arXiv subjects

I. Ferreras

Publications and source records attributed to I. Ferreras.

At least 73 records · Page 4Linked to original sources

arXiv2005

Morphology and evolution of emission line galaxies in the Hubble Ultra Deep Field

We investigate the properties and evolution of a sample of galaxies selected to have prominent emission lines in low-resolution grism spectra of the Hubble Ultra Deep Field (HUDF). These objects, eGRAPES, are late type blue galaxies, characterized by small proper sizes (R_50 < 2 kpc) in the 4350A rest-frame, low masses (5x10^9 M_sun), and a wide range of luminosities and surface brightnesses. The masses, sizes and volume densities of these objects appear to change very little up to a redshift of z=1.5. On the other hand, their surface brightness decreases significantly from z=1.5 to z=0 while their mass-to-light ratio increases two-folds. This could be a sign that most of low redshift eGRAPES have an older stellar population than high redshift eGRAPES and hence that most eGRAPES formed at higher redshifts. The average volume density of eGRAPES is (1.8 \pm 0.3)x10^{-3} Mpc^{-3} between 0.3 < z < 1.5. Many eGRAPES would formally have been classified as Luminous Compact Blue Galaxies (LCBGs) if these had been selected based on small physical size, blue intrinsic color, and high surface brightness, while the remainder of the sample discussed in this paper forms an extension of LCBGs towards fainter luminosities.

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arXiv2005

GOODS, UDF, and the evolution of early-type galaxies

The HST/ACS images of GOODS-South have been used to select a sample of early-type galaxies, based on morphology and on the Kormendy relation. The classification scheme does not use galaxy colours, hence it does not bias against young stellar populations. The 249 galaxies (i_AB<24) paint a complex formation picture. Their stellar populations show gradients which readily rule out a large range of ages among and within galaxies. On the other hand, there is a decrease in the comoving number density, which suggests a strong bias when comparing local and distant early-type galaxies. This bias can be caused either by a significant fraction of non early-type progenitors or by a selection effect (e.g. dust-enshrouded progenitors). The deep images of the UDF were used to determine the structural properties of some of these galaxies. Regarding the distribution of disky/boxy isophotes, we do not find large differences with respect to local systems. However, some early-types with the standard features of a red and dead galaxy reveal interesting residuals, possible signatures of past merging events.

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arXiv2005

The Structure and Star Formation History of Early-Type Galaxies in the UDF/GRAPES Survey

We present a two-pronged approach to the formation of early-type galaxies, using a sample of 18 galaxies at 0.5 < z < 1 from the HST/ACS Ultra Deep Field and GRAPES surveys: 1) We combine slitless low resolution spectroscopy from the GRAPES dataset with simple models of galaxy formation to explore their star formation histories. 2) We also perform an analysis of their surface brightness distribution with the unprecedented details provided by the ACS superb angular resolution and photometric depth. Our spectroscopic analysis reveals that their stellar populations are rather homogeneous in age and metallicity and formed at redshifts z ~ 2-5. Evolving them passively, they become practically indistinguishable from ellipticals at z = 0. Also, their isophotal shapes appear very similar to those observed for nearby ellipticals, in that the percentages of disky and boxy galaxies at z ~ 1 are close to the values measured at z = 0. Moreover,we find that the isophotal structure of z ~ 1 early-type galaxies obeys the correlations already observed among nearby ellipticals, i.e. disky ellipticals have generally higher characteristic ellipticities, and boxy ellipticals have larger half-light radii and are brighter in the restframe B band. In this respect then, no significant structural differences are seen for ellipticals between z = 0 and 1.

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arXiv2005

The HI content of the recently discovered field dwarf galaxy APPLES1

We present observations in a search for neutral hydrogen associated with the recently detected field dwarf galaxy APPLES1, performed with the Parkes radiotelescope. The observed radio spectrum shows no evident (> 3sigma rms) line emission indicating an upper limit for the HI content of the galaxy of ~ 10E6 solar masses and providing an upper value for the M(HI)/L(B) ratio equal to 2.4 solar masses/solar luminosities. The low value of the HI content suggested by the observations, together with the galaxy optical morphology, might indicate that APPLES1 is a dwarf spheroidal. This indication is in contrast with the evidence of recent star formation, which is typical for dwarf irregular galaxies. This may suggest that APPLES1 belongs to the class of mixed dwarf irregular/spheroidal transition-type galaxies. We also conclude that the relatively low neutral gas mass in APPLES1 can be explained by an extended and inefficient star formation process, without the need for a dramatic event such as enhanced star formation or a past encounter with a massive galaxy or galaxy group.

