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While numerical simulations suggest that the strength of the Lyman alpha(Lya) line of star-forming disk galaxies strongly depends on the inclination atwhich they are observed (i.e. from edge-on to face-on, we expect to see achange from an attenuated Lya line to a strong Lya emission line), recentobservations with the Hubble space telescope (HST) have highlighted fewlow-redshift highly inclined (edge-on) disk galaxies that breaks this trend. Weaim to understand how a strong Lya emission line is able to escape from one ofthose inclined disk galaxies, named Mrk1486 (z=0.0338). For that purpose weused a large set of HST imaging and spectroscopic data to investigate both theISM structure and the dominant source of Lya radiation inside Mrk1486.Moreover, we used a 3D Monte Carlo Lya radiation transfer code to study theradiative transfer of Lya and UV continuum photons inside a 3D geometry ofneutral hydrogen (HI) and dust that models the ISM structure at the galaxycenter. The analysis of IFU Halpha spectroscopic data of Mrk1486 indicates thepresence of two bipolar galactic winds of HI gas above and bellow the diskplane of Mrk1486. Furthermore, comparing different diagnostic diagrams (such as[OIII]5007/Hbeta versus [OI]6300/Halpha) to photo- and shock-ionization models,we find that the Lya production of Mrk1486 is dominated by photoionizationinside the galaxy disk. From this perspective, our numerical simulationssucceed in reproducing the strength and spectral shape of the observed Lya lineof Mrk1486 by assuming a scenario in which the Lya photons are produced insidethe disk, travel along the galactic winds and scatter on cool HI materialstoward the observer. As bipolar galactic winds are ubiquitous in star-formingdisk galaxies, this mechanism may explain the origin of strong Lya emissionlines recently observed from highly inclined galaxies at high-redshift.

The Lyman alpha Reference Sample VI: Lyman alpha escape from the edge-on disk galaxy Mrk1486