Relying on infrared surface brightness fluctuactions to trace AGB properties in a sample of elliptical galaxies in the Virgo and Fornax clusters, we assess the puzzling origin of the "UV-upturn" phenomenon, recently traced down to the presence of a hot horizontal branch stellar component. We find that the UV-upturn actually signals a profound change in the c-m diagram of stellar populations in elliptical galaxies, involving both the hot stellar component and red-giant evolution.
The puzzling origin of the ``UV-upturn'' phenomenon, observed in some elliptical galaxies, has recently been settled by identifying hot HB stars as main contributors to galaxy ultraviolet luminosity excess. While a blue HB morphology seems a natural characteristic of metal-poor stellar populations, its appearence in metal-rich systems, often coupled with a poorer rate of planetary nebulae per unit galaxy luminosity, might be calling for an intimate connection between UV excess and AGB properties in early-type galaxies. In this work, we want to briefly assess this issue, relying on infrared sur…
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. 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. We also provide the ingredients necessary for constructing models with arbitrary HB morphologies.
The high-resolution (R = 10^4) near-ultraviolet IUE spectrum of the peculiar post Asymptotic Giant Branch (post-AGB) supergiant HD101584 (F0Iape) has been analyzed. In the wavelength range between 2500 and 3000ang, 291 out of 332 absorption features are identified. The main contribution comes from FeII, CrII and MnII. Most of the absorption features are blends from several absorption lines. This is mainly due to the intrinsic broad line profiles in the spectrum of HD101584 in combination with the many absorption lines which occur in the UV for a F-type spectrum. The list compiled gives for all…
We consider the general theoretical problem of ultraviolet light from old stellar populations (t > 2 Gyr), and the interpretation of galaxy spectra at short wavelengths (lamda < 3200A) The sources believed to be responsible for the observed `ultraviolet upturn phenomenon' (UVX) are Post-AGB (P-AGB), extreme horizontal branch (EHB) and AGB-Manque (AGBM) stars. The production of EHB stars depends on mass loss on the red giant branch, which is poorly understood, making the far-UV flux problematical as an indicator of the gross properties, such as the age, of a given galaxy. We discuss the current…
Early-type galaxies exhibit thermal and molecular resonance emission from dust that is shed and heated through stellar mass loss as a subset of the population moves through the AGB phase of evolution. Because this emission can give direct insight into stellar evolution in addition to galactic stellar mass loss and ISM injection rates, we conducted a program to search for this signature emission with CAM on ISO. We obtained 6-15 micron imaging observations in six narrow bands for nine elliptical galaxies; every galaxy is detected in every band. For wavelengths shorter than 9 microns, the spectr…
Deep near-infrared images obtained with adaptive optics systems on the Gemini North and Canada-France-Hawaii telescopes are used to investigate the bright stellar content and central regions of the nearby elliptical galaxy Maffei 1. Stars evolving on the upper asymptotic giant branch (AGB) are resolved in a field 3 arcmin from the center of the galaxy. The locus of bright giants on the (K, H-K) color-magnitude diagram is consistent with a population of stars like those in Baade's Window reddened by E(H-K) = 0.28 +/- 0.05 mag. This corresponds to A_V = 4.5 +/- 0.8 mag, and is consistent with pr…
Far-ultraviolet radiation is a ubiquitous, if unanticipated, phenomenon in elliptical galaxies and early-type spiral bulges. It is the most variable photometric feature associated with old stellar populations. Recent observational and theoretical evidence shows that it is produced mainly by low-mass, small-envelope, helium-burning stars in extreme horizontal branch and subsequent phases of evolution. These are probably descendents of the dominant, metal rich population of the galaxies. Their lifetime UV outputs are remarkably sensitive to their physical properties and hence to the age and the …
Any model for the formation of elliptical planetary nebulae (PNs) should account for the positive correlation between the mass loss rate and the degree of departure from sphericity of the AGB progenitor's wind. I propose that this correlation results from dust formation above magnetic cool spots. The model deals with elliptical PNs, but not with bipolar PNs. The basic assumption is that a weak dynamo amplifies magnetic fields in AGB stars, such that magnetic cool spots are formed, mainly near the equatorial plane. Enhanced dust formation above these cool spots leads to a higher mass loss rate …
The UV upturn phenomenon observed in elliptical galaxies is attractive for its potential value as an age indicator of old stellar systems. We present our most recent population models for the UV evolution of elliptical galaxies. We confirm that the dominant UV sources are either metal-poor or metal-rich hot horizontal-branch (HB) stars in local giant ellipticals, but we also note that the contribution from post-asymptotic-giant-branch (PAGB) stars overwhelms the UV spectrum at higher redshifts (look-back times). The model UV spectral energy distribution (SED) is therefore strongly affected by …
