Ignacio Ferreras

The elliptical galaxy colour-magnitude relation as a discriminant between the monolithic and merger paradigms

The colour-magnitude relation (CMR) of cluster ellipticals has been widely used to constrain their star formation histories (SFHs) and to discriminate between the monolithic and merger paradigms of elliptical galaxy formation. We investigate the elliptical CMR predicted in the merger paradigm by using a LCDM hierarchical merger model. We first highlight sections of the literature which indicate that the traditional use of fixed apertures to derive colours gives a distorted view of the CMR due to the presence of colour gradients in galaxies. Fixed aperture observations make the CMR steeper and tighter than it really is. We then show that the star formation history (SFH) of cluster ellipticals predicted by the model is quasi-monolithic, with over 95 percent of the total stellar mass formed before a redshift of 1. The quasi-monolithic SFH produces a predicted CMR that agrees well at all redshifts with its observed counterpart once the fixed aperture effect is removed. More importantly, we present arguments to show that the elliptical-only CMR can be used to constrain the SFHs of present-day cluster ellipticals only if we believe a priori in the monolithic collapse model. It is not a meaningful tool for constraining the SFH in the merger paradigm, because a progressively larger fraction of the progenitor set of present-day cluster ellipticals is contained in late-type star forming systems at higher redshift, which cannot be ignored when deriving the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between the two competing theories of elliptical galaxy evolution.

The formation history of the Galactic bulge

The distributions of the stellar metallicities of K giant stars in several fields of the Galactic bulge, taken from the literature and probing projected Galactocentric distances of ∼500 pc to ∼3 kpc, are compared with a simple model of star formation and chemical evolution. Our model assumes a Schmidt law of star formation and is described by only a few parameters that control the infall and outflow of gas and the star formation efficiency. Exploring a large volume of parameter space, we find that very short infall time-scales are needed (0.5 Gyr), with durations of infall and star formation greater than 1 Gyr being ruled out at the 90 per cent confidence level. The metallicity distributions are compatible with an important amount of gas and metals being ejected in outflows, although a detailed quantification of the ejected gas fraction is strongly dependent on a precise determination of the absolute stellar metallicities. We find a systematic difference between the samples of Ibata & Gilmore, at projected distances of 1‐3 kpc, and the sample in Baade’s window (Sadler et al.). This could be caused either by a true metallicity gradient in the bulge or by a systematic offset in the calibration of [Fe/H] between these two samples. This offset does not play an important role in the estimate of infall and formation time-scales, which are mostly dependent on the width of the distributions. The recent bulge data from Zoccali et al. are also analysed, and the subsample with subsolar metallicities still rules out infall time-scales1 Gyr at the 90 per cent confidence level. Hence, the short time-scales we derive based on the observed distribution of metallicities are robust and should be taken as stringent constraints on bulge formation models.

On breaking the age-metallicity degeneracy in early-type galaxies: infall versus star formation efficiency

The correlation between [Mg/Fe] and galaxy mass found in elliptical galaxies sets a strong constraint on the duration of star formation. Furthermore, the colour-magnitude relation restricts the range of ages and metallicities of the stellar populations. We combine these two constraints with a model of star formation and chemical enrichment including infall and outflow of gas to find that the trend towards supersolar [Mg/Fe] in massive ellipticals excludes a pure metallicity sequence as an explanation of the colour-magnitude relation. An age spread is required, attributable either to a range of star formation efficiencies or to a range of infall timescales. We find that the inferred range of stellar ages is compatible with the small scatter and the redshift evolution of the colour-magnitude relation. Two alternative scenarios can explain the data: fixed infall with an efficiency which is linearly dependent on mass, or a fixed efficiency with a mass-dependent infall timescale t~1/sqrt(M). We conclude that the actual scenario may well involve a combination of these two parameters, with mass dependencies which should span the range of those given above.

