Central Elliptical Galaxy Research Articles

A Supermassive Black Hole Fundamental Plane for Ellipticals

We obtain the coefficients of a new fundamental plane for supermassive black holes at the centers of elliptical galaxies, involving measured central black hole mass and photometric parameters that define the light distribution. The galaxies are tightly distributed around this mass fundamental plane, with improvement in the rms residual over those obtained from the MBH-σ and MBH-L relations. This implies a strong multidimensional link between central massive black hole formation and global photometric properties of elliptical galaxies, and provides an improved estimate of black hole mass from galaxy data.

Brownian Motion of Black Holes in Dense Nuclei

We evaluate the Brownian motion of a massive particle (black hole) at the center of a galaxy using N-body simulations. Our galaxy models have power-law central density cusps like those observed at the centers of elliptical galaxies. The simulations show that the black hole achieves a steady state kinetic energy that is substantially different from what would be predicted based on the properties of the galaxy model in the absence of the black hole. The reason appears to be that the black hole responds to stars whose velocities have themselves been raised by the presence of the black hole. Over a wide range of density slopes and black hole masses, the black hole's mean kinetic energy is equal to what would be predicted under the assumption that it is in energy equipartition with stars lying within a distance ~rh/2 from it, where rh is the black hole's influence radius. The dependence of the Brownian velocity on black hole mass is approximately V2 ∝ M, 0.5 γ 2.0, with γ the power-law index of the stellar density profile, ρ ∝ r-γ. This is less steep than the M dependence predicted in a model in which the effect of the black hole on the stellar velocities is ignored. The influence of a stellar mass spectrum on the black hole's Brownian motion is also evaluated and found to be consistent with predictions from Chandrasekhar's theory. We use these results to derive a probability function for the mass of the Milky Way black hole based on a measurement of its proper-motion velocity. Interesting constraints on MBH will require a velocity resolution exceeding 0.5 km s-1.

The central elliptical galaxy in fossil groups and formation of brightest cluster galaxies

Abstract We study the dominant central giant elliptical galaxies in ‘fossil groups’ using deep optical (R-band) and near-infrared (Ks-band) photometry. These galaxies are as luminous as the brightest cluster galaxies (BCGs), raising immediate interest in their link to the formation of BCGs and galaxy clusters. However, despite apparent similarities, the dominant fossil galaxies show non-boxy isophotes, in contrast to the most luminous BCGs. This study suggests that the structure of the brightest group galaxies produced in fossil groups are systematically different to the majority of BCGs. If the fossils do indeed form from the merger of major galaxies including late-types within a group, then their discy nature is consistent with the results of recent numerical simulations of semi-analytical models which suggest that gas rich mergers result in discy isophote ellipticals. We show that fossils form a homogeneous population in which the velocity dispersion of the fossil group is tightly correlated with the luminosity of the dominant elliptical galaxy. This supports the scenario in which the giant elliptical galaxies in fossils can grow to the size and luminosity of BCGs in a group environment. However, the boxy structure of luminous BCGs indicate that they are either not formed as fossils, or have undergone later gas-free mergers within the cluster environment.

Discovery of Ultracompact Dwarf Galaxies in the Virgo Cluster

We have discovered nine ultracompact dwarf galaxies (UCDs) in the Virgo Cluster, extending samples of these objects outside the Fornax Cluster. Using the Two Degree Field (2dF) multifiber spectrograph on the Anglo-Australian Telescope, the new Virgo members were found among 1500 color-selected, starlike targets with 16: 0 < b(j) < 20.2 in a 2 degrees diameter field centered on M87 (NGC 4486). The newly found UCDs are comparable to the UCDs in the Fornax Cluster, with sizes less than or similar to 100 pc, -12.9 < M-B < -10.7, and exhibiting red absorption-line spectra, indicative of an older stellar population. The properties of these objects remain consistent with the tidal threshing model for the origin of UCDs from the surviving nuclei of nucleated dwarf elliptical galaxies disrupted in the cluster core but can also be explained as objects that were formed by mergers of star clusters created in galaxy interactions. The discovery that UCDs exist in Virgo shows that this galaxy type is probably a ubiquitous phenomenon in clusters of galaxies; coupled with their possible origin by tidal threshing, the UCD population is a potential indicator and probe of the formation history of a given cluster. We also describe one additional bright UCD with M-B = -12.0 in the core of the Fornax Cluster. We find no further UCDs in our Fornax Cluster Spectroscopic Survey down to bj 19.5 in two additional 2dF fields extending as far as 3 degrees from the center of the cluster. All six Fornax bright UCDs identified with 2dF lie within 0.degrees 5 (projected distance of 170 kpc) of the central elliptical galaxy NGC 1399.

