arXiv subjects

I. Ferreras

Publications and source records attributed to I. Ferreras.

At least 55 records · Page 3Linked to original sources

arXiv2014

SPIDER X - Environmental effects in central and satellite early-type galaxies through the stellar fossil record

A detailed analysis of how environment affects the star formation history of early-type galaxies (ETGs) is undertaken via high signal to noise ratio stacked spectra obtained from a sample of 20,977 ETGs (morphologically selected) from the SDSS-based SPIDER survey. Two major parameters are considered for the study: the central velocity dispersion (sigma), which relates to local drivers of star formation, and the mass of the host halo, which relates to environment-related effects. In addition, we separate the sample between centrals (the most massive galaxy in a halo) and satellites. We derive trends of age, metallicity, and [alpha/Fe] enhancement, with sigma. We confirm that the major driver of stellar population properties in ETGs is velocity dispersion, with a second-order effect associated to the central/satellite nature of the galaxy. No environmental dependence is detected for satellite ETGs, except at low sigma - where satellites in groups or in the outskirts of clusters tend to be younger than those in the central regions of clusters. In contrast, the trends for centrals show a significant dependence on halo mass. Central ETGs in groups (i.e. with a halo mass >10^12.5 M_Sun) have younger ages, lower [alpha/Fe], and higher internal reddening, than "isolated" systems (i.e. centrals residing in low-mass, <10^12.5 M_Sun, halos). Our findings imply that central ETGs in groups formed their stellar component over longer time scales than "isolated" centrals, mainly because of gas-rich interactions with their companion galaxies.

astro-ph.GA↗

arXiv2014

Constraints on the merging channel of massive galaxies since z~1

(Abridged) We probe the merging channel of massive galaxies over the z=0.3-1.3 redshift window by studying close pairs in a sample of 238 galaxies with stellar mass >1E11Msun, from the deep (m<26.5AB, 3 sigma) SHARDS survey. SHARDS provides medium band photometry equivalent to low-resolution optical spectra (R~50), allowing us to obtain extremely accurate photometric redshifts (|Dz|/(1+z)~0.55%) and to improve the constraints on the age distribution of the stellar populations. A strong correlation is found between the age difference of central and satellite galaxy and stellar mass ratio, from negligible age differences in major mergers to age differences ~4 Gyr for 1:100 minor mergers. However, this correlation is simply a reflection of the mass-age trend in the general population. The dominant contributor to the growth of massive galaxies corresponds to mass ratios mu=Msat/Mcen>0.3, followed by a decrease in the fractional mass growth rate linearly proportional to log mu, at least down to mu~0.01, suggesting a decreasing role of mergers involving low-mass satellites, especially if dynamical friction timescales are taken into account. A simple model results in an upper limit for the average mass growth rate of massive galaxies of DM/M/Dt~ 0.08+-0.02 per Gyr, over the z<1 range, with a ~70% fractional contribution from (major) mergers with mu>0.3. The majority of the stellar mass contributed by mergers does not introduce significantly younger populations, in agreement with the small radial age gradients observed in present-day early-type galaxies.

astro-ph.CO↗

arXiv2014

A panchromatic analysis of starburst galaxy M82: Probing the dust properties

(Abridged) We combine NUV, optical and IR imaging of the nearby starburst galaxy M82 to explore the properties of the dust both in the interstellar medium of the galaxy and the dust entrained in the superwind. The three NUV filters of Swift/UVOT enable us to probe in detail the properties of the extinction curve in the region around the 2175A bump. The NUV colour-colour diagram strongly rules out a Calzetti-type law, which can either reflect intrinsic changes in the dust properties or in the star formation history compared to starbursts well represented by such an attenuation law. We emphasize that it is mainly in the NUV region where a standard Milky-Way-type law is preferred over a Calzetti law. The age and dust distribution of the stellar populations is consistent with the scenario of an encounter with M81 in the recent 400 Myr. The radial gradients of the NUV and optical colours in the superwind region support the hypothesis that the emission in the wind cone is driven by scattering from dust grains entrained in the ejecta. The observed wavelength dependence reveals either a grain size distribution $n(a)\propto a^{-2.5}$, where $a$ is the size of the grain, or a flatter distribution with a maximum size cutoff, suggesting that only small grains are entrained in the supernovae-driven wind.

