arXiv subjects

Laura Greggio

Publications and source records attributed to Laura Greggio.

45 records · Page 3Linked to original sources

arXiv1999

Stellar yields and chemical evolution - The solar neighborhood as a calibrator

Uncertainties in stellar nucleosynthesis and their impact on models of chemical evolution are discussed. Comparing the Type II supernova nucleosynthesis prescriptions from Woosley & Weaver (1995) and Thielemann, Nomoto, & Hashimoto (1996), it turns out that the latter predict higher Mg/Fe ratios that are more favorable in reproducing the observed abundance features of the Milky Way. Provided that chemical evolution models are calibrated on the solar neighborhood, they offer a powerful tool to constrain structure formation. In particular, galaxy formation models that yield star formation histories significantly longer than 1 Gyr fail to reproduce the super-solar Mg/Fe ratios observed in elliptical galaxies.

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arXiv1999

A Near-Infrared Stellar Census of the Blue Compact Dwarf Galaxy VII~Zw~403

We present near-infrared single-star photometry for the low-metallicity Blue Compact Dwarf galaxy VII~Zw~403. We achieve limiting magnitudes of F110W~$\approx$~25.5 and F160W~$\approx$~24.5 using one of the NICMOS cameras with the HST equivalents of the ground-based J and H filters. The data have a high photometric precision (0.1 mag) and are $>95$% complete down to magnitudes of about 23, far deeper than previous ground-based studies in the near-IR. The color-magnitude diagram contains about 1000 point sources. We provide a preliminary transformation of the near-IR photometry into the ground system...

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arXiv1999

The uncertainties in the synthetic indices for stellar populations

The study of line strengths in the spectra of early-type galaxies has proven to be a powerful tool for investigating the age and the metallicity of these systems. When computing models for spectrophotometric narrow-band indices, index calibrations as functions of the relevant stellar parameters (e.g. temperature, gravity and metal content) are used. Thus synthetic indices depend upon these calibrations (called fitting functions), as well as on the stellar evolution ingredients adopted. All these inputs suffer from uncertainties, which impact on the derived value for the indices. In this paper we address this problem quantitatively. We compute synthetic Mg2, Fe5270, Fe5335 and Hbeta indices for Simple Stellar Populations (SSP) of various ages and metallicities, under different prescriptions for the fitting functions. This allows us to estimate the impact of the uncertainties in the fitting functions. By comparing our models to those of other authors computed with the same fitting functions, we estimate the uncertainty associated to the use of different stellar evolution prescriptions. It is found that the modelling of the Horizontal Branch impacts particularly on and Hbeta. In the range of parameters explored, the uncertainties introduced by the use of different fitting functions can be appreciably larger than the error affecting the observational data. This typically occurs at high Z for the metallic line strengths, at low Z for the Hbeta index.

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arXiv1999

A Stellar Population Gradient in VII Zw 403 - Implications for the Formation of Blue Compact Dwarf Galaxies

We present evidence for the existence of an old stellar halo in the Blue Compact Dwarf galaxy VII Zw 403. VII Zw 403 is the first Blue Compact Dwarf galaxy for which a clear spatial segregation of the resolved stellar content into a "core-halo" structure is detected. Multicolor HST/WFPC2 observations indicate that active star formation occurs in the central region, but is strikingly absent at large radii. Instead, a globular-cluster-like red giant branch suggests the presence of an old (> 10 Gyr) and metal poor (<[Fe/H]>=-1.92) stellar population in the halo. While the vast majority of Blue Compact Dwarf galaxies has been recognized to possess halos of red color in ground-based surface photometry, our observations of VII Zw 403 establish for the first time a direct correspondence between a red halo color and the presence of old, red giant stars. If the halos of Blue Compact Dwarf galaxies are all home to such ancient stellar populations, then the fossil record conflicts with delayed-formation scenarios for dwarfs.

