Duilia Fernandes de Mello

Starburst99: Synthesis Models for Galaxies with Active Star Formation

Starburst99 is a comprehensive set of model predictions for spectrophotometric and related properties of galaxies with active star formation. The models are an improved and extended version of the data set previously published by Leitherer & Heckman. We have upgraded our code by implementing the latest set of stellar evolution models of the Geneva group and the model atmosphere grid compiled by Lejeune et al. Several predictions which were not included in the previous publication are shown here for the first time. The models are presented in a homogeneous way for five metallicities between Z = 0.040 and 0.001 and three choices of the initial mass function. The age coverage is 106—109 yr. We also show the spectral energy distributions which are used to compute colors and other quantities. The full data set is available for retrieval at a Web site, which allows users to run specific models with nonstandard parameters as well. We also make the source code available to the community.

WFPC2 Observations of the Hubble Deep Field South

The Hubble Deep Field South (HDF-S) observations targeted a high Galactic latitude field near QSO J2233-606. We present Wide Field Planetary Camera 2 observations of the field in four wide bandpasses centered at roughly 300, 450, 606, and 814 nm. Observations, data reduction procedures, and noise properties of the final images are discussed in detail. A catalog of sources is presented, and the number counts and color distributions of the galaxies are compared with a new catalog of the original Hubble Deep Field (HDF-N) that has been constructed in an identical manner. The two fields are qualitatively similar, with the galaxy number counts for the two fields agreeing to within 20%. The HDF-S has more candidate Lyman break galaxies at z > 2 than the HDF-N. The star formation rate per unit volume computed from the HDF-S, based on the UV luminosity of high-redshift candidates, is a factor of 1.9 higher than from the HDF-N at z ~ 2.7, and a factor of 1.3 higher at z ~ 4.

The Peculiar Type II Supernova 1997D: A Case for a Very Low 56Ni Mass*

SN 1997D in NGC 1536 is possibly the least luminous and energetic Type II supernova discovered to date. The entire light curve is subluminous, never reaching MV=-14.65. The radioactive tail follows the 56Co decay slope. In the case of a nearly complete trapping of the γ-rays, the 56Ni mass derived from the tail brightness is extremely small, ~0.002 M☉. At discovery, the spectra showed a red continuum and line velocities on the order of 1000 km s−1. The luminosity and the photospheric expansion velocity suggest that the explosion occurred about 50 days before discovery and that a plateau probably followed. Model light curves and spectra of the explosion of a 26 M☉ star successfully fitted the observations. Low-mass models are inconsistent with the observations. The radius of the progenitor, constrained by the prediscovery upper limits, is R0 300 R☉. A low explosion energy of ~4 × 1050 ergs is then required in the modeling. The strong Ba II lines in the photospheric spectra are reproduced with a solar abundance and low Teff. A scenario in which the low 56Ni mass observed in SN 1997D is due to fallback of material onto the collapsed remnant of the explosion of a 25-40 M☉ star appears to be favored over the case of the explosion of an 8-10 M☉ star with low 56Ni production.

B Stars as a Diagnostic of Star Formation at Low and High Redshift

We have extended the evolutionary synthesis models by Leitherer et al. by including a new library of B stars generated from the IUE high-dispersion spectra archive. We present the library and show how the stellar spectral properties vary according to luminosity classes and spectral types. We have generated synthetic UV spectra for prototypical young stellar populations varying the IMF and the star formation law. Clear signs of age effects are seen in all models. The contribution of B stars in the UV line spectrum is clearly detected, in particular for greater ages when O stars have evolved. With the addition of the new library we are able to investigate the fraction of stellar and interstellar contributions and the variation in the spectral shapes of intense lines. We have used our models to date the spectrum of the local super-star cluster NGC 1705-1. Photospheric lines of C III λ1247, Si III λ1417, and S V λ1502 were used as diagnostics to date the burst of NGC 1705-1 at 10 Myr. Interstellar lines are clearly seen in the NGC 1705-1 spectrum. Broadening and blueshifts of several resonance lines are stronger in the galaxy spectrum than in our models and are confirmed to be intrinsic of the galaxy. Si II λ1261 and Al II λ1671 were found to be pure interstellar lines with an average blueshift of 78 km s-1 owing to a directed outflow of the interstellar medium. We have selected the star-forming galaxy 1512-cB58 as a first application of the new models to high-z galaxies. This galaxy is at z = 2.723, it is gravitationally lensed, and its high signal-to-noise ratio Keck spectrum shows features typical of local starburst galaxies, such as NGC 1705-1. Models with continuous star formation were found to be more adequate for 1512-cB58 since there are spectral features typical of a composite stellar population of O and B stars. A model with Z = 0.4 Z☉ and an IMF with α = 2.8 reproduces the stellar features of the 1512-cB58 spectrum.

