Multiwavelength analysis of the Lyman-alpha emitting galaxy Haro 2 : relation between the diffuse Lyman-alpha and soft X-ray emissions

• 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.

Subject headings

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Keyword

galaxies: starburst

galaxies: star formation

galaxies: ISM

ultraviolet: galaxies

cosmology: observations

galaxies: individual: Haro 2

Publication and Content Type

ref (subject category)

art (subject category)