arXiv subjects

C. Chiosi

Publications and source records attributed to C. Chiosi.

62 records · Page 4Linked to original sources

arXiv1997

To what Extent are Mg2 and indicators of Mg and Fe Abundances?

The gradients in line strength indices such as Mg2 and (Carollo & Danziger 1994a,b; Carollo et al. 1993), and their increase toward massive system (Worthey et al. 1992, Faber et al. 1992, Davies et al. 1993, Weiss et al. 1995) observed in elliptical galaxies are customarily considered as indicators of gradients in the chemical abundances of Mg and Fe and arguments are given for an enhancement of Mg ($α$-elements in general) with respect to Fe toward the center of these galaxies or going from dwarf to massive ellipticals. In this paper, we present a detailed analysis of the whole problem aimed at understanding how the indices Mg2 and depend on the the chemical abundances and age of the stellar mix of a galaxy. We find that Mg2 and do not simply correlate with the abundances of Mg and Fe and the ratio Mg/Fe, but depend also on the underlaying distribution of stars in different metallicity bins N(Z), or equivalently on the past history of star formation. In practice, inferring the abundance of Mg and Fe and their ratio is hampered by the need of some information about N(Z). Finally, the observational gradients in Mg2 and do not automatically imply gradients in chemical abundances.

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arXiv1997

A new scenario of galaxy evolution under a universal IMF

In this paper, basic observational properties of elliptical galaxies such as the integrated spectra, the chemical abundances and the enhancement of $α$-elements inferred from broad-band colors and line strength indices $Mg_2$ and $< Fe >$ (and their gradients), the color-magnitude relation, the UV fluxes, and the mass-luminosity ratios, are examined in the light of current theoretical interpretations, and attention is called on several points of internal contradiction. Indeed existing models for the formation and evolution of elliptical galaxies are not able to simultaneously account for all of the above observational features. We suggest that the new initial mass function (IMF) by Padoan et al. (1997), which depends on the temperature, density, and velocity dispersion of the medium in which stars are formed, may constitute a break-through in the difficulties in question. Models of elliptical galaxies incorporating the new IMF (varying with time and position inside a galaxy) are presented and discussed at some extent. As a result of the changing IMF, the enhancement of $α$-elements and tilt of the Fundamental Plane are easily explained leaving unaltered the interpretation of the remaining properties under examination. Finally, some implications concerning the relative proportions of visible stars, collapsed remnants (baryonic dark matter), and gas left over by the star forming process are examined.

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arXiv1997

Diagnostic of chemical abundances and ages from the line strength indices Hb, , and Mg2: what do they really imply ?

The line strength indices Hb, Mg2, and , together with their gradients and dependence on the galaxy luminosity (mass), observed in elliptical galaxies are customarily considered as reliable indicators of systematic differences in age and abundances of Mg and Fe. In this paper, we address the question whether or not the indices Mg2 and (and their gradients) are real indicators of chemical abundances and enhancement of these or other effects must be taken into account. We show that this is not the case because the above indices are severely affected by the unknown relative percentage of stars as function of the metallicity. In order to cast the problem, first we provide basic calibrations for the variations dHb, dMg2, and d as a function of the age Dlog(t) (in Gyr), metallicity Dlog(Z/Z_o), and D[Mg/Fe]. Second, we analyze the implications of the gradients in Mg2 and observed across these systems. Finally, the above calibration is used to explore the variations from galaxy to galaxy of the nuclear values of Hb, Mg2, and . The differences dHb, dMg2, and d are converted to differences Dlog(t), Dlog(Z/Z_o), and D[Mg/Fe]. We thoroughly examine the relationship Dlog(t)-Mv, Dlog(Z/Z_o)-Mv, and D[Mg/Fe]-Mv, and advance the suggestion that the duration of the star forming period gets longer or the age of the last episode of stellar activity gets closer to the present at decreasing galaxy mass. This result is discussed in the context of current theories of galaxy formation and evolution. i.e. merger and isolation. In brief, we conclude that none of these can explain the results of our analysis, and suggest that the kind of time and space dependent IMF proposed by Padoan et al. (1997) and the associated models of elliptical galaxies elaborated by Chiosi et al. (1997) should be at work.

