arXiv subjects

D. Thomas

Publications and source records attributed to D. Thomas.

227 records · Page 13Linked to original sources

arXiv2003

Spatially resolved spectroscopy of Coma cluster early -- type galaxies: III. The stellar population gradients

We derive central values and logarithmic gradients for the Hbeta, Mg and Fe indices of 35 early-type galaxies in the Coma cluster. We find that pure elliptical galaxies have on average slightly higher velocity dispersions, lower Hbeta, and higher metallic line-strengths than galaxies with disks (S0). The gradients strongly correlate with the gradients of sigma, but only weakly with the central index values and galaxy velocity dispersion. Using stellar population models with variable element abundance ratios from Thomas, Maraston & Bender (2003a) we derive average ages, metallicities and [alpha/Fe] ratios in the center and at the effective radius. We find that the [alpha/Fe] ratio correlates with velocity dispersion and drives 30% of the Mg-sigma relation, the remaining 70% being caused by metallicity variations. We derive negative metallicity gradients (-0.16 dex per decade) that are significantly flatter than what is expected from gaseous monolithic collapse models, pointing to the importance of mergers in the galaxy formation history. The gradients in age are negligible, implying that no significant residual star formation has occurred either in the center or in the outer parts of the galaxies, and that the stellar populations at different radii must have formed at a common epoch. For the first time we derive the gradients of the [alpha/Fe] ratio and find them very small on the mean. Hence, [alpha/Fe] enhancement is not restricted to galaxy centers but it is a global phenomenon. Our results imply that the Mg-sigma local relation inside a galaxy, unlike the global Mg-sigma relation, must be primarily driven by metallicity variations alone.

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arXiv2002

Kinematics and stellar populations of 17 dwarf early-type galaxies

We present kinematics and stellar population properties of 17 dwarf early-type galaxies in the luminosity range -14> M_B> -19. Our sample fills the gap between the intensively studied giant elliptical and Local Group dwarf spheroidal galaxies. The dwarf ellipticals of the present sample have constant velocity dispersion profiles within their effective radii and do not show significant rotation, hence are clearly anisotropic. The dwarf lenticulars, instead, rotate faster and are, at least partially, supported by rotation. From optical Lick absorption indices, we derive metallicities and element abundances. Combining our sample with literature data of the Local Group dwarf spheroidals and giant ellipticals, we find a surprisingly tight linear correlation between metallicity and luminosity over a wide range: -8> M_B> -22. The alpha/Fe ratios of our dwarf ellipticals are significantly lower than the ones of giant elliptical galaxies, which is in agreement with spectroscopy of individual stars in Local Group dwarf spheroidals. Our results suggest the existence of a clear kinematic and stellar population dichotomy between dwarf and giant elliptical galaxies. This result is important for theories of galaxy formation, because it implies that present-day dwarf ellipticals are not the fossiled building blocks of giant ellipticals.

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arXiv2002

The Chemistry of Extragalactic Globular Clusters

We present preliminary results of VLT/FORS spectroscopy of globular clusters in nearby early-type galaxies. Our project aims at studying the chemistry and determine the ages of globular cluster (sub-)populations. First results indicate that the different galaxies host from little to significant intermediate-age populations, and that the latter have alpha-element over iron ratios closer to solar than the old population that show an alpha-element enhancement similar to the diffuse stellar light.

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arXiv1999

Abundance ratios in hierarchical galaxy formation

The chemical enrichment and stellar abundance ratios of galaxies which form in a hierarchical clustering scheme are calculated. For this purpose I adopt the star formation histories (SFH) as they are delivered by semi-analytic models in Kauffmann (1996}. It turns out that the average SFH of cluster ellipticals does not yield globally alpha-enhanced stellar populations. The star burst that occurs when the elliptical forms in the major merger plays therefore a crucial role in producing alpha-enhancement. Only under the assumption that the IMF is significantly flattened with respect to the Salpeter value during the burst, a Mg/Fe overabundant population can be obtained. In particular for the interpretation of radial gradients in metallicity and alpha-enhancement, the mixing of global and burst populations are of great importance. The model predicts bright field galaxies to be less alpha-enhanced than their counterparts in clusters.

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arXiv1998

Enrichment of the Intracluster Medium

The relevance of galaxies of different luminosity and mass for the chemical enrichment of the intracluster medium (ICM) is analysed. For this purpose, I adopt the composite luminosity function of cluster galaxies from Trentham (1998), which exhibits a significant rise at the very faint end. The model - adopting a universal Salpeter IMF - is calibrated on reproducing the M_Fe/L_tot, M_Fe/M_*, and alpha/Fe ratios observed in clusters. Although the contribution to total luminosity and ICM metals peaks around L* galaxies (M* approx -20), faint objects with M_B>-18 still provide at least 30 per cent of the metals present in the ICM. In consistency with the solar alpha/Fe ratios determined by {ASCA}, the model predicts that 60 per cent of the ICM iron comes from Type Ia supernovae. The predicted slope of the relation between intracluster gas mass and cluster luminosity emerges shallower than the observed one, indicating that the fraction of primordial gas increases with cluster richness.

