Excitation properties of galaxies with the highest [OIII]/[OII] ratios: No evidence for massive escape of ionizing photons

The possibility that star-forming galaxies may leak ionizing photons is at the heart of many present-day studies that investigate the reionization of the Universe. We test this hypothesis on local blue compact dwarf galaxies of very high excitation. We assembled a sample of such galaxies by examining the spectra from Data Releases 7 and 10 of the Sloan Digital Sky Survey. We argue that reliable conclusions cannot be based on strong lines alone, and adopt a strategy that includes important weak lines such as [OI] and the high-excitation HeII and [ArIV] lines. Our analysis is based on purely observational diagrams and on a comparison of photoionization models with well-chosen emission-line ratio diagrams. We show that spectral energy distributions from current stellar population synthesis models cannot account for all the observational constraints, which led us to mimick several scenarios that could explain the data. These include the additional presence of hard X-rays or of shocks. We find that only ionization-bounded models (or models with an escape fraction of ionizing photons lower than 10%) are able to simultaneously explain all the observational constraints.Comment: accepted in Astronomy & Astrophysic

HST observations of the blue compact dwarf SBS 0335-052: a probable young galaxy

We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Ly$_alpha$ and OI 1302 of the extremely metal-deficient (Z~Zsun/41) blue compact dwarf (BCD) galaxy SBS 0335-052. All the star formation in the BCD occurs in six super-star clusters (SSC) with ages =< 3-4 Myr. Dust is clearly present and mixed spatially with the SSCs. There is a supershell of radius ~380 pc, delineating a large supernova cavity. The instantaneous star formation rate is ~0.4 Msun yr^-1. Strong narrow Ly$\alpha$ emission is not observed. Rather there is low intensity broad (FWZI = 20 A) Ly$\alpha$ emission superposed on even broader Ly$\alpha$ absorption by the HI envelope. This broad low-intensity emission is probably caused by resonant scattering of Ly$\alpha$ photons. The BCD appears to be a young galaxy, undergoing its very first burst of star formation. This conclusion is based on the following evidence: 1) the underlying extended low-surface-brightness component is very irregular and filamentary, suggesting that a significant part of the emission comes from ionized gas; 2) it has very blue colors (-0.34 =< (V-I)$_0$ =< 0.16), consistent with gaseous emission colors; 3) the OI 1302 line is not detected in absorption in the GHRS spectrum, setting an upper limit for N(O)/N(H) in the HI envelope of the BCD of more than 3000 times smaller than the value in Orion.Comment: 20 pages and 6 Postscript figures. Submitted to Astrophysical Journa

Observing H2 Emission in Forming Galaxies

We study the H2 cooling emission of forming galaxies, and discuss their observability using the future infrared facility SAFIR. Forming galaxies with mass >10^11 Msun emit most of their gravitational energy liberated by contraction in molecular hydrogen line radiation, although a large part of thermal energy at virialization is radiated away by the H Ly alpha emission. For more massive objects, the degree of heating due to dissipation of kinetic energy is so great that the temperature does not drop below 10^4 K and the gravitational energy is emitted mainly by the Ly alpha emission. Therefore, the total H2 luminosity attains the peak value of about 10^42 ergs/s for forming galaxies whose total mass 10^11 Msun. If these sources are situated at redshift z=8, they can be detected by rotational lines of 0-0S(3) at 9.7 micron and 0-0S(1) at 17 micron by SAFIR. An efficient way to find such H2 emitters is to look at the Ly alpha emitters, since the brightest H2 emitters are also luminous in the Ly alpha emission.Comment: 20 pages, 7 figures, ApJ accepte

Protostellar Collapse with Various Metallicities

The thermal and chemical evolution of gravitationally collapsing protostellar clouds is investigated, focusing attention on their dependence on metallicity. Calculations are carried out for a range of metallicities spanning the local interstellar value to zero. During the time when clouds are transparent to continuous radiation, the temperatures are higher for those with lower metallicity, reflecting lower radiative ability. However, once the clouds become opaque, in the course of the adiabatic contraction of the transient cores, their evolutionary trajectories in the density-temperature plane converge to a unique curve that is determined by only physical constants. The trajectories coincide with each other thereafter. Consequently, the size of the stellar core at the formation is the same regardless of the gas composition of the parent cloud.Comment: 30 pages. The Astrophysical Journal, 533, in pres