The spatial and age distribution of stellar populations in DDO 190

The spatial distribution of stellar populations, the star formation history, and other properties of the dIrr galaxy DDO 190 have been analyzed using color--magnitude diagrams (CMDs) of about 3900 resolved stars and the Ha fluxes of HII regions. From the mean color index of the red giant branch, a mean metallicity [Fe/H]=-2.0 is obtained. The I magnitude of the TRGB has been used to estimate the distance. DDO 190 is 2.9+/-0.2 Mpc from the Milky Way, 2.1 Mpc from the M 94 group (CnV-I), 2.4 Mpc from the M 81 group and 2.9 Mpc from the barycenter of the Local Group, all indicating that it is an isolated, field galaxy. The surface-brightness distribution of the galaxy is well fitted by ellipses of ellipticity e=1-a/b=0.1 and P.A.=82deg. The radial star density distribution follows an exponential law of scale length a=43."4, corresponding to 611 pc. The Holmberg semi-major axis to mu_B=26.5 is estimated to be r^B_(26.5)=3.'0. Stellar populations of different ages in DDO 190 show strong spatial decoupling, the oldest population appearing much more extended than the youngest. Stars younger than 0.1 Gyr occupy only the central 40'' (0.55 kpc); stars younger than a few (~4) Gyr extend out to ~80'' (125 kpc), and for larger galactocentric distances only older stars seem to be present. This behavior is found in all the dIrr galaxies for which spatially extended studies have been performed and could be related with the kinematical history of the galaxy.Comment: To be published in the AJ. 29 pages, 13 figure

The evolutionary status of the low-metallicity blue compact dwarf galaxy SBS 0940+544

We present the results of spectrophotometry and V,R,I, Halpha CCD photometry of the blue compact dwarf (BCD) galaxy SBS 0940+544. Broad-band images taken with the 2.1m KPNO and 1.23m Calar Alto telescopes reveal a compact high-surface-brightness H II region with ongoing star formation, located at the northwestern tip of the elongated low surface brightness (LSB) main body of the BCD. High signal-to-noise 4.5m Multiple Mirror Telescope (MMT) and 10m Keck II telescope long-slit spectroscopy of SBS 0940+544 is used to derive element abundances of the ionized gas in the brightest H II region and to study the stellar population in the host galaxy. The oxygen abundance in the brightest region with strong emission lines is 12+log(O/H) = 7.46-7.50, or 1/29-1/26 solar, in agreement with earlier determinations and among the lowest for BCDs. Hbeta and Halpha emission lines and Hdelta and Hgamma absorption lines are detected in a large part of the main body. Three methods are used to put constraints on the age of the stellar population at different positions along the major axis. They are based on (a) the equivalent widths of the emission lines, (b) the equivalent widths of the absorption lines and (c) the spectral energy distributions (SED). Several scenarios of star formation have been considered. The observed properties in the main body can be reproduced by a continuous star formation process which started not earlier than 100 Myr ago, if a small extinction is assumed. However, the observations can be reproduced equally well by a stellar population forming continuously since 10 Gyr ago, if the star formation rate has increased during the last 100 Myr in the main body of SBS 0940+544 by at least a factor of five. In summary, we find no compelling evidence which favors either a young or an old age of SBS 0940+544.Comment: 20 pages, 15 Postscript figures, accepted for publication in the Astronomy and Astrophysic

Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies. I. SBS 1129+576

Spectroscopy and V,I CCD photometry of the dwarf irregular galaxy SBS 1129+576 are presented for the first time. The CCD images reveal a chain of compact H II regions within the elongated low-surface-brightness (LSB) component of the galaxy. Star formation takes place mainly in two high-surface-brightness H II regions. The mean (V-I) colour of the LSB component in the surface brightness interval between 23 and 26 mag/sq.arcsec is relatively blue ~0.56+/-0.03 mag, as compared to the (V-I)~0.9-1.0 for the majority of known dwarf irregular and blue compact dwarf (BCD) galaxies. Spectroscopy shows that the galaxy is among the most metal-deficient galaxies with an oxygen abundance 12+log(O/H)= 7.36+/-0.10 in the brightest H II region and 7.48+/-0.12 in the second brightest H II region, or 1/36 and 1/28 of the solar value, respectively. Hbeta and Halpha emission lines and Hdelta and Hgamma absorption lines are detected in a large part of the LSB component. We use two extinction-insensitive methods based on the equivalent widths of (1) emission and (2) absorption Balmer lines to put constraints on the age of the stellar populations in the galaxy. In addition, we use two extinction-dependent methods based on (3) the spectral energy distribution (SED) and (4) the (V-I) colour. The observed properties of the LSB component can be reproduced by a stellar population forming continuously since 10 Gyr ago, provided that the star formation rate has increased during the last 100 Myr by a factor of 6 to 50 and no extinction is present. However, the observational properties of the LSB component can be reproduced equally well by continuous star formation which started not earlier than 100 Myr ago and stopped at 5 Myr, if some extinction is assumed.(Abridged)Comment: 17 pages, 12 figures, Accepted for publication in A&

