Spectroscopic study of blue compact galaxies

Abstract

Abstract. This is the third paper of a series dedicated to the study of the star formation rates, star formation histories, metallicities and dust contents of a sample of blue compact galaxies (BCGs). We constrain the stellar contents of 73 blue compact galaxies by analyzing their continuum spectra and the equivalent widths of strong stellar absorption features using a technique of empirical population synthesis based on a library of observed star-cluster spectra. Our results indicate that blue compact galaxies are typically age-composite stellar systems; in addition to young stars, intermediate-age and old stars contribute significantly to the 5870 Å continuum emission of most galaxies in our sample. The stellar populations of blue compact galaxies also span a variety of metallicities. The ongoing episodes of star formation started typically less than a billion years ago. Some galaxies may be undergoing their first global episode of star formation, while for most galaxies in our sample, older stars are found to contribute up to half the optical emission. Our results suggest that BCGs are primarily old galaxies with discontinuous star formation histories. These results are consistent with the results from deep imaging observations of the color-magnitude diagrams of a few nearby BCGs using HST and large ground-based telescopes. The good quality of our population synthesis fits of BCG spectra allow us to estimate the contamination of the Hα, Hβ, Hγ and Hδ Balmer emission lines by stellar absorption. The absorption equivalent widths measured in the synthetic spectra range from typically 1.5 Å for Hα, to 2-5 Å for Hβ, Hγ, and Hδ. The implied accurate measurements of emission-line intensities will be used in a later study to constrain the star formation rates and gas-phase chemical element abundances of blue compact galaxies.

Mapping the properties of blue compact dwarf galaxies: integral field spectroscopy with PMAS

“…I Zw 123 is a relatively wellstudied object: optical surface photometry was published in several papers (Cairós et al 2001a,b;Gil de Paz et al 2003;Caon et al 2005;Amorín et al 2007), and NIR photometry in Cairós et al (2003). A thorough spectroscopic study, including an analysis of its stellar content in terms of population synthesis models, was published in the series of papers by , , Kong et al (2003), Kong (2004) and Shi et al (2005).…”

“…Doublier et al (1997), Doublier et al (1999), Gil de Paz et al (2003 and Gil de Paz & Madore (2005). It is also included in the spectroscopy study of BCDs by , , Kong et al (2003), Kong (2004) and Shi et al (2005), and is the only object in our sample in common with the sample of galaxies studied by means of IFS by Petrosian et al (2002).…”

Oxygen abundance in the Sloan Digital Sky Survey

“…Subtracting the underlying stellar absorption is crucial to study nebular emission lines and the abundances of gaseous material (Kong et al 2003). In this paper we use the method developed by Li et al (2005) to subtract the underlying starlight.…”

Spectrum Fitting Code for LAMOST ExtraGAlactic Surveys (LEGAS)

“…Because only few galaxies can be resolved into individual stars, population synthesis method is often used to analyze stellar population of galaxy and to subtract underlying stellar absorption from nebular emission lines. In our Center (Center for astrophysics, USTC), since 1999, we have used and developed different population synthesis methods, which are used to analyze the spectral properties for galaxies (Kong & Cheng 1999;Kong et al 2000, Kong et al 2003, Li et al 2005, Lu et al 2005. In Figure 2, as an example, we show the spectral fitting results for two galaxies in Kong et al (2003).…”

“…Examples of spectral fitting for two galaxies in Kong et al (2003). OBS is the observed spectrum of galaxy, SYN is the synthesis stellar spectrum from the best matched template, OBS-SYN is the difference between them.…”