astro-ph.GA↗

arXiv2013

Chronos: A NIR spectroscopic galaxy survey. From the formation of galaxies to the peak of activity

Chronos is our response to ESA's call for white papers to define the science for the future L2, L3 missions. Chronos targets the formation and evolution of galaxies, by collecting the deepest NIR spectroscopic data, from the formation of the first galaxies at z~10 to the peak of formation activity at z~1-3. The strong emission from the atmospheric background makes this type of survey impossible from a ground-based observatory. The spectra of galaxies represent the equivalent of a DNA fingerprint, containing information about the past history of star formation and chemical enrichment. The proposed survey will allow us to dissect the formation process of galaxies including the timescales of quenching triggered by star formation or AGN activity, the effect of environment, the role of infall/outflow processes, or the connection between the galaxies and their underlying dark matter haloes. To provide these data, the mission requires a 2.5m space telescope optimised for a campaign of very deep NIR spectroscopy. A combination of a high multiplex and very long integration times will result in the deepest, largest, high-quality spectroscopic dataset of galaxies from z=1 to 12, spanning the history of the Universe, from 400 million to 6 billion years after the big bang, i.e. covering the most active half of cosmic history.

astro-ph.CO↗

arXiv2013

SPIDER VIII - Constraints on the Stellar Initial Mass Function of Early-type Galaxies from a Variety of Spectral Features

We perform a spectroscopic study to constrain the stellar Initial Mass Function (IMF) by using a large sample of 24,781 early-type galaxies from the SDSS-based SPIDER survey. Clear evidence is found of a trend between IMF and central velocity dispersion, sigma0, evolving from a standard Kroupa/Chabrier IMF at 100km/s towards a more bottom-heavy IMF with increasing sigma0, becoming steeper than the Salpeter function at sigma0>220km/s. We analyze a variety of spectral indices, corrected to solar scale by means of semi-empirical correlations, and fitted simultaneously with extended MILES (MIUSCAT) stellar population models. Our analysis suggests that sigma0, rather than [alpha/Fe], drives the IMF variation. Although our analysis cannot discriminate between a single power-law (unimodal) and a low-mass (<0.5MSun) tapered (bimodal) IMF, we can robustly constrain the fraction in low-mass stars at birth, that is found to increase from 20% at sigma0~100km/s, up to 80% at sigma0~300km/s. Additional constraints can be provided with stellar mass-to-light (M/L) ratios: unimodal models predict M/L significantly larger than dynamical M/L, across the whole sigma0 range, whereas a bimodal IMF is compatible. Our results are robust against individual abundance variations. No significant variation is found in Na and Ca in addition to the expected change from the correlation between [alpha/Fe] and sigma0. [Abridged]

astro-ph.CO↗

arXiv2013

A multiwavelength study of the Magellanic-type galaxy NGC 4449 - I. Modelling the spectral energy distribution, the ionization structure and the star formation history

[Abridged] We present an integrated photometric spectral energy distribution (SED) of the Magellanic-type galaxy NGC 4449 from the far-ultraviolet (UV) to the submillimetre, including new observations acquired by the Herschel Space Observatory. We include integrated UV photometry from the Swift Ultraviolet and Optical Telescope using a measurement technique which is appropriate for extended sources with coincidence loss. In this paper, we examine the available multiwavelength data to infer a range of ages, metallicities and star formation rates for the underlying stellar populations, as well as the composition and the total mass of dust in NGC 4449. We present an iterative scheme, which allows us to build an in-depth and multicomponent representation of NGC 4449 `bottom-up', taking advantage of the broad capabilities of the photoionization and radiative transfer code MOCASSIN (MOnte CArlo SimulationS of Ionized Nebulae). We fit the observed SED, the global ionization structure and the emission line intensities, and infer a recent SFR of 0.4 Msolar/yr and a total stellar mass of approximately 1e9 Msolar emitting with a bolometric luminosity of 5.7e9 Lsolar. Our fits yield a total dust mass of 2.9e6 Msolar including 2 per cent attributed to polycyclic aromatic hydrocarbons. We deduce a dust to gas mass ratio of 1/190 within the modelled region. While we do not consider possible additional contributions from even colder dust, we note that including the extended HI envelope and the molecular gas is likely to bring the ratio down to as low as ~ 1/800.