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arXiv1999

The Mini AGN at the Center of the Elliptical Galaxy NGC 4552 with HST

The complex phenomenology shown by the UV-bright, variable spike first detected with the Hubble Space Telescope (HST) at the center of the otherwise normal galaxy NGC 4552 is further investigated with both HST imaging (FOC) and spectroscopy (FOS). HST/FOC images taken in 1991, 1993, and 1996 in the near UV have been analyzed in a homogeneous fashion, showing that the central spike has brightened by a factor ~4.5 between 1991 and 1993, and has decreased its luminosity by a factor ~2.0 between 1993 and 1996. FOS spectroscopy extending from the near UV to the red side of the optical spectrum reveals a strong UV continuum over the spectrum of the underlying galaxy, along with several emission lines in both the UV and the optical ranges. In spite of the low luminosity of the UV continuum of the spike (~3*10^5 Lsolar), the spike is definitely placed among AGNs by current diagnostics based on the emission line intensity ratios, being just on the borderline between Seyferts and LINERs. Line profiles are very broad, and both permitted and forbidden lines are best modelled with a combination of broad and narrow components, with FWHM of ~3000 km s^-1 and ~700 km s^-1, respectively. This evidence argues for the variable central spike being produced by a modest accretion event onto a central massive black hole (BH), with the accreted material having possibly being stripped from a a star in a close fly by with the BH. The 1996 broad Halpha luminosity of this mini-AGN is ~5.6*10^37 erg s^-1, about a factor of two less than that of the nucleus of NGC 4395, heretofore considered to be the faintest known AGN.

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arXiv1999

Central UV Spikes in two Galactic Spheroids

FOS spectra and FOC photometry of two centrally located, UV-bright spikes in the elliptical galaxy NGC 4552 and the bulge-dominated early spiral NGC 2681, are presented. These spectra reveal that such point-like UV sources detected by means of HST within a relatively large fraction ~15% of bulges can be related to radically different phenomena. While the UV unresolved emission in NGC 4552 represents a transient event likely induced by an accretion event onto a supermassive black hole, the spike seen at the center of NGC 2681 is not variable and it is stellar in nature.

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arXiv1999

Objects in NGC 205 Resolved into Stellar Associations by HST Ultraviolet Imaging

We have obtained high resolution UV images with the HST+WFPC2 of the central region of the dwarf elliptical galaxy NGC 205. Our images reveal that many of the hot UV stars previously detected and studied from the ground are actually multiple systems, open clusters and star associations. We have performed photometry of two such clusters and we find our data are consistent with stellar ages of 50 and 100 Myr, respectively. From the number of massive stars in NGC 205 we estimate that the star formation episode in this galaxy has turned ~1,000 Msolar of gas into stars over the last 100 Myr.

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arXiv1998

Fundamental properties of the new dwarf galaxy And VI - alias Pegasus Dwarf - another companion of M31

We present medium deep CCD imaging in B, V, and I of the Pegasus Dwarf galaxy (And VI) which was recently found by Karachentsev & Karachentseva (1998), and independently also by Armandroff et al. (1999). The Calar Alto 2.2m images show a low surface brightness galaxy. Its structure resembles that of the other known dSph companions of M31 And I, II, III, and V. The brightest stars are resolved in all three colors. Color-magnitude diagrams in either B-V or V-I show the tip of the red giant branch which allows us to estimate a true distance modulus of $24.5 \pm 0.2$. The color-magnitude diagrams and the structure show no evidence for recent star formation, thus, a classification as spheroidal dwarf galaxy with a rather old population seems appropriate. The total absolute magnitude of this dwarf is $M_{V,0} = -10.4\pm0.2$.

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arXiv1998

The UV emission of elliptical galaxies

Recent observational and theoretical developments concerning the origin of the ultraviolet emission from ellipticals are reviewed and discussed,with particular, reference to the extensive set of predictions contained in Greggio and Renzini (1990). Strong support to the preferred hot horizontal branch and AGB-manque' scenario for the origin of the UV emission has come from UIT, HUT and HST data. However, the UV sensitivity of such space telescopes has proved insufficient to unambiguously detect the predicted fading away of the UV rising branch with redshift, that we consider the ultimate check for this scenario.

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