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Context. The transport of Ly alpha photons in galaxies is a complex process and the conditions under which Ly alpha photons manage to escape from certain galaxies is still under investigation. The Lyman alpha reference sample (LARS) is a sample of 14 local star-forming galaxies, designed to study Ly alpha in detail and relate it to rest-frame UV and optical emission. Aims. With the aim of identifying rest-frame UV and optical properties, which are typical of Ly alpha emitters (LAEs, galaxies with EW(Ly alpha) > 20 angstrom) at both low and high redshift, we investigated the morphological properties of the LARS galaxies, in particular the ones that exhibit intense Ly alpha radiation. Methods. We measured sizes and morphological parameters in the continuum, Ly alpha, and Ha images. We studied morphology by using the Gini coefficient vs. M20 and asymmetry vs. concentration diagrams. We then simulated LARS galaxies at z similar to 2 and 5.7, performing the same morphological measurements. We also investigated the detectability of LARS galaxies in current deep field observations. The subsample of LAEs within LARS (LARS-LAEs) was stacked to provide a comparison to stacking studies performed at high redshift. Results. LARS galaxies have continuum size, stellar mass, and rest-frame absolute magnitude typical of Lyman break analogues in the local Universe and also similar to 2 < z < 3 star-forming galaxies and massive LAEs. LARS optical morphology is consistent with the one of merging systems, and irregular or starburst galaxies. For the first time we quantify the morphology in Ly alpha images: even if a variety of intrinsic conditions of the interstellar medium can favour the escape of Ly alpha photons, LARS-LAEs appear small in the continuum, and their Ly alpha is compact. LARS galaxies tend to be more extended in Ly alpha than in the rest-frame UV. It means that Ly alpha photons escape by forming haloes around HII regions of LARS galaxies. Conclusions. The stack

Context. The transport of Ly alpha photons in galaxies is a complex process and the conditions under which Ly alpha photons manage to escape from certain galaxies is still under investigation. The Lyman alpha reference sample (LARS) is a sample of 14 local star-forming galaxies, designed to study Ly alpha in detail and relate it to rest-frame UV and optical emission. Aims. With the aim of identifying rest-frame UV and optical properties, which are typical of Ly alpha emitters (LAEs, galaxies with EW(Ly alpha) > 20 angstrom) at both low and high redshift, we investigated the morphological properties of the LARS galaxies, in particular the ones that exhibit intense Ly alpha radiation. Methods. We measured sizes and morphological parameters in the continuum, Ly alpha, and Ha images. We studied morphology by using the Gini coefficient vs. M20 and asymmetry vs. concentration diagrams. We then simulated LARS galaxies at z similar to 2 and 5.7, performing the same morphological measurements. We also investigated the detectability of LARS galaxies in current deep field observations. The subsample of LAEs within LARS (LARS-LAEs) was stacked to provide a comparison to stacking studies performed at high redshift. Results. LARS galaxies have continuum size, stellar mass, and rest-frame absolute magnitude typical of Lyman break analogues in the local Universe and also similar to 2 < z < 3 star-forming galaxies and massive LAEs. LARS optical morphology is consistent with the one of merging systems, and irregular or starburst galaxies. For the first time we quantify the morphology in Ly alpha images: even if a variety of intrinsic conditions of the interstellar medium can favour the escape of Ly alpha photons, LARS-LAEs appear small in the continuum, and their Ly alpha is compact. LARS galaxies tend to be more extended in Ly alpha than in the rest-frame UV. It means that Ly alpha photons escape by forming haloes around HII regions of LARS galaxies. Conclusions. The stack

