of 143 early-type galaxies belonging to the Virgo and Fornax Clusters using the Advanced Camera for Surveys (ACS) on board HST have not found a dichotomy in gamma', but instead a systematic progression from central luminosity deficit to excess relative to the inward extrapolation of the best-fitting global Sersic model. Given that earlier studies also found that the dichotomy persisted when analyzing the deprojected density profile slopes, we investigate the distribution of the three-dimensional luminosity density profiles of the ACS Virgo and Fornax Cluster Survey galaxies. Having fitted the surface brightness profiles with modified Sersic models, we then deproject the galaxies using an Abel integral and measure the inner slopes gamma_3D of the resulting luminosity density profiles at various fractions of the effective radius R_e. We find no evidence of a dichotomy, but rather, a continuous variation in the central luminosity profiles as a function of galaxy magnitude. We introduce a parameter, Delta_3D, that measures the central deviation of the deprojected luminosity profiles from the global Sersic fit, showing that this parameter varies smoothly and systematically along the luminosity function.

The ACS Fornax Cluster Survey. II. The Central Brightness Profiles of Early-Type Galaxies: A Characteristic Radius on Nuclear Scales and the Transition from Central Luminosity Deficit to Excess

We analyze brightness profiles for 143 early-type galaxies in the Virgo and Fornax Clusters, observed with the Advanced Camera for Surveys on the Hubble Space Telescope. Sérsic models are found to provide accurate representations of the global profiles with a notable exception: the observed profiles deviate systematically inside a characteristic "break" radius of Rb ≈ 0.02−0.01+0.025Re, where Re is the effective radius of the galaxy. The sense of the deviation is such that bright galaxies (MB ≲ − 20) typically show central light deficits with respect to the inward extrapolation of the Sérsic model, while the great majority of low- and intermediate-luminosity galaxies (-19.5 ≲ MB ≲ − 15) show central light excesses; galaxies of intermediate luminosities (-20≲ MB ≲ − 19.5) are generally well fitted by Sérsic models over all radii. We show that the slope, γ', of the central surface brightness profiles, when measured at fixed fractions of Re, varies smoothly as a function of galaxy luminosity in a manner that depends sensitively on the choice of measurement radius. We find no evidence for a core/power-law dichotomy, and show that a recent claim of strong bimodality in γ' is likely an artifact of the biased galaxy selection function used in that study. To provide a more robust characterization of the inner regions of galaxies, we introduce a parameter Δ0.02 = log (g/ S) —where g and S are the integrated luminosities inside 0.02Re of the observed profile and of the inward extrapolation of the outer Sérsic model—to describe the central luminosity deficit (Δ0.02 < 0) or excess (Δ0.02 > 0). We find that Δ0.02 varies smoothly over the range of ≈720 in luminosity spanned by the sample galaxies, with again no evidence for a dichotomy. We argue that the central light excesses in MB ≳ − 19 galaxies may be the analogs of the dense central cores predicted by some numerical simulations to form via gas inflows.

Surface Brightness Profiles for a Sample of LMC, SMC, and Fornax Galaxy Globular Clusters

We use Hubble Space Telescope archival images to measure central surface brightness profiles of globular clusters around satellite galaxies of the Milky Way. We report results for 21 clusters around the LMC, five around the SMC, and four around the Fornax dwarf galaxy. The profiles were obtained using a recently developed technique based on measuring integrated light, which is tested on an extensive simulated data set. Our results show that for 70% of the sample, the central photometric points of our profiles are brighter than previous measurements using star counts with deviations as large as 2 mag arcsec-2. About 40% of the objects have central profiles deviating from a flat central core, with central logarithmic slopes continuously distributed between -0.2 and -1.2. These results are compared with those found for a sample of Galactic clusters using the same method. We confirm the known correlation in which younger clusters tend to have smaller core radii, and we find that they also have brighter central surface brightness values. This seems to indicate that globular clusters might be born relatively concentrated, and that a profile with an extended flat core might not be the ideal choice for initial profiles in theoretical models.

Surface Brightness Profiles of Galactic Globular Clusters fromHubble Space TelescopeImages

The Hubble Space Telescope (HST) allows us to study the central surface brightness profiles of globular clusters at unprecedented detail. We have mined the HST archives to obtain 38 WFPC2 images of Galactic globular clusters with adequate exposure times and filters, which we use to measure their central structure. We outline a reliable method to obtain surface brightness profiles from integrated light that we test on an extensive set of simulated images. Most clusters have central surface brightness about 0.5 mag brighter than previous measurements made from ground-based data, with the largest differences around 2 mag. Including the uncertainties in the slope estimates, the surface brightness slope distribution is consistent with half of the sample having flat cores and the remaining half showing a gradual decline from 0 to -0.8 [d log Σ/d log r)]. We deproject the surface brightness profiles in a nonparametric way to obtain luminosity density profiles. The distribution of luminosity density logarithmic slopes shows similar features, with half of the sample between -0.4 and -1.8. These results are in contrast to our theoretical bias that the central regions of globular clusters are either isothermal (i.e., flat central profiles) or very steep (i.e., luminosity density slope approximately -1.6) for core-collapse clusters. With only 50% of our sample having central profiles consistent with isothermal cores, King models appear to represent most globular clusters in their cores poorly.

