N. G. Guseva

The blue compact dwarf galaxy I Zw 18: A comparative study of its low-surface-brightness component

Using HSTand ground-based optical and NIR imaging data �� ��� , we investigate whether the blue compact dwarf (BCD) galaxy I Zw 18 possesses an extended low-surface-brightness (LSB) old stellar population underlying its star-forming regions, as is the case in the majority of BCDs. This question is central to the long-standing debate on the evolutionary state of I Zw 18. We show that the exponential intensity decrease observed in the filamentary LSB envelope of the BCD out to >18 �� (>1.3 kpc assuming a distance of 15 Mpc) is not due to an evolved stellar disc underlying its star-forming regions, but rather, due to extended ionized gas emission. Ionized gas accounts for more than 80% of the line-of-sight emission at a galactocentric distance of ∼0.65 kpc (∼ 3e ffective radii), and for >30% to 50% of the R light of the main body of I Zw 18. Broad-band images reveal, after subtraction of nebular line emission, a relatively smooth stellar host extending slightly beyond the star-forming regions. This unresolved stellar component, though very compact, is not exceptional for intrinsically faint dwarfs with respect to its structural properties. However, being blue over a radius range of ∼5 exponential scale lengths and showing little colour contrast to the star-forming regions, it differs strikingly from the red LSB host of standard BCDs. This fact, together with the comparably blue colours of the faint C component, ∼1.6 kpc away from the main body of I Zw 18, suggests that the formation of I Zw 18 as a whole has occurred within the last 0.5 Gyr, making it a young BCD candidate. Furthermore, we show that the ionized envelope of I Zw 18 is not exceptional among star-forming dwarf galaxies, neither by its exponential intensity fall-off nor by its scale length. However, contrary to evolved BCDs, the stellar LSB component of I Zw 18 is much more compact than the ionized gas envelope. In the absence of an appreciable underlying stellar population, extended ionized gas emission dominates in the outer parts of I Zw 18, mimicking an exponential stellar disc on optical surface brightness profiles.

SBS?0335?052W: The Lowest Metallicity Star-forming Galaxy Known

We present 4 m Kitt Peak telescope and 6.5 m MMT spectrophotometry of the extremely low metallicity galaxy SBS 0335-052W, the western companion of the blue compact dwarf galaxy SBS 0335-052E. These observations have been combined with published 10 m Keck data to derive for the brightest region of SBS 0335-052W an oxygen abundance 12 + log O/H = 7.12 ? 0.03. This makes SBS 0335-052W the lowest metallicity star-forming galaxy known in the local universe. Using a Monte Carlo technique, we fit the spectral energy distribution of SBS 0335-052W to derive the age of the oldest stars contributing to its optical light. We find that star formation in SBS 0335-052W began less than 500 Myr ago, making it a likely nearby young dwarf galaxy.

Balmer and Paschen Jump Temperature Determinations in Low-Metallicity Emission-Line Galaxies

We have used the Balmer and Paschen jumps to determine the temperatures of the H+ zones of a total sample of 47 H II regions. The Balmer jump was used on MMT spectrophotometric data of 22 low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies and of one H II region in the spiral galaxy M101. The Paschen jump was used on spectra of 24 H II emission-line galaxies selected from the Data Release 3 of the Sloan Digital Sky Survey (SDSS). To derive the temperatures, we have used a Monte Carlo technique varying the electron temperature in the H+ zone, the extinction of the ionized gas and that of the stellar population, the relative contribution of the ionized gas to the total emission, and the star formation history to fit the spectral energy distribution of the galaxies. For the MMT spectra, the fit was done in the wavelength range 3200-5200 A, which includes the Balmer discontinuity, and for the SDSS spectra, in the wavelength range 3900-9200 A, which includes the Paschen discontinuity. We find for our sample of H II regions that the temperatures of the O2+ zones determined from the nebular-to-auroral line intensity ratio of doubly ionized oxygen [O III] λλ(4959 + 5007)/λ4363 do not differ, in a statistical sense, from the temperatures of the H+ zones determined from fitting the Balmer and Paschen jumps and the spectral energy distributions (SEDs). We cannot rule out small temperature differences of the order of 3%-5%.

Cometary Blue Compact Dwarf Galaxies

‘Cometary’ Blue Compact Dwarf Galaxies (iI,C BCDs) are characterized by off-center starbursts close to the end of an elongated dwarf irregular (dI)-like host galaxy. This may either represent randomly enhanced star-forming activity of a dI, or may be caused by a set of special properties of such systems or their environment. For a first investigation of this issue, we analyse the nearby iI,C BCDs Markarian 59 and Markarian 71. Using deep ground-based spectrophotometric data and HST images, we derive physical properties, structure and ages of the starburst regions and the underlying stellar host galaxies. The metallicities show small scatter in the vicinity of the star-forming regions and along the major axis of Mrk 59 which suggests effective mixing of heavy elements on kpc scales. The surface brightness profiles of the underlying host galaxies in either iI,C BCD show an exponential decay with a central surface brightness and scale length that are intermediate between typical iE/nE BCDs and dls. Spectral population synthesis models in combination with colour magnitude diagrams and colour profiles yield most likely formation ages of ~2 Gyr for the host galaxies in both iI,C BCDs, with upper limits of ~4 Gyr for Mrk 59 and ~3 Gyr for Mrk 71, i.e. significantly lower than the typical age of several Gyr derived for the host galaxies of iE/nE BCDs. These findings raise the question whether iI,C systems form a distinct class within BCDs with respect to the age and structure of their hosts, or whether they represent an evolutionary stage connecting young i0 BCDs and ‘classical’ iE/nE BCDs. Properties of analogous objects studied in the local universe and at medium redshifts provide some support for this evolutionary hypothesis.

The New Brilliant Blue Compact Dwarf Galaxy: A Test on the Early Stages of Galaxy Evolution

A blue compact dwarf galaxy SBS 0335 - 052 from the Second Byurakan Survey with extremely low heavy element abundance is found. The oxygen abundance in SBS 0335 - 052 is 77 times lower than the solar value and it is 1.7 times lower than that in I Zw 18. The electron temperature of gas is extremely high and equals 24800 K. SBS 0335 - 052 is a probable candidate for a young galaxy.