Alexei Kniazev

Home Research Publications Bookmarks My family

Galactic Structure
HII Galaxies - Galaxies with intense current bursts of star formation:

The heavy element abundances of gas-rich galaxies and their gas-mass fractions are the main parameters characterizing their global evolution. These can be related to both the global properties of galaxies and the membership of galaxies in various elements of the large-scale structure. The knowledge of the metallicity for large homogeneously selected galaxy samples would allow us to address various issues of galaxy chemical evolution on a good statistical basis. My research related to this topic have been concentrated on collaborative projects aimed at creating new samples of such objects, determination of their element abundances as well as the detailed study of some interesting objects from these new samples. I was and/or I am still involved in the following projects:

(i) BCGs from the First and Second Byurakan Surveys. This work was based on the objective-prism photoplates of the Byurakan Surveys. (ii) A new digital KPNO International Spectral Survey (KISS) aimed at creating a deep sample of emission-line galaxies. The technique we employed for this project combines the benefits of a traditional photographic survey with the advantages of using new CCD detectors in astronomy. The spectral range for objective-prism spectra covered 4800-5500 A; was restricted by a specially designed filter that transmits from rest-frame H$\beta$ to just shortward of the strong night-sky line at 5577 A; and this greatly reduces the sky background. (iii) The Hamburg/SAO Survey of Emission-Line Galaxies (HSS-ELG) was based on the digitized photographic objective-prism database of the Hamburg Quasar Survey. The main goal of the HSS-ELG was to search for emission-line galaxies with HII-type spectra in order to create a new deep sample of BCGs in an area of about 3000$^\circ$ of the sky. Another important goal of this work was to search for new extremely low-metallicity galaxies. (iv) Hamburg/SAO Survey for Low Metallicity Galaxies (HSS-LM) is devoted to the search for extremely metal-deficient BCGs and to the creation of a well selected large BCG sample with strong emission lines and oxygen abundance measured with classic T$_{e}$ method, in an area of about 3500$^\circ$ of the sky not covered with previous searches.

I was involved in the observations, reduction and analysis of spectral and photometrical data for these projects. New created samples from these projects were used for various follow-up studies and some of them were done with my participation, namely:
(a) The studies of the environment of low-mass galaxies with active starformation (SF hereafter) and a possible trigger of SF bursts due to gravitational interaction. Finally we concluded that in $\sim$80% (or more) BCGs from the studied sample, the SF bursts are triggered either by tidal action of various strengths from other galaxies, or due to mergers of low-mass galaxies. (b) Searches for and detailed studies of Wolf-Rayet features and broad components of strong emission lines. They are the signatures of collective effects of massive star winds and supernova explosions on to interstellar medium of host galaxies of various metallicities. (c) Heavy element abundance patterns and the metallicity distribution of BCGs. (d) Structural parameters and morphology of BCGs.

One of the main results of all these projects was the discovery of new extremely metal-poor galaxies (XMPGs; with 12 + log O/H $<$ 7.6 or Z $<$ 1/20 solar) in the nearby Universe. These galaxies have received much attention, since some of these objects may be primeval galaxies that are now experiencing their first major burst of star formation and they are local counterparts of high-redshift just formed dwarf galaxies. The first such object discovered in our projects, was the galaxy SBS 0335-052, which is almost as metal-poor as the well-known metal-poor galaxy I Zw 18. I was in the team that discovered this object and analysed spectral and photometric data for SBS 0335-052 and its western companion 0335-052W, also discovered by our team. It was shown that 0335-052W has the same redshift as that of SBS 0335-052 and has an extremely low oxygen abundance. It was argued that both these galaxies are likely to be nearby young dwarf galaxies and the properties of these galaxies were compared with theoretical predictions and properties of high-redshift galaxies which are often considered as primeval galaxies.

Different XMPGs can have differing evolution scenarios, and therefore quite different observational properties. To understand the nature of XMPGs we need to carefully study the group properties of sufficiently large number of such galaxies with reliable estimates of O/H. As a first step of such a project we planed to form the sufficiently large sample of XMPGs. The systematical searches for XMPGs were conducted and, as a result, many new such galaxies were found. At the same time we checked a half-dozen of previously-claimed XMPGs and found that majority of them were not real XMPGs but had poor observational spectral data and should be rejected from the final sample.

For some new discovered XMPGs (additionally to studies of SBS 0335-052 system) we started detailed multi-wavelength studies where we found that SF bursts were triggered by tidal actions with local environment and studied XMPGs are located within voids. Deep multi-band photometry was performed for some of those galaxies with 1.5m (Calar Alto, Spain) and 2.5m (La Silla, Chile) telescopes as well as integrated HI in the 21-cm line radio observations and infrared observations with NTT and VLT telescopes. Part of the data is reduced and some new data are still being reduced.

The Sloan Digital Sky Survey (SDSS) is an ambitious international survey project that will ultimately cover one quarter of the sky through deep, 5-color photometry and spectroscopy. I have begun a systematic search of XMPGs using the spectroscopic database of SDSS. Eight new objects were identified after the analysis of 250,000 galaxy spectra within an area of $\sim$3000 deg$^2$ on the sky. Our oxygen abundance determinations have an accuracy of $\le$ 0.1 dex and are based on the temperature-sensitive [OIII] $\lambda$4363 A; line and on the direct calculation of the electron temperature. For that I developed a new method of oxygen abundance determination using the [OII] $\lambda$7319,7330 A; lines, which is particularly useful for SDSS emission-line spectra with redshifts $\le$ 0.024 since the [OII] $\lambda$3727 A; emission line falls outside of the SDSS wavelength range. In parallel, I started a project to create the SDSS HII-galaxies with Oxygen abundances Catalog (SHOC), which is a listing of strong emission-line galaxies from SDSS Data Release I. One of the goal for both projects is to create a sample XMPGs and reference sample of ELGs for a comparative study of their properties.

Home Research Publications Bookmarks My family
South African Astronomical Observatory Last Updated: 10.December.2005