arXiv subjects

D. Rosa-Gonzalez

Publications and source records attributed to D. Rosa-Gonzalez.

25 records · Page 2Linked to original sources

arXiv2009

Searching for Compact Star Clusters in M81 Using HST/ACS Images

We study the stellar cluster population in M81, using the HST/ACS images in the filters F435W, F606W and F814W that cover a total field of view of approximately 140 square arcmin. We present details about the selection criteria, which were based both on morphological and photometrical features. The extracted sample of stellar clusters shows the presence of two cluster populations, a blue cluster group (young) with 560 objects, and a red cluster group (old) with 120 objects. The young group lacks clusters more massive than 10000 solar masses, that are present in large numbers in its neighbor M82. The luminosity distribution function of the young group follows a power-law distribution with an index of 2.0, whereas that for the red group resembles very much the globular cluster luminosity function in the Milky Way. Assuming an age of 5 Gyr, these red clusters have masses between 0.1 to 10 million solar masses.

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arXiv2008

HST/ACS imaging of M82: A comparison of mass and size distribution functions of the younger nuclear and older disk clusters

We present the results obtained from an objective search for stellar clusters, both in the currently active nuclear starburst region, and in the post-starburst disk of M82. Images obtained with the HST/ACS in F435W(B), F555W(V), and F814W(I) filters were used in the search for the clusters. We detected 653 clusters of which 393 are located outside the central 450 pc in the post-starburst disk of M82. The luminosity function of the detected clusters show an apparent turnover at B=22 mag (M_B=-5.8), which we interpret from Monte Carlo simulations as due to incompleteness in the detection of faint clusters, rather than an intrinsic log-normal distribution. We derived a photometric mass of every detected cluster from models of simple stellar populations assuming a mean age of either an 8 (nuclear clusters) or 100 (disk clusters) million years old. The mass functions of the disk (older) and the nuclear (younger) clusters follow power-laws, the former being marginally flatter (alpha=1.5+/-0.1) than the latter (alpha=1.8+/-0.1). The distribution of sizes (Full Width at Half Maximum) of clusters brighter than the apparent turn-over magnitude (mass>2E+4 Mo) can be described by a log-normal function. This function peaks at 10 pc for clusters more massive than 1E+5 Mo, whereas for lower masses, the peak is marginally shifted to larger values for the younger, and smaller values for the older clusters. The observed trend towards flattening of the mass function with age, together with an over-abundance of older compact clusters, imply that cluster disruption in M82 is both dependent on the mass and size of the clusters.

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arXiv2007

Are the compact star clusters in M82 evolving towards globular clusters?

Recent HST/ACS images of M82 covering the entire galaxy have been used to detect star clusters. The galaxy is known to contain a young population (age < 10 Myr) in its starburst nucleus, surrounded by a post-starburst disk of age < 1 Gyr. We detect more than 650 star clusters in this galaxy, nearly 400 of them in the post-starburst disk. These data have been used to derive the luminosity, mass and size functions separately for the young nuclear, and intermediate-age disk clusters. In this contribution, we discuss the evolutionary status of these clusters, especially, on the chances of some of these clusters surviving to become old globular clusters.

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arXiv2007

XMM-Newton view of galaxy pairs: activation of quiescent black holes?

We report on XMM-Newton observations of three nearby galaxy pairs, AM0707-273, AM1211-465, and AM2040-674. All six galaxies were previously classified as HII galaxies based on optical and IR spectroscopic analysis. All galaxies were detected with XMM-Newton and each member was isolated and analyzed independently. The X-ray spectra reveal strong evidence of AGN activity in the NE member of AM1211-465 pair. We measured a luminosity of 1.94(+0.11/-0.15)x10^42 erg/s in the 2-10 keV band and the presence of a neutral FeK_alpha line with a confidence level of 98.8%. The high nH value, 2.2+/-0.2x10^22 cm^-2, would explain the misclassification of the source. Marginal evidence of AGN nature was found in the X-ray spectra of AM1211-465SW and AM0707-273E. The X-ray emission of the three remaining galaxies can be explained by starburst activity.

