Oxygen abundance in the Sloan Digital Sky Survey

We present two samples of \hii galaxies from the Sloan Digital Sky Survey (SDSS) spectroscopic observations data release 3. The electron temperatures($T_e$) of 225 galaxies are calculated with the photoionized \hii model and $T_e$ of 3997 galaxies are calculated with an empirical method. The oxygen abundances from the $T_e$ methods of the two samples are determined reliably. The oxygen abundances from a strong line metallicity indicator, such as $R_{23}$, $P$, $N2$, and $O3N2$, are also calculated. We compared oxygen abundances of \hii galaxies obtained with the $T_e$ method, $R_{23}$ method, $P$ method, $N2$ method, and $O3N2$method. The oxygen abundances derived with the $T_e$ method are systematically lower by $\sim$0.2 dex than those derived with the $R_{23}$ method, consistent with previous studies based on \hii region samples. No clear offset for oxygen abundance was found between $T_e$ metallicity and $P$, $N2$ and $O3N2$ metallicity. When we studied the relation between N/O and O/H, we found that in the metallicity regime of \zoh > 7.95, the large scatter of the relation can be explained by the contribution of small mass stars to the production of nitrogen. In the high metallicity regime, \zoh > 8.2, nitrogen is primarily a secondary element produced by stars of all masses.Comment: 7 pages, 3 figures. A&A accepte

Star formation history and dust attenuation in galaxies drawn from ultraviolet surveys

We compile a new sample of 115 nearby, non-Seyfert galaxies spanning a wide range of star formation activities, from starburst to nearly dormant, based on ultraviolet (UV) observations with various satellites. We combine these observations with infrared (IR) observations to study the relation between ratio of total far-IR to UV luminosities and UV spectral slope (the `IRX-UV' relation). We show that, at fixed UV spectral slope, quiescent star-forming galaxies in our sample have systematically lower ratio of total far-IR to UV luminosities than starburst galaxies. The strengths of spectral indices sensitive to star formation history, such as the 4000 A break and EW(Ha), correlate well with distance from the mean relation for starburst galaxies in the IRX-UV diagram, while there is little or no correlation between the dust-sensitive Ha/Hb ratio and this distance. This is strong observational evidence that the star formation history is relevant to the `second parameter' affecting the IRX-UV relation. We show that these results can be understood in the framework of the simple model of Charlot & Fall 2000 for the transfer of starlight through the ISM in galaxies. We confirm that, for starburst galaxies, the tight IRX-UV relation can be understood most simply as a sequence in overall dust content. In addition, we find that the broadening of the relation for quiescent star-forming galaxies can be understood most simply as a sequence in the ratio of present to past-averaged star formation rates. We provide simple formulae for estimating the UV attenuation as a function of either the ratio of far-IR to UV luminosities or the UV spectral slope and show that the accuracy of these estimates can be improved if some constraints are available on the ratio of present to past-averaged star formation rates [abridged].Comment: 11 pages, 5 figures, to appear in MNRAS. v2: correction of a typo in the definition of x in fitting formula (7

Quasi-simultaneous five-frequency VLBA observations of PKS 0528+134

We present results of Very Long Baseline Array (VLBA) observations of PKS 0528+134 at five frequencies (2.3, 5.0, 8.4, 15.4, and 22.2 GHz). These quasi-simultaneous data enable us to study the spectral distribution of Very Long Baseline Interferometer (VLBI) components for the first time in this highly variable source, from which the central compact core is identified. Our observations indicate that there are two bendings for the jet motion at parsec scale. We provide an approximate spatial fit to the curved jet trajectory using the Steffen et al. (\cite{Steffen95}) helical model. We further investigate the proper motions of three jet components, which all show superluminal motion. At high frequencies (15.4 and 22.2 GHz) we detected a new component, which is estimated to be related to a radio burst peaking at about 2000.Comment: 11 pages, 5 figures, 3 table

