Jose M. Vilchez

New Improved Calibration Relations for the Determination of Electron Temperatures and Oxygen and Nitrogen Abundances in H II Regions

New improved empirical calibrations for the determination of electron temperatures and oxygen and nitrogen abundances in H II regions from the strong emission lines of oxygen, nitrogen, and sulfur are given. They are derived using spectra of H II regions with measured electron temperatures as calibrating data. Calibration relations are given separately for three classes of H II regions: cool, warm, and hot ones. Criteria for assigning a H II region to one of these classes are suggested. We find that classification ambiguities arise only in the case of hot H II regions with enhanced nitrogen abundances. The derived calibrations provide reliable abundances for H II regions: the mean difference between oxygen abundances determined from the calibrations and Te -based oxygen abundances is ~0.075 dex, while it is ~0.05 dex for nitrogen abundances.

Violent star formation in NGC 2363

This is an electronic version of an article published in The Astrophysical Journal. Gonzalez-Delgado, R.M. et al. Violent star formation in NGC 2363. The Astrophysical Journal 437 (1994): 239-261

MULTIBAND ANALYSIS OF A SAMPLE OF BLUE COMPACT DWARF GALAXIES. I. SURFACE BRIGHTNESS DISTRIBUTION, MORPHOLOGY, AND STRUCTURAL PARAMETERS

Broadband observations in B, V, R, and I have been performed for a sample of 28 galaxies cataloged as blue compact dwarf galaxies (BCDs). Our deep imaging, reaching surface brightness levels of a few percent of the sky brightness, has allowed the detection of underlying emission or low surface brightness features for a substantial fraction of the sample. In this paper we present the first results of the program: deep contour maps in the B band, surface brightness profiles, and color profiles for all the galaxies. The information derived for this sample of galaxies has allowed us to analyze the morphology and the structural components of BCDs. Over 70% of the galaxies show complex profiles that preclude fitting by a single standard law, with extra structure at high to intermediate intensity levels. In 21 galaxies of the sample, an underlying low surface brightness component has been detected.

Multiband Analysis of a Sample of Blue Compact Dwarf Galaxies. II. Spatially Resolved and Integrated Photometry

This is the second in a series of papers devoted to the study of a sample of 28 blue compact dwarf galaxies (BCDs). In the first paper, we presented the broadband observations in B, V, R, and I. Deep contour maps, surface-brightness profiles, and color profiles were used to analyze and discuss their morphology and structure. Here, we present new U-band and Hα observations. We compute isophotal and asymptotic magnitudes of the sample galaxies and total Hα fluxes. Comparison with previous works is done to assess the quality of the data available for BCDs. Finally, we produce an atlas of detailed color and Hα maps, the first systematic atlas of this kind published so far. The high quality of the data allows us to identify the different star-forming knots in the maps and to discriminate between them and the regions where star formation activity has already ceased. We supplement the atlas with a morphological description of each galaxy and group the galaxies in four classes according to the distribution of their star-forming regions.

H II Region Population in a Sample of Nearby Galaxies with Nuclear Activity. I. Data and General Results

This paper presents the distribution of ionized gas in the disk and circumnuclear region of a sample of 55 galaxies with active nuclei, Seyfert galaxies of type 1 and 2, and low-ionization nuclear emission line regions (LINERs). We use CCD narrow-band Hα + [N II] images to map the distribution of H II regions and the morphology of the circumnuclear extended emission associated with the active nucleus. We also present [O III] images for a few of the galaxies. An analysis of the presence of extended emission and of H II regions is carried out twofold, as a function of the level of nuclear activity and of the Hubble type. In this sample, the extended emission is more common in Seyfert type 1 than in type 2; however, disk star formation is more common in Seyfert type 2. One-third of the galaxies have circumnuclear H II regions, but only 8% of these are Seyfert 1. The number surface density of H II regions and the location of the brightest H II region indicate that in Seyfert 2 the star formation is more important in the inner disk, as defined by the radius that contains half the total blue luminosity. In Seyfert 1 the distribution of H II regions is more uniform with distance, and the H II regions are farther away from the nucleus than in Seyfert 2. We discuss the possible connection between the dynamical properties of the galaxy, particularly in the inner parts, and the differences found in Seyfert types 1 and 2.

