The Lick-index spectrophotometric system is investigated in its inherent statistical and operational properties to ease a more appropriate use for astrophysical studies. Non-Gaussian effects in the index standardization procedure suggest that a minim
um S/N ratio has to be reached by spectral data, such as S/N >= 5 px^{-1} for a spectral resolution R~2000. In addition, index (re-)definition in terms of narrow-band color should be preferred over the classical pseudo-equivalent width scheme. The overlapping wavelength range among different indices is also an issue, as it may lead the latter ones to correlate, beyond any strictly physical relationship. The nested configuration of the Fe5335, Fe5270 indices, and the so-called Mg complex (including Mg1, Mg2 and Mgb) is analysed, in this context, by assessing the implied bias when joining entangled features into global diagnostic meta-indices, like the perused [MgFe] metallicity tracer. The perturbing effect of [OIII](5007) and [NI](5199) forbidden gas emission on Fe5015 and Mgb absorption features is considered, and an updated correction scheme is proposed when using [OIII](5007) as a proxy to appraise Hbeta residual emission. When applied to present-day elliptical galaxy population, the revised Hbeta scale leads, on average, to 20-30% younger age estimates. Finally, the misleading role of the christening element in Lick-based chemical analyses is illustrated for the striking case of Fe4531. In fact, while Iron is nominally the main contributor to the observed feature in high-resolution spectra, we have shown that the Fe4531 index actually maximizes its responsiveness to Titanium abundance.
We report on a UV-oriented imaging survey in the fields of the old, metal-rich open clusters, NGC 6791, NGC 6819 and NGC 7142. These three clusters represent both very near and ideal stellar aggregates to match the distinctive properties of the evolv
ed stellar populations, as in elliptical galaxies and bulges of spirals. The CMD of the three clusters is analyzed in detail, with special emphasis to the hot stellar component. We report, in this regard, one new extreme horizontal-branch star candidate in NGC 6791. For NGC 6819 and 7142, the stellar luminosity function points to a looser radial distribution of faint lower Main Sequence stars, either as a consequence of cluster dynamical interaction with the Galaxy or as an effect of an increasing fraction of binary stars toward the cluster core, as actually observed in NGC 6791 too.
We use deep, high quality colour magnitude diagrams obtained with the Hubble Space Telescope to compute a simplified version of the Mironov index [SMI; B/(B+R)] to parametrize the horizontal branch (HB) morphology for 23 globular clusters in the M31
galaxy (Sample-A), all located in the outer halo at projected distances between 10 kpc and 100 kpc. This allows us to compare them with their Galactic counterparts, for which we estimated the SMI exactly in the same way, in the SMI vs. [Fe/H] plane. We find that the majority of the considered M31 clusters lie in a significantly different locus, in this plane, with respect to Galactic clusters lying at any distance from the center of the Milky Way. In particular they have redder HB morphologies at a given metallicity, or, in other words, clusters with the same SMI value are ~0.4 dex more metal rich in the Milky Way than in M31. We discuss the possible origin of this difference and we conclude that the most likely explanation is that many globular clusters in the outer halo of M31 formed ~1-2 Gyr later than their counterparts in the outer halo of the Milky Way, while differences in the cluster-to-cluster distribution of He abundance of individual stars may also play a role. The analysis of another sample of 25 bright M31 clusters (eighteen of them with M_V<= -9.0, Sample-B), whose SMI estimates are much more uncertain as they are computed on shallow colour magnitude diagrams, suggests that extended blue HB tails can be relatively frequent among the most massive M31 globular clusters, possibly hinting at the presence of multiple populations.
The evolutionary properties of the old metal-rich Galactic open cluster NGC6791 are assessed, based on deep UB photometry and 2Mass JK data. For 4739 stars in the cluster, bolometric luminosity and effective temperature have been derived from theoret
ical (U-B) and (J-K) color fitting. The derived H-R diagram has been matched with the UVBLUE grid of synthetic stellar spectra to obtain the integrated SED of the system, together with a full set UV (Fanelli) and optical (Lick) narrow-band indices. The cluster appears to be a fairly good proxy of standard elliptical galaxies, although with significantly bluer infrared colors, a shallower 4000A Balmer break, and a lower Mg2 index. The confirmed presence of a dozen hot stars, along their EHB evolution, leads the cluster SED to consistently match the properties of the most active UV-upturn galaxies, with 1.7+/-0.4% of the total bolometric luminosity emitted shortward of 2500A. The cluster Helium abundance results Y=0.30 +/-0.04, while the Post-MS implied stellar lifetime from star number counts fairly agrees with the theoretical expectations from both the Padova and BASTI stellar tracks. A Post-MS fuel consumption of 0.43 +/- 0.01 M_sun is found for NGC6791 stars, in close agreement with the estimated mass of cluster He-rich white dwarfs. Such a tight figure may lead to suspect that a fraction of the cluster stellar population does actually not reach the minimum mass required to effectively ignite He in the stellar core.