XMM‐NewtonObservation of an X‐Ray Trail between the Spiral Galaxy NGC 6872 and the Central Elliptical Galaxy NGC 6876 in the Pavo Group

We present XMM-Newton observations of a trail of enhanced X-ray emission extending along the full 87 × 4' region between the large spiral galaxy NGC 6872 and the dominant elliptical galaxy NGC 6876 in the Pavo group, the first known X-ray trail associated with a spiral galaxy in a poor galaxy group and, with a projected length of 90 kpc, one of the longest known X-ray trails. The X-ray surface brightness in the trail region is roughly constant beyond ~20 kpc of NGC 6876 in the direction of the spiral galaxy. The trail is hotter (~1 keV) than the undisturbed Pavo IGM (~0.5 keV) and has low metal abundances (0.2 Z☉). The 0.5-2 keV luminosity of the trail, measured using a 67 × 90 kpc rectangular region, is 6.6 × 1040 ergs s-1. We compare the properties of gas in the trail to the spectral properties of gas in the spiral galaxy NGC 6872 and in the elliptical galaxy NGC 6876 to constrain its origin. We suggest that the X-ray trail is either IGM gas gravitationally focused into a Bondi-Hoyle wake, a thermal mixture of ~60% Pavo IGM gas with ~40% galaxy gas that has been removed from the spiral galaxy NGC 6872 by turbulent viscous stripping, or both, due to the spiral galaxy's supersonic motion at angle ξ ~ 40° with respect to the plane of the sky, through the densest region of the Pavo IGM. Assuming ξ = 40° and a filling factor η in a cylindrical volume with radius 33 kpc and projected length 90 kpc, the mean electron density and total hot gas mass in the trail are 1 × 10-3η-1/2 cm-3 and 1.1 × 1010η1/2 M☉, respectively.

On the dynamics of the satellite galaxies in NGC 5044

The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy that is surrounded by ∼160 dwarf satellites. The projected number density profile of this dwarf population deviates within ∼1/3 of the virial radius from a projected Navarro, Frenk and White (NFW) profile, which is assumed to approximate the underlying total matter distribution. By means of a semi-analytic model, we demonstrate that the interplay between gravitation, dynamical friction and tidal mass loss and destruction can explain the observed number density profile. We use only two parameters in our models: the total to stellar mass fraction of the satellite haloes and the disruption efficiency. The disruption efficiency is expressed by a minimum radius. If the tidal radius of a galaxy (halo) falls below this radius, it is assumed to become unobservable. The preferred parameters are an initial total to stellar mass fraction of ∼20 and a disruption radius of 4 kpc. In that model, about 20 per cent of all the satellites are totally disrupted on their orbits within the group environment. Dynamical friction is less important in shaping the inner slope of the number density profile because the reduction in mass by tidal forces lowers the impact of the friction term. The main destruction mechanism is tide. In the preferred model, the total B-band luminosity of all disrupted galaxies is about twice the observed luminosity of the central elliptical galaxy, indicating that a significant fraction of stars are scattered into the intragroup medium. Dwarf galaxy satellites closer to the centre of the NGC 5044 group may exhibit optical evidence of partial tidal disruption. If dynamical friction forces the satellite to merge with the central elliptical, the angular momentum of the satellite tends to be removed at the apocentre passage. Afterwards, the satellite drops radially towards the centre.