Context. Though Ly alpha emission is one of the most used tracers of massive star formation at high redshift, it is strongly affected by neutral gas radiation transfer effects. A correct understanding of these effects is required to properly quantify the star formation rate along the history of the Universe. Aims. We aim to parameterize the escape of Ly alpha photons as a function of the galaxy properties, in order to properly calibrate the Ly alpha luminosity as a tracer of star formation intensity at any age of the Universe. Methods. We have embarked on a program to study the properties of the Ly alpha emission (spectral profile, spatial distribution, relation to Balmer lines intensity,...) in a number of starburst galaxies in the Local Universe. The study is based on Hubble Space Telescope spectroscopic and imaging observations at various wavelengths, X-ray data, and ground-based spectroscopy, complemented with the use of evolutionary population synthesis models. Results. We present here the results obtained for one of those sources, IRAS 08339+6517, a strong Ly alpha emitter in the Local Universe, which is undergoing an intense episode of massive star formation. We have characterized the properties of the starburst, which transformed 1.4 x 10(8) M-circle dot of gas into stars around 5-6 Myr ago. The mechanical energy released by the central super stellar cluster (SSC), located in the core of the starburst, has created a cavity devoid of gas and dust around it, leaving a clean path through which the UV continuum of the SSC is observed, with almost no extinction. While the average extinction affecting the stellar continuum is significantly larger out of the cavity, with E(B - V) = 0.15 on average, we have not found any evidence for regions with very large extinctions, which could be hiding some young, massive stars not contributing to the global UV continuum. The observed soft and hard X-ray emissions are consistent with this scenario, being originated by the inte, AuthorCount:8

Though Lyman alpha emission (Lya) is one of the most used tracers of massive star formation at high redshift, a correct understanding of radiation transfer effects by neutral gas is required to properly quantify the star formation rate along the history of the Universe. We are embarked in a program to study the properties of the Lya emission (spectral profile, spatial distribution, relation to Balmer lines intensity,...) in several local starburst galaxies. We present here the results obtained for IRAS 08339+6517. Using evolutionary population synthesis models, we have characterized the properties of the starburst (UV continuum, Halpha, total infrared and X-ray emissions, etc.), which transformed 1.4e+8 Mo of gas into stars around 5-6 Myr ago. In addition to the central compact emission blob, we have identified a diffuse Lya emission component smoothly distributed over the whole central area of IRAS 08339+6517. This diffuse emission is spatially decoupled from the UV continuum, the Halpha emission or the Halpha/Hbeta ratio. Both locally and globally, the Lya/Halpha ratio is lower than the Case B predictions, even after reddening correction, with an overall Lya escape fraction of only 4%. We conclude that in IRAS 08339+6517 the resonant scattering of Lya photons by an outflowing shell of neutral gas causes their highly-efficient destruction by dust, which explains the low Lya escape fraction measured. These results stress again the importance of a proper correction of scattering and transfer effects when using Lya to derive the star formation rate in high-redshift galaxies., Comment: Accepted for publication in A&A, 17 pages, 13 figures, 8 tables. If problems with quality of images, see https://cloud.cab.inta-csic.es/public.php?service=files&file=%2Fotih%2Ffiles%2Foti_mas%2Firas%2Firas-v53.ack_referee.pdf

Though Lyman alpha emission (Lya) is one of the most used tracers of massive star formation at high redshift, a correct understanding of radiation transfer effects by neutral gas is required to properly quantify the star formation rate along the history of the Universe. We are embarked in a program to study the properties of the Lya emission (spectral profile, spatial distribution, relation to Balmer lines intensity,...) in several local starburst galaxies. We present here the results obtained for IRAS 08339+6517. Using evolutionary population synthesis models, we have characterized the properties of the starburst (UV continuum, Halpha, total infrared and X-ray emissions, etc.), which transformed 1.4e+8 Mo of gas into stars around 5-6 Myr ago. In addition to the central compact emission blob, we have identified a diffuse Lya emission component smoothly distributed over the whole central area of IRAS 08339+6517. This diffuse emission is spatially decoupled from the UV continuum, the Halpha emission or the Halpha/Hbeta ratio. Both locally and globally, the Lya/Halpha ratio is lower than the Case B predictions, even after reddening correction, with an overall Lya escape fraction of only 4%. We conclude that in IRAS 08339+6517 the resonant scattering of Lya photons by an outflowing shell of neutral gas causes their highly-efficient destruction by dust, which explains the low Lya escape fraction measured. These results stress again the importance of a proper correction of scattering and transfer effects when using Lya to derive the star formation rate in high-redshift galaxies., Comment: Accepted for publication in A&A, 17 pages, 13 figures, 8 tables. If problems with quality of images, see https://cloud.cab.inta-csic.es/public.php?service=files&file=%2Fotih%2Ffiles%2Foti_mas%2Firas%2Firas-v53.ack_referee.pdf

