arXiv subjects

J. M. Vilchez

Publications and source records attributed to J. M. Vilchez.

At least 91 records · Page 5Linked to original sources

arXiv2006

On the relation between electron temperatures in the O+ and O++ zones in high-metallicity HII regions

We suggest a new way to establish the relation between the electron temperature t3 within the [OIII] zone and the electron temperature t2 within the [OII] zone in high-metallicity (12+log(O/H) > 8.25) HII regions. The t2 - t3 diagram is constructed by applying our method to a sample of 372 HII regions. We find that the correlation between t2 and t3 is tight and can be approximated by a linear expression. The new t2 - t3 relation can be used to determine t2 and accurate abundances in high-metallicity HII regions with a measured t3. It can also be used in conjunction with the ff relation for the determination of t3 and t2 and oxygen abundances in high-metallicity HII regions where the [OIII]4363 auroral line is not detected. The derived t2 - t3 relation is independent of photoionization models of HII regions.

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arXiv2006

Distribution of extinction and star formation in NGC1569

We investigate the distribution of the intrinsic extinction in NGC1569 using an extinction map derived from the H_alpha/H_beta emission line ratio. We compare the extinction distribution to that of the dust emission traced by SPITZER IRAC(8microns) and MIPS (24microns) maps. The intrinsic extinction shows spatial variations, from zones with negligible extinction to zones with values up to A(V)=0.8mag. We find an extinction shell and establish a relation between this shell and the interstellar expanding structure produced by the stellar winds comig from the Super Star Cluster (SSCs) A and B in the center of the galaxy. We suggest that the extinction shell has been produced by the accumulation of dust at the border of the shell. Although we find a good spatial correlation between the Balmer extinction and infrared emission, there is a spatial displacement between the 8microns and 24microns maxima and the maximum in Balmer extinction which needs further investigation.

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arXiv2006

Distribution of the Extinction and Star Formation in NGC 1569

We investigate spatial the distribution of the intrinsic extinction in the starburst dwarf galaxy NGC1569 creating an extinction map of the whole galaxy derived from the Halpha/Hbeta emission line ratio. We differentiate the extinction in the HII regions from the extinction of the diffuse gas. The intrinsic extinction shows considerable variations over the plane of the galaxy, from negligible extinction up to highest values of A(V)=0.8mag. The extinction map shows small scale clumpy structures possibly due to a clumpy dust distribution. We also identify in this map a shell structure, for which we establish a causal relation with the expanding gas structure produced by the stellar winds coming from the Super Star Clusters (SSC) in the center of the galaxy. The comparison of the spatial profiles of the extinction, dust and gaseous emissions crossing the border of the shell shows a layered structure; the peak of this Halpha distribution lying closest to the SSC A, followed outwards by the peak of the extinction and at a still larger distances by the bulk of the atomic gas. We suggest that the extinction shell has been produced by the SSCs and that it can be explained by the accumulation of dust at the border of this ionized gas structure.

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arXiv2006

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.

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arXiv2005

What is the temperature structure in the giant HII region NGC 588?

We present the results of an exhaustive study of the ionized gas in NGC 588, a giant H II region in the nearby spiral galaxy M33. This analysis uses a high number of diagnostics in the optical and infrared ranges. Four temperature diagnostics obtained with optical lines agree with a gas temperature of 11000K, while the [O III] 5007/88um ratio yields a much lower temperature of 8000K approximately. This discrepancy suggests the presence of large temperature inhomogeneities in the nebula. We investigated the cause of this discrepancy by constructing photoionization models of increasing complexity. In particular, we used the constraints from the Halpha and Hbeta surface brightness distributions and state-of-the-art models of the stellar ionizing spectrum. None of the successive attempts was able to reproduce the discrepancy between the temperature diagnostics, so the thermal balance of NGC 588 remains unexplained. We give an estimate of the effect of this failure on the O/H and Ne/O estimates and show that O/H is known to within +/-0.2 dex

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arXiv2005

Structural Parameters of Dwarf Galaxies in the Coma Cluster: On the Origin of dS0 Galaxies

