Evolution Of Elliptical Galaxies Research Articles

A possible theoretical explanation of metallicity gradients in elliptical galaxies

ABSTRA C T Models of chemical evolution of elliptical galaxies taking into account different escape velocities at different galactocentric radii are presented. As a consequence of this, the chemical evolution develops differently in different galactic regions; in particular, we find that the galactic wind, powered by supernovae (of Type II and I) starts, under suitable conditions, in the outer regions and successively develops in the central ones. The star formation is assumed to stop after the onset of the galactic wind in each region. The main result found in the present work is that this mechanism is able to reproduce metallicity gradients, namely the gradients in the Mg2 index, in good agreement with observational data. We also find that in order to honour the constant ΩhMg=Feiy ratio with galactocentric distance, as inferred from metallicity indices, a variable initial mass function as a function of galactocentric distance is required. This is only a suggestion, as trends on abundances inferred purely from metallicity indices are still uncertain.

Galaxy formation and evolution - I. The Padua TREE-SPH code (PD-SPH)

In this paper we report on pd-sph, the new tree-sph code developed in Padua. The main features of the code are described and the results of a new and independent series of 1D and 3D tests are shown. The paper is mainly dedicated to the presentation of the code and to the critical discussion of its performance. In particular, great attention is devoted to the convergency analysis. The code is highly adaptive in space and time by means of individual smoothing lengths and individual time-steps. At present it contains both dark and baryonic matter, this latter in the form of gas and stars, cooling, thermal conduction, star formation, feedback from Type I and II supernovae, stellar winds, and ultraviolet flux from massive stars, and finally chemical enrichment. New cooling rates that depend on the metal abundance of the interstellar medium are employed, and the differences with respect to the standard ones are outlined. Finally, we show the simulation of the dynamical and chemical evolution of a disc-like galaxy with and without feedback. The code is suitably designed to study in a global fashion the problem of formation and evolution of elliptical galaxies, and in particular to feed a spectrophotometric code from which the integrated spectra, magnitudes and colours (together with their spatial gradients) can be derived.

Surface photometry of early–type galaxies in the Hubble deep field

The detailed surface photometry of a sample of early–type galaxies in the Hubble Deep Field is presented as part of a long–term project aimed to settle strong observational constraints to the theories modelling the evolution of elliptical galaxies from the early stages. The sample has been extracted, in the V606 band, from the database provided by the ESO-STECF-HDF Group (Couch 1996). The selection criteria involve the total magnitude, the number of pixels detected above the background level and an automatic star/galaxy classifier. Moreover, form visual inspection of the frames, we excluded the galaxies showing unambiguous late–type morphology. The analysis of the luminosity and geometrical profiles, carried out on the 162 candidates obeying our selection criteria, resulted in a list of 99 "bona fide” early–type galaxies, for which accurate total magnitudes and effective radii were computed on the basis of the equivalent luminosity profiles. The comparison with the magnitudes given by Williams et al. (1996) indicates that the automated photometry tends to underestimate the total luminosity of the ellipticals. The luminosity profiles of most of galaxies in our sample follow fairly well the de Vaucouleurs law ("Normal” profiles). However, a relevant fraction of galaxies, even following the law in the main body light distribution, exhibit in the inner region a flattening of the luminosity profile not attributable to the PSF ("Flat” profiles) or, in some cases, a complex (multi–nucleus) structure ("Merger” profiles). A statistically significant correlation is found between the shapes of the luminosity profiles and the ellipticity distribution. In particular, the average ellipticity of galaxies belonging to the "Flat” and "Merger” classes is significantly higher than that of the "Normal” galaxies. Finally, even taken into account the relevant uncertainty of the outer position angle profiles, the amount of isophotal twisting of HDF ellipticals turns out to be significantly larger with respect to that of the local samples.

