arXiv subjects

P. R. T. Coelho

Publications and source records attributed to P. R. T. Coelho.

23 records · Page 2Linked to original sources

arXiv2018

J-PLUS: Morphological star/galaxy classification by PDF analysis

Our goal is to morphologically classify the sources identified in the images of the J-PLUS early data release (EDR) into compact (stars) or extended (galaxies) using a suited Bayesian classifier. J-PLUS sources exhibit two distinct populations in the r-band magnitude vs. concentration plane, corresponding to compact and extended sources. We modelled the two-population distribution with a skewed Gaussian for compact objects and a log-normal function for the extended ones. The derived model and the number density prior based on J-PLUS EDR data were used to estimate the Bayesian probability of a source to be star or galaxy. This procedure was applied pointing-by-pointing to account for varying observing conditions and sky position. Finally, we combined the morphological information from g, r, and i broad bands in order to improve the classification of low signal-to-noise sources. The derived probabilities are used to compute the pointing-by-pointing number counts of stars and galaxies. The former increases as we approach to the Milky Way disk, and the latter are similar across the probed area. The comparison with SDSS in the common regions is satisfactory up to r ~ 21, with consistent numbers of stars and galaxies, and consistent distributions in concentration and (g - i) colour spaces. We implement a morphological star/galaxy classifier based on PDF analysis, providing meaningful probabilities for J-PLUS sources to one magnitude deeper (r ~ 21) than a classical boolean classification. These probabilities are suited for the statistical study of 150k stars and 101k galaxies with 15 < r < 21 present in the 31.7 deg2 of the J-PLUS EDR. In a future version of the classifier, we will include J-PLUS colour information from twelve photometric bands.

astro-ph.GA↗

arXiv2016

Self-similarity in the chemical evolution of galaxies and the delay time distribution of SNe Ia

Recent improvements in the age dating of stellar populations and single stars allow us to study the ages and abundance of stars and galaxies with unprecedented accuracy. We here compare the relation between age and α-element abundances for stars in the solar neighborhood to that of local, early-type galaxies. We find both relations to be very similar. Both fall into two regimes with a flat slope for ages younger than ~9 Gyr and a steeper slope for ages older than that value. This quantitative similarity seems surprising, given the different types of galaxies and scales involved. For the sample of early-type galaxies we also show that the data are inconsistent with literature delay time distributions of either single or double Gaussian shape. The data are consistent with a power law delay time distribution. We thus confirm that the delay time distribution inferred for the Milky Way from chemical evolution arguments also must apply to massive early-type galaxies. We also offer a tentative explanation for the seeming universality of the age-[α/Fe] relation as the manifestation of averaging of different stellar populations with varying chemical evolution histories.

astro-ph.GA↗

arXiv2015

Central enhancement of the nitrogen-to-oxygen abundance ratio in barred galaxies

Bar-induced gas inflows towards the galaxy centres are recognized as a key agent for the secular evolution of galaxies. One immediate consequence is the accumulation of gas in the centre of galaxies where it can form stars and alter the chemical and physical properties. We use a sample of nearby face--on disc galaxies with available SDSS spectra to study whether the properties of the ionised gas in the central parts (radii <~0.6-2.1 kpc) of barred galaxies are altered by the presence of a bar, and whether the bar effect is related to bar and/or parent galaxy properties. The distributions of all parameters analysed are different for barred and unbarred galaxies, except for the R23 metallicity tracer and the oxygen abundance (from photoionisation models). The median values point towards (marginally) larger dust content, star formation rate per unit area, electron density and ionisation parameter in the centres of barred galaxies than in the unbarred counterpart. The most remarkable barred/unbarred difference appears in the [NII]6583/Ha line ratio, which is on average ~25% larger in barred galaxies, due to a larger N/O in the centres of these galaxies. We observe an enhancement of the central gas differences in later-type galaxies or galaxies with less massive bulges. However the bar seems to have a lower impact on the central gas properties for galaxies with more massive bulges (M_bulge > 10^10 M_sun) or galaxies with total stellar mass above ~ 10^10.8 M_sun. In conclusion, we find observational evidence that the presence of a galactic bar affects the central ionised gas properties of disc galaxies, where the most striking effect is an enhancement in the N/O abundance ratio, which can be qualitatively interpreted as due to a different origin or evolutionary processes for less and more massive bulges, with the gaseous phase of the former having currently a closer relation with bars.

astro-ph.GA↗

arXiv2015

Abundance patterns in early-type galaxies: is there a 'knee' in the [Fe/H] vs. [alpha/Fe] relation?

Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in accordance with their old ages and short formation timescales. In this contribution we aim to resolve the enrichment histories of ETGs. This means we study the abundance of Fe ([Fe/H]) and the alpha-element groups ([alpha/Fe]) separately for stars older than 9.5 Gyr ([Fe/H]o, [alpha/Fe]o) and for stars between 1.5 and 9.5 Gyr ([Fe/H]i, [alpha/Fe]i). Through extensive simulation we show that we can indeed recover the enrichment history per galaxy. We then analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS selected to be ETGs. We separate out those galaxies for which the abundance of iron in stars grows throughout the lifetime of the galaxy, i.e. in which [Fe/H]o < [Fe/H]i. We confirm earlier work where the [Fe/H] and [alpha/Fe] parameters are correlated with the mass and velocity dispersion of ETGs. We emphasize that the strongest relation is between [alpha/Fe] and age. This relation falls into two regimes, one with a steep slope for old galaxies and one with a shallow slope for younger ETGs. The vast majority of ETGs in our sample do not show the 'knee' in the plot of [Fe/H] vs. [alpha/Fe] commonly observed in local group galaxies. This implies that for the vast majority of ETGs, the stars younger than 9.5 Gyrs are likely to have been accreted or formed from accreted gas. The properties of the intermediate-age stars in accretion-dominated ETGs indicate that mass growth through late (minor) mergers in ETGs is dominated by galaxies with low [Fe/H] and low [alpha/Fe]. The method of reconstructing the stellar enrichment histories of ETGs introduced in this paper promises to constrain the star formation and mass assembly histories of large samples of galaxies in a unique way.

astro-ph.GA↗

arXiv2014

A new library of theoretical stellar spectra with scaled-solar and alpha-enhanced mixtures

Theoretical stellar libraries have been increasingly used to overcome limitations of empirical libraries, e.g. by exploring atmospheric parameter spaces not well represented in the latter. This work presents a new theoretical library which covers 3000 $\leq$ Teff $\leq$ 25000\,K, -0.5 $\leq$ log g $\leq$ 5.5, and 12 chemical mixtures covering 0.0017 $\leq$ Z $\leq$ 0.049 at both scaled-solar and $α$-enhanced compositions. This library complements previous ones by providing: (i) homogeneous computations of opacity distribution functions, models atmospheres, statistical surface fluxes and high resolution spectra; (ii) high resolution spectra with continua slopes corrected by the effect of predicted lines, and; (iii) two families of $α$-enhanced mixtures for each scaled-solar iron abundance, to allow studies of the $α$-enhancement both at `fixed iron' and `fixed Z' cases. Comparisons to observed spectra were performed and confirm that the synthetic spectra reproduce well the observations, although there are wavelength regions which should be still improved. The atmospheric parameter scale of the model library was compared to one derived from a widely used empirical library, and no systematic difference between the scales was found. This is particularly reassuring for methods which use synthetic spectra for deriving atmospheric parameters of stars in spectroscopic surveys.

astro-ph.SR↗