Recent evidence based independently on spectral line strengths and dynamical modelling point towards a non-universal stellar Initial Mass Function (IMF), probably implying an excess of low-mass stars in elliptical galaxies with a high velocity disper
sion. Here we show that a time-independent bottom-heavy IMF is compatible neither with the observed metal-rich populations found in giant ellipticals nor with the number of stellar remnants observed within these systems. We suggest a two-stage formation scenario involving a time-dependent IMF to reconcile these observational constraints. In this model, an early strong star-bursting stage with a top-heavy IMF is followed by a more prolonged stage with a bottom-heavy IMF. Such model is physically motivated by the fact that a sustained high star formation will bring the interstellar medium to a state of pressure, temperature and turbulence that can drastically alter the fragmentation of the gaseous component into small clumps, promoting the formation of low-mass stars. This toy model is in good agreement with the different observational constrains on massive elliptical galaxies, such as age, metallicity, alpha-enhancement, M/L, or the mass fraction of the stellar component in low-mass stars.
We analyse the Tully-Fisher relation at moderate redshift from the point of view of the underlying stellar populations, by comparing optical and NIR photometry with a phenomenological model that combines population synthesis with a simple prescriptio
n for chemical enrichment. The sample comprises 108 late-type galaxies extracted from the FORS Deep Field (FDF) and William Herschel Deep Field (WHDF) surveys at z<1 (median redshift z=0.45). A correlation is found between stellar mass and the parameters that describe the star formation history, with massive galaxies forming their populations early (zFOR~3), with star formation timescales, tau1~4Gyr; although with very efficient chemical enrichment timescales (tau2~1Gyr). In contrast, the stellar-to-dynamical mass ratio - which, in principle, would track the efficiency of feedback in the baryonic processes driving galaxy formation - does not appear to correlate with the model parameters. On the Tully-Fisher plane, no significant age segregation is found at fixed circular speed, whereas at fixed stellar-to-dynamical mass fraction, age splits the sample, with older galaxies having faster circular speeds at fixed Ms/Mdyn. Although our model does not introduce any prior constraint on dust reddening, we obtain a strong correlation between colour excess and stellar mass.
Theoretical studies of structure formation find an inverse proportionality between the concentration of dark matter haloes and virial mass. This trend has been recently confirmed for virial masses Mvir > ~6e12 Msun by the observation of the X-ray emi
ssion from the hot halo gas. We present an alternative approach to this problem, exploring the concentration of dark matter haloes over galaxy scales on a sample of 18 early-type systems. Our c-Mvir relation is consistent with the X-ray analysis, extending towards lower virial masses, covering the range from ~4e11 Msun up to 5e12 Msun. A combination of the lensing analysis along with photometric data allows us to constrain the baryon fraction within a few effective radii, which is compared with prescriptions for adiabatic contraction (AC) of the dark matter haloes. We find that the standard methods for AC are strongly disfavored, requiring additional mechanisms -- such as mass loss during the contraction process -- to play a role during the phases following the collapse of the haloes.
We study the star forming regions in the spiral galaxy NGC4321, taking advantage of the spatial resolution (2.5 arcsec FWHM) of the Swift/UVOT camera and the availability of three UV passbands in the region 1600-3000 A, in combination with optical an
d IR imaging from SDSS, KPNO/Ha and Spitzer/IRAC, to obtain a catalogue of 787 star forming regions out to three disc scale lengths. We determine the properties of the young stellar component and its relationship with the spiral arms. The Ha luminosities of the sources have a strong decreasing radial trend, suggesting more massive star forming regions in the central part of the galaxy. When segregated with respect to NUV-optical colour, blue sources have a significant excess of flux in the IR at 8 micron, revealing the contribution from PAHs, although the overall reddening of these sources stays below E(B-V)=0.2 mag. The distribution of distances to the spiral arms is compared for subsamples selected according to Ha luminosity, NUV-optical colour, or ages derived from a population synthesis model. An offset is expected between these subsamples as a function of radius if the pattern speed of the spiral arm were constant - as predicted by classic density wave theory. No significant offsets are found, favouring instead a mechanism where the pattern speed has a radial dependence.
We have explored the buildup of the local mass-size relation of elliptical galaxies using two visually classified samples. At low redshift we compiled a subsample of 2,656 elliptical galaxies from SDSS, whereas at higher redshift (up to z~1) we extra
cted a sample of 228 object from the HST/ACS images of the GOODS. All the galaxies in our study have spectroscopic data, allowing us to determine the age and mass of the stellar component. Using the fossil record information contained in the stellar populations of our local sample, we do not find any evidence for an age segregation at a given stellar mass depending on the size of the galaxies. At a fixed dynamical mass there is only a <9% size difference in the two extreme age quartiles of our sample. Consequently, the local evidence does not support a scenario whereby the present-day mass-size relation has been progressively established via a bottom-up sequence, where older galaxies occupy the lower part this relation, remaining in place since their formation. We find a trend in size that is insensitive to the age of the stellar populations, at least since z~1. This result supports the idea that the stellar mass-size relation is formed at z~1, with all galaxies populating a region which roughly corresponds to 1/2 of the present size distribution. The fact that the evolution in size is independent of stellar age, together with the absence of an increase in the scatter of the relationship with redshift does not support the puffing up mechanism. The observational evidence, however, can not reject at this stage the minor merging hypothesis. We have made an estimation of the number of minor merger events necessary to bring the high-z galaxies into the local relation compatible with the observed size evolution. Since z=0.8, if the merger mass ratio is 1:3 we estimate ~3+-1 minor mergers and if the ratio is 1:10 we obtain ~8+-2 events.