Time‐dependent Circulation Flows: Iron Enrichment in Cooling Flows with Heated Return Flows

We describe a new type of dynamical model for hot gas in galaxy groups and clusters in which gas moves simultaneously in both radial directions. The observational motivations for this type of flow are compelling. X-ray spectra indicate that little or no gas is cooling to low temperatures. Bubbles of hot gas typically appear in Chandra X-ray images and XMM-Newton X-ray spectra within ~50 kpc of the central elliptical galaxy. These bubbles must be buoyant. Furthermore, the elemental composition and total mass of gas-phase iron observed within ~100 kpc of the center can be understood as the accumulated outflow of most or all of the iron produced by Type Ia supernovae in the central galaxy over time. This gaseous iron has been circulating for many gigayears, unable to cool. As dense inflowing gas cools, it produces a positive central temperature gradient, a characteristic feature of normal cooling flows. This gas dominates the local X-ray spectrum but shares the total available volume with centrally heated, outflowing gas. Circulating flows eventually cool catastrophically if the outflowing gas transports mass but no heat; to maintain the circulation both mass and energy must be supplied to the inflowing gas over a large volume, extending to the cooling radius. The rapid radial recirculation of gas within ~50 kpc results in a flat core in the gas iron abundance, similar to many group and cluster observations. We believe the circulation flows described here are the first gasdynamic, long-term evolutionary models that are in good agreement with all essential features observed in the hot gas: little or no gas cools as required by XMM spectra, the gas temperature increases outward near the center, and the gaseous iron abundance is about solar near the center and decreases outward.

An [ITAL]XMM-Newton[/ITAL] Observation of NGC 1399 Reveals Two Phases of Hot Gas and Supersolar Abundances in the Central Regions

We present an initial analysis of a new XMM-Newton observation of NGC 1399, the central elliptical galaxy of the Fornax group. Spectral fitting of the spatially resolved spectral data of the European Photon Imaging Camera MOS and pn CCDs reveals that a two-temperature (2T) model of the hot gas is favored over single-phase and cooling flow models within the central ~20 kpc. The preference for the 2T model applies whether or not the data are deprojected. The cooler component has a temperature (~0.9 keV) similar to the kinetic temperature of the stars, while the hotter component has a temperature (~1.5 keV) characteristic of the virial temperature of a ~1013 M☉ halo. The two-phase model (and other multitemperature models) removes the "Fe bias" within r ≲ 20 kpc and gives ZFe/Z☉ ≈ 1.5-2. At larger radii, the iron abundance decreases until ZFe/Z☉ ~ 0.5 for r ~ 50 kpc. The Si abundance is supersolar (1.2-1.7 solar) within the central regions, while ZSi/ZFe ≈ 0.8 over the entire region studied. These Fe and Si abundances imply that ≈80% of the Fe mass within r ~ 50 kpc originates from Type Ia supernovae (SNe Ia). This SNe Ia fraction is similar to that inferred for the Sun and therefore suggests a stellar initial mass function similar to the Milky Way.

Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). I. New CFHT MOS Spectroscopy and the Composite Database

We present a comprehensive database of kinematic, photometric, and positional information for 352 objects in the field of M87 (NGC 4486), the central giant elliptical galaxy in the Virgo Cluster; the majority of the tracers are globular clusters associated with that galaxy. New kinematic information comes from multislit observations with the Multi-Object Spectrograph (MOS) of the Canada-France-Hawaii Telescope (CFHT), an investigation that has added 96 new velocities to and confirmed many of the earlier values in a preexisting data set of 256 velocities published elsewhere. The photometry, consisting of magnitudes and colors in the Washington (T1, C-T1) system, is based on CCD observations made at the Cerro Tololo Inter-American Observatory (CTIO) and the Kitt Peak National Observatory (KPNO). The composite database represents the largest compilation of pure Population II dynamical tracers yet identified in any external galaxy; moreover, it extends to larger spatial scales than have earlier investigations. The inclusion of photometric information allows independent study of the distinct red and blue subpopulations of the bimodal globular cluster system of M87. In a companion paper we use this powerful data set to analyze the present dynamical state of the M87 globular cluster system and consider the question of its interaction and formation history.

