The star formation histories of elliptical galaxies across the Fundamental Plane (original) (raw)
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Constraints on the Stellar Populations of Elliptical Galaxies from Ultraviolet Spectra
1996
We present preliminary results from spectral synthesis models of old stellar populations for the spectral range 912-4000Å with ∼ 10Å resolution, which can be used to investigate the UVX phenomenon and to assess ages and abundances. Model spectra incorporating extreme horizontal branch (EHB) and Post-Asymptotic Giant Branch (P-AGB) populations give good matches to the far-UV spectra of galaxies. These models indicate an EHB fraction which is < 10% of the total HB population in all but the most extreme examples of the UVX phenomenon, where the EHB fraction is still ≤ 20%. Once the hot component that gives rise to the UVX phenomenon is accounted for, the mid-UV wavelength range (2200 < λ < 3300Å) provides information about the age and metallicity of the underlying stellar population. The flux in this spectral range arises mainly from stars close to the main sequence turnoff. We compare models with the spectrum of M31 and discuss UV features which should be useful as population diagnostics.
A Far Ultraviolet Analysis of the Stellar Populations in Six Elliptical and S0 Galaxies
Astrophysical Journal, 1997
We have analyzed the far-ultraviolet (FUV) spectra of six elliptical and S0 galaxies in order to characterize their hot stellar populations. The spectra were obtained using the Hopkins Ultraviolet Telescope (HUT) in March 1995. These data, together with the spectra of two galaxies observed with HUT in 1990, represent the only FUV spectra of early type galaxies that extend to the Lyman limit at 912 A and therefore include the "turnover" in the spectral energy distribution below Lyman alpha. Using an extensive new grid of synthetic spectra which match the HUT resolution and cover the relevant parameter space of temperature and gravity, we have constructed synthetic spectral energy distributions by integrating over predicted stellar evolutionary tracks for horizontal branch stars and their progeny. When the models are compared with the HUT data, we find that those with supersolar metal abundances and helium best reproduce the flux across the entire HUT wavelength range, while those with subsolar Z & Y fit less well, partly because of a significant flux deficit shortward of 970 A in the models. We find that AGB-Manque evolution is required in all fits to the HUT spectra, suggesting that all of the galaxies have some subdwarf B star population. At any Z & Y, the models that best match the HUT flux are dominated by stars evolving from a narrow range of envelope mass on the blue end of the horizontal branch.
The far-ultraviolet radiation from elliptical galaxies
1997
Since the discovery of the Ultraviolet Upturn Phenomenon ("UVX") in early-type galaxies it has been clear that the stellar populations of such systems contain an unexpected hot component. Recent work has provided strong circumstantial evidence that the stars radiating at short wavelengths (λ < 2000Å) is in fact due to hot horizontal branch, post-HB stars and post-AGB stars. We summarize the arguments in favour of this hypothesis. We then derive an estimate for the fraction of all HB stars that must be contributing to the UV upturn phenomenon in the strongest UVX galaxy, NGC 1399, and derive a hot star fraction fH ∼ 0.16. The implication is that UVX arises from a minority fraction of the dominant stellar population. We conclude that the mechanism that produces the UVX is not one that can be explained naturally by the presence of an extremely metal-rich or metal-poor population.
The UV emission of elliptical galaxies
arXiv (Cornell University), 1998
The far-ultraviolet is the most rapidly evolving portion of the spectrum in both very young galaxies and very old galaxies. The "UV upturn" in the spectra of elliptical galaxies shortward of 2000Å offers a promising probe of the ages and chemical evolution of very old galaxies. In early-type non-active galaxies with the bluest 1550 − V colors, the bulk of the emission arises from Extreme Horizontal Branch (EHB) stars, along their evolution from the zeroage HB to the white-dwarf cooling curve. The strength of the UV-upturn is governed by the fraction of stars that evolve through the EHB phase, which is in turn governed by age, metallicity, helium abundance, and other parameters such as stellar rotation and binarity that might influence the amount of mass loss on the RGB. Spectral constraints on the nature of the hot stellar population from Astro-2 are reviewed, and new imaging results from the HST Faint Object Camera are presented. Attempts to measure evolution through observations of high-redshift elliptical galaxies in the rest-frame UV are reviewed.