Context. Lyman-alpha emission is commonly used as star formation tracer in cosmological studies. Nevertheless, resonant scattering strongly affects the resulting luminosity, leading to variable and unpredictable escape fractions in different objects. Aims. To understand how the Ly alpha escape fraction depends on the properties of the star-forming regions, we need high spatial resolution multiwavelength studies of nearby Ly alpha emitters, like Haro 2. Methods. We study the Ly alpha emission of Haro 2 in connection with the properties of the young stellar population, the characteristics of the interstellar medium, the distribution and intensity of the Balmer emission lines and the properties of the X-ray emission. We have used HST-STIS spectral images along the major and minor axes of Haro 2 to characterize the Ly alpha emission, as well as FOC UV, WFPC-2 optical and NICMOS near infrared broadband-filter images to analyze the properties of the stellar population. WFPC-2 Ha image and ground-based spectroscopy allow us to study the Balmer emission lines. Finally, Chandra/ACIS X-ray images provide resolved distribution of the X-ray emission at various energy bands. The observational data are analyzed by comparison with the predictions from evolutionary synthesis models to constrain the properties of the star formation episode. Results. The UV, H alpha and far infrared luminosities of the Haro 2 nuclear starburst are well reproduced assuming a young stellar population with ages similar to 3.5-5.0 Myr, affected by differential intestellar extinctions. A significant fraction of the stars are completely obscured in the UV, being identifiable only indirectly by their contribution to the ionization of the gas and to the far infrared emission. The diffuse soft X-ray emission extending over the whole source is attributed to gas heated by the mechanical energy released by the starburst. A compact hard X-ray emission (likely an UltraLuminous X-ray source) has been identified in, AuthorCount:8

In order to use Lyman alpha (Lya) emission as star formation tracer in cosmological studies, we must understand how the resonant scattering affects the escape fraction of the Lya photons. Thus, high spatial resolution multiwavelength studies of nearby Lya emitters, like Haro 2, are highly needed. For that purpose, we have used Chandra X-ray and HST (UV, optical and NIR) images of Haro 2, and STIS and ground-based spectral images along its major and minor axes, to characterize the Lya emission and the properties of the stellar population. The UV, Ha (Halpha) and FIR luminosities of the Haro 2 nuclear starburst are reproduced using evolutionary synthesis models assuming a young stellar population with ages ~3.5-5.0 Myr, affected by differential interstellar extinctions. The observed X-ray emission is attributed to gas heated by the mechanical energy released by the starburst (soft component) and a Ultra-Luminous X-ray source candidate (hard). Both compact and diffuse Lya components are observed. Whereas Lya is spatially decoupled from Balmer lines emission, Balmer decrement and UV continuum, the diffuse Lya component is spatially correlated with the diffuse soft X-ray emission. Moreover, unlike the compact one, diffuse Lya shows luminosities larger than predicted from Ha, assuming case B recombination and dust extinction as derived from Ha/Hbeta. We propose that, whereas the compact Lya emission is associated to the massive stellar clusters and is affected by outflows and dust extinction, the diffuse Lya originates in gas ionized by the hot plasma responsible for the soft X-ray radiation, as suggested by their spatial correlation and by the measured L(Ha)/LsoftX ratios. Calibration of Lya as star formation rate tracer should therefore include both effects (destruction vs. enhancement) to avoid biases in the study of galaxies at cosmological distances., Comment: Accepted for publication in A&A, 18 pages, 8 figures, 9 tables. If problems with quality of images, see http://www.cab.inta-csic.es/users/otih/haro2-v63.clean.pdf