A Possible Model of Persistent X‐Ray–luminous Gas around the Giant Elliptical Galaxy M87

We have investigated, through spherically symmetric hydrodynamical simulation, whether thermal conduction together with stellar activities (mass loss and Type Ia supernovae) can stabilize the X-ray-emitting gas near the center of elliptical galaxies. The configurations of the gas are divided into two distinct states, depending on the amount of intracluster gas surrounding the galaxy and the stellar mass-loss rate in the galaxy. One is a state of thermal balance between the radiative cooling and thermal conduction that can persist for more than 10 Gyr. The gas reaches nearly hydrostatic equilibrium in the galactic potential, with the gas density declining as r-1. The X-ray luminosity can be as bright as that observed in M87, depending on the initial intracluster gas densities. Subsolar Fe abundance of the gas is expected. The other state results in a cooling catastrophe near the center on a timescale of 107-108 yr owing to radiative cooling. This catastrophe arises when (1) the initial intracluster gas density is so low that the thermal energy in the intracluster gas is too low to carry enough energy inward and (2) the stellar mass-loss rate or the initial intracluster gas density is so large that the timescale of cooling outweighs that of conduction.

The Survival of the Core Fundamental Plane against Galactic Mergers

The basic dimensional properties of the centers of elliptical galaxies, such as length scale, luminosity, and velocity dispersion, lie on a fundamental plane similar to that of elliptical galaxies as a whole. The orientation of this plane, and the distribution of core parameters within it, point to a strong correlation of core density with either core or total luminosity, and indicate that low-luminosity ellipticals are much denser than high-luminosity galaxies (Hubble Space Telescope data suggest that this relationship may be as steep as ρc∝L−2). In addition, low-luminosity ellipticals have a much smaller length scale than their high-luminosity counterparts. Since we think that small galaxies are occasionally accreted by big ones, the high density of these galaxies and their likely durability against the time-varying tidal field of the bigger ones suggests that they will survive substantially intact in the cores of larger galaxies and would be easily visible. Their presence would destroy the observed correlation. Motivated by this apparent inconsistency between an observed fact and a simple physical argument, we have developed an effective simulation method and applied it to the problem of the accretion of very dense secondary companions by tenuous primaries. We have studied the accretion of objects of varying luminosity ratios, with sizes and densities drawn from the fundamental plane under the assumption that the mass distribution in the central parts of the galaxies follows the light. The results indicate that in mergers with mass ratios greater than 10, chosen with an appropriate central density dependence on luminosity, the smaller object is only stripped down to the highest density encountered in the primary during the accretion process. Thus, the form of the core fundamental plane suggests that the mass distribution in galaxy centers is different from the light distribution, or that an understanding of secondary survival requires more than the dynamics of visible stars.

Deep Imaging of AX J2019+112: The Luminosity of a “Dark Cluster”

We detect a distant cluster of galaxies centered on the QSO lens and luminous X-ray source AX J2019+112, a.k.a. the Dark Cluster. Using deep V and I Keck images and wide-field Ks imaging from the New Technology Telescope (NTT), a tight red sequence of galaxies is identified within a radius of 0.2 h-1 Mpc of the known z = 1.01 elliptical lensing galaxy. The sequence, which includes the central elliptical galaxy, has a slope in good agreement with the model predictions of Kodama et al. for z ~ 1. We estimate the integrated rest-frame luminosity of the cluster to be LV ≥ 3.2 × 1011 h-2 L☉ (after accounting for significant extinction at the low latitude of this field), more than an order of magnitude higher than previous estimates. The central region of the cluster is deconvolved using the technique of Magain, Courbin, & Sohy, revealing a thick central arc coincident with an extended radio source. All the observed lensing features are readily explained by differential magnification of a radio-loud active galactic nucleus by a shallow elliptical potential. The QSO must lie just outside the diamond caustic, producing two images; the arc is a highly magnified image formed from a region close to the center of the host galaxy, projecting inside the caustic. The mass-to-light ratio within an aperture of 0.4 h-1 Mpc is MX/LV = 224h(M/LV)☉, using the X-ray temperature. The strong lens model yields a compatible value, M/LV = 372h(M/LV)☉, whereas an independent weak-lensing analysis sets an upper limit of M/LV < 520h(M/LV)☉, typical of massive clusters.