Stellar Populations in Gas‐rich Galaxy Mergers. I. Dependence on Star Formation History

We investigate the nature of stellar populations of major galaxy mergers between late-type spirals with considerably abundant interstellar medium by performing numerical simulations designed to solve both the dynamical and chemical evolution of the mergers in a self-consistent manner. We particularly consider that the star formation history of galaxy mergers is a crucial determinant of the nature of stellar populations of merger remnants, and therefore we investigate how the difference in star formation history between galaxy mergers affects the chemical evolution of galaxy mergers. We found that the rapidity of star formation, which is defined as the ratio of the dynamical timescale to the timescale of gas consumption by star formation, is the most important determinant for a number of fundamental characteristics of stellar populations of merger remnants. The main results obtained in this study are the following. 1. A galaxy merger with more rapid star formation becomes elliptical with larger mean metallicity. This is primarily because, in the merger with more rapid star formation, a smaller amount of metal-enriched gas is tidally stripped away during merging, and, consequently, a larger amount of the gas can be converted to stellar components. This demonstrates that the cause of the color-magnitude relation of elliptical galaxies can be closely associated with the details of merging dynamics that depend on the rapidity of star formation in galaxy mergers. 2. A negative metallicity gradient fitted reasonably well by a power law can be reproduced by a dissipative galaxy merger with star formation. The magnitude of the metallicity gradient is larger for an elliptical galaxy formed by a galaxy merger with less rapid star formation. 3. The absolute magnitude of the metallicity gradient correlates with that of the age gradient in galaxy mergers in the sense that a merger remnant with a steeper negative metallicity gradient is more likely to show a steeper age gradient. 4. The outer part of a stellar population is both older and less metal-enriched than the nucleus in an elliptical galaxy formed by a galaxy merger with less rapid star formation. Moreover, the metallicity of the outer part of the gaseous component for some models with less rapid star formation is appreciably smaller than the stellar metallicity. This result implies that the origin of metal-poor hot gaseous X-ray halos in real elliptical galaxies can essentially be ascribed to the dynamics of dissipative galaxy merging. 5. Irrespective of the rapidity of star formation, the epoch of galaxy merging affects both the mean stellar metallicity and the mean stellar age of merger remnants: later galaxy mergers are more likely to become ellipticals with both younger and more metal-enriched stellar populations. This result reflects the fact that in the later mergers, a larger amount of more metal-enriched interstellar gas is preferentially converted into stars during the later period of star formation triggered by galaxy merging. These five results clearly demonstrate that even the chemical evolution of elliptical galaxies can be strongly affected by the details of dynamical evolution of galaxy merging, which are furthermore determined by the rapidity of star formation of galaxy mergers. In particular, tidal stripping of interstellar gas and the total amount of gaseous dissipation during galaxy merging are demonstrated to play vital roles in determining a number of chemical properties of merger remnants. Based on these results, we adopt a specific assumption of luminosity dependence on rapidity of star formation and thereby discuss how successfully the present merger model can reproduce a number of fundamental chemical, photometric, and spectroscopic characteristics of elliptical galaxies.

1.20. The color-magnitude relation of bulges

One of the main aims with respect to the evolution of spiral galaxies is to clarify how the disk and bulge were formed. The study of bulges has usually been focused on how bulges resemble to elliptical galaxies. For the study of the evolution of elliptical galaxies, the color-magnitude relation (C-M relation) gives us a clue to understanding the formation and evolution of ellipticals. In fact, the tight C-M relation of ellipticals (Bower et al. 1992) was interpreted as the sequence of the metallicity difference (Kodama and Arimoto 1997). On the contrary, there are few observations for the C-M relation of bulges. On the analogy of elliptical galaxies, the C-M relation of bulges, if it exists, may be one of the most important clues to understanding the formation process of bulges. The main goal of the present study is to investigate the C-M relation of bulges.

Dynamical Evolution of Elliptical Galaxies with Central Singularities

We study the effect of a massive central singularity on the structure of a triaxial galaxy using N-body simulations. Starting from a single initial model, we grow black holes with various final masses Mh and at various rates, ranging from impulsive to adiabatic. In all cases, the galaxy achieves a final shape that is nearly spherical at the center and close to axisymmetric throughout. However, the rate of change of the galaxy's shape depends strongly on the ratio Mh/Mg of black hole mass to galaxy mass. When Mh/Mg 2%, the galaxy becomes axisymmetric in little more than a crossing time. We propose that the rapid evolution toward axisymmetric shapes that occurs when Mh/Mg > 2% provides a negative feedback mechanism which limits the mass of central black holes by cutting off their supply of fuel.