In this paper we analyze the structural parameters of the dwarf galaxies in the Coma cluster with $-18 \le M_{B} \le -16$ and classify them in two types: those with surface brightness profiles well fitted by a single Sersic law were called dwarf ellipticals (dEs), and those fitted with Sersic-plus-exponential profiles were classified as dwarf lenticulars (dS0s). The comparison of the structural parameters of the dwarf galaxies in the Coma and Virgo clusters shows that they are analogous. Photometrically, the dE and dS0 galaxies in Coma are equivalent, having similar colors and global scales. However, the scale of the innermost parts (bulges) of dS0 galaxies is similar to the bulges of late-type spiral galaxies. In contrast, dEs have larger scales than the bulges of bright galaxies. This may indicate that dS0 and dE galaxies have different origins. While dE galaxies can come from dwarf irregulars (dIs) or from similar processes as bright Es, the origin of dS0 galaxies can be harassed bright late-type spiral galaxies.

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arXiv2004

Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic properties

We performed a compilation of more than 1000 published spectra of HII regions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. The radial distributions of oxygen and nitrogen abundances were derived. The correlations between oxygen abundance and macroscopic properties are examined. There is a significant difference between the L-Z relationship obtained here and that based on the oxygen abundances determined through the R_23-calibrations. The oxygen abundance of NGC 5457 recently determined using direct measurements of Te (Kennicutt, Bresolin & Garnett 2003) agrees with the L-Z relationship derived here, but is in conflict with the L-Z relationship derived with the R_23-based oxygen abundances. The obtained L-Z relation for spirals is compared to that for irregulars. Our sample of galaxies shows evidence that the slope of the O/H-M_B relationship for spirals is slightly more shallow than that for irregulars. The effective oxygen yields were estimated for spiral and irregular galaxies. The effective oxygen yield increases with increasing luminosity from M_B=-11 to M_B=-18 (or with increasing rotation velocity from Vrot=10 km/s to Vrot=100 km/s) and then remains approximately constant. Irregular galaxies from our sample have effective oxygen yields lowered by a factor of 3 at maximum, i.e. irregular galaxies usually keep at least 1/3 of the oxygen they manufactured during their evolution.

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arXiv2004

Chemical abundances in spiral and irregular galaxies. O and N abundances derived from global emission--line spectra

The validity of oxygen and nitrogen abundances derived from the global emission-line spectra of galaxies via the P-method has been investigated using a collection of published spectra of individual HII regions in irregular and spiral galaxies. The conclusions of Kobulnicky, Kennicutt & Pizagno (1999) that global emission-line spectra can reliably indicate the chemical properties of galaxies has been confirmed. It has been shown that the comparison of the global spectrum of a galaxy with a collection of spectra of individual HII regions can be used to distinguish high and low metallicity objects and to estimate accurate chemical abundances in a galaxy. The oxygen and nitrogen abundances in samples of UV-selected and normal nearby galaxies have been determined. It has been found that the UV-selected galaxies occupy the same area in the N/O -- O/H diagram as individual HII regions in nearby galaxies. Finally, we show that intermediate-redshift galaxies systematically deviate from the metallicity -- luminosity trend of local galaxies.

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arXiv2003

ce-61: a Tidal Dwarf Galaxy in the Hercules cluster?

A candidate Tidal Dwarf Galaxy, ce-61, was identified in the merger system IC 1182 in the Hercules supercluster. The multi-wavelength data we obtained so far do not prove, however, that it is kinematically detached from the IC 1182 system and gravitationally bound.

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arXiv2003

Capturing a star formation burst in galaxies infalling onto the cluster Abell 1367

The discovery of a striking astrophysical laboratory in the cluster of galaxies Abell 1367 (Sakai et al. 2002) is confirmed with independent imaging and spectroscopic observations and further investigated in the present analysis. Two giant and ten dwarf/HII galaxies, members to a group, are simultaneously undergoing a burst of star formation. Redshift measurements suggest that the group galaxies are in the process of falling into the cluster at very high speed. We explore two possible mechanisms that could have triggered the short-lived stellar burst we are witnessing: the first, internal to the group itself, via tidal interactions among its members, the hypothesis favoured by Sakai et al. (2002); the second associated with the high velocity infall of the group galaxies into the cluster intergalactic medium. We present evidences in favour and against the two hypotheses.