Intracluster Planetary Nebulae in the Virgo Cluster. I. Initial Results

We report the initial results of a survey for intracluster planetary nebulae in the Virgo Cluster. In two 16' × 16' fields, we identify 69 and 16 intracluster planetary nebula candidates, respectively. In a third 16' × 16' field near the central elliptical galaxy M87, we detect 75 planetary nebula candidates, of which a substantial fraction are intracluster in nature. By examining the number of the planetaries detected in each field and the shape of the planetary nebula luminosity function, we show that (1) the intracluster starlight of Virgo is distributed nonuniformly and varies between subclumps A and B, (2) the Virgo Cluster core extends ~3 Mpc in front of M87, and thus is elongated along the line of sight, and (3) a minimum of 22% of Virgo's stellar luminosity resides between the galaxies in our fields, and the true number may be considerably larger. We also use our planetary nebula data to argue that the intracluster stars in Virgo are likely derived from a population that is of moderate age and metallicity.

M87, Globular Clusters, and Galactic Winds: Issues in Giant Galaxy Formation

We have used the high-resolution camera at the Canada-France-Hawaii Telescope to obtain VRI photometry of the globular clusters in the innermost 140'' of the M87 halo. The results are used to discuss several issues concerning the formation and evolution of globular cluster systems in supergiant elliptical galaxies like M87. Our principal results are as follows: (1) From our deep R-band photometry of the cluster population, we find no significant change in the globular cluster luminosity function (GCLF) with galactocentric radius, for cluster masses M 105 M☉. This result places constraints on current theoretical predictions of the rate of cluster evolution by tidal shocking and evaporation, indicating that the main effects of dynamical evolution may be felt only on lower mass clusters (105 M☉) that are below the faint limit of most current observations. (2) Combining our V - I color indices with other data in the literature, we derive the metallicity gradient and mean metallicity of the clusters from r = 9'' out to r 500'' (~0.7–35 kpc). Within the core radius rc = 1' of the globular cluster system, the metallicity distribution is uniform, but at larger radii the mean metallicity declines steadily as Z/Z☉ ~ r-0.9. (3) The various options for explaining the existence of high specific frequency galaxies like M87 are evaluated. We argue that alternatives involving secondary evolution (such as the formation of many globular clusters during mergers, or the existence of a population of intergalactic globular clusters centered on the same location as the central elliptical galaxy) are capable of modifying the characteristics of the globular cluster system in distinctive ways, but are not likely to be the primary explanation for high-SN galaxies. (4) We offer a new explanation for the large observed SN range among the brightest cluster elliptical galaxies (BCGs). It is suggested that these central supergiant E galaxies formed in an exceptionally turbulent or high-density environment which favored a very rapid initial star formation burst. As a result, a much higher than average fraction of the protogalactic gas was driven out in a galactic wind not long after the first globular clusters were formed, thus artificially boosting the specific frequency of the remaining galaxy. From a total sample of 30 BCGs, we derive empirical scaling relations that relate to this hypothesis. Our analysis favors the view that BCGs began forming at redshifts z 5, distinctly earlier than most other galaxies.

