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Register a new userStellar populations in the centres of brightest cluster galaxies
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Added by
Susan Ilani Loubser
Publication date
2009
fields
Physics
and research's language is
English
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This paper is part of a series devoted to the study of the stellar populations in brightest cluster galaxies (BCGs), aimed at setting constraints on the formation and evolution of these objects. We have obtained high signal-to-noise ratio, long-slit spectra of 49 BCGs in the nearby Universe. Here, we derive Single Stellar Population (SSP)-equivalent ages, metallicities and alpha-abundance ratios in the centres of the galaxies using the Lick/IDS system of absorption line indices. We systematically compare the indices and derived parameters for the BCGs with those of large samples of ordinary elliptical galaxies in the same mass range. We find no significant differences between the index-velocity dispersion relations of the BCG data and those of normal ellipticals, but we do find subtle differences between the derived SSP-parameters. The BCGs show, on average, higher metallicity ([Z/H]) and alpha-abundance ([E/Fe]) values. We analyse possible correlations between the derived parameters and the internal properties of the galaxies (velocity dispersion, rotation, luminosity) and those of the host clusters (density, mass, distance from BCG to X-ray peak, presence of cooling flows), with the aim of dissentangling if the BCG properties are more influenced by their internal or host cluster properties. The SSP-parameters show very little dependence on the mass or luminosity of the galaxies, or the mass or density of the host clusters. Of this sample, 26 per cent show luminosity-weighted ages younger than 6 Gyr, probably a consequence of recent - if small - episodes of star formation. In agreement with previous studies, the BCGs with intermediate ages tend to be found in cooling-flow clusters with large X-ray excess.
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We present the stellar population and velocity dispersion gradients for a sample of 24 brightest cluster galaxies (BCGs) in the nearby Universe for which we have obtained high quality long-slit spectra at the Gemini telescopes. With the aim of studying the possible connection between the formation of the BCGs and their host clusters, we explore the relations between the stellar population gradients and properties of the host clusters as well as the possible connections between the stellar population gradients and other properties of the galaxies. We find mean stellar population gradients (negative {Delta}[Z/H]/log r gradient of -0.285{pm}0.064; small positive {Delta}log (age)/log r gradient of 0.069{pm}0.049; and null {Delta}[E/Fe]/log r gradient of -0.008{pm}0.032) that are consistent with those of normal massive elliptical galaxies. However, we find a trend between metallicity gradients and velocity dispersion (with a negative slope of -1.616{pm}0.539) that is not found for the most massive ellipticals. Furthermore, we find trends between the metallicity gradients and K-band luminosities (with a slope of 0.173{pm}0.081) as well as the distance from the BCG to the X-ray peak of the host cluster (with a slope of -7.546{pm}2.752). The latter indicates a possible relation between the formation of the cluster and that of the central galaxy.
We present an examination of the kinematics and stellar populations of a sample of 3 Brightest Group Galaxies (BGGs) and 3 Brightest Cluster Galaxies (BCGs) in X-ray groups and clusters. We have obtained high signal-to-noise Gemini/GMOS (Gemini South Multi-Object Spectrograph) long-slit spectra of these galaxies and use Lick indices to determine ages, metallicities and alpha-element abundance ratios out to at least their effective radii. We find that the BGGs and BCGs have very uniform masses, central ages and central metallicities. Examining the radial dependence of their stellar populations, we find no significant velocity dispersion, age, or alpha-enhancement gradients. However, we find a wide range of metallicity gradients, suggesting a variety of formation mechanisms. The range of metallicity gradients observed is surprising given the homogeneous environment these galaxies probe and their uniform central stellar populations. However, our results are inconsistent with any single model of galaxy formation and emphasize the need for more theoretical understanding of both the origins of metallicity gradients and galaxy formation itself. We postulate two possible physical causes for the different formation mechanisms.
We present deep J and Ks band photometry of 20 high redshift galaxy clusters between z=0.8-1.5, 19 of which are observed with the MOIRCS instrument on the Subaru Telescope. By using near-infrared light as a proxy for stellar mass we find the surprising result that the average stellar mass of Brightest Cluster Galaxies (BCGs) has remained constant at ~9e11MSol since z~1.5. We investigate the effect on this result of differing star formation histories generated by three well known and independent stellar population codes and find it to be robust for reasonable, physically motivated choices of age and metallicity. By performing Monte Carlo simulations we find that the result is unaffected by any correlation between BCG mass and cluster mass in either the observed or model clusters. The large stellar masses imply that the assemblage of these galaxies took place at the same time as the initial burst of star formation. This result leads us to conclude that dry merging has had little effect on the average stellar mass of BCGs over the last 9-10 Gyr in stark contrast to the predictions of semi-analytic models, based on the hierarchical merging of dark matter haloes, which predict a more protracted mass build up over a Hubble time. We discuss however that there is potential for reconciliation between observation and theory if there is a significant growth of material in the intracluster light over the same period.
This paper is part of a series devoted to the investigation of a large sample of brightest cluster galaxies (BCGs), their properties and the relationships between these and the properties of the host clusters. In this paper, we compare the stellar population properties derived from high signal-to-noise, optical long-slit spectra with the GALEX ultraviolet (UV) colour measurements for 36 nearby BCGs to understand the diversity in the most rapidly evolving feature in old stellar systems, the UV-upturn. We investigate: (1) the possible differences between the UV-upturn of BCGs and those of a control sample of ordinary ellipticals in the same mass range, as well as possible correlations between the UV-upturn and other general properties of the galaxies; (2) possible correlations between the UV-upturn and the properties of the host clusters; (3) recently proposed scenarios where helium-sedimentation in the cluster centre can produce an enhanced UV-upturn. We find systematic differences between the UV-colours of BCGs and ordinary ellipticals, but we do not find correlations between these colours and the properties of the host clusters. Furthermore, the observations do not support the predictions made by the helium-sedimentation model as an enhancer of the UV-upturn.
We use a combination of deep optical and near-infrared light profiles for a morphologically diverse sample of Virgo cluster galaxies to study the radially-resolved stellar populations of cluster galaxies over a wide range of galaxy structure. We find that, in the median, the age gradients of Virgo galaxies are either flat (lenticulars and Sa-Sb spirals) or positive (ellipticals, Sbc+Sc spirals, gas-rich dwarfs, and irregulars), while all galaxy types have a negative median metallicity gradient. Comparison of the galaxy stellar population diagnostics (age, metallicity, and gradients thereof) against structural and environmental parameters also reveals that the ages of gas-rich systems depend mainly on their atomic gas deficiencies. Conversely, the metallicities of Virgo gas-poor galaxies depend on their concentrations, luminosities, and surface brightnesses. The stellar population gradients of all Virgo galaxies exhibit no dependence on either their structure or environment. We interpret these stellar population data for Virgo galaxies in the context of popular formation and evolution scenarios, and suggest that gas-poor giants grew hierarchically (through dissipative starbursts), gas-poor dwarfs have descended from at least two different production channels (e.g., environmental transformation and merging), while spirals formed inside-out, but with star formation in the outskirts of a significant fraction of the population having been quenched due to ram pressure stripping. (Abridged)
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