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arXiv2003

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

We analyze J, H and Ks near-infrared data for 9 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, as well as color maps of the sample galaxies. The morphology of the BCDs in the 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 fit by ellipses. We derive accurate optical near--infrared host galaxy colors for eight of the sample galaxies; these colors are typical of an evolved stellar population. Interestingly, optical near--infrared 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.

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arXiv2003

On emission-line spectra obtained from evolutionary synthesis models I. Dispersion in the ionising flux and Lowest Luminosity Limits

(abriged) Stellar clusters with the same general physical properties (e.g., total mass, age, and star-formation mode) may have very different stellar mass spectra due to the incomplete sampling of the underlying mass function; such differences are especially relevant in the high-mass tail due to the smaller absolute number of massive stars. The dispersion in the number of massive stars also produces a dispersion in the properties of the corresponding ionising spectra. In this paper, we lay the bases for the future analysis of this effect by evaluating the dispersion in the ionising fluxes of synthetic spectra. As an important consequence, we found that the intensities of synthetic fluxes at different ionisation edges are strongly correlated, a fact suggesting that no additional dispersion will result from the inclusion of sampling effects in the analysis of diagnostic diagrams; this is true for HII regions on all scales. Additionally, we find convincing suggestions that the He II lines are strongly affected by sampling, and so cannot be used to constrain the evolutionary status of stellar clusters. We also establish the range of applicability of synthesis models set by the Lowest Luminosity Limit for the ionising flux, that is the lowest limit in cluster mass for which synthesis models can be applied to predict ionising spectra. This limit marks the boundary between the situations in which the ionising flux is better modeled with a single star as opposed to a star cluster; this boundary depends on the metallicity and age, ranging from 10^3 to more than 10^6 Mo. As a consequence, synthesis models should not be used to try to account for the properties of clusters with smaller masses.

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arXiv2002

Non-confirmation of reported HI clouds without optical counterparts in the Hercules Cluster

21 cm HI line observations were made with the Arecibo Gregorian telescope of 9 HI clouds in the Hercules Cluster which were reported as tenative detections in a VLA HI study of the cluster (Dickey 1997) and for which our deep CCD imaging failed to find any optical counterparts. No sensitive observations could be made of one of these (sw-174) due to the presence of a close-by strong continuum source. The other 8 tentative HI detections were not reconfirmed by the Arecibo HI measurements. The CCD images did reveal faint, low surface brightness counterparts near the centres of two other VLA HI sources invisible on the Palomar Sky Survey, sw-103 and sw-194.

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arXiv2002

Spectroscopy of Dwarf HII Galaxies in the Virgo Cluster. I. Data, Chemical Abundances and Ionization Structure

Long-slit spectroscopy has been obtained for a sample of 22 blue dwarf galaxies selected in the direction of the Virgo Cluster as part of a larger sample of Virgo blue dwarf galaxies for which deep Halpha imaging has been collected. Most of the galaxies in the present sample appear classified as BCDs or dwarf Irregulars in the Virgo Cluster Catalog. Line fluxes, Hbeta equivalent width and extinction coefficients, spatial emission profiles, ionization structure and physical conditions are presented for the galaxies of the sample. Chemical abundances have been derived either using a direct determination of the electron temperature or after detailed examination of the predictions of different empirical calibrations. The oxygen abundance derived for the sample of Virgo dwarf galaxies span in the range 7.6 < 12 + log O/H < 8.9, and the corresponding nitrogen to oxygen abundance ratio cover from values typical of low metallicity field BCD galaxies up to near solar.

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arXiv2002

The r'-band luminosity function of Abell1367: a comparison with Coma

We made a large (approximately 1degr x 1degr) r'-band imaging survey of the central regions of the two nearby clusters of galaxies, Abell1367 and Coma. The data, presented as a catalog, are used to construct the r'-band luminosity function (LF) of galaxies in these two clusters, by subtracting the Yasuda et al. (2001) galaxy counts from our cluster counts. Our Coma luminosity function is consistent with previous determinations, i.e. providing a faint end slope alpha = -1.47_-0.09^+0.08, significantly steeper than the one we find for Abell1367 (alpha = -1.07_-0.16^+0.20). The counts in Abell1367 show a relative minimum at r' ~ 19, followed by a steep increase faintward. The difference between the two clusters appears significant, given the consistency of the experimental conditions in the two clusters. Whereas for Coma we find a significant increase of the slope of the LF outwards, no such effect is found for Abell1367.