In order to use Lyman alpha (Lya) emission as star formation tracer in cosmological studies, we must understand how the resonant scattering affects the escape fraction of the Lya photons. Thus, high spatial resolution multiwavelength studies of nearby Lya emitters, like Haro 2, are highly needed. For that purpose, we have used Chandra X-ray and HST (UV, optical and NIR) images of Haro 2, and STIS and ground-based spectral images along its major and minor axes, to characterize the Lya emission and the properties of the stellar population. The UV, Ha (Halpha) and FIR luminosities of the Haro 2 nuclear starburst are reproduced using evolutionary synthesis models assuming a young stellar population with ages ~3.5-5.0 Myr, affected by differential interstellar extinctions. The observed X-ray emission is attributed to gas heated by the mechanical energy released by the starburst (soft component) and a Ultra-Luminous X-ray source candidate (hard). Both compact and diffuse Lya components are observed. Whereas Lya is spatially decoupled from Balmer lines emission, Balmer decrement and UV continuum, the diffuse Lya component is spatially correlated with the diffuse soft X-ray emission. Moreover, unlike the compact one, diffuse Lya shows luminosities larger than predicted from Ha, assuming case B recombination and dust extinction as derived from Ha/Hbeta. We propose that, whereas the compact Lya emission is associated to the massive stellar clusters and is affected by outflows and dust extinction, the diffuse Lya originates in gas ionized by the hot plasma responsible for the soft X-ray radiation, as suggested by their spatial correlation and by the measured L(Ha)/LsoftX ratios. Calibration of Lya as star formation rate tracer should therefore include both effects (destruction vs. enhancement) to avoid biases in the study of galaxies at cosmological distances., Comment: Accepted for publication in A&A, 18 pages, 8 figures, 9 tables. If problems with quality of images, see http://www.cab.inta-csic.es/users/otih/haro2-v63.clean.pdf

To derive the history of star formation in the Universe a set of calibrated star formation rate tracers at different wavelengths is required. The calibration has to consistently take into account the effects of extinction, star formation regime (short or long-lived) and evolutionary state to avoid biases at different redshift ranges. We use evolutionary synthesis models optimized for intense episodes of star formation in order to compute a consistent calibration of the most usual star formation rate tracers at different energy ranges, from X-ray to radio luminosities. Nearly-instantaneous and continuous star formation regimes, and the effect of interstellar extinction are considered, as well as the effect of metallicity on the calibration of the different estimators. A consistent calibration of a complete set of star formation rate tracers is presented, computed for the most usual star-forming regions conditions: evolutionary state, star formation regime, interstellar extinction and initial mass function. We discuss the validity of the different tracers in different star formation scenarios and compare our predictions with previous calibrations of general use. Nearly-instantaneous and continuous star formation regimes must be distinguished. While the Star Formation Strength (\msun) should be used for the former, the more common Star Formation Rate (\msun yr$^{-1}$) is only valid for episodes forming stars at a constant rate during extended periods for time. Moreover, even for the latter, the evolutionary state should be taken into account, since most SFR tracers stabilize only after 100 Myr of evolution., Comment: Accepted for publication in A&A, webtool in http://www.laeff.cab.inta-csic.es/research/sfr/, 19 pages, 10 figures, 14 tables. New version including language style revision

To derive the history of star formation in the Universe a set of calibrated star formation rate tracers at different wavelengths is required. The calibration has to consistently take into account the effects of extinction, star formation regime (short or long-lived) and evolutionary state to avoid biases at different redshift ranges. We use evolutionary synthesis models optimized for intense episodes of star formation in order to compute a consistent calibration of the most usual star formation rate tracers at different energy ranges, from X-ray to radio luminosities. Nearly-instantaneous and continuous star formation regimes, and the effect of interstellar extinction are considered, as well as the effect of metallicity on the calibration of the different estimators. A consistent calibration of a complete set of star formation rate tracers is presented, computed for the most usual star-forming regions conditions: evolutionary state, star formation regime, interstellar extinction and initial mass function. We discuss the validity of the different tracers in different star formation scenarios and compare our predictions with previous calibrations of general use. Nearly-instantaneous and continuous star formation regimes must be distinguished. While the Star Formation Strength (\msun) should be used for the former, the more common Star Formation Rate (\msun yr$^{-1}$) is only valid for episodes forming stars at a constant rate during extended periods for time. Moreover, even for the latter, the evolutionary state should be taken into account, since most SFR tracers stabilize only after 100 Myr of evolution., Comment: Accepted for publication in A&A, webtool in http://www.laeff.cab.inta-csic.es/research/sfr/, 19 pages, 10 figures, 14 tables. New version including language style revision

