4 research outputs found
Discovery of a massive variable star with Z=Zo/36 in the galaxy DDO 68
The Local Volume dwarf galaxy DDO 68, from the spectroscopy of its two
brightest HII regions (Knots 1 and 2) was designated as the second most
metal-poor star-forming galaxy [12+log(O/H)=7.14]. In the repeated spectral
observations in 2008 January with the 6-m telescope (BTA) of the HII region
Knot 3 [having 12+log(O/H)=7.10+-0.06], we find a strong evidence of a
transient event related to a massive star evolution. From the follow-up
observation with the higher spectral resolution in 2008 February, we confirm
this phenomenon, and give parameters of its emission-line spectrum comprising
of Balmer HI and HeI lines. The luminosities of the strongest transient lines
(Ha, Hb) are of a few 10^36 erg s^-1. We also detected an additional continuum
component in the new spectrum of Knot 3, which displays the spectral energy
distribution raising to ultraviolet. The estimate of the flux of this continuum
leads us to its absolute V-band magnitude of ~-7.1. Based on the spectral
properties of this transient component, we suggest that it is related to an
evolved massive star of luminous blue variable type with Z=Zo/36. We briefly
discuss observational constraints on parameters of this unique (in the aspect
of the record low metallicity of the progenitor massive star) event and propose
several lines of its study.Comment: 6 pages, 5 Postscript figures, to appear in MNRAS Letters in June
2008 issu
Stellar metallicity of star-forming galaxies at z ~ 3
The stellar metallicity is a direct measure of the amount of metals present
in a galaxy, as a large part of the metals lie in its stars. In this paper we
investigate new stellar metallicity indicators suitable for high-z galaxies
studying the stellar photospheric absorption lines in the rest frame
ultraviolet, hence sampling predominantly young hot stars. We defined these new
indicators based on the equivalent widths (EW) of selected features using
theoretical spectra created with the evolutionary population synthesis code
Starburts99. We used them to compute the stellar metallicity for a sample of
UV-selected galaxies at z > 3 from the AMAZE survey using very deep (37h per
object) VLT/FORS spectra. Moreover, we applied the new metallicity indicators
to eight additional high redshift galaxies found in literature. We then
compared stellar and gas-phase metallicities measured from the emission lines
for all these galaxies, finding that within the errors the two estimates are in
good agreement, with possible tendency to have stellar metallicities lower than
the gas phase ones. For the first time, we are able to study the stellar
mass-metallicity relation at z > 3. We find that the metallicity of young, hot
stars in galaxies at z \sim 3 have similar values of the aged stars in local
SDSS galaxies, contrary to what observed for the gas phase metallicity.Comment: 14 pages, 8 figures, accepted for publication in A&