Spectroscopic Study of Blue Compact Galaxies II. Spectral Analysis and Correlations

This is the second paper in a series studying the star formation rates, stellar components, metallicities, and star formation histories and evolution of a sample of blue compact galaxies. We analyzed spectral properties of 97 blue compact galaxies, obtained with the Beijing Astronomical Observatory (China) 2.16 m telescope, with spectral range 3580\AA -- 7400\AA. We classify the spectra according to their emission lines: 13 of the total 97 BCGs sample are non-emission line galaxies (non-ELGs); 10 have AGN-like emission (AGNs), and 74 of them are star-forming galaxies (SFGs). Emission line fluxes and equivalent widths, continuum fluxes, the 4000 \AA Balmer break index and equivalent widths of absorption lines are measured from the spectra. We investigate the emission line trends in the integrated spectra of the star-forming galaxies in our sample, and find that: 1) The equivalent widths of emission lines are correlated with the galaxy absolute blue magnitude $M_B$; lower luminosity systems tend to have larger equivalent widths. 2) The equivalent width ratio [NII]6583/H$\alpha$ is anti-correlated with equivalent width H$\alpha$; a relationship is given that can be used to remove the [NII] contribution from blended H$\alpha$ + [NII]6548, 6583. 3) The [OII], H$\beta$, H$\gamma$ and H$\alpha$ fluxes are correlated; those can be used as star formation tracers in the blue. 4) The metallicity indices show trends with galaxy absolute magnitude and attenuation by dust; faint, low-mass BCGs have lower metallicity and color excess.Comment: 25 pages, 9 color ps-figures, accepted for publication in Astronomy and Astrophysic

New insights into the stellar content and physical conditions of star-forming galaxies at z = 2-3 from spectral modelling

We have used extensive libraries of model and empirical galaxy spectra (assembled respectively from the population synthesis code of Bruzual and Charlot and the fourth data release of the Sloan Digital Sky Survey) to interpret some puzzling features seen in the spectra of high redshift star-forming galaxies. We show that a stellar He II 1640 emission line, produced in the expanding atmospheres of Of and Wolf-Rayet stars, should be detectable with an equivalent width of 0.5-1.5AA in the integrated spectra of star-forming galaxies, provided the metallicity is greater than about half solar. Our models reproduce the strength of the He II 1640 line measured in the spectra of Lyman break galaxies for established values of their metallicities. With better empirical calibrations in local galaxies, this spectral feature has the potential of becoming a useful diagnostic of massive star winds at high, as well as low, redshifts. We also uncover a relationship in SDSS galaxies between their location in the [O III]/Hb vs. [N II]/Ha diagnostic diagram (the BPT diagram) and their excess specific star formation rate relative to galaxies of similar mass. We infer that an elevated ionisation parameter U is at the root of this effect, and propose that this is also the cause of the offset of high redshift star-forming galaxies in the BPT diagram compared to local ones. We further speculate that higher electron densities and escape fractions of hydrogen ionising photons may be the factors responsible for the systematically higher values of U in the H II regions of high redshift galaxies. The impact of such differences on abundance determinations from strong nebular lines are considered and found to be relatively minor.Comment: 14pages, Accepted for MNRA

Spectroscopic study of blue compact galaxies IV. Star formation rates and gas depletion timescales