Gravitational Lensing in Low-Redshift Clusters of Galaxies: The Arclike Object in Abell 3408 and Its Lensing Interpretation

We analyze the seldom discussed lensing effects expected in low-z clusters (z = 0.05-0.15), using as an example the bright arc (z=0.073) discovered by Campusano & Hardy near the central elliptical galaxy of the cluster Abell 3408 (z=0.042). The photometric and spectroscopic observations for both the central elliptical and the arc are presented. The mass distribution in A3408 is modeled by scaled versions of two representative distributions derived from studies of clusters at higher redshifts: (i) a minimum mass case where the mass distribution follows the light profile of the central elliptical galaxy and (ii) a maximum mass case where a typical massive dark halo is added to the previous case. The observed arc is well reproduced by both models, but rather small magnifications of the source galaxy are implied. The source galaxy is tentatively identified in both the lensing and nonlensing scenarios as being a spiral. The smaller lensed spiral (14.6 h -->50−1 kpc, MB = -18.2) predicted by the dark halo model appears to fit the observations marginally better. Furthermore, we found that only the dark halo model predicts a measurable amount of weak shear in the images of faint background galaxies. We conclude that observations, under very good seeing conditions, of weak shear in faint background galaxies in the direction of low-redshift galaxy clusters are possible. The combination of the latter with X-ray data can provide a powerful tool for probing the mass distribution in the very central region of galaxy clusters.

The Properties of Poor Groups of Galaxies. II. X‐Ray and Optical Comparisons

We use ROSAT PSPC data to study the X-ray properties of a sample of 12 poor groups that have extensive membership information. Diffuse X-ray emission is detected in nine of these groups. In all but one of the X-ray-detected groups, the X-ray emission is centered on a luminous elliptical galaxy. Fits to the surface brightness profiles of the X-ray emission suggest the presence of two X-ray components in these groups. The first component is centered on the central elliptical galaxy and is extended on scales of 20-40 h-1 kpc. The location and extent of this component, combined with its X-ray temperature (~0.7- 0.9 keV) and luminosity (~1041-42 h-2 ergs s-1), favor an origin in the interstellar medium of the central galaxy. Alternatively, the central component may be the result of a large-scale cooling flow. The second X-ray component is detected out to a radius of at least ~100-300 h-1 kpc. This component follows the same relationships found among the X-ray temperature (T), X-ray luminosity (LX), and optical velocity dispersion (σr) of rich clusters. This result suggests that the X-ray-detected groups are low-mass versions of clusters and that the extended gas component can properly be called the intragroup medium, by analogy to the intracluster medium in clusters. The failure to detect an intragroup medium in the three groups with very low velocity dispersions is consistent with their predicted X-ray luminosities and temperatures based on the relationships derived for clusters and X-ray-detected groups. The best-fit value of β derived from the σr-T relationship for groups and clusters is ~0.99 ± 0.08, implying that the galaxies and the hot gas trace the same potential with equal energy per unit mass and that the groups are dynamically relaxed. We also find a trend for the position angle of the optical light in the central elliptical galaxy to align with the position angle of the large-scale X-ray emission. This trend is consistent with that found for some rich clusters containing cD galaxies. The alignment of the central galaxy with the extended X-ray emission suggests that the formation and/or evolution of the central galaxy is linked to the shape of the global group potential. One possible scenario is that the central galaxy formed via galaxy-galaxy mergers early in the lifetime of the group and has not been subject to significant dynamical evolution recently.

Discovery of an Abundance Gradient in the Central Region of the Virgo Cluster

We present the ASCA results of spatially resolved X-ray spectra in the central regions of the Virgo cluster, near to the central elliptical galaxy M87 and 40' northwest of M87 (M87 NW). Since the spectra of the M87 region is complex, an adequate fit requires at least two thin thermal-plasma components. The temperatures of the hot and cool components are approximately 3.0 and 1.3 keV, respectively, and the temperature of these components are both nearly constant over the central 10' radius from the center of M87. The spectrum of M87 NW, on the other hand, can be well fitted by a single thin thermal-plasma model of ∼ 2.7 keV temperature. The abundances of Fe, Si, and S rise towards the center of M87, though the ratios of Si/Fe and S/Si remain constant. We also found, contrary to the Einstein Observatory's results, that the abundance ratio of oxygen to iron (O/Fe) is smaller than the solar value.

