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First Detection of Lyman Continuum Escape from a Local Starburst Galaxy

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First Detection of Lyman Continuum Escape from a Local Starburst Galaxy A&A 448, 513–524 (2006) Astronomy DOI: 10.1051/0004-6361:20053788 & c ESO 2006 Astrophysics First detection of Lyman continuum escape from a local starburst galaxy I. Observations of the luminous blue compact galaxy Haro 11 with the Far Ultraviolet Spectroscopic Explorer (FUSE) N. Bergvall1, E. Zackrisson1, B.-G. Andersson2, D. Arnberg1,J.Masegosa3, and G. Östlin4 1 Dept. of Astronomy and Space Physics, Box 515, 75120 Uppsala, Sweden e-mail: [nils.bergvall;erik.zackrisson]@astro.uu.se 2 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA e-mail: [email protected] 3 Instituto de Astrofisica de Andalucia, Granada, Spain e-mail: [email protected] 4 Stockholm Observatory, SCFAB, 106 91 Stockholm, Sweden e-mail: [email protected] Received 7 July 2005 / Accepted 2 November 2005 ABSTRACT Context. The dominating reionization source in the young universe has not yet been identified. Possible candidates include metal poor dwarf galaxies with starburst properties. Aims. We selected an extreme starburst dwarf, the Blue Compact Galaxy Haro 11, with the aim of determining the Lyman continuum escape fraction from UV spectroscopy. Methods. Spectra of Haro 11 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). A weak signal shortwards of the Lyman break is identified as Lyman continuum (LyC) emission escaping from the ongoing starburst. From profile fitting to weak metal lines we derive column densities of the low ionization species. Adopting a metallicity typical of the H II regions of Haro 11, these data correspond to a hydrogen column density of ∼1019cm−2. This relatively high value indicates that most of the LyC photons escape through transparent holes in the interstellar medium. We then use spectral evolutionary models to constrain the escape fraction of the produced LyC photons. Results. Assuming a normal Salpeter initial mass function we obtain a Lyman continuum escape fraction of fesc ∼ 4–10%. We argue that in a hierarchical galaxy formation scenario, the upper limit we derive for the escape rate allows for a substantial contribution to cosmic reionization by starburst dwarf galaxies at high redshifts. Key words. galaxies: evolution – galaxies: formation – galaxies: starburst – galaxies: dwarfs – ultraviolet: galaxies – cosmology: diffuse radiation 1. Introduction about what could have been the major ionization sources of the IGM during the reionization epoch – stars, normal galax- According to WMAP data, the epoch of reionization started at ies, AGNs or some exotic mechanism (Yan & Windhorst 2004; ≈ aredshiftofz 20 (Kogut et al. 2003). Observations of the Meiksin & White 2003). The rapid decline of the space den- high opacity in the Lyman continuum (hereafter LyC) emission sity of quasars at redshifts z > 3 (Madau et al. 1999; Fan et al. of galaxies (Becker et al. 2001; Fan et al. 2002) and the re- 2001) makes it likely that their contribution to the LyC produc- sults from the recent studies of the intergalactic medium (IGM) tion rate required for the reionization is insignificant, although around SDSS quasars (e.g. Mesinger & Haiman 2004) indi- it has been proposed that miniquasars fed by intermediate-mass cate that the final stage of the epoch of reionization may have black holes may have been important (Madau et al. 2004). occurred at redshifts z ∼ 6−7. There is still no consensus At lower redshifts, observations of a sample of Lyman- Based on observations made with the NASA-CNES-CSA Far break galaxies at z ∼ 3.4 (Steidel et al. 2001) seem to show that Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by galaxies could account for a significant fraction of the photon the Johns Hopkins University under NASA contract NAS-32985. flux required for reionization. The observations indicate a Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20053788 514 N. Bergvall et al.: First detection of Lyman continuum escape from a local starburst galaxy contribution of LyC photons a factor of ∼5 higher than for would be a possibility. Hirashita et al. seem to find that the es- quasars at the same epoch. These results have however not been cape fraction is higher in galaxies with properties similar to confirmed by other similar studies. On the contrary, it has been Haro 11 (e.g. the BCG ESO 338-IG04). Therefore an alter- argued (Fernández-Soto et al. 2003) that a maximum of 4% of native, or complementary, explanation for the low flux of the the LyC flux could escape from luminous galaxies at redshifts recombination lines, may be that a significant fraction of the 1.9 < z < 3.5. This is also close to the upper limit of the es- LyC photons is in fact escaping from the galaxy. This is cape fraction of low redshift galaxies (Leitherer et