Description of Collections and Encyclopaedias on the Web using XML

Michard, Alain; Pham-Dac, Giang

doi:10.1023/a:1009076406097

Cited by 7 publications

(5 citation statements)

References 1 publication

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“…twists are also smaller than for core galaxies, but the text shows that the relative amounts of light involved in the twist phenomenon are the same in both types of galaxy. The strengths of the color gradients superficially appear to depend on the profile type of the galaxy, as already suggested by Figure 11 of Carollo et al (1997) and by high-resolution groundbased images of galaxy centers (Michard 1998); however, this effect appears to be largely due to the greater prevalence of dust in power-law galaxies. Figure 10 plots the color gradients as a function of the limiting logarithmic profile slope at the resolution limit, 0 ; the symbol type encodes the central dust indices given in Table 3, with open symbols corresponding to galaxies with dust absorption greater than 0.02 mag.…”

Section: Central Color Gradientssupporting

confidence: 53%

The Centers of Early-Type Galaxies withHubble Space Telescope. V. New WFPC2 Photometry

et al. 2005

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We present observations of 77 early-type galaxies imaged with the PC1 CCD of the Hubble Space Telescope (HST ) WFPC2. ''Nuker-law'' parametric fits to the surface brightness profiles are used to classify the central structure into ''core'' or ''power-law'' forms. Core galaxies are typically rounder than power-law galaxies. Nearly all power-law galaxies with central ellipticities ! 0:3 have stellar disks, implying that disks are present in powerlaw galaxies with < 0:3 but are not visible because of unfavorable geometry. A few low-luminosity flattened core galaxies also have disks; these may be transition forms from power-law galaxies to more luminous core galaxies, which lack disks. Several core galaxies have strong isophote twists interior to their break radii, although power-law galaxies have interior twists of similar physical significance when the photometric perturbations implied by the twists are evaluated. Central color gradients are typically consistent with the envelope gradients; core galaxies have somewhat weaker color gradients than power-law galaxies. Nuclei are found in 29% of the core galaxies and 60% of the power-law galaxies. Nuclei are typically bluer than the surrounding galaxy. While some nuclei are associated with active galactic nuclei (AGNs), just as many are not; conversely, not all galaxies known to have a low-level AGN exhibit detectable nuclei in the broadband filters. NGC 4073 and 4382 are found to have central minima in their intrinsic starlight distributions; NGC 4382 resembles the double nucleus of M31. In general, the peak brightness location is coincident with the photocenter of the core to a typical physical scale of <1 pc. Five galaxies, however, have centers significantly displaced from their surrounding cores; these may be unresolved asymmetric double nuclei. Finally, as noted by previous authors, central dust is visible in about half of the galaxies. The presence and strength of dust correlates with nuclear emission; thus, dust may outline gas that is falling into the central black hole. The prevalence of dust and its morphology suggest that dust clouds form, settle to the center, and disappear repeatedly on $10 8 yr timescales. We discuss the hypothesis that cores are created by the decay of a massive black hole binary formed in a merger. Apart from their brightness profiles, there are no strong differences between core galaxies and power-law galaxies that demand this scenario; however, the rounder shapes of core, their lack of disks, and their reduced color gradients may be consistent with it.

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Section: Central Color Gradientssupporting

confidence: 53%

The Centers of Early-Type Galaxies withHubble Space Telescope. V. New WFPC2 Photometry

et al. 2005

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“…This correlation has been investigated here, taking determinations of core/power-law and values of γ and a(4)/a from the work of Pellegrini (1999), Rest et al (2001), Ravindranath et al (2002), Michard (1998), Faber et al (1997 and Bender et al (1989). For galaxies appearing in more than one paper, preference was in the order of the papers as listed, with the following exceptions.…”

Section: Scatter As a Function Of Core Profilementioning

confidence: 99%

Correlations of near-infrared, optical and X-ray luminosity for early-type galaxies