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arXiv1997

Model color-magnitude diagrams for Hubble Space Telescope observations of Local Group dwarf galaxies

In this paper, we discuss a method to conduct a quantitative study of the star formation history (SFH) of Local Group (LG) galaxies using (HST) data. This method has proven to be successful in the analysis of the SFH of the same kind of galaxies using ground-based observations. It is based on the comparison of observed CMDs with a set of model CMDs. The latter are computed assuming different evolutionary scenarios, and include a detailed simulation of observational effects. HST CMDs are ~3 mags deeper than typical ground-based CMDs, allowing the observation, for all LG galaxies, of a part of the CMD that up till now had remained accessible only for the very nearest galaxies. A very important feature that will become accessible is the HB+red-clump. The distribution of stars along this structure is quite sensitive to age and metallicity and should provide a very important improvement in the time resolution of the SFH for stars older than ~2-3 Gyr. We show and discuss four model CMDs which would be comparable with CMDs from deep HST observations. These model CMDs represent the following evolutionary scenarios corresponding to a wide range of dwarf galaxy sub-types from dI to dE: A) a constant SFR from 15Gyr ago to the present time; B) as A), but with the SFR stopped 0.5 Gyr ago; C) a constant SFR in the age range 10-9Gyr and D) as C) but in the age range 15-12 Gyr. In all four cases a range of metallicity from Z=0.0001 to Z=0.004 has been assumed. The present analysis is just a first qualitative approach to what one may expect to find in the CMDs of LG galaxies. However a complete set of model CMDs must be computed to analize the data for each galaxy, using the crowding effects derived for that particular galaxy.

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arXiv1997

Probing the Galaxy I. The galactic structure towards the galactic pole

Observations of (B-V) colour distributions towards the galactic poles are compared with those obtained from synthetic colour-magnitude diagrams to determine the major constituents in the disc and spheroid. The disc is described with four stellar sub-populations: the young, intermediate, old, and thick disc populations, which have respectively scale heights of 100 pc, 250 pc, 0.5 kpc, and 1.0 kpc. The spheroid is described with stellar contributions from the bulge and halo. The bulge is not well constrained with the data analyzed in this study. A non-flattened power-law describes the observed distributions at fainter magnitudes better than a deprojected R^{1/4}-law. Details about the age, metallicity, and normalizations are listed in Table 1. The star counts and the colour distributions from the stars in the intermediate fields towards the galactic anti-centre are well described with the stellar populations mentioned above. Arguments are given that the actual solar offset is about 15 pc north from the galactic plane.

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arXiv1996

Probing the Age of Elliptical Galaxies

In this paper we address the question whether age and metallicity effects can be disentangled with the aid of the broad-band colours and spectral indices from absorption feature strengths, so that the age of elliptical galaxies can be inferred. The observational data under examination are the indices Hbeta and [MgFe], and the velocity dispersion Sigma for the sample of galaxies of Gonzales (1993), supplemented by the ultra-violet data, i.e. the colour (1550-V), of Burstein et al. (1988). The analysis is performed with the aid of chemo-spectro-photometric models of elliptical galaxies with infall of primordial gas (aimed at simulating the collapse phase of galaxy formation) and the occurrence of galactic winds. The galaxy models are from Tantalo et al. (1995).

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arXiv1996

Spectro-photometric Evolution of Elliptical Galaxies. II. Models with infall

In this paper we present new chemo-spectro-photometric models of elliptical galaxies in which infall of primordial gas is allowed to occur. They aim to simulate the collapse of a galaxy made of two components, i.e. luminous material and dark matter. The mass of the dark component is assumed to be constant in time, whereas that of the luminous material is supposed to accrete at a suitable rate. They also include the effect of galactic winds powered by supernova explosions and stellar winds from massive, early-type stars. The models are constrained to match a number of properties of elliptical galaxies, i.e. the slope and mean colours of the colour-magnitude relation (CMR), V versus (V--K), the UV excess as measured by the colour (1550--V) together with the overall shape of the integrated spectral energy distribution (ISED) in the ultraviolet, the relation between the Mg2 index and (1550--V), the mass to blue luminosity ratio M/Lb as a function of the B luminosity, and finally the broad-band colours (U--B), (B--V), (V--I), (V--K), etc.

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arXiv1995

Bright Stars and Recent Star Formation in the Irregular Magellanic Galaxy NGC2366

The stellar content of the Im galaxy NGC 2366 is discussed on the basis of CCD BVR photometry. The three brightest blue and red stars have been used to estimate its distance, obtaining a balue of 2.9 Mpc. The spatial distribution of the young stellar population is discussed in the light of the integrated color indices and the color-magnitude diagrams of different zones of the galaxy. A generalized star formation burst seems to have taken place about 50 Myr ago. The youngest stars are preferentially formed in the South-West part of the bar, where the giant HII complex NGC 2363 is located, being younger and bluer. The bar seems to play a role favouring star formation in one of its extremes. Self-propagation however, does not seem to be triggering star formation at large scale. A small region, populated by very young stars has also been found at the East of the galaxy.

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