Using objects found in a systematic survey of the galactic Plane in the 1612-MHz OH line, we discuss in detail two ``sequences'' of post-AGB evolution, a red and a blue. We argue that the red and the blue groups separate by initial mass at 4Msun, based on evolutionary-sequence turn-off colours, spectral energy distributions, outflow velocities and scaleheight. The higher-mass (blue) objects may have earlier AGB termination. The lower-mass (red) objects undergo very sudden reddening for IRAS colour R21\sim1.2; these sources must all undergo a very similar process at AGB termination. The transit…
We have used the adaptive optics system on the Canada-France-Hawaii Telescope to study a field 6 arcmin from the center of the heavily obscured elliptical galaxy Maffei 1. Our near diffraction-limited H and K' images reveal an excess population of objects with respect to a background field, and we conclude that the asymptotic giant branch tip (AGB-tip) in Maffei 1 occurs at K = 20 +/- 0.25. Assuming that stars at the AGB-tip in Maffei 1 have the same intrinsic luminosity as those in the bulge of M31, then the actual distance modulus of Maffei 1 is 28.2 +/- 0.3 if A_V = 5.1, which corresponds t…
We present far-ultraviolet (far-UV) images of the rich galaxy cluster ZwCl1358.1+6245, taken with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). When combined with archival HST observations, our data provide a measurement of the UV-to-optical flux ratio in 8 early-type galaxies at z=0.33. Because the UV flux originates in a population of evolved, hot, horizontal branch (HB) stars, this ratio is potentially one of the most sensitive tracers of age in old populations -- it is expected to fade rapidly with lookback time. We find that the UV emission in these g…
I review the current understanding of the origin and nature of ``warm'' ionized gas in giant elliptical galaxies in the light of results of recent imaging and spectroscopic surveys. CCD imaging surveys have revealed that emission-line disks or filaments are (as a rule) associated with dust absorption, even in the X-ray brightest systems. This strongly suggests that the origin of this ionized gas is generally not through ``cooling flows''; galaxy interactions are favored instead. Using data from a new spectrophotometric survey of ``normal'' elliptical galaxies covering the whole optical range, …
PG1159-035 is the prototype of the PG1159 spectral class which consists of extremely hot hydrogen-deficient (pre-) white dwarfs. It is also the prototype of the GW Vir variables, which are non-radial g-mode pulsators. The study of PG1159 stars reveals insight into stellar evolution and nucleosynthesis during AGB and post-AGB phases. We perform a quantitative spectral analysis of PG1159-035 focusing on the abundance determination of trace elements. We have taken high-resolution ultraviolet spectra of PG1159-035 with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. They…
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 (< 2000 Angstrom) 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 d…
We study the red giant populations of two dE galaxies, AM 1339-445 and AM 1343-452, with the aim of investigating the number and luminosity of any upper asymptotic giant branch (AGB) stars present. The galaxies are members of the Centaurus A group (D~3.8 Mpc) and are classified as outlying (R~350 kpc) satellites of Cen A. The analysis is based on near-IR photometry for individual red giant stars, derived from images obtained with ISAAC on the VLT. The photometry, along with optical data derived from WFPC2 images retrieved from the HST science archive, enable us to investigate the stellar popul…
We examined effects of the ultraviolet background radiation (UVB) on the colour--magnitude relation (CMR) of elliptical galaxies in clusters of galaxies in the hierarchical clustering scenario by using a semi-analytic model of galaxy formation. In our model the UVB photoionizes gas in dark haloes and suppresses the cooling of the diffuse hot gas onto galaxy discs. By using a semi-analytic model without the effect of the UVB, Kauffmann & Charlot found that the CMR can be reproduced by strong supernova heating because such supernova feedback suppresses the chemical enrichment in galaxies especia…
We present the GALEX UV photometry of the elliptical galaxies in Abell clusters at moderate redshifts (z < 0.2) for the study of the look-back time evolution of the UV upturn phenomenon. The brightest elliptical galaxies (M_r < -22) in 12 remote clusters are compared with the nearby giant elliptical galaxies of comparable optical luminosity in the Fornax and Virgo clusters. The sample galaxies presented here appear to be quiescent without signs of massive star formation or strong nuclear activity, and show smooth, extended profiles in their UV images indicating that the far-UV (FUV) light is m…
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 pa…