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arXiv2006

Thermal Emission from HII Galaxies: Discovering the Youngest Systems

We studied the radio properties of very young massive regions of star formation in HII galaxies, with the aim of detecting episodes of recent star formation in an early phase of evolution where the first supernovae start to appear. Our sample consists of 31 HII galaxies, characterized by strong Hydrogen emission lines, for which low resolution VLA 3.5cm and 6cm observations were obtained. The radio spectral energy distribution has a range of behaviours; 1) there are galaxies where the SED is characterized by a synchrotron-type slope, 2) galaxies with a thermal slope, and, 3) galaxies with possible free-free absorption at long wavelengths. The latter SEDs were found in a few galaxies and represent a signature of heavily embedded massive star clusters closely related to the early stages of massive star formation. Based on the comparison of the star formation rates determined from the recombination lines and those determined from the radio emission we find that SFR(Ha) is on average five times higher than SFR(1.4GHz). We confirm this tendency by comparing the ratio between the observed flux at 20 cm and the expected one, calculated based on the Ha star formation rates, both for the galaxies in our sample and for normal ones. This analysis shows that this ratio is a factor of 2 smaller in our galaxies than in normal ones, indicating that they fall below the FIR/radio correlation. These results suggest that the emission of these galaxies is dominated by a recent and massive star formation event in which the first supernovae (SN) just started to explode. We conclude that the systematic lack of synchrotron emission in those systems with the largest equivalent width of Hb can only be explained if those are young starbursts of less than 3.5Myr of age.

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arXiv2002

Sub-au imaging of water vapour clouds around four Asymptotic Giant Branch stars

We present MERLIN maps of the 22-GHz H2O masers around four low-mass late-type stars (IK Tau U Ori, RT Vir and U Her), made with an angular resolution of ~ 15 milliarcsec and a velocity resolution of 0.1 km s-1. The H2O masers are found in thick expanding shells with inner radii ~ 6 to 16 au and outer radii four times larger. The expansion velocity increases radially through the H2O maser regions, with logarithmic velocity gradients of 0.5--0.9. IK Tau and RT Vir have well-filled H2O maser shells with a spatial offset between the near and far sides of the shell, which suggests that the masers are distributed in oblate spheroids inclined to the line of sight. U Ori and U Her have elongated poorly-filled shells with indications that the masers at the inner edge have been compressed by shocks; these stars also show OH maser flares. MERLIN resolves individual maser clouds, which have diameters of 2 -- 4 au and filling factors of only ~ 0.01 with respect to the whole H2O maser shells. The CSE velocity structure gives additional evidence the maser clouds are density bounded. Masing clouds can be identified over a similar timescale to their sound crossing time (~2 yr) but not longer. The sizes and observed lifetimes of these clouds are an order of magnitude smaller than those around red supergiants, similar to the ratio of low-mass:high-mass stellar masses and sizes. This suggests that cloud size is determined by stellar properties, not local physical phenomena in the wind.

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arXiv2001

An Empirical Calibration of Star Formation Rate Estimators

(Abridged) The observational determination of the behaviour of the star formation rate (SFR) with look-back time or redshift has two main weaknesses: 1- the large uncertainty of the dust/extinction corrections, and 2- that systematic errors may be introduced by the fact that the SFR is estimated using different methods at different redshifts. To assess the possible systematic differences among the different SFR estimators and the role of dust, we have compared SFR estimates using H$α$, SFR(H$α$), [OII]$λ$3727Å, SFR(OII), UV, SFR(UV) and FIR, SFR(FIR) luminosities of a sample comprising the 31 nearby star forming galaxies having high quality photometric data in the UV, optical and FIR. We review the different "standard" methods for the estimation of the SFR and find that while the standard method provides good agreement between SFR(H$α$) and SFR(FIR), both SFR(OII) and SFR(UV) are systematically higher than SFR(FIR), irrespective of the extinction law. We show that the excess in the SFR(OII) and SFR(UV) is mainly due to an overestimate of the extinction resulting from the effect of underlying stellar Balmer absorptions in the measured emission line fluxes. Taking this effect into consideration in the determination of the extinction brings the SFR(OII) and SFR(UV) in line with the SFR(FIR) and simultaneously reduces the internal scatter of the SFR estimations. Based on these results we have derived "unbiased" SFR expressions for the SFR(UV), SFR(OII) and SFR(H$α$). We have used these estimators to recompute the SFR history of the Universe using the results of published surveys.

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