The starburst galaxy NGC 7714

This is an electronic version of an article published in The Astrophysical Journal. Gonzalez-Delgado, R.M. et al. The starbust galaxy NGC 7714. The Astrophysical Journal 439 (1995): 604-622

The Star Formation History and Metal Content of the Green Peas. New Detailed GTC-OSIRIS Spectrophotometry of Three Galaxies

We present deep broadband imaging and long-slit spectroscopy of three compact, low-mass starburst galaxies at redshift z ~ 0.2-0.3, also referred to as Green Peas (GP). We measure physical properties of the ionized gas and derive abundances for several species with high precision. We find that the three GPs display relatively low extinction, low oxygen abundances, and remarkably high nitrogen-to-oxygen ratios. We also report on the detection of clear signatures of Wolf-Rayet (W-R) stars in these galaxies. We carry out a pilot spectral synthesis study using a combination of both population and evolutionary synthesis models. Their outputs are in qualitative agreement, strongly suggesting a formation history dominated by starbursts. In agreement with the presence of W-R stars, these models show that these GPs currently undergo a major starburst producing between ~4% and ~20% of their stellar mass. However, as models imply, they are old galaxies that formed most of their stellar mass several Gyr ago. The presence of old stars has been spectroscopically verified in one of the galaxies by the detection of Mg I λλ5167, 5173 absorption lines. Additionally, we perform a surface photometry study based on Hubble Space Telescope data, which indicates that the three galaxies possess an exponential low surface brightness envelope. If due to stellar emission, the latter is structurally compatible with the evolved hosts of luminous blue compact dwarf (BCD)/H II galaxies, suggesting that GPs are identifiable with major episodes in the assembly history of local BCDs. These conclusions highlight the importance of these objects as laboratories for studying galaxy evolution at late cosmic epochs.

Oxygen abundances in the most oxygen-rich spiral galaxies

Oxygen abundances in the spiral galaxies expected to be richest in oxygen are estimated. The new abundance determinations are based on the recently discovered ff-relation between auroral and nebular oxygen line fluxes in HII regions. We find that the maximum gas-phase oxygen abundance in the central regions of spiral galaxies is 12+log(O/H)~8.75. This value is significantly lower than the previously accepted value. The central oxygen abundance in the Milky Way is similar to that in other large spirals.

Deep Near-Infrared Mapping of Young and Old Stars in Blue Compact Dwarf Galaxies

We analyze J, H, and Ks near-infrared data for nine blue compact dwarf (BCD) galaxies, selected from a larger sample that we have already studied in the optical. We present contour maps, surface brightness and color profiles, and color maps of the sample galaxies. The morphology of the BCDs in the near-infrared (NIR) has been found to be basically the same as in the optical. The inner regions of these systems are dominated by the starburst component. At low surface brightness levels the emission is due to the underlying host galaxy; the latter is characterized by red, radially constant colors and isophotes well fitted by ellipses. We derive accurate optical-NIR host galaxy colors for eight of the sample galaxies; these colors are typical of an evolved stellar population. Interestingly, optical-NIR color maps reveal the presence of a complex, large-scale absorption pattern in three of the sample galaxies. We study the applicability of the Sersic law to describe the surface brightness profiles of the underlying host galaxy and find that, because of the limited surface brightness interval over which the fit can be made, the derived Sersic parameters are very sensitive to the selected radial interval and to errors in the sky subtraction. Fitting an exponential model gives generally more stable results and can provide a useful tool to quantify the structural properties of the host galaxy and compare them with those of other dwarf classes, as well as with those of star-forming dwarfs at higher redshifts.

Bidimensional Spectroscopic Mapping and Chemical Abundances of the Star-forming Dwarf Galaxy I Zw 18

We present bidimensional, long-slit spectroscopic mapping of the prototypical low-metallicity dwarf galaxy I Zw 18. A total of eight spaced slit positions were observed, covering most of the body of the galaxy and part of the extended envelope of the diffuse ionized halo seen in deep Hα images of I Zw 18. Spectroscopic maps of the galaxy have been produced for Hα, Hβ equivalent width, [O III], [O II], reddening, excitation, and line-free continuum. Chemical abundances have been derived for 16 positions in I Zw 18, including zones sampling part of the extended ionized envelope of the galaxy. Our results confirm and extend previous findings that I Zw 18 presents a substantially homogeneous chemical composition over the whole galaxy. The chemical abundances of I Zw 18 as derived from a σ-weighted average of all the observed positions give 12 + log O/H = 7.19 ± 0.07 and 12 + log N/H = 5.64 ± 0.12 for oxygen and nitrogen, respectively, and He/H = 0.073 ± 0.004 for helium, a value consistent with some previous determinations and one that would lead to a helium mass fraction of Y = 0.228 ± 0.009. However, we notice that when the bright central knots are excluded from the mean, a slightly larger helium abundance is derived, yielding Y = 0.242 ± 0.010. We argue that underlying stellar absorption in the helium lines is present in the spectra of the central bright knots of I Zw 18 and will be affecting the derivation of the helium abundance for these central knots. The implications of our results for the chemical evolution models and for the overall enrichment history of the I Zw 18 galaxy are discussed.