Infall models of elliptical galaxies: further evidence for a top-heavy initial mass function

Chemical and photometric models of elliptical galaxies with infall of primordial gas, and subsequent ejection of processed material via galactic winds, are described. Ensuring that these models are consistent with the present-day colour-luminosity relation and the measured intracluster medium (ICM) abundances, we demonstrate that the initial mass function (IMF) must be significantly flatter (i.e., x<=0.80) than the canonical Salpeter slope (i.e., x=1.35). Such a ``top-heavy'' IMF is in agreement with the earlier conclusions based upon closed-box models for elliptical galaxy evolution. On the other hand, the top-heavy IMF, in conjunction with these semi-analytic infall models, predicts an ICM gas mass which exceeds that observed by up to a factor three, in contrast with the canonical closed-box models. Time and position-dependent IMF formalisms may prove to be a fruitful avenue for future research, but those presently available in the literature are shown to be inconsistent with several important observational constraints.

Galactic winds and the photochemical evolution of elliptical galaxies: the classic model revisited

We consider the simultaneous chemical, photometric, and gaseous thermal energy evolution of elliptical galaxies. The evolution of chemical abundances in the intracluster medium (ICM) is set by the differing time-scales for gas ejection, via supernovae (SNe)-driven winds, from dwarf, normal, and giant ellipticals, and is monitored concurrently. Emphasis is placed upon the influence of, and sensitivity to, the underlying stellar initial mass function (IMF), star formation efficiency, Supernovae Type Ia rates, supernovae remnant (SNR) dynamics, and the most recent advances in stellar nucleosynthesis. Unlike many previous studies, we adhere to a wide range of optical (e.g. colour-metallicity-luminosity relationship) and X-ray (e.g. recent ASCA ICM abundance measurements) observational constraints. IMFs biased towards high-mass stars, at least during the early phases of star formation, are implicated in order to satisfy all the observational constraints.

Chaos and Mixing in Triaxial Stellar Systems

We investigate the timescales for stochasticity and chaotic mixing in a family of triaxial potentials that mimic the distribution of light in elliptical galaxies. Some of the models include central point masses designed to represent nuclear black holes. Most of the boxlike orbits are found to be stochastic, with mean Liapunov times that are 3-6 times the period of the long-axis orbit. In models with large cores or small black holes, the stochastic orbits mimic regular box orbits for hundreds of oscillations at least. However a small core radius or significant black hole mass causes most of the stochastic orbits to diffuse through phase space on the same timescale, visiting a significant fraction of the volume beneath the equipotential surface. We estimate timescales for chaotic mixing in the more strongly stochastic models by evolving ensembles of 10^4 points until their distribution reaches a nearly steady state. Mixing initially takes place rapidly, with characteristic times of 10-30 dynamical times, as the phase points fill a region similar in shape to that of a box orbit. Subsequent mixing is slower, with characteristic times of hundreds of orbital times. Mixing rates were found to be enhanced by the addition of modest force perturbations. The consequences for the structure and evolution of elliptical galaxies are discussed.

Multiphase Chemical Evolution in Elliptical Galaxies

Recent ASCA results show that the iron abundance of the X-ray-emitting hot gas of elliptical galaxies is less than the solar abundance. The observed low iron abundance is inconsistent with the predictions of previous chemical evolution models. In order to solve this problem, we present a simple model of chemical evolution for elliptical galaxies after the galactic-wind period, assuming that the gases ejected from stars do not mix with the circumferential gas. The ejected gas components evolve separately according to their birth time and origin. We have investigated their evolution qualitatively. The gas Components originated from supernova remnant shells cool and drop out of the hot gas faster than the other components because of their high density and metal abundance. As a result, supernovae cannot heat all the gas of an elliptical galaxy effectively, contrary to previous results. If the metal abundance of mass-loss gas is not uniform, the mass-loss gas with higher abundance also easily drops out, and the average abundance of the hot gas can decrease. We believe that this is a hint toward solving the low- abundance problem.