al. 1995; the possibility that we study in this paper. Hurwitz et al. 1997; Deharveng et al. 2001; Heckman et al. H II regions in nearby galaxies appear to leak 40–50% of 2001). The lack of a strong trend with redshift indicates that the LyC photons produced by the hot stars into the diffuse galaxies at higher redshifts also have low escape fractions, in- interstellar medium (Ferguson et al. 1996; Oey et al. 1997). sufficient to keep the intergalactic gas ionized. Alternative ion- Although it cannot be completely excluded that the gas may ization sources that have been discussed are massive pregalac- be ionized by single O stars located in situ, a comparison tic Pop III stars and young globular clusters. Most probably the of the observed emission line strengths with theoretical mod- reionization is ruled by different sources at different redshifts. It els (Iglesias-Páramo & Muñoz-Tuñón 2002; Wood & Mathis is therefore important to establish observational constraints on 2004) give support to the leakage interpretation. An even more the LyC fluxes from the various candidates at different epochs. extreme case of a density bounded H II region around a mas- Our information about the galaxy luminosity/mass function sive star cluster was reported by Leitherer et al. (1996). They at the epoch of reionization is very meager, and it is not at all estimate that about 75% of the LyC radiation is leaking into clear what mass scale would dominate the LyC photon produc- the diffuse ambient medium. Depending on the distribution of tion rate. In a galaxy formation scenario where mergers are of the gas, the location where these photons are absorbed may be major importance one would expect starburst dwarf galaxies to very different from case to case. In some situations the diffuse be frequent among the first generation of galaxies. Indeed the ionized gas may be much more extended than the H II region number density of so called Luminous Compact Blue Galaxies producing the photons and will be ignored by the observer who (LCBGs) appears to increase with redshift (Lilly et al. 1998; consequently will underestimate the star formation rate. Mallén-Ornelas et al. 1999). It is therefore important to inves- The other alternative for the discrepancy between theory tigate if this type of galaxy could have been an important ion- and observations of the Balmer emission line strengths is that ization source at high redshift. Dwarf galaxies are however too the dust is unevenly distributed across the starburst region so fainttobeobservedathighredshifts. We are therefore depen- that the younger regions are more reddened than the older ones dent on theoretical models and observations of nearby galaxies (Calzetti et al. 1994). This could reduce the equivalent width that can be used as templates of the first generation of dwarf of Hα with up to a factor of ≈2. The only way to really dis- galaxies. Recent models do not give much hope for Lyman tinguish between a global leakage into to intergalactic medium 7 escape from galaxies of masses larger than 10 solar masses and the other two alternatives is to search for leaking photons (Kitayama et al. 2004) but these predictions have to be tested shortward of the Lyman limit. observationally. In previous studies of starburst dwarfs (Bergvall 1985; We have searched for local dwarfs with properties similar Bergvall & Östlin 2002; Östlin et al. 1999, 2001) we have to young dwarf galaxies, that could be used as test objects. A focused on a few luminous (−19 > M > −21) metal poor few such studies have already been carried out but only pro- B blue compact galaxies (BCGs). They have properties similar to vided upper limits to the escape fraction (e.g., Giallongo et al. the LBGs at intermediate redshifts (e.g. Guzman et al. 2003). 1997; Devriendt et al. 1998; Shull et al. 1999). However, pre- Here we report on such a study using FUSE, the Far Ultraviolet dicted equivalent widths of the Balmer emission lines in star- Spectroscopic Explorer. bursts are in some cases larger than observed (e.g. Bergvall 1985; Bresolin et al. 1999; Mas-Hesse & Kunth 1999; Moy et al. 2001; Stasinska et al. 2001). Several possible explana- 1.1. The target galaxy tions to this were discussed by Mas-Hesse & Kunth (1999). One possibility is that the most massive stars remain hidden in The target galaxy of our choice is Haro 11 (=ESO 350-IG38) at h m s ◦ their parental clouds until most of the fuel has been consumed. α2000 = 00 36 52.5 , δ2000 = −33 49 49 . From the emission- This would mimic a truncation of the IMF at high masses. line spectrum we derive a heliocentric radial velocity of v = −1 Mas-Hesse and Kunth also discussed the possibility that a large 0 6180 km s , corresponding to a distance of 86 Mpc, as- − − fraction of the ionizing photons are absorbed by dust in the suming a Hubble constant of 72 km s 1 Mpc 1.Wehavepre- H II regions.
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