Bipolar planetary nebulae (PNe), as well as extreme elliptical PNe are formed through the influence of a stellar companion. But half of all PN progenitors are not influenced by any stellar companion, and, as I show here, are expected to rotate very slowly on reaching the upper asymptotic giant branch; hence they expect to form spherical PNe, unless they are spun-up. But since most PNe are not spherical, I argue that about 50 percents of AGB stars are spun-up by planets, even planets having a mass as low as 0.01 times the mass of Jupiter, so they form elliptical PNe. The rotation by itself will…
Recent observational and theoretical developments concerning the origin of the ultraviolet emission from ellipticals are reviewed and discussed,with particular, reference to the extensive set of predictions contained in Greggio and Renzini (1990). Strong support to the preferred hot horizontal branch and AGB-manque' scenario for the origin of the UV emission has come from UIT, HUT and HST data. However, the UV sensitivity of such space telescopes has proved insufficient to unambiguously detect the predicted fading away of the UV rising branch with redshift, that we consider the ultimate check …
In this work we investigate the stellar population, metallicity distribution and ionized gas in the elliptical galaxy NGC 5044, using long-slit spectroscopy and a stellar population synthesis method. We found differences in the slope of metal-line profiles along the galaxy which suggests an enhancement of alpha elements, particularly towards the central region. The presence of a non-thermal ionization source, such as a low-luminosity AGN and/or shock ionization, is implied by the large values of the ratio (N II])Ha observed in all sampled regions. However, the emission lines observed in the ex…
In this paper, we show that the optically bright stars above the tip of the first red giant branch (TRGB) in the color magnitude (CM) diagrams of nearby dwarf elliptical (dE) galaxies, commonly interpreted as indication of the existence of an intermediate-age population, may, in certain circumstances, be an artifact of observational effects on a pure old population system. For this purpose, model CM diagrams have been computed simulating the observational effects that can be found in different regions of a Local Group dE galaxy. The starting synthetic CM diagram represents a pure old system, w…
We are using optical/IR surface brightness fluctuations (SBFs) to validate the latest stellar population synthesis models and to understand the stellar populations of ellipticals. Integrated light and spectra measure only the first moment of the stellar luminosity function (Σn_i * L_i). Since SBFs also depend on the second moment (Σn_i * L_i^2), they provide novel information, in particular about the reddest, most luminous RGB and AGB stars, which are the most difficult stars to model. SBFs can also provide useful new constraints on the age/metallicity of unresolved stellar populations in elli…
We present the deepest near-UV image of M32 to date, which for the first time resolves hot HB stars in an elliptical galaxy. Given the near-solar metallicity of M32, much larger than that of globular clusters, the existence of an extended HB is a striking example of the 2nd parameter effect, and, most importantly, provides direct evidence that hot HB stars and their progeny are the major contributors to the UV upturn phenomenon observed in ellipticals. Our image, obtained with STIS, detects ~8000 stars in a 25"x25" field, centered 7.7" from the galaxy nucleus. These stars span a range of 21-28…
We present preliminary results from spectral synthesis models of old stellar populations for the spectral range 912-4000 A with ~ 10A 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…
The bright stellar content near the center of the Local Group elliptical galaxy M32 is investigated with 0.12 arcsec FWHM H and K images obtained with the Gemini Mauna Kea telescope. Stars with K = 15.5, which are likely evolving near the tip of the asymptotic giant branch (AGB), are resolved to within 2 arcsec of the nucleus, and it is concluded that the peak stellar brightness near the center of M32 is similar to that in the outer regions of the galaxy. Moreover, the projected density of bright AGB stars follows the visible light profile to within 2 arcsec of the nucleus, indicating that the…
Results from IRAS and recent X-ray and optical surveys are reviewed to discuss the properties and nature of the interstellar medium in elliptical galaxies. As to the dust component, there is a strong contrast with the situation among spiral galaxies in that masses of dust in ellipticals as derived from optical extinction are an order of magnitude LOWER than those derived from IRAS data. This dilemma can be resolved by assuming an extra, extended, diffusely distributed component of dust which is not detectable in optical data. Bona-fide global correlations among ISM components ---and between io…
We present images, integrated photometry, surface-brightness and color profiles for a total of 1034 nearby galaxies recently observed by the GALEX satellite in its far-ultraviolet (FUV; 1516A) and near-ultraviolet (NUV; 2267A) bands. (...) This data set has been complemented with archival optical, near-infrared, and far-infrared fluxes and colors. We find that the integrated (FUV-K) color provides robust discrimination between elliptical and spiral/irregular galaxies and also among spiral galaxies of different sub-types. Elliptical galaxies with brighter K-band luminosities (i.e. more massive)…
