High‐Ionization Emission in Metal‐deficient Blue Compact Dwarf Galaxies

Abstract

Primordial stars are expected to be very massive and hot, producing copious amounts of hard ionizing radiation. The best place to study hard ionizing radiation in the local universe is in very metal-deficient blue compact dwarf (BCD) galaxies. We have carried out a MMT spectroscopic search for [Ne v] k3426 (ionization potential of 7.1 ryd), [Fe v] k4227 (ionization potential of 4 ryd), and He ii k4686 (ionization potential of 4 ryd) emission in a sample of 18 BCDs. We have added data from previous work and from the Data Release 3 of the Sloan Digital Sky Survey. In total, we have assembled a BCD high-ionization sample with [Ne v] emission in four galaxies, [Fe v] emission in 15 galaxies, and He ii emission in 465 galaxies. With this large sample, we have reached the following conclusions. There is a general trend of higher [Ne v], [Fe v], and He ii emission at lower metallicities. However, metallicity is not the only factor that controls the hardness of the radiation. High-mass X-ray binaries and main-sequence stars are probably excluded as the main sources of the very hard ionizing radiation responsible for [Ne v] emission. The most likely source of [Ne v] emission is probably fast radiative shocks moving with velocities k450 km s À1 through a dense interstellar medium with an electron number density of several hundreds cm À3 and associated with supernova explosions of the most massive stars. These have masses of $50-100 M and are formed in very compact super-star clusters (SSCs). The softer ionizing radiation required for He ii emission is likely associated with less massive evolved stars and/or radiative shocks moving through a less dense interstellar medium.

Gemini GMOS spectroscopy of HeII nebulae in M 33

“…As discussed previously, the HeII-ionizing sources of M 33 HBW673, M 33 BCLMP651 and LMC N44C are still unknown. In several studies of nebular HeII emission in BCDs, Izotov and collaborators (e.g., Thuan & Izotov 2005;Izotov et al 2006a) find the presence of nebular HeIIλ4686 emission even at large distances from young clusters, indicating that mechanisms other than stellar photoionization (e.g., shocks or high-mass X-ray binaries) seem to be present, at least in some objects. A similar result was found by Monreal-Ibero et al (2010).…”

“…The nebular HeII λ4686 emission line is often observed in the integrated spectra of blue compact dwarf galaxies (BCDs; e.g., Kunth & Sargent 1981;de Mello et al 1998;Izotov et al 2006a;Kehrig et al 2008), together with other signatures of high ionization (e.g., [Ne V] and [Fe V] emission) (Thuan & Izotov 2005). It is often seen in Wolf-Rayet (WR) galaxies, especially in lowmetallicity systems (e.g., Schaerer et al 1999;López-Sánchez & Esteban 2010).…”

“…They find that the region occupied by the likely ionizing cluster is associated to the nebular emission in 4686 Å indicating that the hard ionizing radiation responsible for the nebular HeII λ 4686 emission can be related to the WR stars. However, although WR and O stars could be responsible for most of the HeII emission in BCDs and WR galaxies (e.g., Schaerer 1996), other processes are apparently needed to explain this high ionization in a significant fraction of them (e.g., Guseva et al 2000;Thuan & Izotov 2005) Models of HeII λ4686 as a diagnostic have already been computed, both without (e.g., Schaerer 1996Schaerer , 2002Smith et al 2002) and with supernovae remnants (SNRs; Cerviño et al 2002). The metal-poor sensitivity is a primary motivation to develop diagnostics for relatively unevolved starbursts, and strong nebular HeIIλ1640, 4686 emission is expected to be one of the best signatures of massive Population III stars (e.g., Schaerer 2002;Schaerer 2003 and references therein).…”

Star formation sustained by gas accretion

“…Despite repeated efforts to find galaxies more metal poor (e.g., Terlevich et al 1991;Izotov et al 1999; Kunth and Östlin 2000) the 1 % lower limit metallicity remains today Kunth and Lebouteiller 2011). Several explanations have been put forward to account for this minimum metallicity: the self-enrichment of the HII region used for measuring (Kunth and Sargent 1986), the metal abundance of the proto-galactic cloud (Kunth and Lebouteiller 2011), the metallicity threshold set by the ejecta from population III stars (Audouze and Silk 1995;Thuan and Izotov 2005), technical difficulties for metallicity determinations below a threshold (Papaderos et al 2008), and others. None of them seem to be fully compelling.…”

Searching for Pop III stars and galaxies at high redshift

“…3.1, AGN or shocks) and nebular He ii is observed in some normal H ii regions (see e.g. Thuan & Izotov 2005) or in other peculiar objects (cf. Fosbury et al 2003 anddiscussion in Schaerer 2003b). galaxies with very high W(Lyα), the excess of Lyman continuum flux seen in some LBGs, and the nature of Lyα blobs.…”