Evolutionary Status of Dwarf ``Transition'' Galaxies

We present deep B, R and Halpha imaging of 3 dwarf galaxies: NGC3377A, NGC4286, and IC3475. Based on previous broadband imaging and HI studies, these mixed-morphology galaxies were proposed by Sandage & Hoffman (1991) to be, respectively, a gas-rich low surface brightness Im dwarf, a nucleated dwarf that has lost most of its gas and is in transition from Im to dS0,N, and the prototypical example of a gas-poor ``huge low surface brightness'' early-type galaxy. From the combination of our broadband and Halpha imaging with the published information on the neutral gas content of these three galaxies, we find that (1) NGC3377A is a dwarf spiral; (2) NGC3377A and NGC4286 have comparable amounts of ongoing star formation, as indicated by their Halpha emission, while IC3475 has no detected HII regions to a very low limit; (3) the global star formation rates are at least a factor of 20 below that of 30 Doradus for NGC3377A and NGC4286; (4) while the amount of star formation is comparable, the distribution of star forming regions is very different between NGC3377A and NGC4286; (5) given their current star formation rates and gas contents, both NGC3377A and NGC4286 can continue to form stars for more than a Hubble time; (6) both NGC3377A and NGC4286 have integrated total B-R colors that are redder than the integrated total B-R color for IC3475, and thus it is unlikely that either galaxy will ever evolve into an IC3475 counterpart; and (7) IC3475 is too blue to be a dE. We thus conclude that we have not identified potential precursors to galaxies such as IC3475, and unless signifcant changes occur in the star formation rates, neither NGC3377A nor NGC4286 will evolve into a dwarf elliptical or dwarf spheroidal within a Hubble time.Comment: 34 pages, 6 jpg figures, 3 postscript figures, and 4 tables, uses AASTeX, ApJ, in pres

The N/O Plateau of Blue Compact Galaxies: Monte Carlo Simulations of the Observed Scatter

Chemical evolution models and Monte Carlo simulation techniques have been combined for the first time to study the distribution of blue compact galaxies on the N/O plateau. Each simulation comprises 70 individual chemical evolution models. For each model, input parameters relating to a galaxy's star formation history (bursting or continuous star formation, star formation efficiency), galaxy age, and outflow rate are chosen randomly from ranges predetermined to be relevant. Predicted abundance ratios from each simulation are collectively overplotted onto the data to test its viability. We present our results both with and without observational scatter applied to the model points. Our study shows that most trial combinations of input parameters, including a simulation comprising only simple models with instantaneous recycling, are successful in reproducing the observed morphology of the N/O plateau once observational scatter is added. Therefore simulations which include delay of nitrogen injection are no longer favored over those which propose that most nitrogen is produced by massive stars, if only the plateau morphology is used as the principal constraint. The one scenario which clearly cannot explain plateau morphology is one in which galaxy ages are allowed to range below 250 Myr. We conclude that the present data for the N/O plateau are insufficient by themselves for identifying the portion of the stellar mass spectrum most responsible for cosmic nitrogen production.Comment: 41 pages, 15 figures; accepted by ApJ, to appear Aug. 20, 200

The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. II. Photometric Study of the Intermediate-Age Star Cluster BS 90

We present the results of our investigation of the intermediate-age star cluster BS 90, located in the vicinity of the HII region N 66 in the SMC, observed with HST/ACS. The high-resolution data provide a unique opportunity for a very detailed photometric study performed on one of the rare intermediate-age rich SMC clusters. The complete set of observations is centered on the association NGC 346 and contains almost 100,000 stars down to V ~28 mag. In this study we focus on the northern part of the region, which covers almost the whole stellar content of BS 90. We construct its stellar surface density profile and derive structural parameters. Isochrone fits on the CMD of the cluster results in an age of about 4.5 Gyr. The luminosity function is constructed and the present-day mass function of BS 90 has been obtained using the mass-luminosity relation, derived from the isochrone models. We found a slope between -1.30 and -0.95, comparable or somewhat shallower than a typical Salpeter IMF. Examination of the radial dependence of the mass function shows a steeper slope at larger radial distances, indicating mass segregation in the cluster. The derived half-mass relaxation time of 0.95 Gyr suggests that the cluster is mass segregated due to its dynamical evolution. From the isochrone model fits we derive a metallicity for BS 90 of [Fe/H]=-0.72, which adds an important point to the age-metallicity relation of the SMC. We discuss our findings on this relation in comparison to other SMC clusters.Comment: Accepted for Publication in ApJ, 12 pages emulateapj TeX style, 10 figure

An investigation of the luminosity-metallicity relation for a large sample of low-metallicity emission-line galaxies