Recent morphological studies of galaxies by the {\it Hubble Space Telescope (HST)} have revealed that actively star-forming galaxies at intermediate and high redshifts ($z=0.5-2.0$) have very clumpy and irregular distributions of stars. It is however unclear whether and how these clumpy galaxies evolve into the present spiral and elliptical galaxies with regular shapes. We here propose that spatial distributions of AGB stars, probing the different mean age and metallicity of the underlying stellar population, can provide vital clues to the evolution of these clumpy galaxies, in particular, tho…
The proto-planetary Red Rectangle nebula is powered by HD 44179, a spectroscopic binary (P = 318 d), in which a luminous post-AGB component is the primary source of both luminosity and current mass loss. Here, we present the results of a seven-year, eight-orbit spectroscopic monitoring program of HD 44179, designed to uncover new information about the source of the Lyman/far-ultraviolet continuum in the system as well as the driving mechanism for the bipolar outflow producing the current nebula. Our observations of the H-alpha line profile around the orbital phase of superior conjunction revea…
Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we have obtained deep far-ultraviolet images of the compact elliptical galaxy M32. When combined with earlier near-ultraviolet images of the same field, these data enable the construction of an ultraviolet color-magnitude diagram of the hot horizontal branch (HB) population and other hot stars in late phases of stellar evolution. We find few post-asymptotic giant branch (PAGB) stars in the galaxy, implying that these stars either cross the HR diagram more rapidly than expected, and/or that they spend a significant fr…
This series of papers comprises a systematic exploration of the hypothesis that the far ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the Zero Age Horizontal Branch through to a point late in post-HB evolution or a point on the white dwarf cooling track. We use the term `Extreme Horizontal Branch' (EHB) to refer to HB sequences of constant mass that do not reach the thermally-pulsing stage on the AGB. These m…
Extremely hot horizontal branch (HB) stars and their progeny are widely considered to be responsible for the "ultraviolet upturn" (or UVX) phenomenon observed in elliptical galaxies and the bulges of spirals. Yet, the precise evolutionary channels that lead to the production of these stars remain the source of much debate. In this review, we discuss two key physical ingredients that are required in order for reliable quantitative models of the UV output of stellar populations to be computed, namely, the mass loss rates of red giant branch stars and the helium enrichment "law" at high metallici…
We have extended the study of the young star clusters observed in the merger remnant galaxy NGC 7252 by obtaining K band photometry for these clusters. Our K band data significantly complement the optical photometry and spectroscopy in the literature: K band data are fundamental to study the Asymptotic Giant Branch (AGB) population of these clusters, since the AGB phase transition (occuring between the age of 200 Myr and 1 Gyr) causes abrupt changes in the near-infrared luminosity of the clusters while producing only small changes in the optical. Therefore, the e.g. V-K colour is ideal to stud…
The asymptotic giant branch (AGB) phase of stellar evolution is common to most stars of low and intermediate mass. Most of the carbon and nitrogen in the Universe is produced by AGB stars. The final fate of the AGB envelopes are represented by planetary nebulae (PN). By studying PN abundances and compare them with the yields of stellar evolution is possible to quantify carbon and nitrogen production, and to study cosmic recycling in galactic and Magellanic Cloud populations. In this paper we present the latest results in PN chemical abundance analysis and their implication to the chemical evol…
The stellar population, metallicity distribution and ionized gas in the elliptical galaxies NGC 6868 and NGC 5903 are investigated in this paper by means of long-slit spectroscopy and stellar population synthesis. Lick indices in both galaxies present a negative gradient indicating an overabundance of Fe, Mg, Na and TiO in the central parts with respect to the external regions. Concerning the emitting gas conspicuously detected in NGC 6868, we test three hypotheses as ionizing source: an H II region, post-AGB stars and an Active Galactic Nucleus (AGN). Diagnostic diagrams involving the ratios …
The traditional view of elliptical galaxies has been that they formed in a single, rapid burst of star formation at high redshift, and have evolved quiescently since that time. In opposition to this traditional view is evidence that at least some elliptical galaxies have formed from the merger of two disk galaxies. What has not been clear is which process is the dominant formation mechanism for the large majority of elliptical galaxies. This question has significant implications for cosmological models, as different models make different predictions for the formation mechanism and epoch of ell…