Exploring the star formation history of elliptical galaxies: beyond simple stellar populations with a new line strength estimator

We study the stellar populations of a sample of 14 elliptical galaxies in the Virgo cluster. Using spectra with high signal-to-noise ratio (S/N greater than or similar to 100 A-1) we propose an alternative approach to the standard side-band method to measure equivalent widths (EWs). Our boosted median continuum is shown to map the EWs more robustly than the side-band method, minimizing the effect from neighbouring absorption lines and reducing the uncertainty at a given S/N. Our newly defined line strengths are more successful at disentangling the age-metallicity degeneracy. We concentrate on Balmer lines (H beta, H gamma, H delta), the G band and the combination [MgFe] as the main age and metallicity indicators. We go beyond the standard comparison of the observations with simple stellar populations (SSPs) and consider four different models to describe the star formation histories, either with a continuous star formation rate or with a mixture of two different SSPs. These models improve the estimates of the more physically meaningful mass-weighted ages. Composite models are found to give more consistent fits among individual line strengths and agree with an independent estimate using the spectral energy distribution. A combination of age- and metallicity-sensitive spectral features allows us to constrain the average age and metallicity. For a Virgo sample of elliptical galaxies our age and metallicity estimates correlate well with stellar mass or velocity dispersion, with a significant threshold around 5 x 1010 M(circle dot) above which galaxies are uniformly old and metal rich. This threshold is reminiscent of the one found by Kauffmann et al. in the general population of Sloan Digital Sky Survey galaxies at a stellar mass 3 x 1010 M(circle dot). In a more speculative way, our models suggest that it is formation epoch and not formation time-scale what drives the mass-age relationship of elliptical galaxies.

Massive Elliptical Galaxies: From Cores to Halos

In the context of recent observational results that show massive ellipticals were in place at high redshifts, we reassessthestatusofmonolithiccollapseinaCDMuniverse.Usingasampleofover2000galaxiesfromtheSloan Digital SkySurvey,bycomparing thedynamical mass andstellar mass (estimated fromcolors)wefindthat ellipticals have ‘‘cores’’ that are baryon-dominated within their half-light radius. These galaxies correspond to 3 � peaks in the spherical collapse model if the total mass in the halo is assumed to be 20 times the dynamical mass within the halflight radius. This value yields stellar mass‐to‐total massratios of 8%, compared to a cosmological baryon fraction of 18% derived from the first 3 years ofWMAPobservations alone. We further develop a method for reconstructing the concentration halo parameterc of the progenitors of these galaxies by utilizing adiabatic contraction. Although the analysis is done within the framework of monolithic collapse, the resulting distribution ofc is lognormal with a peakvalueofc � 3 10anda distributionwidth similartothe results ofN-bodysimulations. We alsoderive scaling relationsbetweenstellaranddynamicalmassandthevelocitydispersion,andfindthatthesearesufficienttorecover the tilt of the fundamental plane. Subject headingg dark matter — galaxies: elliptical and lenticular, cD — galaxies: evolution — galaxies: formation — galaxies: fundamental parameters

Setting new constraints on the age of the Universe

There are three independent techniques for determining the age of the Universe: via cosmochronology of long-lived radioactive nuclei, via stellar modelling and population synthesis of the oldest stellar populations, and, most recently, via the precision cosmology that has become feasible with the mapping of the acoustic peaks in the cosmic microwave background. We demonstrate that all three methods give completely consistent results, and enable us to set rigorous bounds on the maximum and minimum ages that are allowed for the Universe. We present new constraints on the age of the Universe by performing a multiband colour analysis of bright cluster ellipticals over a large redshift range (0.3<z<0.9), which allows us to infer the ages of their stellar populations over a wide range of possible formation redshifts and metallicities. Applying a prior to Hubbles constant of H-0 = 72 +/-8 km s(-1) Mpc(-1) we find the age of the Universe to be 13.0(-2.0)(+3.0) Gyr (1 sigma), in agreement with the estimates from Type Ia supernovae, as well as with the latest uranium decay estimates, which yield an age for the Milky Way of 12.5 +/-3 Gyr. If we combine the results from cluster ellipticals with the analysis of the angular power spectrum of the cosmic microwave background and with the observations of Type Ia supernovae at high redshift, we find a similar age: 13.2(-0.8)(+1.2) Gyr. Without the assumption of any priors, universes older than 18 Gyr are ruled out by the data at the 90 per cent confidence level.