A Comparison of Three Elliptical Galaxy Photochemical Evolution Codes

view Abstract Citations (12) References (54) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Comparison of Three Elliptical Galaxy Photochemical Evolution Codes Gibson, Brad K. Abstract Working within the classic supernovae-driven wind framework for elliptical galaxy evolution, We perform a systematic investigation into the discrepancies between the predictions of three contemporary codes (by Arimoto & Yoshii, Bressan et al., and Gibson). By being primarily concerned with reproducing the present-day color-metallicity-luminosity (CML) relations among elliptical galaxies, the approaches taken in the theoretical modeling have managed to obscure many of the hidden differences between the codes. Targeting the timescale for the onset of the initial galactic wind, t_GW_, as a primary "difference" indicator, We demonstrate exactly how and why each code is able to claim successful reproduction of the CML relations, despite possessing apparently incompatible input ingredients. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177678 arXiv: arXiv:astro-ph/9605074 Bibcode: 1996ApJ...468..167G Keywords: GALAXIES: ABUNDANCES; GALAXIES: ELLIPTICAL AND LENTICULAR; CD; GALAXIES: EVOLUTION; METHODS: MISCELLANEOUS; Astrophysics E-Print: 30 pages, LaTeX, 3 figures available from http://msowww.anu.edu.au/~gibson/publications.html, to be published in Sept 1996 ApJ, Part I full text sources arXiv | ADS |

On the Effects of Bursts of Massive Star Formation during the Evolution of Elliptical Galaxies

We consider the hypothesis that the formation of elliptical galaxies includes a phase in which star formation is mostly restricted to massive stars, with the bias towards high mass stars increasing with elliptical galaxy mass. The mass fraction of stars in this top-heavy mode of star formation is constrained by requiring the resulting stellar remnants to account for the observed increase in the mass-to-light ratio of ellipticals with increasing galaxy mass. We then consider the implications of this population of massive stars for the intracluster medium and the extragalactic background at various wavelengths. The mass and abundance ratios of metals produced by our proposed population of massive stars are consistent with the observations of the mass and abundance ratios of metals in the hot gas of galaxy clusters for most of the standard range of IMF slopes and SN II yields. The predicted energy density produced by this stellar population approaches current limits on the extragalactic background at both optical wavelengths, into which the ultraviolet radiation of the massive stars is likely to be redshifted, and far-infared wavelengths, at which starlight reprocessed by dust associated with the starburst will be observed. In either case, the background is predicted to be significantly clustered since massive ellipticals are clustered.

A Tolman Surface Brightness Test for Universal Expansion and the Evolution of Elliptical Galaxies in Distant Clusters

We use the intercept of the elliptical galaxy radius-surface brightness (SB) relation at a fixed metric radius as the standard condition for the Tolman SB test of the universal expansion. We use surface photometry in the optical and near-infrared of elliptical galaxies in Abell 2390 (z = 0.23) and Abell 851 (z = 0.41) and compare them to the Coma Cluster at z ≈ 0. The photometric data for each cluster are well described by the Kormendy relation r -->e -->∑eA, where A = -0.9 in the optical and A = -1.0 in the near-infrared. The scatter about this near-infrared relation is only 0.076 in log re at the highest redshift, which is much smaller than at low redshifts, suggesting a remarkable homogeneity of the cluster elliptical population at z = 0.41. We use the intercept of these fixed-slope correlations at Re = 1 kpc (assuming H0 = 75 km s-1 Mpc-1, Ω0 = 0.2, and Λ0 = 0, where the results are only weakly dependent on the cosmology) to construct the Tolman SB test for these three clusters. The data are fully consistent with universal expansion if we assume simple models of passive evolution for elliptical galaxies but are inconsistent with a nonexpanding geometry (the tired-light cosmology) at the 5 σ confidence level at z = 0.41. These results suggest luminosity evolution in the rest-frame K band of 0.36 ± 0.14 mag from z = 0.41 to the present and are consistent with the elliptical galaxies' having formed at high redshift. The SB intercept in elliptical galaxy correlations is thus a powerful tool for investigating models of their evolution for significant look-back times.