Near-infrared (2.2 $μm$) observations of a sample of 48 elliptical galaxies in the Coma cluster have been carried out and used to study the near-ir fundamental plane (FP) of ellipticals in this cluster. An rms scatter of 0.072 dex was found for this relation, similar to that of its optical counterpart, using the same sample of galaxies. This corresponds to an uncertainty of 18% in distances to individual galaxies derived from this relation. The sensitivity of the near-infrared FP to the star formation or changes in metallicity and stellar population among the ellipticals was explored and found…
The discovery of an excess of light in the far-ultraviolet (UV) in 1969 in elliptical galaxies was a major surprise. While it is now clear that this UV excess (UV-upturn) is probably caused by an old population of helium-burning stars. Han et al (2002, 2003) proposed a binary model for the formation of hot subdwarfs (helium burning stars) and the model can reproduce the observations in our Galaxy. By applying the binary model to the study of evolutionary population synthesis, we have obtained an {\it a priori} model for the UV-upturn of elliptical galaxies. The model shows that the UV-upturn i…
The rest-frame UV contains the most sensitive indicators of age for elliptical galaxies. While the near-UV flux from young ellipticals isolates the main sequence turnoff, the far-UV flux in old ellipticals is dominated by hot horizontal branch (HB) stars. This evolved population was first revealed by early UV observations showing a sharp flux increase shortward of rest-frame 2500 A, subsequently dubbed the "UV upturn." The phenomenon has since been characterized in many local ellipticals, and measurements at intermediate redshifts are now underway. Once ellipticals reach ages of 5-10 Gyr, stel…
The discovery of an excess of light in the far-ultraviolet (UV) spectrum in elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. This has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In p…
We have obtained high resolution UV images with the HST+WFPC2 of the central region of the dwarf elliptical galaxy NGC 205. Our images reveal that many of the hot UV stars previously detected and studied from the ground are actually multiple systems, open clusters and star associations. We have performed photometry of two such clusters and we find our data are consistent with stellar ages of 50 and 100 Myr, respectively. From the number of massive stars in NGC 205 we estimate that the star formation episode in this galaxy has turned ~1,000 Msolar of gas into stars over the last 100 Myr.
The discovery of a flux excess in the far-ultraviolet (UV) spectrum of elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes, rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. We have developed an evolutionary population synthesis model for the far-UV excess of elliptical galaxies based on the binar…
PG1159 stars are hot hydrogen-deficient post-AGB stars with effective temperatures within a range from 75000 K up to 200000 K. These stars are probably the result of a late helium-shell flash that had occurred during their first descent from the AGB. The lack of hydrogen is caused by flash-induced envelope mixing and burning of H in deeper regions. Now the former intershell matter is seen on the surface of the stars. Hence the stellar atmospheres show metal abundances drastically different from the solar values. Our sample comprises ten PG1159 stars with effective temperatures between 85000 K …
The excess of far-ultraviolet (far-UV) radiation in elliptical galaxies has remained one of their most enduring puzzles. In contrast, the origin of old blue stars in the Milky Way, hot subdwarfs, is now reasonably well understood: they are hot stars that have lost their hydrogen envelopes by various binary interactions. Here, we review the main evolutionary channels that produce hot subdwarfs in the Galaxy and present the results of binary population synthesis simulations that reproduce the main properties of the Galactic hot-subdwarf population. Applying the same model to elliptical galaxies,…
We acquired spectra of 24 LMC PNe in the 1150--3000 Årange in order to determine carbon and other ionic abundances. The sample more than doubles the number of LMC PNe with good quality UV spectra in this wavelength range, and whose optical images are available in the {\it HST} archive. The {\it Space Telescope Imaging Spectrograph} was used with a very large aperture to obtain virtually slit-less spectra, thus the monochromatic images in the major nebulae emission lines are also available. The analysis of the data shows extremely good quality spectra. This paper presents the emission lines ide…
In this paper we present new chemo-spectro-photometric 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 g…
We present BVRI CCD surface photometry for the central (6'.35 X 6'.35) regions of the dwarf elliptical galaxies NGC 185 and NGC 205 in the Local Group. Surface brightness profiles of NGC 185 (R < 225") and NGC 205 (R < 186") show excess components in the central regions. The colors of NGC 185 get bluer inward at R < 25", while they remain constant at R >= 25". The colors of NGC 205 get bluer inward at 1" < R < 50", and remain flat outside. Our photometry, supplemented by the photometry based on the far-ultraviolet and visual images of the HST archive data, shows that there is an inversion of c…
