We present two pilot studies for the search and characterization of accretion events in star-forming dwarf galaxies. Our strategy consists of two complementary approaches: i) the direct search for stellar substructures around dwarf galaxies through deep wide-field imaging, and ii) the characterization of the chemical properties in these systems up to large galacto-centric distances. We show our results for two star-forming dwarf galaxies, the starburst irregular NGC 4449, and the extremely metal-poor dwarf DDO 68.

Heliocentric (UVW) and galactocentric (VR VΦ VZ) space motions were derived for 38 RR Lyrae (RRL) and 79 blue horizontal branch (BHB) stars in a 200-sq degree area near the North Galactic Pole (NGP). A kinematic analysis of the 26 RRL and 52 BHB stars whose height (Z) above the plane is < 8 kpc shows that the sample is not homogeneous. Our BHB sample shows zero galactic rotation and roughly isotropic velocity dispersions. whereas the RRL sample shows a definite retrograde rotation and non-isotropic velocity dispersions. The combined BHB and RRL sample shows a smaller retrograde rotation that is similar to that found by Majewski et al. (1996) for a sample of subdwarfs in SA 57 at the NGP. There are significantly more RRL with negative W-velocity (streaming down) than positive W-velocity, whereas the numbers of BHB stars are comparable. This indicates the presence near the NGP of an accreted halo component that is rich in RRL (probably Oosterhoff type I) stars.

These results are presented in detail in a forthcoming paper (Kinman et al. 2007).

The value of the $\alpha$ ratio, the number of PNe per unit bolometric luminosity in a galaxy, is computed using stellar population synthesis models covering the whole range of Hubble types of galaxies.

Model predictions are compared with the PNe counts in the Local Group, which indicate a fairly constant value of $\alpha$ – between 1 and 6 PNe per 10$^7$ solar luminosities – along the Hubble sequence.

New GSC-II proper motions of RR Lyrae and Blue Horizontal Branch (BHB) stars near the North Galactic Pole are used to show that the Galactic Halo 5 kpc above the Plane has a significantly retrograde galactic rotation.

A recognized problem when searching for primeval galaxies at cosmological distances is a definition of a firm selection criterion (alternative to a direct but extremely time-demanding measure of z) to single out high-redshift candidates from the plethora of other more or less peculiar objects (i.e. AGN, starburst galaxies etc.) at lower distances.

The case of the HST “Deep Field” (HDF) observations (Williams et al. 1996) is especially relevant in this regard as galaxies up to z = 3.2 have been detected in the field (Steidel et al. 1996).

A grid of synthetic spectral energy distributions, representative of old stellar populations, has been used to derive colors in different photometric systems, and to compare the theoretical predictions with observational data for galactic globular clusters.

We discuss the results of a cosmological test involving mean colors of the early-type galaxy population in distant clusters for tracing the World model. A global approach to the cosmological problem is attempted deriving the allowed combinations for the fundamental parameters (Ho, qo, λo), and the redshift of galaxy formation (zf).

In this contribution we give a progress report for our work intending to approach in a more complete way the problem of a quantitative calibration of the Mg2 index (Faber et al. 1977, A.J., 82, 941; Buzzoni, Gariboldi & Mantegazza 1991 submitted to A.J.). We have first investigated empirically the relationship between the index and the fundamental parameters for a wide set of Galactic standard stars deriving a detailed calibration for dwarfs and giants. This allowed to build up synthetic models for stellar populations exploring Mg2 in the galaxies with varying overall distinctive parameters of the populations.

In this contribution we present observations for a set of three clusters of galaxies at intermediate redshift (z ~ 0.2) selected from the revised Abell catalog (Abell et al. 1989, Ap.J. Suppl., 70, 1) and observed in the Gunn photometric system at the ESO 3.6m telescope in La Silla (Chile) during various runs between 1986 and 1990. This is part of a more extended project addressing the study of the distant clusters, as outlined in more detail in Molinari et al. (1990, M.N.R.A.S., 246, 576).

A new color-magnitude diagram for M 3 is presented. 10,000 stars have been measured down to V = 22 with an internal accuracy better than 0.03 mag to get complete and very accurate samples over well defined areas.

More than 10,000 stars have been measured down to V = 22 in two different areas. In the first, with 3.5 < r < 6.0 arcmin, photometric completeness has been achieved down to V = 21.5 and an algorithm to correct for losses due to unrecoverable crowding and blending has been experimentally computed. In the second, within a square field of 15 × 15 arcmin, completeness has been extended only to V = 18, well below the horizontal branch.

Many tests made on the data guarantee an internal photometric accuracy better than 0.03 mag at V = 21. Therefore, both the total population of each branch and the relative star-number ratios are “bona fide” representatives of the corresponding evolutionary time-scales. Here we simply present: 1) the color-magnitude diagram (see Fig. 1) obtained from the reduction of a wide collection of Palomar plates; 2) a table which presents the contribution of the various branches to the integrated cluster light; 3) the preliminary indication that, within the annulus we have considered, the blue stragglers seem to be slightly less centrally concentrated than the subgiants in the same magnitude interval.