Aims This is the third paper of a series devoted to the study of the stellar content of early-type galaxies The goal of the series is to set constraints on the evolutionary status of these objects Methods We present line-strength gradients for 22 spectral indices measured in a sample of 82 early-type galaxies in different environments, including the high-density cores of the Coma cluster, the Virgo cluster, poor groups, and isolated field galaxies Using new evolutionary population synthesis models, we derive age and metallicity gradients, and compare the mean values with the predictions of different galaxy formation models We explore the behaviour of individual chemical species by deriving the metallicity gradient with different indicators Results We find that the strength of the metallicity gradient inferred from stellar population models depends on the specific Lick index employed In particular, metallicity gradients obtained with CN2 and C4668 combined with H beta are steeper than those measured using Ca4227 or Fe4383 The correlation of the metallicity gradients with other parameters also depends on the specific index employed If the metallicity gradient is obtained using CN2 and Mgb, then it correlates with the central age of the galaxies On the contrary, if Fe4383 or Ca4227 is used, the metallicity gradient correlates with the velocity dispersion gradient Conclusions This may suggest that several mechanisms have helped to set the age and metallicity gradients in early-type galaxies While we do not find any correlation between the metallicity gradient and the central velocity dispersion for galaxies in low-density environments, we find a marginal correlation between the metallicity gradient and the mass for galaxies in the centre of the Coma cluster We also find a trend for which galaxies in denser environments show a steeper metallicity gradient than galaxies in less dense environments We interpret these results in light of the different mechanisms proposed to explain the observed changes between galaxies as a function of environment

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Stellar populations of early-type galaxies in different environments - III. Line-strength gradients

Aims. This is the second paper of a series devoted to study the stell ar content of early-type galaxies. The goal of the series is t o set constraints on the evolutionary status of these objects Methods. We use a new set of models which include an improved stellar library (MILES) to derive simple stellar population (SSP)-equivalent parameters in a sample of 98 early-type galaxies. The sample contains galaxies in the field, poor groups, and galaxies in t he Virgo and Coma clusters. Results. We find that low-density environment galaxies span a larger r ange in SSP age and metallicity than their counterparts in high density

Stellar populations of early-type galaxies in different environments II

New developments in understanding the stellar populations of ! elliptical nuclei are discussed. Variations in iron-peak spectral j features have recently been detected, which are much smaller than \\ changes in CN, Mg, and Na. A study is presently in progress to I determine whether this pattern is consistent with lockstep enhancements j in all heavy elements. Present evidence on the ultraviolet spectra of | ellipticals now suggests that the source of the excess UV light is not I young OB stars. A new correlation is also pointed out between UV j excess and mean line strength. If real, this trend is further evidence j against significant numbers of young OB stars in elliptical galaxies. j To first approximation, only two physical parameters are needed to j characterize the spectra of old stellar populations, namely stellar I age (or rate of star formation versus time) and composition. My j remarks today concern these two quantities as they apply to ellipticalI galaxy populations. Consider composition first. It has long been known that E-galaxy spectra exhibit a sequence in line strength and color, exemplified both by variations among galaxies and by radial gradients within j galaxies. As with the phenomenologically similar trend among globular \\ clusters, this sequence has conventionally been interpreted in terms of metallicity variations. Attempts to calibrate the sequence in terms of iron abundance have typically shown that the strongest-lined ellipticals are between two and four times more metal-rich than M32 (Faber 1973; Oke and Schwarzschild 1975; Larson and Tinsley 1974; Cohen 1979). A weakness of these previous calibrations, however, is the fact that they depend largely on the very large line-strength changes seen only in CN, Mg, and Na. No iron-peak features have as yet been used. Counter to the usual metallicity interpretation, a few authors have proposed instead that only the elements N, Mg, and Na are changing

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The stellar content of elliptical nuclei