astro-ph.CO↗

arXiv2012

Systematic variation of the stellar Initial Mass Function with velocity dispersion in early-type galaxies

An essential component of galaxy formation theory is the stellar initial mass function (IMF), that describes the parent distribution of stellar mass in star forming regions. We present observational evidence in a sample of early-type galaxies (ETGs) of a tight correlation between central velocity dispersion and the strength of several absorption features sensitive to the presence of low-mass stars. Our sample comprises ~40,000 ETGs from the SPIDER survey (z<0.1). The data, extracted from the Sloan Digital Sky Survey, are combined, rejecting both noisy data, and spectra with contamination from telluric lines, resulting in a set of 18 stacked spectra at high signal-to-noise ratio (S/N> 400 per A). A combined analysis of IMF-sensitive line strengths and spectral fitting is performed with the latest state-of the art population synthesis models (an extended version of the MILES models). A significant trend is found between IMF slope and velocity dispersion, towards an excess of low-mass stars in the most massive galaxies. Although we emphasize that accurate values of the IMF slope will require a detailed analysis of chemical composition (such as [a/Fe] or even individual element abundance ratios), the observed trends suggest that low-mass ETGs are better fit by a Kroupa-like IMF, whereas massive galaxies require bottom-heavy IMFs, exceeding the Salpeter slope at velocity dispersions above 200km/s.

astro-ph.CO↗

arXiv2012

SPIDER VII - Revealing the Stellar Population Content of Massive Early-type Galaxies out to 8Re

Radial trends of stellar populations in galaxies provide a valuable tool to understand the mechanisms of galaxy growth. In this paper, we present the first comprehensive analysis of optical-optical and optical-NIR colours, as a function of galaxy mass, out to the halo region (8Re) of early-type galaxies (ETGs). We select a sample of 674 massive ETGs (M*>3x10^10MSun) from the SDSS-based SPIDER survey. By comparing with a large range of population synthesis models, we derive robust constraints on the radial trends in age and metallicity. Metallicity is unambiguously found to decrease outwards, with a measurable steepening of the slope in the outer regions (Re 10^11MSun). Furthermore, when segregating with respect to large scale environment, the age gradient is more significant in ETGs residing in higher density regions. These results shed light on the processes leading from the formation of the central core to the growth of the stellar envelope of massive galaxies. The fact that the populations in the outer regions are older and more metal-poor than in the core suggests a process whereby the envelope of massive galaxies is made up of accreted small satellites (i.e. minor mergers) whose stars were born during the first stages of galaxy formation.

astro-ph.CO↗

arXiv2011

On the Radial Stellar Content of Early-Type Galaxies as a Function of Mass and Environment

Using optical-optical and optical-NIR colors, we analyze the radial dependence of age and metallicity inside massive (M* > 10^10.5 MSun), low-redshift (z<0.1), early-type galaxies (ETGs), residing in both high-density group regions and the field. On average, internal color gradients of ETGs are mainly driven by metallicity, consistent with previous studies. However, we find that group galaxies feature positive age gradients, Nabla_t, i.e. a younger stellar population in the galaxy center, and steeper metallicity gradients, compared to the field sample, whose Nabla_t ranges from negative in lower mass galaxies, to positive gradients at higher mass. These dependencies yield new constraints to models of galaxy formation and evolution. We speculate that age and metallicity gradients of group ETGs result from (either gas-rich or minor-dry) mergers and/or cold-gas accretion, while field ETGs exhibit the characteristic flatter gradients expected from younger, more metal-rich, stars formed inside--out by later gas-cooling.