With this third paper of a series we present Johnson-Gunn B,g,V,r,i,z multicolour photometry for 79 objects, including a significant fraction of the faintest galaxies around NGC5044, assessing group membership on the basis of apparent morphology and
low-resolution optical spectroscopy to estimate redshift for 21 objects. Together, dEs and Ims provide the bulk of the galaxy luminosity function, around M(g)sim-18.0, while the S0 and dSph components dominate, respectively, the bright and faint-end tails of the distribution. This special mix places the NGC 5044 group just midway between the high-density cosmic aggregation scale typical of galaxy clusters, and the low-density environment of looser galaxy clumps like our Local Group. The bright mass of the 136 member galaxies with available photometry and morphological classification, amounts to a total of 2.3x10^{12}M_sun while current SFR within the group turns to be about or higher than 23M_sun/yr. In this regard, a drift toward bluer integrated colours is found to be an issue for dEs pointing to a moderate but pervasive star-formation activity even among nominally quiescent stellar systems. Through Lick narrow-band index analysis, dwarf ellipticals are found to share a sub-solar metallicity (-1.0 < [Fe/H] <-0.5) with a clear decoupling between Iron and alpha elements, as already established also for high-mass systems. Both dEs and dS0s are consistent with an old age, about one Hubble time, although a possible bias, toward higher values of age, may be induced by the gas emission affecting the Hbeta strength.
The problem of chemo-photometric evolution of late-type galaxies is dealt with relying on prime physical arguments of energetic self-consistency between chemical enhancement of galaxy mass, through nuclear processing inside stars, and luminosity evol
ution of the system. Chemical enhancement is assessed in terms of the so-called yield metallicity, that is the metal abundance of processed mass inside stars, as constrained by the galaxy photometric history.
We have investigated the relevant trend of the bolometric correction (BC) at the cool-temperature regime of red giant stars and its possible dependence on stellar metallicity. Our analysis relies on a wide sample of optical-infrared spectroscopic obs
ervations, along the 3500A-2.5micron wavelength range, for a grid of 92 red giant stars in five (3 globular + 2 open) Galactic clusters, along the -2.2<[Fe/H]<+0.4 metallicity range. Bolometric magnitudes have been found within an internal accuracy of a few percent. Making use of our new database, we provide a set of fitting functions for the V and K BC vs. Teff and broad-band colors, valid over the interval 3300K<Teff<5000K, especially suited for Red Giants. No evident drift of both BC(V) and BC(K) with [Fe/H] is found. Things may be different, however, for the B-band correction, given a clear (B-V) vs. [Fe/H] correlation in place for our data, with metal-poor stars displaying a bluer (B-V) with respect to the metal-rich sample, for fixed Teff. Our empirical bolometric scale supports the conclusion that (a) BC(K) from the most recent studies are reliable within <0.1 mag over the whole color/temperature range considered in this paper, and (b) the same conclusion apply to BC(V) only for stars warmer than ~3800K. At cooler temperatures the agreement is less general, and MARCS models are the only ones providing a satisfactory match to observations.
We present a new homogeneous set of metallicity estimates based on Lick indices for 245 old globular clusters of the M31 galaxy comprised in the Revised Bologna Catalog. The metallicity distribution of the M31 globular clusters is briefly discussed a
nd compared with that of the Milky Way. Simple parametric statistics suggests that the [Fe/H] distribution is likely not unimodal. The strong correlation between metallicity and kinematics found in previous studies is confirmed. The most metal-rich GCs tend to be packed at the center of the system and share the galactic rotation as traced by the HI disk. Although the velocity dispersion around the curve increases with decreasing metallicity, also clusters with [Fe/H]<-1.0 display a clear rotational pattern, at odds with their Milky Way counterparts.
We outline a new method to derive a snapshot metallicity estimate of stellar systems (providing one resolves at least the brightest part of the CMD) just on the basis of low-resolution (i.e., 6-8A FWHM) spectroscopy of a small stellar sample. Our met
hod relies on the Fe5270 Lick index measurements and takes advantage of the special behavior of this spectral feature, that reaches its maximum strength among the ubiquitous component of K-type giants. This makes the Fe5270(max} estimate a robust and model-independent tracer of cluster [Fe/H], being particularly insensitive to the age of the stellar population. A comparison of the Fe5270(max) distribution derived from globular and open clusters, as well as from the field giant population in the Galaxy disk, confirms a tight correlation of the index maximum vs. cluster [Fe/H] allover the entire metallicity range for stellar population with [Fe/H] >~ -2.0. Relying on a theoretical calibration of the feature, we trust to effectively infer cluster metallicity within a typical uncertainty of 0.1-0.2dex, depending on RGB luminosity sampling of the observations.
Relying on infrared surface brightness fluctuactions to trace AGB properties in a sample of elliptical galaxies in the Virgo and Fornax clusters, we assess the puzzling origin of the UV-upturn phenomenon, recently traced down to the presence of a hot
horizontal branch stellar component. We find that the UV-upturn actually signals a profound change in the c-m diagram of stellar populations in elliptical galaxies, involving both the hot stellar component and red-giant evolution.
