Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation

We present V and I photometry of the resolved stars in the most metal-deficient blue compact dwarf galaxy known, I Zw 18 (Zsun/50), using Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest ones ever obtained for this galaxy. The resulting I vs. V-I color-magnitude diagram (CMD) reaches limiting magnitudes V=I=29 mag. It reveals a young stellar population of blue main-sequence (MS) stars (age <30 Myr) and blue and red supergiants (10 Myr<age<100 Myr), but also an older evolved population of asymptotic giant branch (AGB) stars (100 Myr<age<500 Myr). We derive a distance to I Zw 18 in the range 12.6 Mpc - 15 Mpc from the brightness of its AGB stars, with preferred values in the higher range. The red giant branch (RGB) stars are conspicuous by their absence, although, for a distance of I Zw 18 <15 Mpc, our imaging data go ~ 1-2 mag below the tip of the RGB. Thus, the most evolved stars in the galaxy are not older than 500 Myr and I Zw 18 is a bona fide young galaxy. Several star formation episodes can be inferred from the CMDs of the main body and the C component. There have been respectively three and two episodes in these two parts, separated by periods of ~ 100-200 Myr. In the main body, the younger MS and massive post-MS stars are distributed over a larger area than the older AGB stars, suggesting that I Zw 18 is still forming from the inside out. In the C component, different star formation episodes are spatially distinct, with stellar population ages decreasing from the northwest to the southeast, also suggesting the ongoing build-up of a young galaxy.Comment: 29 pages, 13 Postscript figures, accepted for publication in the Astrophysical Journa

HST observations of the blue compact dwarf SBS 0335-052: a probable young galaxy

We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Ly$_alpha$ and OI 1302 of the extremely metal-deficient (Z~Zsun/41) blue compact dwarf (BCD) galaxy SBS 0335-052. All the star formation in the BCD occurs in six super-star clusters (SSC) with ages =< 3-4 Myr. Dust is clearly present and mixed spatially with the SSCs. There is a supershell of radius ~380 pc, delineating a large supernova cavity. The instantaneous star formation rate is ~0.4 Msun yr^-1. Strong narrow Ly$\alpha$ emission is not observed. Rather there is low intensity broad (FWZI = 20 A) Ly$\alpha$ emission superposed on even broader Ly$\alpha$ absorption by the HI envelope. This broad low-intensity emission is probably caused by resonant scattering of Ly$\alpha$ photons. The BCD appears to be a young galaxy, undergoing its very first burst of star formation. This conclusion is based on the following evidence: 1) the underlying extended low-surface-brightness component is very irregular and filamentary, suggesting that a significant part of the emission comes from ionized gas; 2) it has very blue colors (-0.34 =< (V-I)$_0$ =< 0.16), consistent with gaseous emission colors; 3) the OI 1302 line is not detected in absorption in the GHRS spectrum, setting an upper limit for N(O)/N(H) in the HI envelope of the BCD of more than 3000 times smaller than the value in Orion.Comment: 20 pages and 6 Postscript figures. Submitted to Astrophysical Journa

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

Extinction law variations and dust excitation in the spiral galaxy NGC 300

We investigate the origin of the strong radial gradient in the ultraviolet-to-infrared ratio in the spiral galaxy NGC 300, and emphasize the importance of local variations in the interstellar medium geometry, concluding that they cannot be neglected with respect to metallicity effects. This analysis is based upon a combination of maps from GALEX and Spitzer, and from the ground (UBVRI, Halpha and Hbeta). We select ionizing stellar clusters associated with HII regions of widely varying morphologies, and derive their fundamental parameters from population synthesis fitting of their spectral energy distributions, measured to eliminate local backgrounds accurately. From these fits, we conclude that the stellar extinction law is highly variable in the line of sight of young clusters of similar ages. In the particular model geometry that we consider most appropriate to the sampled regions, we checked that our findings are not significantly altered by the correct treatment of radiative transfer effects. The variations are systematic in nature: extinction laws of the Milky Way or LMC type are associated with compact HII regions (the compacity being quantified in two different ways), while clusters surrounded by diffuse HII regions follow extinction laws of the 30 Doradus or SMC type. The Calzetti starburst attenuation law, although most often degenerate with the 30 Doradus extinction law, overpredicts ionizing photon fluxes by large amounts. We also find that the extinction law variations are correlated with the column density of dust species emitting in the near- and mid-infrared. Finally, we briefly discuss the nebular to stellar extinction ratios, and the excitation of aromatic band carriers, invalidating their claimed association with cold dust.Comment: accepted for publication in ApJ -- figure 6 abridged her