(abridged) We present 8.2m VLT spectroscopic observations of 28 HII regions in 16 emission-line galaxies and 3.6m ESO telescope spectroscopic observations of 38 HII regions in 28 emission-line galaxies. These emission-line galaxies were selected mainly from the Data Release 6 (DR6) of the Sloan Digital Sky Survey (SDSS) as metal-deficient galaxy candidates. We collect photometric and high-quality spectroscopic data for a large uniform sample of star forming galaxies including new observations. Our aim is to study the luminosity-metallicity (L-Z) relation for nearby galaxies, especially at its low-metallicity end and compare it with that for higher-redshift galaxies. From our new observations we find that the oxygen abundance in 61 out of the 66 HII regions of our sample ranges from 12+logO/H=7.05 to 8.22. Our sample includes 27 new galaxies with 12+logO/H<7.6 which qualify as extremely metal-poor star-forming galaxies (XBCDs). Among them are 10 HII regions with 12+logO/H<7.3. The new sample is combined with a further 93 low-metallicity galaxies with accurate oxygen abundance determinations from our previous studies, yielding in total a high-quality spectroscopic data set of 154 HII regions. 9000 more galaxies with oxygen abundances, based mainly on the Te-method, are compiled from the SDSS. Our data set spans a range of 8 mag with respect to its absolute magnitude in SDSS g (-12>Mg>-20) and nearly 2 dex in its oxygen abundance (7.0<12+logO/H<8.8), allowing us to probe the L-Z relation in the nearby universe down to the lowest currently studied metallicity level. The L-Z relation established on the basis of the present sample is consistent with previous ones obtained for emission-line galaxies.Comment: 27 pages, 9 figures, accepted for publication in Astronomy and Astrophysic

The chemical composition of metal-poor emission-line galaxies in the Data Release 3 of the Sloan Digital Sky Survey

We have re-evaluated empirical expressions for the abundance determination of N, O, Ne, S, Cl, Ar and Fe taking into account the latest atomic data and constructing an appropriate grid of photoionization models with state-of-the art model atmospheres. Using these expressions we have derived heavy element abundances in the $\sim$ 310 emission-line galaxies from the Data Release 3 of the Sloan Digital Sky Survey (SDSS)with an observed Hbeta flux F(Hbeta)> 1E-14 erg s^{-1} cm^{-2} and for which the [O III] 4363 emission line was detected at least at a 2sigma level, allowing abundance determination by direct methods. The oxygen abundance 12 + log O/H of the SDSS galaxies lies in the range from ~ 7.1 (Zsun/30) to 8.5 (0.7 Zsun). The SDSS sample is merged with a sample of 109 blue compact dwarf (BCD) galaxies with high quality spectra, which contains extremely low-metallicity objects. We use the merged sample to study the abundance patterns of low-metallicity emission-line galaxies. We find that extremely metal-poor galaxies (12 + log O/H < 7.6, i.e. Z < Zsun/12) are rare in the SDSS sample. The alpha element-to-oxygen abundance ratios do not show any significant trends with oxygen abundance, in agreement with previous studies, except for a slight increase of Ne/O with increasing metallicity, which we interpret as due to a moderate depletion of O onto grains in the most metal-rich galaxies. The Fe/O abundance ratio is smaller than the solar value, by up to 1 dex at the high metallicity end. (abridged)Comment: 17 pages, 12 figures. Accepted for publication in the Astronomy and Astrophysic

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

Abundances in the Neutral Interstellar Medium of I Zw 18 from FUSE Observations

We report on new FUSE far-UV spectroscopy of the most metal-poor blue compact dwarf galaxy I Zw 18. The new data represent an improvement over previous FUSE spectra by a factor of 1.7 in the signal-to-noise. Together with a larger spectral coverage (917-1188 angstroms), this allows us to characterize absorption lines in the interstellar medium with unprecedented accuracy. The kinematics averaged over the large sampled region shows no clear evidence of gas inflows or outflows. The H I absorption is interstellar with a column density of 2.2 (+0.6,-0.5} * 10^21 cm^(-2). A conservative 3 sigma upper limit of 5.25 * 10^(14) cm^(-2) is derived for the column density of diffuse H_2. From a simultaneous fitting of metal absorption lines in the interstellar medium, we infer the following abundances: [Fe/H] = -1.76 +/- 0.12, [O/H] = -2.06 +/- 0.28, [Si/H] = -2.09 +/- 0.12, [Ar/H] = -2.27 +/- 0.13, and [N/H] = -2.88 +/- 0.11. This is in general several times lower than in the H II regions. The only exception is iron, whose abundance is the same. The abundance pattern of the interstellar medium suggests ancient star-formation activity with an age of at least a Gyr that enriched the H I phase. Around 470 SNe Ia are required to produce the iron content. A more recent episode that started 10 to several 100 Myr ago is responsible for the additional enrichment of alpha-elements and nitrogen in the H II regions.Comment: 48 pages including 3 tables (Latex) and 7 figures (postscript). Accepted for publication in the Astrophysical Journa

Evaluation of the structural-phase characteristics of a supersaturated ultrafine-grained Au-Co solid solution by diffractometry in hard synchrotron radiation