Differences in the stellar populations of galaxies can be used to quantify the effect of environment on the star formation history. We target a sample of early-type galaxies from the Sloan Digital Sky Survey in two different environmental regimes: cl
ose pairs and a general sample where environment is measured by the mass of their host dark matter halo. We apply a blind source separation technique based on principal component analysis, from which we define two parameters that correlate, respectively, with the average stellar age (eta) and with the presence of recent star formation (zeta) from the spectral energy distribution of the galaxy. We find that environment leaves a second order imprint on the spectra, whereas local properties - such as internal velocity dispersion - obey a much stronger correlation with the stellar age distribution.
The low-mass end of the stellar Initial Mass Function (IMF) is constrained by focusing on the baryon-dominated central regions of strong lensing galaxies. We study in this letter the Einstein Cross (Q2237+0305), a z=0.04 barred galaxy whose bulge act
s as lens on a background quasar. The positions of the four quasar images constrain the surface mass density on the lens plane, whereas the surface brightness (H-band NICMOS/HST imaging) along with deep spectroscopy of the lens (VLT/FORS1) allow us to constrain the stellar mass content, for a range of IMFs. We find that a classical single power law (Salpeter IMF) predicts more stellar mass than the observed lensing estimates. This result is confirmed at the 99% confidence level, and is robust to systematic effects due to the choice of population synthesis models, the presence of dust, or the complex disk/bulge population mix. Our non-parametric methodology is more robust than kinematic estimates, as we do not need to make any assumptions about the dynamical state of the galaxy or its decomposition into bulge and disk. Over a range of low-mass power law slopes (with Salpeter being Gamma=+1.35) we find that at a 90% confidence level, slopes with Gamma>0 are ruled out.
The kinematics of stars and planetary nebulae in early type galaxies provide vital clues to the enigmatic physics of their dark matter halos. We fit published data for fourteen such galaxies using a spherical, self-gravitating model with two componen
ts: (1) a Sersic stellar profile fixed according to photometric parameters, and (2) a polytropic dark matter halo that conforms consistently to the shared gravitational potential. The polytropic equation of state can describe extended theories of dark matter involving self-interaction, non-extensive thermostatistics, or boson condensation (in a classical limit). In such models, the flat-cored mass profiles widely observed in disc galaxies are due to innate dark physics, regardless of any baryonic agitation. One of the natural parameters of this scenario is the number of effective thermal degrees of freedom of dark matter (F_d) which is proportional to the dark heat capacity. By default we assume a cosmic ratio of baryonic and dark mass. Non-Sersic kinematic ideosyncrasies and possible non-sphericity thwart fitting in some cases. In all fourteen galaxies the fit with a polytropic dark halo improves or at least gives similar fits to the velocity dispersion profile, compared to a stars-only model. The good halo fits usually prefer F_d values from six to eight. This range complements the recently inferred limit of 7<F_d<10 (Saxton & Wu), derived from constraints on galaxy cluster core radii and black hole masses. However a degeneracy remains: radial orbital anisotropy or a depleted dark mass fraction could shift our models preference towards lower F_d; whereas a loss of baryons would favour higher F_d.
(Abridged) We present a detailed study of the stellar populations of a volume-limited sample of early-type galaxies from SDSS, across a range of environments -- defined as the mass of the host dark matter halo. The stellar populations are explored th
rough the SDSS spectra, via projection onto a set of two spectral vectors determined from Principal Component Analysis. We find the velocity dispersion of the galaxy to be the main driver behind the different star formation histories of early-type galaxies. However, environmental effects are seen to play a role (although minor). Galaxies populating the lowest mass halos have stellar populations on average ~1Gyr younger than the rest of the sample. The fraction of galaxies with small amounts of recent star formation is also seen to be truncated when occupying halos more massive than 3E13Msun. The sample is split into satellite and central galaxies for a further analysis of environment. Satellites are younger than central galaxies of the same stellar mass. The younger satellite galaxies in 6E12Msun halos have stellar populations consistent with the central galaxies found in the lowest mass halos of our sample (i.e. 1E12Msun). This result is indicative of galaxies in lower mass halos being accreted into larger halos.
(Abridged) We study the stellar populations of 14 elliptical galaxies in the Virgo cluster. We propose an alternative approach to the standard side-band method to measure equivalent widths (EWs). Our Boosted Median Continuum maps the EWs more robustl
y than the side-band method, minimising the effect from neighbouring absorption lines and reducing the age-metallicity degeneracy. We concentrate on Balmer lines (Hbeta,Hgamma,Hdelta), the G band and the 4000A break as age-sensitive indicators, and on the combination [MgFe] as the main metallicity indicator. We go beyond the standard comparison of the observations with simple stellar populations (SSP) and consider various models to describe the star formation histories, either with a continuous star formation rate or with a mixture of two different SSPs. Composite models are found to give more consistent fits among individual line strengths and agree with an independent estimate using the spectral energy distribution. Our age and metallicity estimates correlate well with stellar mass or velocity dispersion, with a significant threshold around 5E10 Msun above which galaxies are uniformly old and metal rich. In a more speculative way, our models suggest that it is formation **epoch** and not formation timescale what drives the Mass-Age relationship of elliptical galaxies.
