How can large-scale twisted magnetic structures naturally emerge from buoyancy instabilities?

We consider the three-dimensional instability of a layer of horizontal magnetic field in a polytropic atmosphere where, contrary to previous studies, the field lines in the initial state are not unidirectional. We show that if the twist is initially concentrated inside the unstable layer, the modifications of the instability reported by several authors (see e.g. Cattaneo et al. (1990)) are only observed when the calculation is restricted to two dimensions. In three dimensions, the usual interchange instability occurs, in the direction fixed by the field lines at the interface between the layer and the field-free region. We therefore introduce a new configuration: the instability now develops in a weakly magnetised atmosphere where the direction of the field can vary with respect to the direction of the strong unstable field below, the twist being now concentrated at the upper interface. Both linear stability analysis and non-linear direct numerical simulations are used to study this configuration. We show that from the small-scale interchange instability, large-scale twisted coherent magnetic structures are spontaneously formed, with possible implications to the formation of active regions from a deep-seated solar magnetic field

Modeling the radial abundance distribution of the transition galaxy ngc 1313

NGC 1313 is the most massive disk galaxy showing a flat radial abundance distribution in its interstellar gas, a behavior generally observed in magellanic and irregular galaxies. We have attempted to reproduce this flat abundance distribution using a multiphase chemical evolution model, which has been previously used sucessfully to depict other spiral galaxies along the Hubble morphological sequence. We found that it is not possible to reproduce the flat radial abundance distribution in NGC 1313, and at the same time, be consistent with observed radial distributions of other key parameters such the surface gas density and star formation profiles. We conclude that a more complicated galactic evolution model including radial flows, and possibly mass loss due to supernova explosions and winds, is necessary to explain the apparent chemical uniformity of the disk of NGC 1313Comment: 14 paginas, 4 figures, to be published in ApJ, apri

The stellar populations of spiral disks.II Measuring and modeling the radial distribution of absorption spectral indices

The radial distributions of the Mg2 and Fe5270 Lick spectral indices have been measured to large radial distances on the disks of NGC 4303 and NGC 4535 using an imaging technique based on interference filters. These data, added to those of NGC 4321 previously published in Paper I of this series are used to constraint chemical (multiphase) evolutionary models for these galaxies. Because the integrated light of a stellar disk is a time average over the history of the galaxy weighted by the star formation rate, these constraints complement the information on chemical gradients provided by the study of HII regions which, by themselves, can only provide the alpha-elements abundance accumulate over the life of the galaxy. The agreement between the observations and the model predictions shown here lends confidence to the models which are then used to describe the time evolution of galaxy parameters such as star formation rates, chemical gradients, and gradients in the mean age of the stellar population.Comment: to be published in Astrophysical Journa

Metallicities of 0.3<z<1.0 Galaxies in the GOODS-North Field

We measure nebular oxygen abundances for 204 emission-line galaxies with redshifts 0.3<z<1.0 in the Great Observatories Origins Deep Survey North (GOODS-N) field using spectra from the Team Keck Redshift Survey (TKRS). We also provide an updated analytic prescription for estimating oxygen abundances using the traditional strong emission line ratio, R_{23}, based on the photoionization models of Kewley & Dopita (2003). We include an analytic formula for very crude metallicity estimates using the [NII]6584/Halpha ratio. Oxygen abundances for GOODS-N galaxies range from 8.2< 12+log(O/H)< 9.1 corresponding to metallicities between 0.3 and 2.5 times the solar value. This sample of galaxies exhibits a correlation between rest-frame blue luminosity and gas-phase metallicity (i.e., an L-Z relation), consistent with L-Z correlations of previously-studied intermediate-redshift samples. The zero point of the L-Z relation evolves with redshift in the sense that galaxies of a given luminosity become more metal poor at higher redshift. Galaxies in luminosity bins -18.5<M_B<-21.5 exhibit a decrease in average oxygen abundance by 0.14\pm0.05 dex from z=0 to z=1. This rate of metal enrichment means that 28\pm0.07% of metals in local galaxies have been synthesized since z=1, in reasonable agreement with the predictions based on published star formation rate densities which show that ~38% of stars in the universe have formed during the same interval. (Abridged)Comment: AASTeX, 49 pages, 16 figures, accepted for publication in The Astrophysical Journa

Stellar Populations Found in the Central kpc of Four Luminous Compact Blue Galaxies at Intermediate Redshift

We investigate the star formation history of the central regions of four Luminous Compact Blue Galaxies (LCBGs). LCBGs are blue (B-V<0.6), compact (MU_B<21.5 mag arcsec^-2) galaxies with absolute magnitudes M_B brighter than -17.5. The LCBGs analyzed here are located at 0.436<z<0.525. They are among the most luminous (M_B < -20.5), blue (B-V < 0.4) and high surface brightness (MU_B < 19.0 mag arcsec^-2) of this population. The observational data used were obtained with the HST/STIS spectrograph, the HST/WF/PC-2 camera and the HST/NICMOS first camera. We find evidence for multiple stellar populations. One of them is identified as the ionizing population, and the other one corresponds to the underlying stellar generation. The estimated masses of the inferred populations are compatible with the dynamical masses, which are typically 2--10x 10^9 M_sun. Our models also indicate that the first episodes of star formation the presented LCBGs underwent happened between 5 and 7 Gyr ago. We compare the stellar populations found in LCBGs with the stellar populations present in bright, local HII galaxies, nearby spheroidal systems and Blue Compact Dwarf Galaxies. It turns out that the underlying stellar populations of LCBGs are similar yet bluer to those of local HII galaxies. It is also the case that the passive color evolution of the LCBGs could convert them into local Spheroidal galaxies if no further episode of star formation takes place. Our results help to impose constraints on evolutionary scenarios for the population of LCBGs found commonly at intermediate redshifts.Comment: 35 pages, 10 Figures. Accepted for publication in AJ. Compile with pdflatex. Contains png figure