A synchrotron radiation study of immiscible Au-Co alloys obtained by consolidating a heterogeneous mixture of components and subsequent severe plastic deformation was performed. Namely, the estimates of the crystal lattice parameter, the average size of the coherent scattering regions and lattice strains in mechanically alloyed supersaturated solid solutions were made using obtained diffraction patterns and diffraction spectra. The effect of the temperature regime of deformation processing on the listed characteristics is shown, when the transition from cold deformation to cryogenic is carried out. © 2020 American Institute of Physics Inc. All rights reserved.The work was done at the shared research center SSTRC on the basis of the VEPP-4 - VEPP-2000 complex at BINP SB RAS, using equipment supported by project RFMEFI62119X0022. The SR XRD performed at the station "Diffractometry in the "hard" X-ray range" of the 4th synchrotron radiation channel of the VEPP-3 accelerator of the Siberian Center of Synchrotron and Terahertz Radiation of the Budker Institute of Nuclear Physics by Alexey I. Ancharov. Obtaining and deformation processing of the investigated materials were carried out on the basis of M.N. Mikheev Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences, Yekaterinburg. The research was carried out within the state assignment of Ministry of Science and Higher Education of the Russian Federation (theme “Рressure” No. АААА-А18-118020190104-3), supported in part by RFBR (project No. 19-32-60039)

The Extreme Outer Regions of Disk Galaxies: I. Chemical Abundances of HII Regions

We present the first results of an ongoing project to investigate the present-day chemical abundances of the extreme outer parts of galactic disks, as probed by the emission line spectra of a new sample of HII regions. The galaxies studied here, NGC628, NGC1058 and NGC6946, are all late-type spiral galaxies, characterized by larger than average HI-to-optical sizes. Our deep Halpha images have revealed the existence of recent massive star formation, traced by HII regions, out to, and beyond, two optical radii in these galaxies (defined by the B-band 25th magnitude isophote). Optical spectra of these newly-discovered HII regions are used to investigate their densities, ionization parameters, extinctions and in particular their oxygen and nitrogen abundances. Our measurements reveal gas-phase abundances of O/H~10-15% of the solar value, and N/O~20-25% of the solar value, at radii of 1.5-2 R25. Clear evidence also exists for diminished dust extinction (Av~0-0.2) at large radii. The combination of our measurements of outer disk HII region abundances with those for inner disk HII regions published in the literature is a powerful probe of the shape of abundance gradients over unprecedented radial baselines. Within the limits of the current dataset, the radial abundance variations are consistent with single log-linear relationships, although the derived slopes can often differ considerably from those found if only inner disk HII regions are used to define the fit. Interestingly, both the mean level of enrichment and the ratio of N/O measured in extreme outer galactic disks are similar to those values measured in some high redshift damped Lyman-alpha absorbers, suggesting that outer disks at the present epoch are relatively unevolved. (abridged)Comment: 36 pages, 10 embedded postscript files, 3 jpeg files, 7 postscript tables; accepted for publication in the Astronomical Journal (August issue

SN 2009E: a faint clone of SN 1987A

In this paper we investigate the properties of SN 2009E, which exploded in a relatively nearby spiral galaxy (NGC 4141) and that is probably the faintest 1987A-like supernova discovered so far. Spectroscopic observations which started about 2 months after the supernova explosion, highlight significant differences between SN 2009E and the prototypical SN 1987A. Modelling the data of SN 2009E allows us to constrain the explosion parameters and the properties of the progenitor star, and compare the inferred estimates with those available for the similar SNe 1987A and 1998A. The light curve of SN 2009E is less luminous than that of SN 1987A and the other members of this class, and the maximum light curve peak is reached at a slightly later epoch than in SN 1987A. Late-time photometric observations suggest that SN 2009E ejected about 0.04 solar masses of 56Ni, which is the smallest 56Ni mass in our sample of 1987A-like events. Modelling the observations with a radiation hydrodynamics code, we infer for SN 2009E a kinetic plus thermal energy of about 0.6 foe, an initial radius of ~7 x 10^12 cm and an ejected mass of ~19 solar masses. The photospheric spectra show a number of narrow (v~1800 km/s) metal lines, with unusually strong Ba II lines. The nebular spectrum displays narrow emission lines of H, Na I, [Ca II] and [O I], with the [O I] feature being relatively strong compared to the [Ca II] doublet. The overall spectroscopic evolution is reminiscent of that of the faint 56Ni-poor type II-plateau supernovae. This suggests that SN 2009E belongs to the low-luminosity, low 56Ni mass, low-energy tail in the distribution of the 1987A-like objects in the same manner as SN 1997D and similar events represent the faint tail in the distribution of physical properties for normal type II-plateau supernovae.Comment: 19 pages, 9 figures (+7 in appendix); accepted for publication in A&A on 3 November 201