Spectroscopic study of blue compact galaxies V. oxygen abundance and the metallicity-luminosity relation

This is the fifth paper in a series studying the stellar components, star formation histories, star formation rates and metallicities of a blue compact galaxy (BCG) sample. Based on our high-quality ground-based spectroscopic observations, we have determined the electron temperatures, electron densities, nitrogen abundances and oxygen abundances for 72 star-forming BCGs in our sample, using different oxygen abundance indicators. The oxygen abundance covers the range 7.15 < 12 + log (O/H)< 9.0, and nitrogen is found to be mostly a product of secondary nucleosynthesis for 12 + log (O/H)>8.2 and apparently a product of primary nucleosynthesis for 12 + log (O/H)< 8.2. To assess the possible systematic differences among different oxygen abundance indicators, we have compared oxygen abundances of BCGs obtained with the Te method, R23 method, P method, N2 method and O3N2 method. The oxygen abundances derived from the Te method are systematically lower by 0.1-0.25 dex than those derived from the strong line empirical abundance indicators, consistent with previous studies based on region samples. We confirm the existence of the metallicity-luminosity relation in BCGs over a large range of abundances and luminosities. Our sample of galaxies shows that the slope of the metallicity-luminosity relation for the luminous galaxies (~-0.05) is slightly shallower than that for the dwarf galaxies (~-0.17). An offset was found in the metallicity-luminosity relation of the local galaxies and that of the intermediate redshift galaxies. It shows that the metallicity-luminosity relation for the emission line galaxies at high redshift is displaced to lower abundances, higher luminosities, or both.Comment: 11 pages, 4 figure

The redshift evolution of oxygen and nitrogen abundances in emission-line SDSS galaxies

The oxygen and nitrogen abundance evolutions with redshift and galaxy stellar mass in emission-line SDSS galaxies are investigated. This is the first such study for nitrogen abundances, and it provides an additional constraint for the study of the chemical evolution of galaxies. We have devised a criterion to recognize and exclude from consideration AGNs and star-forming galaxies with large errors in the line flux measurements. To select star-forming galaxies with accurate line fluxes measurements, we require that, for each galaxy, the nitrogen abundances derived with various calibrations based on different emission lines agree. Using this selection criterion, subsamples of star-forming galaxies have been extracted from catalogs of the MPA/JHU group. We found that the galaxies of highest masses, those with masses > 10^11.2 M_sun, have not been enriched in both oxygen and nitrogen over the last 3 Gyr: they have formed their stars in the so distant past that these have returned their nucleosynthesis products to the interstellar medium before z=0.25. The galaxies in the mass range from 10^11.0 M_sun to 10^11.2 M_sun do not show an appreciable enrichment in oxygen, but do show some enrichment in nitrogen: they also formed their stars before z=0.25 but later in comparison to the galaxies of highest masses; these stars have not returned nitrogen to the interstellar medium before z=0.25 because they have not had enough time to evolve. This suggests that stars with lifetimes of 2-3 Gyr contribute to the nitrogen production. Finally, galaxies with masses < 10^11 M_sun show enrichment in both oxygen and nitrogen during the last 3 Gyr: they have undergone appreciable star formation and have converted up to 20% of their mass into stars over this period.Comment: 43 pages, 15 figures, accepted for publication in the Astrophysical Journa

A spectroscopic study of component C and the extended emission around I Zw 18

Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are used to investigate the stellar population and the evolutionary status of I Zw 18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18. Hydrogen H$\alpha$ and H$\beta$ emission lines are detected in the spectra of I Zw 18C, implying that ionizing massive stars are present. High signal-to-noise Keck II spectra of different regions in I Zw 18C reveal H$\gamma$, H$\delta$ and higher order hydrogen lines in absorption. Several techniques are used to constrain the age of the stellar population in I Zw 18C. Ages derived from two different methods, one based on the equivalent widths of the H$\alpha$, H$\beta$ emission lines and the other on H$\gamma$, H$\delta$ absorption lines are consistent with a 15 Myr instantaneous burst model. We find that a small extinction in the range $A_V$ = 0.20 -- 0.65 mag is needed to fit the observed spectral energy distribution of I Zw 18C with that model. In the case of constant star formation, all observed properties are consistent with stars forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available observational constraints for I Zw 18C, including those obtained from Hubble Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18 should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized gas emission around I Zw 18. H$\alpha$ emission is detected as far as 30" from it. To a B surface brightness limit of ~ 27 mag arcsec$^{-2}$ we find no observational evidence for extended stellar emission in the outermost regions, at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the Astrophysical Journa