A good correlation has been found in star-forming galaxies between the soft X-ray and the far infrared or radio luminosities. In this work we analyze the relation between the soft X-ray and far infrared luminosities as predicted by evolutionary population synthesis models, aiming first to test the validity of the soft X-ray luminosity as a star formation rate estimator, using the already known calibration of the FIR luminosity as a proxy, and second proposing a calibration based on the predictions of evolutionary synthesis models. We have computed the soft X-ray and far infrared luminosities expected for a massive starburst as a function of evolutionary state, efficiency of the conversion of mechanical energy into soft X-ray luminosity, star formation regime (instantaneous or extended) and dust abundance, and have compared the predictions with observational values for a sample of 62 star-forming galaxies taken from the literature. The observational Lsoftx/Lfir ratios are consistent with the predictions by the models under realistic assumptions (young starbursts, efficiency in the re-processing of mechanical energy of few percent), confirming the direct relation of the diffuse soft X-ray emission with the star formation episode. The soft X-ray emission of the diffuse, extended gas surrounding massive star forming regions can be used as a Star Formation Rate estimator. The empirical calibrations presented in the literature are supported by the predictions of evolutionary synthesis models and by the analysis of a larger sample of star-forming galaxies, but it is biased towards galaxies dominated by relatively unevolved starbursts., Comment: 8 pages, 6 figures. Accepted by Astronomy and Astrophysics. New version including language style revision and 3 new references

A good correlation has been found in star-forming galaxies between the soft X-ray and the far infrared or radio luminosities. In this work we analyze the relation between the soft X-ray and far infrared luminosities as predicted by evolutionary population synthesis models, aiming first to test the validity of the soft X-ray luminosity as a star formation rate estimator, using the already known calibration of the FIR luminosity as a proxy, and second proposing a calibration based on the predictions of evolutionary synthesis models. We have computed the soft X-ray and far infrared luminosities expected for a massive starburst as a function of evolutionary state, efficiency of the conversion of mechanical energy into soft X-ray luminosity, star formation regime (instantaneous or extended) and dust abundance, and have compared the predictions with observational values for a sample of 62 star-forming galaxies taken from the literature. The observational Lsoftx/Lfir ratios are consistent with the predictions by the models under realistic assumptions (young starbursts, efficiency in the re-processing of mechanical energy of few percent), confirming the direct relation of the diffuse soft X-ray emission with the star formation episode. The soft X-ray emission of the diffuse, extended gas surrounding massive star forming regions can be used as a Star Formation Rate estimator. The empirical calibrations presented in the literature are supported by the predictions of evolutionary synthesis models and by the analysis of a larger sample of star-forming galaxies, but it is biased towards galaxies dominated by relatively unevolved starbursts., Comment: 8 pages, 6 figures. Accepted by Astronomy and Astrophysics. New version including language style revision and 3 new references