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arXiv2005

Evolution of field early-type galaxies: The view from GOODS/CDFS

(Abridged) We explore the evolution of field early-type galaxies on a sample extracted from GOODS/CDFS. The galaxies are selected by means of a non-parametric analysis followed by visual inspection. We exclude those galaxies which are not consistent with an evolution into the Kormendy relation. The final set comprises 249 galaxies with a median redshift z=0.7. The distribution of number counts versus apparent magnitude suggests a substantial decrease of the comoving number density with redshift. The majority of the galaxies feature passively evolving old stellar populations. One third of those in the upper half of the redshift distribution have blue colors, in contrast to only 10% in the lower redshift subsample. An adaptive binning of the color maps is performed to explore the internal color distribution. We find that most blue galaxies in our sample feature blue cores whereas most of the red early-types are passively evolving stellar populations with red cores. The color gradients and scatter do not evolve with redshift and are compatible with the observations at z=0 assuming a radial dependence of the metallicity within each galaxy. This work emphasizes the need for a careful sample selection, as we found that most of those galaxies which were visually classified as early types -- but then rejected based on the Kormendy relation -- feature blue colors characteristic of recent star formation.

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arXiv2005

Hot Cores : Probes of High-Redshift Galaxies

The very high rates of second generation star formation detected and inferred in high redshift objects should be accompanied by intense millimetre-wave emission from hot core molecules. We calculate the molecular abundances likely to arise in hot cores associated with massive star formation at high redshift, using several independent models of metallicity in the early Universe. If the number of hot cores exceeds that in the Milky Way Galaxy by a factor of at least one thousand, then a wide range of molecules in high redshift hot cores should have detectable emission. It should be possible to distinguish between independent models for the production of metals and hence hot core molecules should be useful probes of star formation at high redshift.

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arXiv2004

Discovery of a Solitaire Dwarf Galaxy in the APPLES Survey

During the APPLES parallel campaign, the HST Advanced Camera for Surveys has resolved a distant stellar system, which appears to be an isolated dwarf galaxy. It is characterized by a circularly symmetric distribution of stars with an integrated magnitude m(F775W) = 20.13 +- 0.02, a central surface brightness of ~ 21.33 +- 0.18 mag/arcsec^2 and a half-light radius of ~ 1.8 arcsec. The ACS and VLT spectra show no evidence of ionized gas and appear dominated by a 3 Gyr old stellar population. The OB spectral type derived for two resolved stars in the grism data and the systemic radial velocity of ~ 670 km/s measured from the VLT data give a fiducial distance of ~ 9 +- 2 Mpc. These findings, with the support of the spatial morphology, would classify the system among the dwarf spheroidal (dSph) galaxies. Following the IAU rules, we have named this newly discovered galaxy APPLES 1. An intriguing peculiarity of APPLES 1 is that the properties (age and metallicity) of the stellar content so far detected are similar to those of dSph galaxies in the Local Group, where star formation is thought to be driven by galaxy interactions and mergers. Yet, APPLES 1 seems not to be associated with a major group or cluster of galaxies. Therefore, APPLES 1 could be the first example of a field dSph galaxy with self-sustained and regulated star formation and, therefore, would make an interesting test case for studies of the formation and evolution of unperturbed dSph galaxies.

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arXiv2004

The star formation history of intermediate redshift late-type galaxies

We combine the latest observations of disk galaxy photometry and rotation curves at moderate redshift from the FORS Deep Field (FDF) with simple models of chemical enrichment. Our method describes the buildup of the stellar component through infall of gas and allows for gas and metal outflows. In this framework, we keep a minimum number of constraints and we search a large volume of parameter space, looking for the models which best reproduce the photometric observations in the observed redshift range (0.5<z<1). We find the star formation efficiency to correlate well with vMAX so that massive disks are more efficient in the formation of stars and have a smaller spread in stellar ages. This trend presents a break at around vMAX 140km/s. Galaxies on either side of this threshold have significantly different age distributions. This break has been already suggested by several authors in connection with the contribution from either gravitational instabilities or supernova-driven turbulence to star formation. No clear trend is seen between galaxy mass and infall timescale or gas outflows. The model presented in this paper suggests massive disks have formation histories resembling those of early-type galaxies, with highly efficient and short-lived bursts, in contrast with low-mass disks, which have a more extended star formation history. One option to explain the observed shallow slope of the Tully-Fisher relation at intermediate redshift could be small episodes of star formation in low-mass disks.