Line Strength Gradients in Elliptical and Brightest Cluster Galaxies

view Abstract Citations (119) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Line Strength Gradients in Elliptical and Brightest Cluster Galaxies Fisher, David ; Franx, Marijn ; Illingworth, Garth Abstract Line strengths and their gradients in Mg, Fe, and Hβ have been determined for seven elliptical and nine brightest cluster galaxies (BCGs) in order to study their stellar populations and investigate their relationship to one another. We find that BCGs follow the same relationship between central Mg b line strength and central velocity dispersion found for elliptical galaxies. Brightest cluster galaxies are in agreement with the known trend toward more massive elliptical galaxies having larger [Mg/Fe] ratios, while the internal gradients within our BCG and E galaxies are consistent with a roughly constant [Mg/Fe] ratio. We find that a correlation exists between the central [Mg/Fe] ratio and average Hβ line strength in the sense that both BCG and elliptical galaxies with larger [Mg/Fe] ratios have lower <Hβ> strengths. For our sample, Hβ is the best predictor of [Mg/Fe] ratio. If the dominant contribution to the Hβ feature is from turnoff stars then this relation predicts that more massive elliptical galaxies are older than less massive ones. If, however, the main source of the H index is from horizontal-branch stars, then the observed {[Mg/Fe],HP} relation could be the result of more massive elliptical galaxies having flatter IMFs for high-mass stars than less massive elliptical galaxies. The <Hβ> line strengths of the objects in our sample span a range of values. The BCGs generally have low global Hβ line strengths, which, under the assumption that the Hβ feature can be used as an age discriminant, indicates that the bulk of these systems underwent their last major episode of star formation ≳ 8-10 Gyr ago. For both the elliptical galaxies and BCGs we find that within a galaxy, the Hβ profile is flat for objects whose Hβ absorption can be reliably measured. In the presence of a declining metallicity gradient this suggests that the centers of elliptical galaxies and BCGs are ∼1-3 Gyr younger than their outer regions. The metal line strength gradients for the elliptical galaxies and BCGs are similar and consistent with an `average gradient of Δ[Fe/H]/Δ log r = -0.25±0.1, corresponding to a reduction in the mean metallicity of the stellar population by a factor of ∼2 over a factor of 10 in radius. No strong correlations are found between the metallicity gradient sizes and either kinematic or line strength parameters of the galaxies. For the galaxy NGC 4073, the brightest member of the poor cluster MKW 4, we find enhanced Hβ absorption in the region coincident with the counterrotating stellar core. These observations are consistent with a scenario in which the accreted object was a younger system and/or a recent episode of star formation was triggered by an interaction. Publication: The Astrophysical Journal Pub Date: July 1995 DOI: 10.1086/175945 Bibcode: 1995ApJ...448..119F Keywords: GALAXIES: ABUNDANCES; GALAXIES: ELLIPTICAL AND LENTICULAR; CD; GALAXIES: STELLAR CONTENT full text sources ADS | data products SIMBAD (18) NED (17)

Non-stationary disks in the centers of elliptical galaxies

Abstract A model for the disk formation in central region of rotating triaxial elliptical galaxies is proposed. The disks form as a result of the matter accretion from the periphery of the rotating galaxy.

Scanning Fabry-Perot observations of multiple profiles of faint and extended gas in the center of elliptical galaxies

AbstractSome elliptical galaxies possess an extended gaseous disk which is usually detected from Hα and [Nil] emission. 3-D ”Cigale” observations of two of these objects, NGC 1052 and NGC 404, show the evidence of peculiar dynamics a nd in one case multiple profiles of faint extended gas which are probably from different origins. In NGC 1052, the decoupled kinematics of the gas and the stellar systems favors the hypothesis of an external origin of the interstellar matter, while in NGC 404, coherence of the stars and gas axis, as well as the low gas velocity dispersion favors an inner origin of the gas from the stars themselves.