A New Superwind Wolf-Rayet Galaxy Mrk 1259

We report the discovery of a starburst-driven wind (superwind) from the starburst nucleus galaxy Mrk 1259. The estimated number ratio of Wolf-Rayet (WR) to O stars amounts to ~0.09. While the nuclear emission-line region is due to usual photoionization by massive stars, the circumnuclear emission-line regions show anomalous line ratios that can be due to cooling shocks. Since the host galaxy seems to be a face-on disk galaxy and the excitation conditions of the circumnuclear emission-line regions show the spatial symmetry, we consider that we are seeing the superwind nearly from a pole-on view. Cooling shock models may explain the observed emission line ratios of the circumnuclear regions although a factor of 2 overabundance of nitrogen is necessary. All these suggest that the high-mass enhanced starburst occurred ~5X10^6 years ago in the nuclear region of Mrk 1259.Comment: To be published in the Astrophysical Journal Letters, 15 pages, 4 figure

The Mass-Metallicity Relation at z~2

We use a sample of 87 rest-frame UV-selected star-forming galaxies with mean spectroscopic redshift z=2.26 to study the correlation between metallicity and stellar mass at high redshift. Using stellar masses determined from SED fitting to 0.3-8 micron photometry, we divide the sample into six bins in stellar mass, and construct six composite H-alpha+[NII] spectra from all of the objects in each bin. We estimate the mean oxygen abundance in each bin from the [NII]/H-alpha ratio, and find a monotonic increase in metallicity with increasing stellar mass, from 12+log(O/H) = 2.7e9 Msun to 12+log(O/H) = 8.6 for galaxies with = 1e11 Msun. We use the empirical relation between star formation rate density and gas density to estimate the gas fractions of the galaxies, finding an increase in gas fraction with decreasing stellar mass. These gas fractions combined with the observed metallicities allow the estimation of the effective yield y_eff as a function of stellar mass; in constrast to observations in the local universe which show a decrease in y_eff with decreasing baryonic mass, we find a slight increase. Such a variation of metallicity with gas fraction is best fit by a model with supersolar yield and an outflow rate ~4 times higher than the star formation rate. We conclude that the mass-metallicity relation at high redshift is driven by the increase in metallicity as the gas fraction decreases through star formation, and is likely modulated by metal loss from strong outflows in galaxies of all masses. There is no evidence for preferential loss of metals from low mass galaxies as has been suggested in the local universe. [Abridged]Comment: 18 pages, 9 figures, 2 tables; accepted for publication in Ap

The oxygen abundance calibrations and N/O abundance ratios of ~40,000 SDSS star-forming galaxies

Using a large sample of 38,478 star-forming galaxies selected from the Second Data Release of the Sloan Digital Sky Survey database (SDSS-DR2), we derive analytical calibrations for oxygen abundances from several metallicity-sensitive emission-line ratios: [N II]/H_alpha, [O III]/[N II], [N II]/[O II], [N II]/[S II], [S II]/H_alpha, and [O III]/H_beta. This consistent set of strong-line oxygen abundance calibrations will be useful for future abundance studies. Among these calibrations, [N II]/[O II] is the best for metal-rich galaxies due to its independence on ionization parameter and low scatter. Dust extinction must be considered properly at first. These calibrations are more suitable for metal-rich galaxies (8.4<12+log(O/H)<9.3), and for the nuclear regions of galaxies. The observed relations are consistent with those expected from the photoionization models of Kewley & Dopita (2002). However, most of the observational data spread in a range of ionization parameter q from 1*10^7 to 8*10^7 cm s^{-1}, corresponding to logU= -3.5 to -2.5, narrower than that suggested by the models. We also estimate the (N/O) abundance ratios of this large sample of galaxies, and these are consistent with the combination of a "primary" and a dominant "secondary" components of nitrogen.Comment: 17 pages, 10 figures, 1 table. ApJ in pres

Scaling Relations of Dwarf Galaxies without Supernova-Driven Winds

Nearby dwarf galaxies exhibit tight correlations between their global stellar and dynamical properties, such as circular velocity, mass-to-light ratio, stellar mass, surface brightness, and metallicity. Such correlations have often been attributed to gas or metal-rich outflows driven by supernova energy feedback to the interstellar medium. We use high-resolution cosmological simulations of high-redshift galaxies with and without energy feedback, as well as analytic modeling, to investigate whether the observed correlations can arise without supernova-driven outflows. We find that the simulated dwarf galaxies exhibit correlations similar to those observed as early as z~10, regardless of whether supernova feedback is included. We also show that the correlations can be well reproduced by our analytic model that accounts for realistic gas inflow but assumes no outflows, and star formation rate obeying the Kennicutt-Schmidt law with a critical density threshold. We argue that correlations in simulated galaxies arise due to the increasingly inefficient conversion of gas into stars in low-mass dwarf galaxies rather than supernova-driven outflows. We also show that the decrease of the observed effective yield in low-mass objects, often used as an indicator of gas and metal outflows, can be reasonably reproduced in our simulations without outflows. We show that this trend can arise if a significant fraction of metals in small galaxies is spread to the outer regions of the halo outside the stellar extent via mixing. In this case the effective yield can be significantly underestimated if only metals within the stellar radius are taken into account. Measurements of gas metallicity in the outskirts of gaseous disks of dwarfs would thus provide a key test of such explanation.Comment: accepted for publication in ApJ, 19 pages, 12 figures, uses emulateapj