The X-ray emission properties and the dichotomy in the central stellar cusp shapes of early-type galaxies

The Hubble Space Telescope has revealed a dichotomy in the central surface brightness profiles of early-type galaxies, which have subsequently been grouped into two families: core, boxy, anisotropic systems; and cuspy (‘power-law’), discy, rotating ones. Here we investigate whether a dichotomy is also present in the X-ray properties of the two families. We consider both their total soft emission (LSX,tot), which is a measure of the galactic hot gas content, and their nuclear hard emission (LHX,nuc), mostly coming from Chandra observations, which is a measure of the nuclear activity. At any optical luminosity, the highest LSX,tot values are reached by core galaxies; this is explained by their being the central dominant galaxies of groups, subclusters or clusters, in many of the log LSX,tot (erg s−1) ≳ 41.5 cases. The highest LHX,nuc values, similar to those of classical active galactic nuclei (AGNs), in this sample are hosted only by core or intermediate galaxies; at low luminosity AGN levels, LHX,nuc is independent of the central stellar profile shape. The presence of optical nuclei (also found by HST) is unrelated to the level of LHX,nuc, even though the highest LHX,nuc are all associated with optical nuclei. The implications of these findings for galaxy evolution and accretion modalities at the present epoch are discussed.

Spherical models for early-type galaxies with cuspy mass densities

Spherical mass density models are used to fit the central surface brightness profiles of early-type galaxies which are generated from Nuker law parameters obtained from the literature. The mass density and the corresponding potential are in an analytical form. It is shown that only a few mass density components are necessary to obtain a good fit and that for all power-law galaxies and for the core galaxies that we consider, most or all of the mass density components must have cusps to provide good fits. The applied quadratic programming fitting allows for a method of deprojection, which is reliable and convenient. The results can be used directly for further dynamical modelling.

Fundamental galaxy parameters for radio-loud active galactic nuclei and the black hole-radio power connection

We have determined the central velocity dispersions and surface brightness profiles for a sample of powerful radio galaxies in the redshift range 0.06 < z < 0.31, which were selected on the basis of their young radio source. The optical hosts follow the fundamental plane of elliptical galaxies, showing that young radio sources reside in normal ellipticals, as do other types of radio galaxies into which these objects are believed to evolve. As young radio sources are relatively straightforward to select and the contributions of the active galactic nuclei (AGN) light to the optical spectra are minimal, these objects can readily be used to study the evolution of the fundamental plane of elliptical galaxies out to z = 1, independent of optical selection effects. The black hole masses, M b h , of the objects in our sample have been determined using the tight empirical relation of M b h with central velocity dispersion, σ e , and for literature samples of classical radio galaxies and optically selected ellipticals. Only the optically selected inactive galaxies are found to exhibit a correlation between M b h and radio luminosity. In contrast, the radio powers of the AGN in the samples do not correlate with M b h at all, with objects at a given black hole mass ranging over seven orders of magnitude in radio power. It is unlikely that beaming of the radio emission is responsible for the range in radio powers. We have been able to tie in the local population of powerful radio sources with its parent population of inactive elliptical galaxies: the local black hole mass function has been determined using the elliptical galaxy luminosity function and the Faber-Jackson and M b h -σ relations. This was combined with the fraction of radio-loud black holes as a function of M b h , as determined from the optically selected galaxy sample, to derive the local volume-density of radio galaxies and the distribution of their black hole masses. These are shown to be consistent with the local radio luminosity function and the distribution of black hole masses as found in the radio-selected samples, and confirms that elliptical galaxies comprise the large majority of the radio-loud population of active galaxies.

Interpreting Central Surface Brightness and Color Profiles in Elliptical Galaxies

Hubble Space Telescope imagery has revealed dust features in the central regions of many (50%-80%) nearby bright elliptical galaxies. If these features are an indication of an underlying smooth diffuse dust distribution, then the interpretation of central surface brightness and color profiles in elliptical galaxies becomes significantly more difficult. In this Letter, diagnostics for constraining the presence of such an underlying central dust distribution are presented. We show that easily detectable central color gradients and flattened central surface brightness profiles can be induced by even small amounts of smoothly distributed dust (~100 M☉). Conversely, combinations of flat surface brightness profiles and flat color gradients or steep surface brightness profiles and steep color gradients are unlikely to be caused by dust. Taken as a whole, these results provide a simple observational tautology for constraining the existence of smooth diffuse dust distributions in the central regions of elliptical galaxies.