astro-ph.CO↗

arXiv2008

Stellar Populations of Late-Type Bulges at z=1 in the HUDF

We combine the exceptional depth of the Hubble Ultra Deep Field (HUDF) images and the deep GRism ACS Program for Extragalactic Science (GRAPES) grism spectroscopy to explore the stellar populations of 34 bulges belonging to late-type galaxies at z=0.8-1.3. The sample is selected based on the presence of a noticeable 4000A break in their GRAPES spectra, and by visual inspection of the HUDF images. The HUDF images are used to measure bulge color and Sersic index. The narrow extraction of the GRAPES data around the galaxy center enables us to study the spectrum of the bulges in these late-type galaxies, minimizing the contamination from the disk of the galaxy. We use the low resolution (R~50) spectral energy distribution (SED) around the 4000A break to estimate redshifts and stellar ages. The SEDs are compared with models of galactic chemical evolution to determine the stellar mass, and to characterize the age distribution. We find that, (1) the average age of late-type bulges in our sample is ~1.3 Gyr with stellar masses in the range log(M)=6.5-10 solar. (2) Late-type bulges are younger than early-type galaxies at similar redshifts and lack a trend of age with respect to redshift, suggesting a more extended period of star formation. (3) Bulges and inner disks in these late-type galaxies show similar stellar populations, and (4) late-type bulges are better fitted by exponential surface brightness profiles. The overall picture emerging from the GRAPES data is that, in late-type galaxies at z=1, bulges form through secular evolution and disks via an inside-out process.

astro-ph↗

arXiv2007

Predicting spectral features in galaxy spectra from broad-band photometry

We explore the prospects of predicting emission line features present in galaxy spectra given broad-band photometry alone. There is a general consent that colours, and spectral features, most notably the 4000 A break, can predict many properties of galaxies, including star formation rates and hence they could infer some of the line properties. We argue that these techniques have great prospects in helping us understand line emission in extragalactic objects and might speed up future galaxy redshift surveys if they are to target emission line objects only. We use two independent methods, Artifical Neural Neworks (based on the ANNz code) and Locally Weighted Regression (LWR), to retrieve correlations present in the colour N-dimensional space and to predict the equivalent widths present in the corresponding spectra. We also investigate how well it is possible to separate galaxies with and without lines from broad band photometry only. We find, unsurprisingly, that recombination lines can be well predicted by galaxy colours. However, among collisional lines some can and some cannot be predicted well from galaxy colours alone, without any further redshift information. We also use our techniques to estimate how much information contained in spectral diagnostic diagrams can be recovered from broad-band photometry alone. We find that it is possible to classify AGN and star formation objects relatively well using colours only. We suggest that this technique could be used to considerably improve redshift surveys such as the upcoming FMOS survey and the planned WFMOS survey.

astro-ph↗

arXiv2007

UV-optical colours as probes of early-type galaxy evolution

We have studied ~2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range 0 < z < 0.11. Combining GALEX UV photometry with corollary optical data from the SDSS, we find that, at a 95 percent confidence level, at least ~30 percent of galaxies in this sample have UV to optical colours consistent with some recent star formation within the last Gyr. In particular, galaxies with a NUV - r colour less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semi-analytical LCDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have ~1 to 3 percent of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ~300 to 500 Myrs. We also find that monolithically evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colours (NUV - r < 5.5) seen in a significant fraction (~30 percent) of our observed sample.

astro-ph↗

arXiv2007

Mapping the distribution of luminous and dark matter in strong lensing galaxies

We present the distribution of luminous and dark matter in a set of strong lensing (early-type) galaxies. By combining two independent techniques - stellar population synthesis and gravitational lensing - we can compare the baryonic and dark matter content in these galaxies within the regions that can be probed using the images of the lensed background source. Two samples were studied, extracted from the CASTLES and SLACS surveys. The former probes a wider range of redshifts and allows us to explore the mass distribution out to ~5Re. The high resolution optical images of the latter (using HST/ACS) are used to show a pixellated map of the ratio between total and baryonic matter. We find dark matter to be absent in the cores of these galaxies, with an increasing contribution at projected radii R>Re. The slopes are roughly compatible with an isothermal slope (better interpreted as an adiabatically contracted NFW profile), but a large scatter in the slope exists among galaxies. There is a trend suggesting most massive galaxies have a higher content of dark matter in the regions probed by this analysis.

astro-ph↗

arXiv2007

Meso-structure in three strong-lensing systems

We map substructure in three strong lensing systems having particularly good image data: the galaxy lens MG J0414+053 and the clusters SDSS J1004+411 and ACO 1689. Our method is to first reconstruct the lens as a pixelated mass map and then substract off the symmetric part (in the galaxy case) or a projected NFW (for the cluster lenses). In all three systems we find extended irregular structures, or meso-structures, having of order 10% of the total mass. In J0414+053, the meso-structure suggests a tidal tail connecting the main lens with a nearby galaxy; however this interpretation is tentative. In the clusters the identification of meso-structure is more secure, especially in ACO 1689 where two independent sets of lensed images imply very similar meso-structure. In all three cases the meso-structures are correlated with galaxies but much more extended and massive than the stellar components of single galaxies. Such extended structures cannot plausibly persist in such high-density regions without being mixed; the crossing times are too short. The meso-structures therefore appear to be merging or otherwise dynamically evolving systems.