A good correlation has been found in star-forming galaxies between the soft X-ray and the far infrared or radio luminosities. In this work we analyze the relation between the soft X-ray and far infrared luminosities as predicted by evolutionary population synthesis models, aiming first to test the validity of the soft X-ray luminosity as a star formation rate estimator, using the already known calibration of the FIR luminosity as a proxy, and second proposing a calibration based on the predictions of evolutionary synthesis models. We have computed the soft X-ray and far infrared luminosities expected for a massive starburst as a function of evolutionary state, efficiency of the conversion of mechanical energy into soft X-ray luminosity, star formation regime (instantaneous or extended) and dust abundance, and have compared the predictions with observational values for a sample of 62 star-forming galaxies taken from the literature. The observational Lsoftx/Lfir ratios are consistent with the predictions by the models under realistic assumptions (young starbursts, efficiency in the re-processing of mechanical energy of few percent), confirming the direct relation of the diffuse soft X-ray emission with the star formation episode. The soft X-ray emission of the diffuse, extended gas surrounding massive star forming regions can be used as a Star Formation Rate estimator. The empirical calibrations presented in the literature are supported by the predictions of evolutionary synthesis models and by the analysis of a larger sample of star-forming galaxies, but it is biased towards galaxies dominated by relatively unevolved starbursts., Comment: 8 pages, 6 figures. Accepted by Astronomy and Astrophysics. New version including language style revision and 3 new references

The study of massive star formation needs some basic tools, among which reliable SFR tracers are crucial ones. We are presently revising the calibration of SFR tracers at different wavelength ranges using last generation evolutionary population synthesis codes. The FIR luminosity produced by the heated dust in star forming regions is commonly used to characterize a starburst. The X-ray luminosity has been found to show a narrow correlation with it, and therefore it has been proposed as an additional tracer of potential interest for high $z$ galaxies, whose FIR emission is redshifted to the sub-mm range. In this communication we analyze the evolution of the X-ray to FIR luminosities ratio as predicted by the CMHK population synthesis models, both for constant stellar formation and instantaneous bursts. The results are compared to some sample data taken from the literature. The conclusion drawn from the comparison is that the empirical calibration of the soft X-ray luminosity seems a valid SFR tracer only for starbursts around a rather short period of time., Comment: 2 pages, 4 EPS figures, uses asp2006.sty, to be published in the proceedings of the workshop 'Pathways through an eclectic Universe', J. H. Knapen, T. J. Mahoney, and A. Vazdekis (Eds.), ASP Conf. Ser., 2007

The study of massive star formation needs some basic tools, among which reliable SFR tracers are crucial ones. We are presently revising the calibration of SFR tracers at different wavelength ranges using last generation evolutionary population synthesis codes. The FIR luminosity produced by the heated dust in star forming regions is commonly used to characterize a starburst. The X-ray luminosity has been found to show a narrow correlation with it, and therefore it has been proposed as an additional tracer of potential interest for high $z$ galaxies, whose FIR emission is redshifted to the sub-mm range. In this communication we analyze the evolution of the X-ray to FIR luminosities ratio as predicted by the CMHK population synthesis models, both for constant stellar formation and instantaneous bursts. The results are compared to some sample data taken from the literature. The conclusion drawn from the comparison is that the empirical calibration of the soft X-ray luminosity seems a valid SFR tracer only for starbursts around a rather short period of time., Comment: 2 pages, 4 EPS figures, uses asp2006.sty, to be published in the proceedings of the workshop 'Pathways through an eclectic Universe', J. H. Knapen, T. J. Mahoney, and A. Vazdekis (Eds.), ASP Conf. Ser., 2007

The study of massive star formation needs some basic tools, among which reliable SFR tracers are crucial ones. We are presently revising the calibration of SFR tracers at different wavelength ranges using last generation evolutionary population synthesis codes. The FIR luminosity produced by the heated dust in star forming regions is commonly used to characterize a starburst. The X-ray luminosity has been found to show a narrow correlation with it, and therefore it has been proposed as an additional tracer of potential interest for high $z$ galaxies, whose FIR emission is redshifted to the sub-mm range. In this communication we analyze the evolution of the X-ray to FIR luminosities ratio as predicted by the CMHK population synthesis models, both for constant stellar formation and instantaneous bursts. The results are compared to some sample data taken from the literature. The conclusion drawn from the comparison is that the empirical calibration of the soft X-ray luminosity seems a valid SFR tracer only for starbursts around a rather short period of time., Comment: 2 pages, 4 EPS figures, uses asp2006.sty, to be published in the proceedings of the workshop 'Pathways through an eclectic Universe', J. H. Knapen, T. J. Mahoney, and A. Vazdekis (Eds.), ASP Conf. Ser., 2007