HI in very metal-poor galaxies: the SBS 0335-052 system

We present Giant Metrewave Radio Telescope (GMRT), HI 21cm observations of SBS 0335-052E and SBS 0335-052W, a close pair of dwarf galaxies, which are further unusual in being the most metal-poor star-forming galaxies known. We present images at several angular resolutions, ranging from ~40 to 4 arcsec. These images show that SBS 0335-052 is a strongly interacting system, with a faint diffuse HI bridge seen at low resolution, and elongated tails seen at the higher resolutions. The overall morphology suggests that the pair represents a major merger of extremely gas-rich galaxies. The low-resolution velocity field is dominated by the velocity difference between the two galaxies and the velocity gradient along the tidal features. However, for SBS 0335-052W at least, at high angular resolution, one sees a central velocity field that could be associated with the spin of the original undisturbed disc. The highest angular resolution HI images show that the ionized superbubble, identified by Thuan, Izotov & Lipovetsky (1997), in the Hubble Space Telescope (HST) images of SBS 0335-052E, is extended along one of the diffuse tidal features, and that there is a high-density HI clump at the other end of the superbubble. The star formation in SBS 0335-052E occurs mainly in a group of superstar clusters (SSCs) with a clear age gradient; the age decreases as one approaches the dense HI clump. We suggest that this propagating star formation is driven by the superbubble expanding into a medium with a tidally-produced density gradient. The high pressures associated with the compressed material would also naturally explain why current star formation is mainly concentrated in superstar clusters.Comment: Accepted for publication in MNRA

New southern blue compact dwarf galaxies in the 2dF Galaxy Redshift Survey

Aiming to find new extremely metal-deficient star-forming galaxies we extracted from the Two-Degree Field Galaxy Redshift Survey (2dFGRS) 100K Data Release 14 emission-line galaxies with relatively strong [OIII] 4363 emission. Spectroscopic and photometric studies of this sample and, in addition, of 7 Tololo and 2 UM galaxies were performed on the basis of observations with the ESO 3.6m telescope. All sample galaxies qualify with respect to their photometric and spectroscopic properties as blue compact dwarf (BCD) galaxies. Additionally, they show a good overlap with a comparison sample of 100 well-studied emission-line galaxies on the 12+log(O/H) vs. log(Ne/O), log(Ar/O) and log(Fe/O) planes. From the analysis of the 2dFGRS subsample we report the discovery of two new extremely metal-deficient BCDs with an oxygen abundance 12+log(O/H) < 7.6 and of another seven galaxies with 12+log(O/H) < 7.8. Furthermore, we confirm previous oxygen abundance determinations for the BCDs Tol 1304-353, Tol 2146-391, UM 559 and UM 570 to be 12+log(O/H) < 7.8.Comment: 26 pages, 65 figures, 5 tables, uses psfig.sty, Accepted for publication in Astronomy and Astrophysics. The paper with high-resolution figures can be downloaded at http://www.uni-sw.gwdg.de/~papade/Publications/Papaderos2006_2dF.pd