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arXiv2004

Spectral dating of high-redshift stellar populations

(Abridged) Age derivation techniques for unresolved stellar populations at high redshifts are explored using the NUV spectrum of LBDS~53W091 and LBDS~53W069. The photometry and morphology of these galaxies suggest they are early-type systems,a feature that makes them ideal test beds for the analysis of their ages and metallicities. In the analysis that is based on simple stellar population models,we find a significant degeneracy between the derived ages and metallicities both in optical+NIR photometric and NUV spectroscopic analyses. This degeneracy is not so strong for LBDS~53W069. However even in this case the stellar age cannot be constrained better than to a range roughly encompassing one third of the age of the Universe at the observed redshift. We have explored several independent population synthesis models and consistently found similar results. Broadband photometry straddling the rest-frame 4000A break is also subject to a strong age-metallicity degeneracy. The use of realistic chemical enrichment assumptions significantly helps in disentangling the degeneracy. Based on this method, we derive the average stellar age for both galaxies around 3.6-3.8 Gyr with better constraints on the youngest possible ages. From the observational point of view, the most efficient (and feasible) way to set limits on unresolved stellar populations comprises a combination of Balmer absorption lines along with either low SNR rest frame NUV spectroscopy or accurate optical and NIR photometry.

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arXiv2003

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 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. We find that very short infall timescales are needed, with durations of infall and star formation greater than 1 Gyr being ruled out at the 90% 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 metallities. The recent bulge data from Zoccali et al. are also analyzed, and the subsample with subsolar metallicities still rules out infall timescales around 1 Gyr at the 90% confidence level. Hence, the short timescales we derive based on the observed distribution of metallicities are robust and should be taken as stringent constraints on bulge formation models.

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arXiv2002

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.

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arXiv2001

Backwards galaxy formation with only a few parameters

We present a simple phenomenological model of star formation in galaxies that describes the process with a set of a few parameters. The star formation efficiency and the fraction of gas and metals ejected in outflows are assumed to be the main drivers of star formation. The Tully-Fisher and the Faber-Jackson relations in different passbands are used as constraints in the analysis of disk and elliptical galaxies, respectively. We find that a steep correlation between efficiency and maximum rotation velocity is needed in disk systems, whereas elliptical galaxies can be explained by a high star formation efficiency, uncorrelated with the central velocity dispersion. Gas outflows are not important in disks, whereas a correlation of outflows with galaxy mass must be invoked in early-type galaxies to account for the observed colors.

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arXiv2001

How old is the Universe ? 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 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 conservative prior to Hubble's constant of H0=70+-15 km/s/Mpc, we find the age of the universe to be 13.2(+3.6 -2.0) Gyr (1-sigma), in agreement both with the estimates from type Ia supernovae, as well as with the latest uranium decay estimates. Without the assumption of any priors, universes older than 18 Gyr are ruled out by the data at the 90 % confidence level.

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arXiv2001

A backwards approach to the formation of disk galaxies I. Stellar and gas content

A simple chemical enrichment code is described where the two basic mechanisms driving the evolution of the ages and metallicities of the stellar populations are the star formation efficiency and the fraction of gas ejected from the galaxy. Using the observed Tully-Fisher relation in different passbands as a constraint, it is found that a steep correlation between the maximum disk rotational velocity and star formation efficiency must exist either for a linear or a quadratic Schmidt law. Outflows do not play a major role. The redshift evolution of disk galaxies is explored, showing that a significant change in the slope of the Tully-Fisher relation is expected because of the different age distributions of the stellar components in high and low-mass disk galaxies. The slope measured in the rest frame B,K bands is found to change from 3(B); 4(K) at z=0 up to 4.5(B); 5(K) at z~1, with a slight dependence on formation redshift.