Deep far-infrared (FIR) imaging data obtained with ISOPHOT at 90 mum, 150 mum, and 200 mum detected the thermal emission from cold dust in the northern shell region of NGC5128 (Centaurus A), where previously neutral hydrogen and molecular gas has been found. A somewhat extended FIR emission region is present in both the 150 mum and 200 mum map, while only an upper flux limit could be derived from the 90 mum data. The FIR spectral energy distribution can be reconciled with a modified blackbody spectrum with very cold dust color temperatures and emissivity indices in the range 13K beta > 1, respectively, where the data favor the low temperature end. A representative value for the associated dust mass is M(Dust) approximate to 7 x 10(4) M(.), which together with the HI gas mass gives a gas-to-dust ratio of approximate to300, close the average values of normal inactive spiral galaxies. This value, in conjunction with the atomic to molecular gas mass ratio typical for a spiral galaxy, indicates that the interstellar medium (ISM) from the inner part of a captured disk galaxy is likely the origin of the outlying gas and dust. These observations are in agreement with recent theoretical considerations that in galaxy interactions leading to stellar shell structures the less dissipative clumpy component of the ISM from the captured galaxy can lead to gaseous shells. Alternatively, the outlying gas and dust could be a rotating ring structure resulting from an interaction or even late infall of tidal material of a merger in the distant past. With all three components ( atomic gas, molecular gas, dust) of the ISM present in the northern shell region, local star formation may account for the chains of young blue stars surrounding the region to the east and north. The dust cloud may also be involved in the disruption of the large scale radio jet before entering the brighter region of the northern radio lobe.

https://www.aanda.org/articles/aa/pdf/2004/07/aa0314.pdf

First detection of cold dust in the northern shell of NGC5128 (Centaurus A)

We present new radio and optical images of the nearest radio galaxy Centaurus A and its host galaxy NGC 5128. We focus our investigation on the northern transition region, where energy is transported from the similar to 5 kpc (similar to 5 arcmin) scales of the northern inner lobe (NIL) to the similar to 30 kpc (similar to 30 arcmin) scales of the northern middle lobe (NML). Our Murchison Widefield Array observations at 154 MHz and our Parkes radio telescope observations at 2.3 GHz show diffuse radio emission connecting the NIL to the NML, in agreement with previous Australia Telescope Compact Array observations at 1.4 GHz. Comparison of these radio data with our wide-field optical emission-line images show the relationship between the NML radio emission and the ionized filaments that extend north from the NIL, and reveal a new ionized filament to the east, possibly associated with a galactic wind. Our deep optical images show clear evidence for a bipolar outflow from the central galaxy extending to intermediate scales, despite the non-detection of a southern radio counterpart to the NML. Thus, our observational overview of Centaurus A reveals a number of features proposed to be associated with active galactic nucleus feedback mechanisms, often cited as likely to have significant effects in galaxy evolution models. As one of the closest galaxies to us, Centaurus A therefore provides a unique laboratory to examine feedback mechanisms in detail.

The jet/wind outflow in Centaurus A: A local laboratory for AGN feedback

For a number of places in the bulge of M31, and for two places in M32, photometric scans from lambda = 3300 A to lambda = 10,600 A have been obtained with the multichannel spectrometer on the 5-m Hale telescope. The scans show that in both objects the color temperature (particularly shortward of 5000 A) decreases toward the center, and that the strength of the CN bands increases toward the center in both objects, in agreement with other earlier observations. The new data can all be interpreted in terms of an increase of heavy-element abundance toward the center in both objects by a factor probably less than 2, and by an excess of heavy elements in M31 compared with M32 by a factor probably greater than 2, in qualitative agreement with earlier conclusions of other observers.

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Absolute spectrophotometry in M31 and M32

For a number of places in the bulge of M31 and for two places in M32 photometric scans from 3300 A to 10,600 A have been obtained with the multichannel spectrometer on the 5-meter Hale telescope. The scans show that in both objects the color temperature (particularly shortwards of 5000 A) decreases towards the center and that the strength of the CN bands increases towards the center in both objects in agreement with earlier observations. The new data can all be interpreted in terms of an increase of heavy element abundance towards the center in both objects by a factor probably less than 2 and by an excess of heavy elements in M31 compared to M32 by a factor probably greater than 2, in qualitative agreement with earlier conclusions.

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J. B. Oke

Papers

Absolute spectrophotometry in M31 and M32

Absolute spectrophotometry in M31 and M32