Extreme Feedback and the Epoch of Reionization: Clues in the Local Universe

Abstract

The source responsible for reionizing the universe at z > 6 remains uncertain. While an energetically adequate population of star-forming galaxies may be in place, it is unknown whether a large enough fraction of their ionizing radiation can escape into the intergalactic medium. Attempts to measure this escape fraction in intensely star-forming galaxies at lower redshifts have largely yielded upper limits. In this paper, we present new Hubble Space Telescope Cosmic Origins Spectrograph and archival Far-Ultraviolet Spectroscopic Explorer (FUSE) far-UV spectroscopy of a sample of 11 Lyman Break Analogs (LBAs), a rare population of local galaxies that strongly resemble the high-z Lyman Break galaxies. We combine these data with Sloan Digital Sky Survey optical spectra and Spitzer photometry. We also analyze archival FUSE observations of 15 typical UV-bright local starbursts. We find evidence of small covering factors for optically thick neutral gas in three cases. This is based on two independent pieces of evidence: a significant residual intensity in the cores of the strongest interstellar absorption-lines tracing neutral gas and a small ratio of extinction-corrected Hα to UV plus far-IR luminosities. These objects represent three of the four LBAs that contain a young, very compact (∼10 2 pc), and highly massive (∼10 9 M ) dominant central object (DCO). These three objects also differ from the other galaxies in showing a significant amount of blueshifted Lyα emission, which may be related to the low covering factor of neutral gas. All four LBAs with DCOs in our sample show extremely high velocity outflows of interstellar gas, with line centroids blueshifted by about 700 km s −1 and maximum outflow velocities reaching at least 1500 km s −1 . We show that these properties are consistent with an outflow driven by a powerful starburst that is exceptionally compact. We speculate that such extreme feedback may be required to enable the escape of ionizing radiation from star-forming galaxies.

Using Lyman-αto detect galaxies that leak Lyman continuum

“…Heckman et al 2011;Jones et al 2013). Saturated lines imply a high covering fraction of neutral gas along the line of sight.…”

“…Our sample was selected from Bergvall et al (2006), Heckman et al (2011), Leitet et al (2013). To this, we added Tol 1214−277 and GP1219+15, which show a Lyα profile that is highly suggestive of Lyman-continuum leakage according to our predictions in Sects.…”

“…Heckman et al (2011) have identified three LyC-leaking candidates among their eight Lyman-break analogues (LBAs) that were observed with HST/COS. They defined the LyC-leaking candidates as those that show incomplete covering of the far-UV source by neutral gas clouds (derived from the UV LIS absorption lines), and whose UV morphology is dominated by a central compact source.…”

“…We then compare our diagnostics with two other indirect diagnostics of continuum leaking that have been proposed in the literature, which are (a) a low covering fraction of the interstellar absorption lines that are in a low-ionisation state (LIS; e.g. Heckman et al 2011;Jones et al 2013); …”

Analyzing low signal-to-noise FUSE spectra

“…Five local FUSE galaxies were observed in this pilot study, and they were all found to be essentially black in the C ii λ1036 Å line. Grimes et al (2009) and Heckman et al (2011) increased the sample size with a factor ≈3 using FUSE archival data. Neither found strong evidence of any large leakage, although three of the galaxies were found to have rather high upper limits, f esc < 10−18%.…”