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arXiv2000

How young are early-type cluster galaxies ? Quantifying the young stellar component in a rich cluster at z=0.41

We present a new method of quantifying the mass fraction of young stars in galaxies by analyzing near-ultraviolet (NUV)-optical colors. We focus our attention on early-type cluster galaxies, whose star formation history is at present undetermined. Rest-frame NUV (F300W) and optical (F702W) images of cluster Abell 851 (z=0.41) using HST/WFPC2 allow us to determine a NUV-optical color-magnitude relation, whose slope is incompatible with a monolithic scenario for star formation at high redshift. A degeneracy between a young stellar component and its fractional mass contribution to the galaxy is found, and a photometric analysis comparing the data with the predictions for a simple two-stage star formation history is presented. The analysis shows that some of the early-type galaxies may have fractions higher than 10% of the total mass content in stars formed at z~0.5. An increased scatter is found in the color-magnitude relation at the faint end, resulting in a significant fraction of faint blue early-type systems. This would imply that less massive galaxies undergo more recent episodes of star formation, and this can be explained in terms of a positive correlation between star formation efficiency and luminosity.

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arXiv2000

On breaking the age-metallicity degeneracy in early-type galaxies: Outflows versus Star Formation Efficiency

A simple model of chemical enrichment in cluster early-type galaxies is presented where the main parameters driving the formation of the stellar component are reduced to four: infall timescale (tf), formation epoch (zF), star formation efficiency (Ceff) and fraction of gas ejected in outflows (Bout). We find that only variations in Bout or Ceff can account for the colour-magnitude relation, so that the most luminous galaxies had low values of ejected gas and high efficiencies. The combination of chemical enrichment tracks with population synthesis models is used to explore the correlation between mass-to-light ratios and masses. A significant slope mismatch is found between stellar and total M/L ratios, which cannot be explained by an age spread and implies a non-linear correlation between total and stellar mass. The sequences driven by star formation efficiency and outflows are shown to predict different trends at high redshift. Measurement of the dependence of the tilt of the fundamental plane on redshift will break the degeneracy between outflows and star formation efficiency, which will enable us to determine whether the colour-magnitude relation is controlled by age or metallicity.

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arXiv2000

The role of outflows and star formation efficiency in the evolution of early-type cluster galaxies

A phenomenological model for chemical enrichment in early-type galaxies is presented, in which the process of star formation is reduced to a set of four parameters: star formation efficiency (Ceff), fraction of ejected gas in outflows (Bout), formation redshift (zF) and infall timescale (Tf). Out of these four parameters, only variations of Bout or Ceff can account for the color-magnitude relation. A range of outflows results in a metallicity sequence, whereas a range of star formation efficiencies will yield a mixed age + metallicity sequence. The age-metallicity degeneracy complicates the issue of determining which mechanism contributes the most (i.e. outflows versus efficiency). However, the determination of the slope of the correlation between mass-to-light ratio and mass in clusters at moderate or high redshift will allow us to disentangle age and metallicity.

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arXiv1999

Probing the evolution of early-type cluster galaxies through chemical enrichment

A simple chemical enrichment model for cluster early-type galaxies is described in which the mechanisms considered in the evolutionary model are infall of primordial gas, outflows and a possible variation in the star formation efficiency. We find that - within the framework of our models - only outflows can generate a suitable range of metallicities. The chemical enrichment tracks can be combined with the latest population synthesis models to simulate clusters over a wide redshift range, for a set of toy models. The color-magnitude relation of local clusters is used as a constraint, fixing the correlation between absolute luminosity and ejected fraction of gas from outflows. It is found that the correlations between color or mass-to-light ratios and absolute luminosity are degenerate with respect to most of the input parameters. However, a significant change between monolithic and hierarchical models is predicted for redshifts $z\simgt 1$. The comparison between predicted and observed mass-to-light ratios yield an approximate linear bias between total and stellar masses: $M_{\rm Tot}\propto M_{\rm St}^{1.15\pm 0.08}$ in early-type galaxies. If we assume that outflows constitute the driving mechanism for the colors observed in cluster early type galaxies, the metallicity of the intracluster medium (ICM) can be linked to outflows. The color-magnitude constraint requires faint $M_V\sim -16$ galaxies to eject 85% of their gas, which means that most of the metals in the ICM may have originated in these dwarf galaxies.

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