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 vari…
Using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on board the Hubble Space Telescope, we have obtained F110W (J-band) and F160W (H-band) images of three fields in NGC3379, a nearby normal giant elliptical galaxy. These images resolve individual red giant stars, yielding the first accurate color-magnitude diagrams for a normal luminous elliptical. The photometry reaches 1 magnitude below the red giant branch tip with errors of < 0.2 mags in F110W-F160W. A strong break in the luminosity function at F160W = 23.68 +/- 0.06 is identified as the tip of the red giant branch (RGB)…
Taking advantage of the exceptional capabilities of ACS on board of HST, we derive Surface Brightness Fluctuation (SBF) measurements in the B and I bands from images of six elliptical galaxies with $1500 \leq cz \leq 3500$. Given the low S/N ratio of the SBF signal in the blue band images, the reliability of the measurements is verified both with numerical simulations and experimental data tests. This paper presents the first published B- and I-band SBF measurements for distant ($\geq$ 20 Mpc) galaxies, essential for the comparisons of the models to observations of normal ellipticals. By compa…
Broad-band near-infrared images are used to probe the photometric properties of the brightest asymptotic giant branch (AGB) stars within 2 arcminutes of the centers of the dwarf elliptical galaxies NGC 147, NGC 185, and NGC 205. Based on the peak brightness and color of the M giant sequences, ages of 1 Gyr and 0.1 Gyr are predicted for the most recent significant star forming events in NGC 185 and NGC 205, respectively. The brightest AGB stars in NGC 147 indicate that the most recent significant star-forming activity in that galaxy occured \~3 Gyr in the past. The specific frequency of C stars…
Massive galaxies, such as nearby ellipticals, have relatively low number densities, yet they host the majority of the stellar mass in the universe. Understanding their origin is a central problem of galaxy formation. Age dating of stellar populations found in modern ellipticals, and observations of star formation in high redshift galaxies, allow us to determine roughly when these systems formed. These age diagnostics however do not tell us what triggered star formation, or how galaxies form as opposed to simply when. Recent analyses of the structures of z > 2 ultraviolet selected galaxies reve…
The mean ages of early-type galaxies obtained from the analysis of optical spectra, give a mean age of 8 Gyr at z = 0, with 40% being younger than 6 Gyr. Independent age determinations are possible by using infrared spectra (5-21 microns), which we have obtained with the Infrared Spectrograph on the Spitzer Observatory. This age indicator is based on the collective mass loss rate of stars, where mass loss from AGB stars produces a silicate emission feature at 9-12 microns. This feature decreases more rapidly than the shorter wavelength continuum as a stellar population ages, providing an age i…
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 2000 Angstroms 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 zero-age HB to the white-dwarf cooling curve. The strength of the UV-upturn is governed by the fraction of stars that evolve through the E…
The far infrared background is a sink for the hidden aspects of galaxy formation. At optical wavelengths, ellipticals and spheroids are old, even at $z \sim 1.$ Neither the luminous formation phase nor their early evolution is seen in the visible. We infer that ellipticals and, more generally, most spheroids must have formed in dust-shrouded starbursts. In this article, we show how separate tracking of disk and spheroid star formation enables us to infer that disks dominate near the peak in the cosmic star formation rate at $z \lapproxeq 2$ and in the diffuse ultraviolet/optical/infrared backg…
We resolve stars of the nearest giant elliptical galaxy NGC 5128 using VLT with FORS1 and ISAAC. We construct deep U, V and Ks color-magnitude and color-color diagrams in two different halo fields (in the halo and in the north-eastern diffuse shell). In the outer, shell field, at ~14 kpc from the center of the galaxy, there is a significant recent star formation with stars as young as 10 Myr, approximately aligned with the prominent radio and x-ray jet from the nucleus of the host AGN. Ionized gas filaments are evident in ultraviolet images near the area where neutral HI and CO molecular gas w…
PEGASE is a new spectrophotometric evolution model for starbursts and evolved galaxies of the Hubble sequence. Its main originality is the extension to the near-infrared (NIR) of the atlas of synthetic spectra of Rocca-Volmerange & Guiderdoni (1988) with a revised stellar library including cold star parameters and stellar tracks extended to the TP-AGB and the post-AGB phase. The NIR is coherently linked to the visible and the ultraviolet, so that the model is continuous on an exceptionally large wavelength range from 220 A up to 5 microns. Moreover, a precise algorithm allows to follow very ra…