HI and star formation in the most metal-deficient galaxies

We present Giant Metrewave Radio Telescope (GMRT) observations for three (viz., DDO 68, SDSS J2104-0035 and UGC 772) of the six most metal-deficient actively star-forming galaxies known. Although there is a debate as to whether these galaxies are undergoing their first episode of star formation or not, they are `young' in the sense that their ISM is chemically unevolved. In this regard, they are the nearest equivalents of young galaxies in the early Universe. All three galaxies, that we have observed, have irregular HI morphologies and kinematics, which we interpret as either due to tidal interaction with neighbouring galaxies, or the consequences of a recent merger. The remaining three of the six most metal-deficient galaxies are also known to have highly disturbed HI distributions and are interacting. It is interesting because these galaxies were chosen solely on the basis of their metallicity and not for any particular signs of interaction. In this sense (i.e., their gas has not yet had time to settle into a regular disc), one could regard these extremely metal deficient (XMD) galaxies as `young'. The current star formation episode is likely to have been triggered by interaction/merger. It is also possible that the tidal interaction has lead to enhanced mixing with metal-poor gas in outer disc, and hence to a low gas-phase metallicity in the central star-forming regions. We also find that in general these galaxies do not show a one-to-one correspondence between regions of high HI column density and regions with current star formation. However, to the extent that one can define a threshold density, its value (~10^{21} atoms cm^{-2}) is similar to that in galaxies with much higher metallicity.Comment: 11 pages, 11 figures. Accepted for publication in MNRA

Deep multiband surface photometry on star forming galaxies: II. A volume limited sample of 21 emission lines galaxies

We present deep surface photometry of a volume--limited sample of 21 UM emission line galaxies in broadband optical UBVRI and near infra-red (NIR) HKs filters. The sample comprises 19 blue compact galaxies (BCGs) and two spirals. For some targets the exposure times are the deepest to date. For the BCG UM462 we observe a previously undetected second disk component beyond a surface brightness level of mu_B=26 mag arcsec^{-2}. This is a true low surface brightness component with central surface brightness mu_0=24.1 mag arcsec^{-2} and scale length h_r=1.5 kpc. All BCGs are dwarfs, with M_B>=-18, and very compact, with an average scale length of h_r~1 kpc. We separate the burst and host populations for each galaxy and compare them to stellar evolutionary models with and without nebular emission contribution. We also measure the A_{180} asymmetry in all filters and detect a shift from optical to NIR in the average asymmetry of the sample. This shift seems to be correlated with the morphological class of the BCGs. Using the color-asymmetry relation, we identify five BCGs in the sample as mergers, which is confirmed by their morphological class. Though clearly separated from normal galaxies in the concentration-asymmetry parameter space, we find that it is not possible to distinguish luminous starbursting BCGs from the merely star forming low luminosity BCGs.Comment: 48 pages, 39 figures, submitte

Oxygen and nitrogen abundances of HII regions in six spiral galaxies

Spectroscopic observations of 63 HII regions in six spiral galaxies (NGC 628, NGC 783, NGC 2336, NGC 6217, NGC 7331, and NGC 7678) were carried out with the 6-meter telescope (BTA) of Russian Special Astrophysical Observatory with the Spectral Camera attached to the focal reducer SCORPIO in the multislit mode with a dispersion of 2.1A/pixel and a spectral resolution of 10A. These observations were used to estimate the oxygen and nitrogen abundances and the electron temperatures in HII regions through the recent variant of the strong line method (NS calibration). The parameters of the radial distribution (the extrapolated central intercept value and the gradient) of the oxygen and nitrogen abundances in the disks of spiral galaxies NGC 628, NGC 783, NGC 2336, NGC 7331, and NGC 7678 have been determined. The abundances in the NGC 783, NGC 2336, NGC 6217, and NGC 7678 are measured for the first time. Galaxies from our sample follow well the general trend in the luminosity - central metallicity diagram for spiral and irregular galaxies.Comment: 11 pages, 7 figures, 6 tables. Accepted for publication in MNRA

Relation Between the Thickness of Stellar Disks and the Relative Mass of Dark Halo in Galaxies

We consider a thickness of stellar disks of late-type galaxies by analyzing the R and K_s band photometric profiles for two independent samples of edge-on galaxies. The main goal is to verify a hypotesis that a thickness of old stellar disks is related to the relative masses of the spherical and disk components of galaxies. We confirm that the radial-to-vertical scale length ratio for galactic disks increases (the disks become thinner) with the increasing of total mass-to-light ratio of the galaxies, which characterize the contribution of dark halo to the total mass, and with the decreasing of central deprojected disk brightness (surface density). Our results are in good agreement with numerical models of collisionless disks evolved from subcritical velocity dispersion state to a marginally stable equilibrium state. This suggests that in most galaxies the vertical stellar velocity dispersion, which determine the equilibrium disk thickness, is close to the minimum value, that ensures disk stability. The thinnest edge-on disks appear to be low brightness galaxies (after deprojection) in which a dark halo mass far exceeds a mass of the stellar disk.Comment: 13 pages. To be Published in Astronomy Letters, v.28(2002