Searching for Wolf-Rayet Stars in I Zw 18: the Origin of He II Emission*

I Zw 18 is the most metal-poor star-forming galaxy known and is an ideal laboratory to probe stellar evolution theory at low metallicities. Using archival Hubble Space Telescope WFPC2 imaging and Faint Object Spectrograph spectroscopy, we were able to improve previous studies. We constructed a continuum-free He II λ4686 map, which was used to identify Wolf-Rayet (W-R) stars recently found by ground-based spectroscopy and to locate diffuse nebular emission. Most of the He II λ4686 emission is associated with the northwest stellar cluster clearly displaced from the surrounding shell-like [O III] and Hα emission. We found evidence for He II sources, compatible with five to nine WNL stars and/or compact nebular He II λ4686 emission, as well as residual diffuse emission. Only one of them is outside the northwest cluster. We have done an extensive comparison between our results and the recent ground-based data used by Izotov et al. and Legrand et al. to identify WN and WC stars in I Zw 18. The differences between the various data may be understood in terms of varying slit locations, continuum fits, and contamination by nebular lines. We have calculated evolutionary tracks for massive stars and synthesis models at the appropriate metallicity (Z ≈ 0.02 Z☉). These single-star models predict a mass limit MWR ≈ 90 M☉ for W-R stars to become WN and WC/WO. For an instantaneous burst model with a Salpeter initial mass function extending up to Mup ≈ 120-150 M☉, our model predictions are in reasonable agreement with the observed equivalent widths. Our model is also able to fully reproduce the observed equivalent widths of nebular He II λ4686 emission due to the presence of WC/WO stars. This quantitative agreement and the spatial correlation of nebular He II λ4686 with the stellar cluster and the position of W-R stars shown from the ground-based spectra further supports the hypothesis that W-R stars are responsible for nebular He II emission in extragalactic H II regions.

STAR FORMATION IN THE H I BRIDGE BETWEEN M81 AND M82

We present multiwavelength observations of stellar features in the H I tidal bridge connecting M81 and M82 in the region called Arps loop. We identify eight young star-forming regions from Galaxy Evolution Explorer ultraviolet (UV) observations. Four of these objects are also detected at Hα. We determine the basic star-formation history of Arps loop using F475W and F814W images obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope. We find both a young ( 1 Gyr) stellar population with a similar spatial distribution and a metallicity Z ~ 0.004. We suggest that the old stellar population was formed in the stellar disk of M82 and/or M81 and ejected into the intergalactic medium during a tidal passage (~ 200-300 Myr ago), whereas the young UV-bright stars have formed in the tidal debris. The UV luminosities of the eight objects are modest and typical of small clusters or OB associations. The tidal bridge between M81 and M82 therefore appears to be intermediate between the very low levels of star formation seen in the Magellanic bridge and actively star-forming tidal tails associated with major galaxy mergers.

Holmberg IX: The Nearest Young Galaxy*

We present a study of the M81 companion dwarf irregular galaxy Holmberg IX using deep images taken with the Wide Field Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope. Based on color-magnitude diagrams the resolved stellar population toward Holmberg IX contains numerous stars with ages of 200 Myr as well as older red giant stars. By charting the spatial distribution of the red giant stars and considering their inferred metallicities, we conclude that most of these older stars are associated with M81 or its tidal debris. At least 20% of the stellar mass in Holmberg IX was produced in the last ~200 Myr, giving it the youngest stellar population of any nearby galaxy. The location of Holmberg IX, its high gas content, and its youthful stellar population suggest that it is a tidal dwarf galaxy, perhaps formed during the last close passage of M82 around M81.