Heavy element abundances in blue compact galaxies

We present high-quality ground-based spectroscopic observations of 54 supergiant H II regions in 50 low-metallicity blue compact galaxies with oxygen abundances 12 + log O/H between 7.1 and 8.3. We use the data to determine abundances for the elements N, O, Ne, S, Ar and Fe. We also analyze Hubble Space Telescope (HST) Faint Object Spectrograph archival spectra of 10 supergiant H II regions to derive C and Si abundances in a subsample of 7 BCGs. The main result of the present study is that none of the heavy element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12 + log O/H < 7.6 (Z < Zsun/20). This constancy implies that all these heavy elements have a primary origin and are produced by the same massive (M > 10Msun) stars responsible for O production. The dispersion of the C/O and N/O ratios in these galaxies is found to be remarkably small, being only +/-0.03 dex and +/-0.02 dex respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest-metallicity BCGs (secondary N production is negligible at these low metallicities). The absence of a time-delayed production of C and N is consistent with the scenario that galaxies with 12 + log O/H < 7.6 are undergoing now their first burst of star formation, and that they are therefore young, with ages not exceeding 40 Myr. If very low metallicities BCGs are indeed young, this would argue against the commonly held belief that C and N are produced by intermediate-mass (3Msun < M < 9Msun) stars at very low metallicities, as these stars would not have yet completed their evolution in these lowest metallicity galaxies.Comment: 37 pages, 5 EPS figures, to appear in ApJ, February 199

Towards an Understanding of the Mid-Infrared Surface Brightness of Normal Galaxies

We report a mid-infrared color and surface brightness analysis of IC 10, NGC 1313, and NGC 6946, three of the nearby galaxies studied under the Infrared Space Observatory Key Project on Normal Galaxies. Images with < 9 arcsecond (170 pc) resolution of these nearly face-on, late-type galaxies were obtained using the LW2 (6.75 mu) and LW3 (15 mu) ISOCAM filters. Though their global I_nu(6.75 mu)/I_nu(15 mu) flux ratios are similar and typical of normal galaxies, they show distinct trends of this color ratio with mid-infrared surface brightness. We find that I_nu(6.75 mu)/I_nu(15 mu) ~< 1 only occurs for regions of intense heating activity where the continuum rises at 15 micron and where PAH destruction can play an important role. The shape of the color-surface brightness trend also appears to depend, to the second-order, on the hardness of the ionizing radiation. We discuss these findings in the context of a two-component model for the phases of the interstellar medium and suggest that star formation intensity is largely responsible for the mid-infrared surface brightness and colors within normal galaxies, whereas differences in dust column density are the primary drivers of variations in the mid-infrared surface brightness between the disks of normal galaxies.Comment: 19 pages, 6 figures, uses AAS LaTeX; to appear in the November Astronomical Journa

On the maximum value of the cosmic abundance of oxygen and the oxygen yield

We search for the maximum oxygen abundance in spiral galaxies. Because this maximum value is expected to occur in the centers of the most luminous galaxies, we have constructed the luminosity - central metallicity diagram for spiral galaxies, based on a large compilation of existing data on oxygen abundances of HII regions in spiral galaxies. We found that this diagram shows a plateau at high luminosities (-22.3 < M_B < -20.3), with a constant maximum value of the gas-phase oxygen abundance 12+log(O/H) ~ 8.87. This provides strong evidence that the oxygen abundance in the centers of the most luminous metal-rich galaxies reaches the maximum attainable value of oxygen abundance. Since some fraction of the oxygen (about 0.08 dex) is expected to be locked into dust grains, the maximum value of the true gas+dust oxygen abundance in spiral galaxies is 12+log(O/H) ~ 8.95. This value is a factor of ~ 2 higher than the recently estimated solar value. Based on the derived maximum oxygen abundance in galaxies, we found the oxygen yield to be about 0.0035, depending on the fraction of oxygen incorporated into dust grains.Comment: 8 pages, 5 figures, accepted for publication in MNRA