astro-ph↗

arXiv2006

Principal Component Analysis as a tool to explore star formation histories

Principal Component Analysis (PCA) is a well-known multivariate technique used to decorrelate a set of vectors. PCA has been extensively applied in the past to the classification of stellar and galaxy spectra. Here we apply PCA to the optical spectra of early-type galaxies, with the aim of extracting information about their star formation history. We consider two different data sets: 1) a reduced sample of 30 elliptical galaxies in Hickson compact groups and in the field, and 2) a large volume-limited (z<0.1) sample of ~7,000 galaxies from the Sloan Digital Sky Survey. Even though these data sets are very different, the homogeneity of the populations results in a very similar set of principal components. Furthermore, most of the information (in the sense of variance) is stored into the first few components in both samples. The first component (PC1) can be interpreted as an old population and carries over 99% of the variance. The second component (PC2) is related to young stars and we find a correlation with NUV flux from GALEX. Model fits consistently give younger ages for those galaxies with higher values of PC2.

astro-ph↗

arXiv2006

A Principal Component Analysis approach to the Star Formation History of elliptical galaxies in Compact Groups

(Abridged) Environmental differences in the stellar populations of early-type galaxies are explored using principal component analysis (PCA), focusing on differences between elliptical galaxies in Hickson Compact Groups (HCGs) and in the field. The method is model-independent and relies on variations between the observed spectra. The projections (PC1,PC2) reveal a difference with respect to environment, with a wider range in PC1 and PC2 in the group sample. We define a spectral parameter (zeta=0.36PC1-PC2) which simplifies this result to a single number: field galaxies have a very similar value of zeta, whereas HCG galaxies span a wide range in this parameter. We obtain a strong correlation between the values of zeta and the mass fraction in younger stars, so that some group galaxies present a higher fraction of them. PCA is more sensitive than other methods based on a direct analysis of observables such as the structure of the surface brightness profile or the equivalent width of absorption lines. The latter do not reveal any significant variation between field and compact group galaxies. Our results imply that the presence of young stars only amounts to a fraction of a percent in its contribution to the total variance.

astro-ph↗

arXiv2006

The progenitor set of present-day early-type galaxies

We present a comprehensive theoretical study, within a fully realistic semi-analytical framework, of the photometric properties of early-type progenitors in the redshift range 0 0.7) spirals have ~75-95 percent chance of being a progenitor, while the corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is ~50-75 percent. Finally, we explore the correspondence between the true progenitor set of present-day early-types and the commonly used `red-sequence', defined as the set of galaxies within the part of the colour-magnitude space which is dominated by early-type objects. While large members (M_V<-22) of the `red sequence' trace the progenitor set accurately in terms of numbers and mass, the relationship breaks down severely at fainter luminosities (M_V>-21). Hence the red sequence is generally not a good proxy for the progenitor set of early-type galaxies.

astro-ph↗

arXiv2006

On the globular cluster formation history of NGC 5128

We deduce the globular cluster formation history of the nearby elliptical NGC 5128 by using a phenomenological chemical enrichment model to 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. We test whether the observed MDF can be reproduced using a single starburst at high redshift followed by passive evolution (monolithic scenario). Our results strongly indicate that this scenario is incapable of reproducing the observed MDF of the NGC 5128 GC system. At least one more major starburst is required. A persistent feature of our best-fit models is a strong GC formation episode in the first 1-2 Gyrs, followed by a second formation episode typically after 10-12 Gyrs. Roughly one-third of the stellar mass in the GC system is created in this late starburst, producing clusters that are potentially less than 1 Gyr old. Although we do not have adequate resolution in the model or in the observed MDF to make a reliable claim about the possibility of additional GC formation episodes, our results suggest that the GCs in this galaxy did not form solely in a monolithic collapse and seem to support observational evidence which indicates that NGC 5128 is a merger remnant.

astro-ph↗