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arXiv2002

On the origin of nitrogen

The problem of the origin of nitrogen is considered within the framework of an empirical approach. The oxygen abundances and nitrogen to oxygen abundances ratios are derived in HII regions of a number of spiral galaxies through the recently suggested P-method using more than six hundred published spectra. The N/O - O/H diagram for HII regions in irregular and spiral galaxies is constructed. It is found that the N/O values in HII regions of spiral galaxies of early morphological types are higher than those in HII regions with the same metallicity in spiral galaxies of late morphological types. This suggests a long-time-delayed contribution to the nitrogen production. The N/O ratio of a galaxy can then be used as an indicator of the time that has elapsed since the bulk of star formation occurred, or in other words of the nominal "age" of the galaxy as suggested by Edmunds and Pagel more than twenty years ago. The scatter in N/O values at a given O/H can be naturally explained by differences in star formation histories in galaxies. While low-metallicity dwarf galaxies with low N/O do not contain an appreciable amount of old stars, low-metallicity dwarf galaxies with an appreciable fraction of old stars have high N/O. Consideration of planetary nebulae in the Small Magellanic Cloud and in the Milky Way Galaxy suggests that the contribution of low-mass stars to the nitrogen production is significant, confirming the conclusion that there is a long-time-delayed contribution to the nitrogen production.

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arXiv2002

Spectrophotometric Observations of Blue Compact Dwarf Galaxies: Mrk 370

We present results from a detailed spectrophotometric analysis of the blue compact dwarf galaxy (BCD) Mrk 370, based on deep UBVRI broad-band and Halpha narrow-band observations, and long-slit and two-dimensional spectroscopy of its brightest knots. The spectroscopic data are used to derive the internal extinction, and to compute metallicities, electronic density and temperature in the knots. By subtracting the contribution of the underlying older stellar population, modeled by an exponential function, removing the contribution from emission lines, and correcting for extinction, we can measure the true colors of the young star-forming knots. We show that the colors obtained this way differ significantly from those derived without the above corrections, and lead to different estimates of the ages and star-forming history of the knots. Using predictions of evolutionary synthesis models, we estimate the ages of both the starburst regions and the underlying stellar component. We found that we can reproduce the colors of all the knots with an instantaneous burst of star formation and the Salpeter initial mass function with an upper mass limit of 100 solar masses. The resulting ages range between 3 and 6 Myrs. The colors of the low surface brightness component are consistent with ages larger than 5 Gyr. The kinematic results suggest ordered motion around the major axis of the galaxy.

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arXiv2002

A Deep Halpha Survey of Galaxies in the Two Nearby Clusters Abell1367 and Coma: The Halpha Luminosity Functions

We present a deep wide field Halpha imaging survey of the central regions of the two nearby clusters of galaxies Coma and Abell1367, taken with the WFC at the INT2.5m telescope. We determine for the first time the Schechter parameters of the Halpha luminosity function (LF) of cluster galaxies. The Halpha LFs of Abell1367 and Coma are compared with each other and with that of Virgo, estimated using the B band LF by Sandage et al. (1985) and a L(Halpha) vs M_B relation. Typical parameters of phi^* ~ 10^0.00+-0.07 Mpc^-3, L^* ~ 10^41.25+- 0.05 erg sec^-1 and alpha ~ -0.70+-0.10 are found for the three clusters. The best fitting parameters of the cluster LFs differ from those found for field galaxies, showing flatter slopes and lower scaling luminosities L^*. Since, however, our Halpha survey is significantly deeper than those of field galaxies, this result must be confirmed on similarly deep measurements of field galaxies. By computing the total SFR per unit volume of cluster galaxies, and taking into account the cluster density in the local Universe, we estimate that the contribution of clusters like Coma and Abell1367 is approximately 0.25% of the SFR per unit volume of the local Universe.

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