The Lyman alpha reference sample

Abstract

Context. Recent numerical simulations suggest that the strength of the Lyman alpha (Lyα) line of star-forming disk galaxies strongly depends on the inclination at which they are observed: from edge-on to face-on, we expect to see a change from a strongly attenuated Lyα line to a strong Lyα emission line. Aims. We aim to understand how a strong Lyα emission line is able to escape from the low-redshift highly inclined (edge-on) disk galaxy Mrk 1486 (z ∼ 0.0338). To our knowledge, this work is the first observational study of Lyα transport inside an edge-on disk galaxy. Methods. Using a large set of HST imaging and spectroscopic data, we investigated the interstellar medium (ISM) structure and the dominant source of Lyα radiation inside Mrk 1486. Moreover, using a 3D Monte Carlo Lyα radiation transfer code, we studied the radiative transfer of Lyα and UV continuum photons inside a 3D geometry of neutral hydrogen (HI) and dust that models the ISM structure at the galaxy center. Our numerical simulations predicted the Lyα line profile that we then compared to the one observed in the HST/COS spectrum of Mrk 1486. Results. While a pronounced Lyα absorption line emerges from the disk of Mrk 1486, very extended Lyα structures are observed at large radii from the galaxy center: a large Lyα-halo and two very bright Lyα regions located slightly above and below the disk plane. The analysis of IFU Hα spectroscopic data of Mrk 1486 indicates the presence of two bipolar outflowing halos of HI gas at the same location as these two bright Lyα regions. Comparing different diagnostic diagrams (such as [OIII] 5007 /Hβ versus [OI] 6300 /Hα) to photo-and shock-ionization models, we find that the Lyα production of Mrk 1486 is dominated by photoionization inside the galaxy disk. From this perspective, our numerical simulations succeed in reproducing the strength and shape of the observed Lyα emission line of Mrk 1486 by assuming a scenario in which the Lyα photons are produced inside the galaxy disk, travel along the outflowing halos, and scatter on cool HI materials toward the observer. Conclusions. Extended bipolar galactic winds are frequently observed from star-forming disk galaxies. Given the advantage Lyα photons take of such outflowing HI materials to easily escape from Mrk 1486, this mechanism may explain the origin of strong Lyα emission lines frequently observed from highly inclined galaxies at high-redshift. This therefore challenges the robustness of the expected viewing-angle effect on the Lyα properties of star-forming disk galaxies.

Unlocking the Full Potential of Extragalactic Lyα through Its Polarization Properties

“…Blandford & Rees 1974;Suchkov et al 1994). More recently, observations of LARS 05 (Duval et al 2016) nicely illustrate how Lyα photons scatter off a bipolar outflow that burst out of an edge-on disk galaxy. In this section, we focus on predicting spectra and polarization of scattered Lyα radiation emerging from simplified representations of bipo- We model the bipolar outflows as follows: it contains a spherical cloud with either N HI = {10 17 , 10 19 , 10 21 } cm −2 and T = 10 4 K that resides in a fully ionized environment.…”

“…We repeat the previous analysis ( § 3.3.1), but now obscure the central static sphere (the obscured region is indicated with grey dashed lines in Figure 6). This represents a case in which the biconical outflows are separated by for example a dusty galactic disk as in LARS 05 (see Duval et al 2016) or in M82 (Lynds & Sandage 1963;Gallagher & Smith 1999).…”

Magnifying the Early Episodes of Star Formation: Super Star Clusters at Cosmological Distances*

“…Verhamme et al (2012) show directional variation in the Lyα escape from a simulated disk galaxy (see also Zheng & Wallace 2014). This directional variation is reduced significantly in the presence of outflows (e.g., Duval et al 2016) and/or for non-disk geometries, including modified shell models (Behrens et al 2014) and/or clumpy ISM models (Gronke & Dijkstra 2014), where the Lyα escape fraction varies by only a factor of order unity and thus do not provide the variations required to explain the observed flux ratio. However, the theoretical studies have considered systems with ∼kpc extents, and ignored observational aperture effects.…”

Metal-enriched galactic outflows shape the mass–metallicity relationship

“…Duval et al (2016), (3) France et al (2010), (4) Fox et al (2013), (5) Fox et al (2014), (6) Hayes et al (2014), (7) Heckman et al (2015), (8) James et al (2014), (9) Leitherer et al (2013), (10) Östlin et al (2014), (11) Pardy et al (2014), (12) Rivera-Thorsen et al (2015),…”