High-Redshift Milli-Jansky Radio Galaxies

The study of radio galaxies selected at mJy flux levels has the potential to resolve two important issues in observational cosmology provided redshifts can be determined or reliably estimated for complete samples of such sources. First, the deep flux limit, combined with the shape of the radio luminosity function means that the redshift distribution of such samples provides a much more powerful test of the existence of a high-redshift cutoff for radio sources (Dunlop &amp; Peacock 1990) than can be provided by further studies of brighter radio samples. Second, as a consequence of selection from bright radio surveys, the detailed study of galaxies at z &gt; 2 has to date been confined to objects of extreme radio power (e.g. 4C41.17, Chambers et al. 1990; B2 0902+34, Eales et al. 1993), and it has now become clear that the ultraviolet-infrared properties of such sources are strongly contaminated by processes connected to the AGN (Eales &amp; Rawlings 1993; Dunlop &amp; Peacock 1993). Being 100-1000 times less radio luminous than these extreme sources, mJy radio galaxies at comparable redshifts should provide much more representative probes of the formation and evolution of elliptical galaxies in general.

Observational Tests for Ultraviolet Sources in Elliptical Galaxies

An understanding of the empirical correlation between the strength of the UV upturn and the metallicity of luminous elliptical galaxies (Burstein and coworkers) is vital in the study of the evolution of elliptical galaxies. We support a modestly old (15-20 Gyr), metal-rich hypothesis ([Fe/H] 0) for the UV sources in UV upturn galaxies as an alternative to the old (20 Gyr), metal-poor ([Fe/H] -1.0) hypothesis of Park & Lee. This preliminary result of our population synthesis is based on a Gaussian mass distribution on the horizontal branch and a complete library of stellar evolution tracks, both of which are crucial in producing UV light in the metal-rich systems. The metal-rich hypothesis has an advantage over the metal-poor one not only because it explains the UV upturn-metallicity relation but also because it requires neither an uncomfortably large age nor a large age span for elliptical galaxies. We suggest several observational tests to constrain the metallicity of the UV sources.

Evolution of Infrared-selected Galaxies in Z approximately 0.4 Clusters

view Abstract Citations (130) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of Infrared-selected Galaxies in Z approximately 0.4 Clusters Stanford, S. A. ; Eisenhardt, P. R. M. ; Dickinson, Mark Abstract Wide-field optical and near-IR (JHK) imaging is presented for two rich galaxy clusters: Abell 370 at z = 0.374 and Abell 851 (Cl 0939+47) at z = 0.407. The new data are combined to produce colors sampling the 0.55-1.65 micron range in the rest frame. Galaxy catalogs selected from the near-IR images are 90% complete to a limiting magnitude approximately 1.5 mag below K^*^. The resulting samples contain ~100 probable member galaxies per cluster in the central ~2 Mpc. Comparison with Hubble Space Telescope (HST) wide-field planetary camera images yields subsamples of ~70 galaxies in each cluster with morphological types. Analysis of the complete samples and the HST subsamples gives the following results: 1. The optical - K colors of the cluster galaxies are bounded by a red envelope of E/S0's which follow a color-magnitude relation with a slope similar to the corresponding color-magnitude relation in present-epoch E/S0's. 2. The z ~ 0.4 E/S0's are bluer than those in the Bower, Lucey, & Ellis (1992) Coma sample in the optical-K color by 0.13 mag for Abell 370 and by 0.18 mag for Abell 851. Our estimate of the systematic uncertainty indicates this color difference is significant at the 2-3 σ level. If real, the bluing of the E/S0 populations at moderate redshift is consistent with that calculated from the Bruzual & Charlot (1993) models of passive elliptical galaxy evolution. 3. In both clusters the intrinsic scatter of the known E/S0's about their optical - K color-magnitude relation is small (~0.06 mag) and not significantly different from that of Coma E/S0's as given by Bower et al. (1992), indicating that the galaxies within each cluster formed at the same time at an early epoch. These results support the paradigm in which the stellar population in cluster E/S0's is a function only of the galaxy mass and the look-back time. 4. Although the passive-evolution models fit the z ~ 0.4 early-type galaxies in the optical - K color, they fail to reproduce the observations at intervening wavelengths. When normalized at rest frame 1.6 micron, the z ~ 0.4 E/S0's are fainter than the model at rest frame 0.9 microns, and brighter than the model (and than Coma E/S0's) at rest frame 1.2 rim. Such color differences are difficult to explain in terms of age or conventional metallicity effects. 5. The disk galaxies in Abell 851 are bluer and represent a greater fraction of the total sample compared to those in Abell 370. X-ray maps and large-field optical images indicate that Abell 370 is "older" than Abell 851, in the sense of showing a more relaxed distribution of galaxies and hot gas. In agreement with the apparent difference in cluster age, our results suggest the disk galaxies of Abell 370 have had more time in the cluster environment, relative to the disks in Abell 851, to redden and/or fade into obscurity. Publication: The Astrophysical Journal Pub Date: September 1995 DOI: 10.1086/176162 arXiv: arXiv:astro-ph/9504007 Bibcode: 1995ApJ...450..512S Keywords: GALAXIES: CLUSTERS: INDIVIDUAL NAME: ABELL 370; GALAXIES: CLUSTERS: INDIVIDUAL NAME: ABELL 851; GALAXIES: EVOLUTION; GALAXIES: PHOTOMETRY; Astrophysics E-Print: uuencoded gzipped tar file containing latex files of manuscript (42 pages) plus tables (9 pages) full text sources arXiv | ADS | data products SIMBAD (324) NED (313) MAST (1) ESA (1)