Star formation in the intragroup medium and other diagnostics of the evolutionary stages of compact groups of galaxies

Context. Compact groups of galaxies are entities that have high densities of galaxies and serve as laboratories to study galaxy interactions, intergalactic star formation and galaxy evolution. Aims. The main goal of this study is to search for young objects in the intragroup medium of seven compact groups of galaxies: HCG 2, 7, 22, 23, 92, 100 and NGC 92 as well as to evaluate the stage of interaction of each group. Methods. We used Fabry-Perot velocity fields and rotation curves together with GALEX NUV and FUV images and optical R-band and HI maps. Results. (i) HCG 7 and HCG 23 are in early stages of interaction; (ii) HCG 2 and HCG 22 are mildly interacting; and (iii) HCG 92, HCG 100 and NGC 92 are in late stages of evolution. We find that all three evolved groups contain populations of young blue objects in the intragroup medium, consistent with ages < 100 Myr, of which several are younger than < 10 Myr. We also report the discovery of a tidal dwarf galaxy candidate in the tail of NGC 92. These three groups, besides containing galaxies that have peculiar velocity fields, also show extended HI tails. Conclusions. Our results indicate that the advanced stage of evolution of a group, together with the presence of intragroup HI clouds, may lead to star formation in the intragroup medium. A table containing all intergalactic HII regions and tidal dwarf galaxies confirmed to date is appended.

Far-Ultraviolet Imaging of the Hubble Deep Field-North: Star Formation in Normal Galaxies at z < 1*

We present far-ultraviolet (FUV) imaging of the Hubble Deep Field-North (HDF-N) taken with the Solar Blind Channel of the Advanced Camera for Surveys (ACS SBC) and the FUV MAMA detector of the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. The full WFPC2 deep field has been observed at 1600 A. We detect 134 galaxies and one star down to a limit of FUV_(AB) ~ 29. All sources have counterparts in the WFPC2 image. Redshifts (spectroscopic or photometric) for the detected sources are in the range 0 < z < 1. We find that the FUV galaxy number counts are higher than those reported by GALEX, which we attribute at least in part to cosmic variance in the small HDF-N field of view. Six of the 13 Chandra sources at z < 0.85 in the HDF-N are detected in the FUV, and those are consistent with starbursts rather than active galactic nuclei. Cross-correlating with Spitzer sources in the field, we find that the FUV detections show general agreement with the expected L_(IR)/L_(UV) versus β relationship. We infer star formation rates (SFRs), corrected for extinction using the UV slope, and find a median value of 0.3 M_☉ yr^(-1) for FUV-detected galaxies, with 75% of detected sources having SFR < 1 M_☉ yr^(-1). Examining the morphological distribution of sources, we find that about half of all FUV-detected sources are identified as spiral galaxies. Half of morphologically selected spheroid galaxies at z < 0.85 are detected in the FUV, suggesting that such sources have had significant ongoing star formation in the epoch since z ~ 1.

Star-forming Galaxies at Intermediate Redshifts: Morphology, Ages, and Sizes

We present an analysis of the deepest near-UV image obtained with the Hubble Space Telescope using the WFPC2 (F300W) as part of the parallel observations of the Hubble Ultra Deep Field campaign. The U-band 10 σ limiting magnitude measured over 0.2 arcsec2 is mAB = 27.5, which is 0.5 mag deeper than that in the Hubble Deep Field–North. We matched the U-band catalogs with those from the ACS (BViz) taken during the Great Observatories Origins Deep Survey observations of the Chandra Deep Field–South and obtained photometric redshifts for 306 matched objects. We find that the UV-selected galaxies span all the major morphological types at 0.2 100 Myr. The average half-light radius (rest-frame 1200–1800 A) of the UV-selected galaxies at 0.66 < zphot < 1.5 is 026 ± 001 (2.07 ± 0.08 kpc). The UV-selected galaxies are on average fainter (MB = -18.43 ± 0.13) than Lyman break galaxies (LBGs; MB = -23 ± 1). Our sample includes early-type galaxies that are presumably massive and forming stars only in their cores, as well as starburst-type systems that are more similar to the LBGs, although much less luminous. This implies that even the starbursts in our sample are either much less massive than LBGs or forming stars at a much lower rate, or both. The low surface brightness galaxies have no overlap with the LBGs and form an interesting new class of their own.