The Astrophysical Journal Supplement Series, 1994
In this paper we present new chemo-spectrophotometric models of elliptical galaxies in which infall of primordial gas is allowed to occur. They aim to simulate the collapse of a galaxy made of two components, i.e. luminous material and dark matter. The mass of the dark component is assumed to be constant in time, whereas that of the luminous material is supposed to accrete at a suitable rate. They also include the effect of galactic winds powered by supernova explosions and stellar winds from massive, early-type stars. The models are constrained to match a number of properties of elliptical galaxies, i.e. the slope and mean colours of the colour-magnitude relation (CMR), V versus (V-K), the UV excess as measured by the colour (1550-V) together with the overall shape of the integrated spectral energy distribution (ISED) in the ultraviolet, the relation between the Mg 2 index and (1550-V), the mass to blue luminosity ratio M/L B as a function of the B luminosity, and finally the broad-band colours (U-B), (B-V), (V-I), (V-K), etc. The CMR is interpreted as a mass-metallicity sequence of old, nearly coeval objects, whose mean age is 15 Gyr. Assuming the law of star formation to be proportional to M k g (t) with k = 1, the rate of star formation as function of time starts small, grows to a maximum, and then declines thus easily avoiding the excess of metal-poor stars found by BCF with the closed-box scheme (the analog of the G-Dwarf Problem in the solar vicinity). Owing to their stellar content, infall models can easily reproduce all the basic data of the galaxies under examination. As far as the UV excess is concerned, the same sources proposed by BCF are found to hold also with the infall scheme. H-HB and AGB manqué stars of high metallicity play the dominant role, and provide a robust explanation of the correlation between the (1550-V) colour and the luminosity, mass and metallicity of the galaxies. Furthermore, these models con-⋆ Send offprint requests to: C. Chiosi firm the potential of the (1550-V) colour as an age indicator in cosmology as already suggested by BCF. In the rest frame of a massive and metal-rich elliptical galaxy, this colour suffers from one major variation: at the onset of the so-called H-HB and AGB-manqué stars (age about 5.6 Gyr). This transition occurs at reasonably small redshifts and therefore could be detected with the present-day instrumentation.
The Astrophysical Journal, 1995
We present an analysis of the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is that the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. Such objects have Zero Age Horizontal Branch envelope masses M 0 env < 0:05M. Local examples of EHB stars exist in some globular clusters and in the Galactic disk field and serve both as a guide and constraint. We re-derive the broad-band UV colors 1500 V and 2500 V for globular clusters and elliptical galaxies from the available satellite data and investigate color-color and color-line strength correlations. There are several important distinctions between clusters and galaxies. They do not occupy a single Mg 2-color sequence. Clusters can be bluer than any galaxy in 15 V and 25 V , implying larger hot star populations, but galaxies are significantly bluer than clusters in 15 25 at a given 15 V. We attribute this primarily to the effect of metal abundance on the mid-UV (2500 A) light. It also implies that the UVX in galaxies is not produced by metal poor subpopulations similar to the clusters. We develop a simple spectral synthesis formulation for all phases of single star evolution from the ZAMS to the white dwarf cooling track that requires only one or two parameters for each choice of age and abundance. We provide the ingredients necessary for constructing models with arbitrary HB morphologies in the age range 2 < t < 20 Gyr and for 6 metallicities in the range 2:26 < [Fe=H] < 0:58; we also consider the effect of enhanced Y in metal rich models. UV properties of the models are predicted using the Kurucz (1991) atmospheres. The maximum lifetime UV output is produced by EHB stars with M 0 env 0:02M , and can be up to 30 times higher than for post-asymptotic-giant-branch (P-AGB) stars. The ultraviolet output of old populations is governed primarily by the distribution of M 0 env , P(M 0 env), on the ZAHB. The UV output is not very sensitive to [Fe=H] or to Y , but it can change very rapidly with M 0 env. Thus, it is extremely sensitive to the precise nature of giant branch mass loss. Because this process is not well understood physically, we choose to leave mass loss as an implicit free parameter. Our models use simple descriptions of P(M 0 env) to bracket the colors produced from any real distribution of stars. Our models accurately predict the range of UV colors observed for the globular clusters, given known constraints on their age, abundances, and HB morphologies. Clusters with "blue HB" morphologies do not require the hotter EHB stars to explain their UV colors, although a small EHB population is consistent with our models. The largest known population of these stars in a cluster, as a fraction of the total HB, is 20% in ! Cen. For [Fe=H] > 0:5, however, blue HB stars will be rare. As a consequence, we find that models with [Fe=H] > 0 which do not contain EHB stars cannot reproduce the colors of most of the galaxies. However, only small EHB fractions are required: < 5% for the bulk of the E galaxies and 20% for those with the strongest UVX. These results are independent of the assumed [Fe/H]. The EHB fraction required for most galaxies is comparable to the fraction of hot subdwarfs in the Galactic disk. Most of these are EHB stars, and their existence considerably strengthens the case for EHB populations as the source of elliptical galaxy UV light. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is 70% and 20% for moderate UVX and strong UVX systems, respectively. We find that 25 V , but not 15 V , is sensitive to the age and abundance, though these cannot always be cleanly distinguished. The galaxy colors place strong limits of h[Fe=H]i > 0:5 and < 15% on the contribution of globular cluster-type populations to the V light. Galaxy colors are consistent with solar-abundance models with ages in the range 6-14 Gyr. However, the 25 V colors of the galaxies other than the strong UVX systems are too blue to be consistent with [Fe=H] > 0:2 for any age. This may be additional evidence that [Mg/Fe] > 0 in elliptical galaxies. UV colors for M32 are consistent with the solar abundance, intermediate age (4-6 Gyr) population inferred from optical/IR observations.
