| 1. |
- Guaita, Lucia, et al.
(författare)
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The Lyman alpha reference sample IV. Morphology at low and high redshift
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 576
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Tidskriftsartikel (refereegranskat)abstract
- 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 of LARS-LAE Ly alpha images is peaked in the centre, indicating that the conditions, which make a galaxy an LAE, tend to produce a concentrated surface brightness profile. On the other hand, the stack of all LARS galaxies is shallower and more extended. This can be caused by the variety of dust and HI amount and distribution, which produces a more complex, patchy, and extended profile, like the one observed for Lyman break galaxies that can contribute to the stack. We cannot identify a single morphological property that controls whether a galaxy emits a net positive Ly alpha flux. However, the LARS-LAEs have continuum properties consistent with merging systems.
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| 2. |
- Oti-Floranes, H., et al.
(författare)
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Multiwavelength analysis of the Lyman-alpha emitting galaxy Haro 2 : relation between the diffuse Lyman-alpha and soft X-ray emissions
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 546, s. A65-
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Tidskriftsartikel (refereegranskat)abstract
- 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 a star-forming condensation to the southeast. Both compact and diffuse Ly alpha emission components are observed along the major and minor axes in STIS spectral images. Ly alpha is spatially decoupled from Balmer lines emission, Balmer decrement and UV continuum. However, the diffuse Ly alpha component is spatially correlated with the diffuse soft X-ray emission. Moreover, unlike the compact Ly alpha emission, diffuse Ly alpha shows luminosities larger than predicted from H alpha, assuming case B recombination and considering the dust extinction as derived from H alpha/H beta. Conclusions. The Ly alpha emission closely associated to the massive stellar clusters is strongly affected by the properties of the surrounding neutral gas (presence of outflows, dust abundance), leading to even a range of escape fractions at different locations within the same starburst. On the other hand, we propose that the diffuse Ly alpha emission 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(H alpha)/L0.4-2.4keV ratios. Calibration of Ly alpha as star formation rate tracer should therefore include both effects (destruction vs. enhancement) to avoid biases in the study of galaxies at cosmological distances.
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| 3. |
- Oti-Floranes, H., et al.
(författare)
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Physical properties and evolutionary state of the Lyman alpha emitting starburst galaxy IRAS 08339+6517
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 566, s. A38-
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Tidskriftsartikel (refereegranskat)abstract
- 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 interstellar medium heated by the release of mechanical energy in the first case, and by a large number of active high-mass X-ray binaries (HMXBs) in the second. In addition to the central compact emission blob, we have identified a diffuse Ly alpha emission component smoothly distributed over the whole central area of IRAS 08339+6517. This diffuse emission is spatially decoupled from the UV continuum, the H alpha emission, or the H alpha/H beta ratio. Both locally and globally, the Ly alpha/H alpha ratio is lower than the Case B predictions, even after reddening correction, with an overall Ly alpha escape fraction of only 4%. Conclusions. We conclude that in IRAS 08339+6517 the Ly alpha photons resonantly scattered by an outflowing shell of neutral gas are being smoothly redistributed over the whole central area of the galaxy. Their increased probability of being destroyed by dust would explain the low Ly alpha escape fraction measured. In any case, in the regions where the diffuse Ly alpha emission shows the largest Ly alpha/H alpha ratios, no additional sources of Ly alpha emission are required, like ionization by hot plasma as proposed for Haro 2, another galaxy in our sample. These results stress again the importance of a proper correction of scattering and transfer effects when using Ly alpha to derive the star formation rate in high-redshift galaxies.
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| 4. |
- Oskinova, L. M., et al.
(författare)
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ULX contribution to stellar feedback : an intermediate-mass black hole candidate and the population of ULXs in the low-metallicity starburst galaxy ESO338-4
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 627
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Tidskriftsartikel (refereegranskat)abstract
- Context. X-ray radiation from accreting compact objects is an important part of stellar feedback. The metal-poor galaxy ESO 338-4 has experienced vigorous starburst during the last <40 Myr and contains some of the most massive super star clusters in the nearby Universe. Given its starburst age and its star-formation rate, ESO 338-4 is one of the most efficient nearby manufactures of neutron stars and black holes, hence providing an excellent laboratory for feedback studies. Aims. We aim to use X-ray observations with the largest modern X-ray telescopes XMM-Newton and Chandra to unveil the most luminous accreting neutron stars and black holes in ESO 338-4. Methods. We compared X-ray images and spectra with integral field spectroscopic observations in the optical to constrain the nature of strong X-ray emitters. Results. X-ray observations uncover three ultraluminous X-ray sources (ULXs) in ESO 338-4. The brightest among them, ESO 338 X-1, has X-ray luminosity in excess of 10(40) erg s(-1). We speculate that ESO 338-4 X-1 is powered by accretion on an intermediate-mass (greater than or similar to 300 M-circle dot)black hole. We show that X-ray radiation from ULXs and hot superbubbles strongly contributes to He II ionization and general stellar feedback in this template starburst galaxy.
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