Ellis¹,

O’Sullivan²

2006

Monthly Notices of the Royal Astronomical Society

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The relation between X‐ray luminosity and near‐infrared (NIR) luminosity for early‐type galaxies has been examined. NIR luminosities should provide a superior measure of stellar mass compared to optical luminosities used in previous studies, especially if there is significant star formation or dust present in the galaxies. However, we show that the X‐ray–NIR relations are remarkably consistent with the X‐ray–optical relations. This indicates that the large scatter of the relations is dominated by scatter in the X‐ray properties of early‐type galaxies, and is consistent with early‐types consisting of old, quiescent stellar populations. We have investigated scatter in terms of environment, surface brightness profile, Mg2, Hβ, Hγ line strength indices, spectroscopic age and nuclear Hα emission. We found that galaxies with high Mg2 index, low Hβ and Hγ indices or a ‘core’ profile have a large scatter in LX, whereas galaxies with low Mg2, high Hβ and Hγ indices or ‘power‐law’ profiles generally have LX < 1041 erg s−1. There is no clear trend in the scatter with environment or nuclear Hα emission.

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“…Scorza & Bender (1995) have shown that discy ellipticals follow the same trend as observed for spirals and S0s that discs with smaller scale lengths have higher central surface brightness (e.g., Kent 1985). This strongly suggests that discy ellipticals are two-component systems that form a continuing sequence in the Hubble diagram from S0s to galaxies with smaller disc-to-bulge ratios (see also Michard 1984;Capaccioli, Caon & Rampazzo 1990).…”

Section: Introductionmentioning

confidence: 74%

Self-consistent, axisymmetric two-integral models of elliptical galaxies with embedded nuclear discs

Bosch

Zeeuw

1996

Monthly Notices of the Royal Astronomical Society

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Ionized gas discs in the nuclei of ellipticals have proven to be excellent tools for the determination of the central mass density in these galaxies. The recent discovery with the Hubble Space Telescope of small stellar discs embedded in the nuclei of a number of ellipticals and S0s might be of similar importance. We construct two-integral axisymmetric models for such systems. The models consist of a spheroidal bulge with a central density cusp, and a disc described by a strongly flattened exponential spheroid. We use the Hunter & Qian (1993) method to calculate the even part of the phase-space distribution function (DF), and specify the odd part by means of a simple parameterization. We consider both local stability against axisymmetric perturbations, as well as global stability against bar forming modes, and find that our models are stable as long as the discs are not too flat and/or compact. The margin of stability is derived as a function of disc scalelength and central surface density. Its location agrees well with the observed values of these disc parameters. This suggests that discs build up their mass until they become marginally stable.We investigate the photometric as well as the kinematic signatures of nuclear discs, including their velocity profiles (VPs), and study the influence of seeing convolution. In particular, we study to what extent these kinematic signatures can be used to determine the central density of the galaxy, and to test for the presence of massive black holes. We consider nuclear discs that are either dynamically coupled to or decoupled from the host elliptical, including counter-rotating discs. The latter are models for the counter-rotating cores observed in a number of galaxies, which are often found to exhibit discy isophotes in the central region. The counter-rotation is only detectable when the disc light contributes significantly to the central velocity profiles. We find that in this case the observed velocity dispersion will show a central decrease.The rotation curve of a nuclear disc gives an excellent measure of the central massto-light ratio whenever the VPs clearly reveal the narrow, rapidly rotating component associated with the nuclear disc. Steep cusps and seeing convolution both result in central VPs that are dominated by the bulge light, and these VPs barely show the presence of the nuclear disc, impeding measurements of the central rotation velocities of the disc stars. However, if a massive BH is present, the disc component of the VP can be seen in the wing of the bulge part, and measurements of its mean rotation provide a clear signature of the presence of the BH. This signature is insensitive to the uncertainties in the velocity anisotropy, which often lead to ambiguity in the interpretation of a central rise in velocity dispersion as due to a central BH.

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Description of Collections and Encyclopaedias on the Web using XML

Cited by 7 publications

References 1 publication

The Centers of Early-Type Galaxies withHubble Space Telescope. V. New WFPC2 Photometry

The Centers of Early-Type Galaxies withHubble Space Telescope. V. New WFPC2 Photometry

Correlations of near-infrared, optical and X-ray luminosity for early-type galaxies

Self-consistent, axisymmetric two-integral models of elliptical galaxies with embedded nuclear discs

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