Type Ia supernovae and the formation history of early-type galaxies

Using the standard prescription for the rates of supernovae Type II and Type Ia, we compare the predictions of a simple model of star formation in galaxies with the observed radial gradients of abundance ratios in a sample of early-type galaxies to infer the relative contribution of each type of supernova. The data suggest a correlation between the fractional contribution of Type Ia to the chemical enrichment of the stellar populations (1 - ξ) and central velocity dispersion of order 1 - ξ ∼ 0.16 log σ 0 + 0.40, so that the Type Ia contribution in stars ranges from a negligible amount in massive (σ 0 ∼ 300 km s - 1 ) galaxies up to 10 per cent in low-mass (∼100 km s - 1 ) elliptical galaxies. Our model is parametrized by a star-formation time-scale (t S F ) that controls the duration of the starburst. A correlation with galaxy radius as a power law (t S F r β ) translates into a radial gradient of the abundance ratios. The data imply a wide range of formation scenarios for a simple model that fixes the luminosity profile, ranging from inside out (β = 2), to outside in formation (β = -1), as is consistent with numerical simulations of elliptical galaxy formation. An alternative scenario that links t S F to the dynamical time-scale favours inside-out formation over a smaller range 0.4 < β < 0.6. In both cases, massive galaxies are predicted to have undergone a more extended period of star formation in the outer regions with respect to their low-mass counterparts.

Feedback Processes in Early-Type Galaxies

We present a simple phenomenological model of feedback in early-type galaxies that tracks the evolution of the interstellar medium gas mass, metallicity, and temperature. Modeling the star formation rate as a Schmidt law with a temperature-dependent efficiency, we find that intermittent episodes of star formation are common in moderate-size ellipticals. Our model is applicable in the case in which the thermalization time from supernovae is sufficiently long that spatial variations are relatively unimportant, an appropriate assumption for the empirical parameters adopted here, but one that can only be demonstrated conclusively through more detailed numerical studies. The departure from a standard scenario of passive evolution implies significantly younger luminosity-weighted ages for the stellar populations of low-mass galaxies at moderate redshifts, even though the more physically meaningful mass-weighted ages are changed only slightly. Secondary bursts of star formation also lead to a natural explanation of the large scatter in the near-UV-optical relation observed in clusters at moderate redshift and account for the population of E+A galaxies that display a spheroidal morphology. As the late-time formation of stars in our model is due to the gradual cooling of the interstellar medium, which is heated to temperatures ~1 keV by the initial burst of supernovae, our conclusions do not rely on any environmental effects or external mechanisms. Furthermore, a simple estimate of the X-ray emission from this supernova-heated gas leads to an LX-LB correlation that is in good agreement with observed values. Thus, feedback processes may be essential to understanding the observed properties of early-type galaxies from the optical to the X-ray.

On the globular cluster formation history of NGC 5128

We deduce the globular cluster formation history of the nearby elliptical galaxy, NGC 5128, by using a chemical enrichment model to accurately reproduce its observed metallicity distribution function (MDF). We derive the observed MDF using recently obtained U and B photometry of the NGC 5128 GC system, with (U - B) used as the metallicity indicator. Our results indicate that the GC system in this galaxy could be the product of two major GC formation episodes. The initial formation episode occured 11-12 Gyr ago creating 65-75 percent of the mass in the GC system. This was followed by a second late formation episode which peaked 2-4 Gyr ago and produced the remaining 25-35 percent of GC mass.

Using bright ellipticals as dark energy cosmic clocks

The age of the Universe has been increasingly constrained by different techniques, such as the observations of type Ia supernovae (SNIa) at high redshift or dating the stellar populations of globular clusters. In this paper, we present a complementary approach using the colours of the brightest elliptical galaxies in clusters over a wide redshift range (z (less than or similar to) 1). We put new and independent bounds on the dark energy equation of state parametrized by a constant pressure-to-density ratio w(Q) and by a parameter (xi) which determines the scaling between the matter and dark energy densities. We find that accurate estimates of the metallicities of the stellar populations in moderate and high-redshift cluster galaxies can pose stringent constraints on the parameters that describe dark energy. Our results are in good agreement with the analysis of dark energy models using SNIa data as a constraint. Accurate estimates of the metallicities of stellar populations in cluster galaxies at z less than or similar to 2 will make this approach a powerful complement to studies of cosmological parameters using high-redshift SNIa.