An Ultraviolet/Optical Atlas of Bright Galaxies
The Astrophysical Journal Supplement Series, 2001
We present wide-Ðeld imagery and photometry of 43 selected nearby galaxies of all morphological types at ultraviolet and optical wavelengths. The ultraviolet (UV) images, in two broad bands at 1500 and 2500
The Astrophysical Journal, 2007
We present the results from the analysis of optical spectra of 31 Hα-selected regions in the extended UV (XUV) disks of M 83 (NGC 5236) and NGC 4625 recently discovered by GALEX. The spectra were obtained using IMACS at Las Campanas Observatory 6.5m Magellan I telescope and COSMIC at the Palomar 200-inch telescope, respectively for M 83 and NGC 4625. The line ratios measured indicate nebular oxygen abundances (derived from the R23 parameter) of the order of Z ⊙ /5-Z ⊙ /10 1 . For most emission-line regions analyzed the line fluxes and ratios measured are best reproduced by models of photoionization by single stars with masses in the range 20-40 M ⊙ and oxygen abundances comparable to those derived from the R23 parameter. We find indications for a relatively high N/O abundance ratio in the XUV disk of M 83. Although the metallicities derived imply that these are not the first stars formed in the XUV disks, such a level of enrichment could be reached in young spiral disks only 1 Gyr after these first stars would have formed. The amount of gas in the XUV disks allow maintaining the current level of star formation for at least a few Gyr.
A Photometric Study of Giant Ellipticals and Their Stellar Halos With VST
Galaxies, 2017
Observations of diffuse starlight in the outskirts of galaxies are thought to be a fundamental source of constraints on the cosmological context of galaxy assembly in the ΛCDM model. Such observations are not trivial because of the extreme faintness of such regions. In this work, we investigated the photometric properties of six massive early-type galaxies (ETGs) in the VST Elliptical GAlaxies Survey (VEGAS) sample (NGC 1399, NGC 3923, NGC 4365, NGC 4472, NGC 5044, and NGC 5846) out to extremely low surface brightness levels with the goal of characterizing the global structure of their light profiles for comparison to state-of-the-art galaxy formation models. We carried out deep and detailed photometric mapping of our ETG sample taking advantage of deep imaging with VST/OmegaCAM in the g and i bands. By fitting the light profiles, and comparing the results to simulations of elliptical galaxy assembly, we have identified signatures of a transition between relaxed and unrelaxed accreted components and can constrain the balance between in situ and accreted stars. The very good agreement of our results with predictions from theoretical simulations demonstrates that the full VEGAS sample of ∼ 100 ETGs will allow us to use the distribution of diffuse light as a robust statistical probe of the hierarchical assembly of massive galaxies.
2013
Aims. We revisit the scaling relations and star-forming histories of local elliptical galaxies using a novel selection method applied to the Sloan Digital Sky Survey DR7. Methods. We combine two probability-based automated spectroscopic and morphological classifications of ∼ 600000 galaxies with z < 0.25 to isolate true elliptical galaxies. Our sample selection method does not introduce artificial cuts in the parameters describing the galaxy but instead it associates to every object a weight measuring the probability of being in a given spectro-morphological class. Thus the sample minimizes the selection biases. Results. We show that morphologically defined ellipticals are basically distributed in 3 spectral classes, which dominate at different stellar masses. The bulk of the population (∼ 50%) is formed by a well defined class of galaxies with old stellar populations that formed their stars at very early epochs in a short episode of star formation. They dominate the scaling relations of elliptical galaxies known from previous works and represent the canonical elliptical class. At the low mass end, we find a population of slightly larger ellipticals, with smaller velocity dispersions at fixed stellar mass, which seem to have experienced a more recent episode of star formation probably triggered by gas-rich minor mergers. The high mass end tends to be dominated by a third spectral class, slightly more metal rich and with more efficient stellar formation than the reference class. This third class contributes to the curvature of the mass-size relation at high masses reported in previous works. Our method is therefore able to isolate typical spectra of elliptical galaxies following different evolutive pathways.