This is the fourth paper in a series studying star formation rates, stellar components, metallicities, and star formation histories of a blue compact galaxy (BCG) sample. Using Ha, [OII]3727, IR, radio (1.4GHz) luminosities and neutral hydrogen gas masses, we estimated star formation rates(SFR) and gas depletion timescales of 72 star-forming BCGs. The SFRs of the BCGs in our sample span nearly four orders of magnitude, from approximately 10^-2 to 10^2M_sun/yr, with a median SFR of about 3M_sun/yr. The typical gas depletion timescale of BCGs is about one billion years. We found that subtracting underlying stellar absorption is very important to calculate both dust extinction and SFR of galaxies. Otherwise, the intrinsic extinction will be overestimated, the SFRs derived from [OII] and Ha will be underestimated (if the underlying stellar absorption and the internal extinction were not corrected from the observed luminosity) or overestimated (if an overestimated internal extinction were used for extinction correction). After both the underlying stellar absorption and the dust extinction were corrected, a remarkably good correlation emerges among Ha, [OII], IR and radio SFR indicators. Finally, we find a good correlation between the measured SFR and the absolute blue magnitude, metallicity, interstellar extinction of BCGs. Our results indicate that faint, low-mass BCGs have lower star formation rates.Comment: 11 pages, 9 figures, be published in Astronomy and Astrophysics, v.425, p.417-427 (2004

Age Estimations of M31 Globular Clusters from Their Spectral Energy Distributions

This paper presents accurate spectral energy distributions (SEDs) of 16 M31 globular clusters (GCs) confirmed by spectroscopy and/or high spatial-resolution imaging, as well as 30 M31 globular cluster candidates detected by Mochejska et al. Most of these candidates have m_V > 18, deeper than previous searches, and these candidates have not yet been confirmed to be globular clusters. The SEDs of these clusters and candidates are obtained as part of the BATC Multicolor Survey of the Sky, in which the spectrophotometrically-calibrated CCD images of M31 in 13 intermediate-band filters from 4000 to 10000 A were observed. These filters are specifically designed to exclude most of the bright and variable night-sky emission lines including the OH forest. In comparison to the SEDs of true GCs, we find that some of the candidate objects are not GCs in M31. SED fits show that theoretical simple stellar population (SSP) models can fit the true GCs very well. We estimate the ages of these GCs by comparing with SSP models. We find that, the M31 clusters range in age from a few ten Myr to a few Gyr old, as well as old GCs, confirming the conclusion that has been found by Barmby et a, Williams & Hodge, Beasley et al., Burstein et al. and Puzia et al. in their investigations of the SEDs of M31 globular clusters.Comment: Accepted for Publication in A&Ap, 13 pages, 6 figure

Spectroscopic study of blue compact galaxies III. Empirical population synthesis

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 A 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. The absorption equivalent widths measured in the synthetic spectra range from typically 1.5 A for Halpha, to 2 -- 5 A for Hbeta, Hgamma, and Hdelta. 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.Comment: 23 pages, 5 figures, accepted for publication in Astronomy and Astrophysic

Observation of periodic variable stars towards the galactic spiral arms by EROS II

We present the results of a massive variability search based on a photometric survey of a six square degree region along the Galactic plane at ($l = 305^\circ$, $b = -0.8^\circ$) and ($l = 330^\circ$, $b = -2.5^\circ$). This survey was performed in the framework of the EROS II (Exp\'erience de Recherche d'Objets Sombres) microlensing program. The variable stars were found among 1,913,576 stars that were monitored between April and June 1998 in two passbands, with an average of 60 measurements. A new period-search technique is proposed which makes use of a statistical variable that characterizes the overall regularity of the flux versus phase diagram. This method is well suited when the photometric data are unevenly distributed in time, as is our case. 1,362 objects whose luminosity varies were selected. Among them we identified 9 Cepheids, 19 RR Lyrae, 34 Miras, 176 eclipsing binaries and 266 Semi-Regular stars. Most of them are newly identified objects. The cross-identification with known catalogues has been performed. The mean distance of the RR Lyrae is estimated to be $\sim 4.9 \pm 0.3$ kpc undergoing an average absorption of $\sim 3.4 \pm 0.2$ magnitudes. This distance is in good agreement with the one of disc stars which contribute to the microlensing source star population.Our catalogue and light curves are available electronically from the CDS, Strasbourg and from our Web site http://eros.in2p3.fr.Comment: 15 pages, 11 figures, accepted in A&A (april 2002