Extragalactic Planetary Nebulae

It is a great pleasure for me to be able to start off this “Joint Discussion” on Extragalactic Planetary Nebulae. Many astronomers appear somewhat ambivalent in their attitudes towards planetary nebulae. A few of us regard planetaries as the spectacular “last hurrah” before stars slide off into oblivion as white dwarfs. Others think of them as soaring butterflies that evolved from stellar caterpillars. Finally a few less romantic astronomers simply ask “what can planetary nebulae do for us?” Perhaps surprisingly the answer is that they can do a great deal for the progress of various branches of astronomy. At this meeting George Jacoby will, for example, show us how planetary nebulae can be used to measure extragalactic distances. Mike Dopita, Ziaohui Hui and Ken Freeman will tell us that planetaries are powerful probes of the kinematics of galaxies. Furthermore Henry Ferguson will show how planetary nebulae can be used to probe the effects of metallicity on stellar evolution in elliptical galaxies.

UV Bright Stars and Planetary Nebulae in Elliptical Galaxies

AbstractFar UV observations and optical studies of planetary nebula luminosity functions (PNLFs) offer complementary views of the late phases of stellar evolution in elliptical galaxies and spiral galaxy bulges. UV spectroscopy reveals that the hot stellar population is composite, with a mix of temperatures that varies from galaxy to galaxy. This changing mix is most likely due to changes in the relative numbers of stars that channel through the Post-Asymptotic Giant Branch (PAGB), Post-Early-AGB (PEAGB) and Extreme Horizontal Branch (EHB) phases of evolution. EHB stars appear to dominate the integrated λ &lt; 2000 Å flux from galaxies with the strongest far-UV emission, but are too faint to resolve individually in even the nearest galaxies. Far UV images of M31 and M32 reveal a population of hot stars that are much brighter, but do not account for the majority of the far-UV flux. The sources detected are most likely low-mass PAGB stars (0.55 &lt; M/M⊙ &lt; 0.59). In contrast, the PNLF probes the PAGB star mass function at values greater than ∼ 0.6 M⊙. For a given galaxy the relative numbers of stars in these different branches of evolution are determined by the age and chemical evolution of the galaxy and by the physics of mass loss on the red giant branch. We review current constraints on the mass function of hot evolved stars in elliptical galaxies, highlight a few puzzles, and outline where future observations might contribute.

Chemodynamic and Photometric Evolution of Elliptical Galaxies

In many hydrodynamical models of elliptical galaxies the absence of information about the detailed evolution of stars and the spectro-photometric output by hot gas and stars leads to a missing link between the data we obtain in observations (photons) and the internal behavior (dynamics) of a galaxy.

Spectrophotometric evolution of elliptical galaxies. 1: Ultraviolet excess and color-magnitude-redshift relations

Spectrophotometric evolution of elliptical galaxies. 1: Ultraviolet excess and color-magnitude-redshift relations