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arXiv1998

Constraints on galaxy formation from alpha-enhancement in luminous elliptical galaxies

We explore the formation of alpha-enhanced and metal-rich stellar populations in the nuclei of luminous ellipticals under the assumption of two extreme galaxy formation scenarios based on hierarchical clustering, namely a fast clumpy collapse and the merger of two spirals. We investigate the parameter space of star formation time-scale, IMF slope, and stellar yields. In particular, the latter add a huge uncertainty in constraining time-scales and IMF slopes. We find that -- for Thielemann, Nomoto & Hashimoto nucleosynthesis -- in a fast clumpy collapse scenario an [alpha/Fe] overabundance of approx. 0.2 dex in the high metallicity stars can be achieved with a Salpeter IMF and star formation time-scales of the order 10^9 yr. The scenario of two merging spirals which are similar to our Galaxy, instead, fails to reproduce alpha-enhanced abundance ratios in the metal-rich stars, unless the IMF is flattened during the burst ignited by the merger. This result is independent of the burst time-scale. We suggest that abundance gradients give hints to distinguish between the two extreme formation scenarios considered in this paper.

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arXiv1998

Star formation histories in early-type galaxies

I discuss the formation of alpha-enhanced metal-rich stellar populations in the nuclei of luminous ellipticals. Based on hierarchical clustering, different galaxy formation scenarios, which imply different star formation histories, are considered. In contrast to the fast clumpy collapse mode, the late merger of two spiral galaxies fails to reproduce significantly $α$-enhanced abundance ratios, unless the IMF is flattened. Following the star formation history predicted by semi-analytic models of hierarchical clustering for the average elliptical, solar abundance ratios are obtained with Salpeter IMF. According to the models, bright ellipticals in the field are expected to have significantly lower Mg/Fe ratios than their counterparts in a cluster.

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arXiv1997

Stellar Yields and Chemical Evolution I: Abundance Ratios and Delayed Mixing in the Solar Neighbourhood

We analyse two recent computations of type II supernova nucleosynthesis by Woosley & Weaver (1995, WW95) and Thielemann, Nomoto, & Hashimoto (1996, TNH96), focusing on the ability to reproduce the observed [Mg/Fe]-ratios in various galaxy types. We show that the yields of oxygen and total metallicity are in good agreement. However, TNH96-models produce more magnesium in the intermediate and less iron in the upper mass range of type II supernovae than WW95-models. To investigate the significance of these discrepancies for chemical evolution, we calculate Simple Stellar Population-yields for both sets of models and different IMF slopes. We conclude that the Mg-yields of WW95 do not suffice to explain the [Mg/Fe] overabundance neither in giant elliptical galaxies and bulges nor in metal-poor stars in the solar neighbourhood and the galactic halo. Calculating the chemical evolution in the solar neighbourhood according to the standard infall-model (e.g. Matteucci & Greggio 1986; Timmes, Woosley, & Weaver 1995; Yoshii, Tsujimoto, & Nomoto 1995), we find that using WW95 and TNH96 nucleosynthesis, the solar magnesium abundance is underestimated by 29 and 7 per cent, respectively. We include the relaxation of the instantaneous mixing approximation in chemical evolution models by splitting the gas component into two different phases. In additional simulations of the chemical evolution in the solar neighbourhood, we discuss various timescales for the mixing of the stellar ejecta with the interstellar medium. We find that a delay of the order of $10^{8}$ years leads to a better fit of the observational data in the [Mg/Fe]-[Fe/H] diagram without destroying the agreement with solar element abundances and the age-metallicity relation.

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arXiv1995

Predicting Big Bang Deuterium

We present new upper and lower bounds to the primordial abundances of deuterium and helium-3 based on observational data from the solar system and the interstellar medium. Independent of any model for the primordial production of the elements we find (at the 95\% C.L.): $1.5 \times 10^{-5} \le (D/H)_P \le 10.0 \times 10^{-5}$ and $(^3He/H)_P \le 2.6\times 10^{-5}$. When combined with the predictions of standard big bang nucleosynthesis, these constraints lead to a 95\% C.L. bound on the primordial abundance of deuterium: $(D/H)_{best} = (3.5^{+2.7}_{-1.8})\times 10^{-5}$. Measurements of deuterium absorption in the spectra of high redshift QSOs will directly test this prediction. The implications of this prediction for the primordial abundances of helium-4 and lithium-7 are discussed, as well as those for the universal density of baryons.

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arXiv1995

Big Bang Nucleosynthesis in Crisis?

A new evaluation of the constraint on the number of light neutrino species (N_nu) from big bang nucleosynthesis suggests a discrepancy between the predicted light element abundances and those inferred from observations, unless the inferred primordial 4He abundance has been underestimated by 0.014 +/- 0.004 (1 sigma) or less than 10% (95%C.L.) of 3He survives stellar processing. With the quoted systematic errors in the observed abundances and a conservative chemical evolution parameterization, the best fit to the combined data is N_nu = 2.1 +/- 0.3 (1 sigma) and the upper limit is N_nu < 2.6 (95% C.L.). The data are inconsistent with the Standard Model (N_nu = 3) at the 98.6% C.L.

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arXiv1995

Post Big Bang Processing of the Primordial Elements

We explore the Gnedin-Ostriker suggestion that a post-Big-Bang photodissociation process may modify the primordial abundances of the light elements. We consider several specific models and discuss the general features that are necessary (but not necessarily sufficient) to make the model work. We find that with any significant processing, the final D and $^3$He abundances, which are independent of their initial standard big bang nucleosynthesis (SBBN) values, rise quickly to a level several orders of magnitude above the observationally inferred primordial values. Solutions for specific models show that the only initial abundances that can be photoprocessed into agreement with observations are those that undergo virtually no processing and are already in agreement with observation. Thus it is unlikely that this model can work for any non-trivial case unless an artificial density and/or photon distribution is invoked.

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