Large Scale Morphological Segregation in Optically Selected Galaxy Redshift Catalogs

We present the results of an exhaustive analysis of the morphological segregation of galaxies in the CfA and SSRS catalogs through the scaling formalism. Morphological segregation between ellipticals and spirals has been detected at scales up to 15-20 h$^{-1}$ Mpc in the CfA catalog, and up to 20-30 h$^{-1}$ Mpc in the SSRS catalog. Moreover, it is present not only in the densest areas of the galaxy distribution, but also in zones of moderate density.Comment: 9 pages, (1 figure included), uuencode compressed Postscript, (accepted for publication in ApJ Letters), FTUAM-93-2

Shape and kinematics of elliptical galaxies: evolution due to merging at z < 1.5

[EN]Aims. We investigate the evolution in the shape and kinematics of elliptical galaxies in a cosmological framework. Methods. We identified relaxed, elliptical-like objects (ELOs) at redshifts z = 0, z = 0.5, z = 1 and z = 1.5 within a set of hydrodynamic, self-consistent simulations completed for a concordance cosmological model. Results. The population of elliptical systems that we analysed evolve systematically with time becoming rounder in general by z = 0 and also more velocity dispersion supported. We found that this is due primarily to major dry mergers where only a modest amount of angular momentum is involved in the merger event. Despite the general trend, in a significant number of cases the merger event involves a relatively high amount of specific angular momentum, which causes the system in general to acquire higher rotational support and/or a more oblate shape. These evolutionary patterns persist when we study our systems in projection, in simulating true observations, and thus should be evident in future observations.Peer reviewe

A tidal extension in the Ursa Minor dwarf spheroidal galaxy

We report the detection of main-sequence and blue horizontal-branch stars of the Ursa Minor dwarf spheroidal galaxy beyond its tidal radius, indicating the existence of a possible tidal extension in this satellite of the Milky Way. This tidal extension could spread out well beyond the area covered in our survey (R>80 arcmin),as suggested by the presence of a ``break'' to a shallower slope observed in its density profile. The V-band surface brightness for this possible tidal extension range from 29.8 to 31.5 mag arcsec^-2. The area covered in our survey (~1.65 deg^2) is not enough to discriminate if this extra-tidalpopulation is part of a tidal tail or an extended halo around the galaxy. The existence of this tidal extension in Ursa Minor indicates that this satellite is currently undergoing a tidal disruption process by the Milky Way. We discuss the possibility of a tidal origin for the high mass-to-light ratio observed in this galaxy on the basis on our result and recent theoretical simulations of the tidal disruption of dwarf satellites in the Galactic halo.Comment: 13 pages, 3 figures, accepted for publication in Astrophysical Journal Letter

Is the Fast Evolution Scenario for Virialized Compact Groups Really Compelling? The Role of a Dark Massive Group Halo

We report on results of N-body simulations aimed at testing the hypothesis that galaxies in X-ray emitting (i.e., virialized) Compact Groups are not tidally stripped when they are embedded in a common, massive, quiescent dark matter halo. To disentangle the effects of interactions from spurious effects due to an incorrect choice of the initial galaxy model configurations, these have been chosen to be tidally-limited King spheres, representing systems in quasi-equilibrium within the tidal field of the halo. The potential of the halo has been assumed to be frozen and the braking due to dynamical friction neglected. Our results confirm the hypothesis of low rates of tidal stripping and suggest a scenario for virialized Compact Group evolution in their quiescent phases with only very moderate tidally induced galaxy evolution can be generally expected. This implies the group stability, provided that the dynamical friction timescales in these systems are not much shorter than the Hubble time. We discuss briefly this possibility, in particular taking account of the similarity between the velocity dispersions of a typical virialized Compact Groups and the internal velocity dispersion of typical member galaxies. A number of puzzling observational data on Compact Groups can be easily explained in this framework. Other observations would be better understood as the result of enhanced merging activity in the proto-group environment, leading to virialized Compact Group formation through mergers of lower mass halos, as predicted by hierarchical scenarios of structure formation.Comment: 18 pages, 1 postscript file, 2 tables, to be published in ApJLet

Confidence limits of evolutionary synthesis models III. On time-integrated quantities

Evolutionary synthesis models are a fundamental tool to interpret the properties of observed stellar systems. In order to achieve a meaningful comparison between models and real data, it is necessary to calibrate the models themselves, i.e. to evaluate the dispersion due to the discreteness of star formation as well as the possible model errors. In this paper we show that linear interpolations in the log M - log t_k plane, that are customary in the evaluation of isochrones in evolutionary synthesis codes, produce unphysical results. We also show that some of the methods used in the calculation of time-integrated quantities (kinetic energy, and total ejected masses of different elements) may produce unrealistic results. We propose alternative solutions to solve both problems. Moreover, we have quantified the expected dispersion of these quantities due to stochastic effects in stellar populations. As a particular result, we show that the dispersion in the 14N/12C ratio increases with time.Comment: 11 pages, 8 figures, accepted by A&

Radial Velocity of the Phoenix Dwarf Galaxy: Linking Gas and Hi Gas

We present the first radial velocity measurement of the stellar component of the Local Group dwarf galaxy Phoenix, using FORS1 at the VLT UT1 (ANTU) telescope. From the spectra of 31 RGB stars, we derive an heliocentric optical radial velocity of Phoenix Vo=-52 +/- 6 \kms. On the basis of this velocity, and taking into account the results of a series of semi-analytical and numerical simulations, we discuss the possible association of the HI clouds observed in the Phoenix vicinity. We conclude that the characteristics of the HI cloud with heliocentric velocity --23 \kms are consistent with this gas having been associated with Phoenix in the past, and lost by the galaxy after the last event of star formation in the galaxy, about 100 Myr ago. Two possible scenarios are discussed: the ejection of the gas by the energy released by the SNe produced in that last event of star formation, and a ram-pressure stripping scenario. Both in the SNe ejection case and in the ram-pressure sweeping scenario, the distances and relative velocities imply that the HI cloud is not gravitationally bound to Phoenix, since this would require a Phoenix total mass about an order of magnitude larger than its total estimated mass. Finally, we discuss the possibility that Phoenix may be a bound Milky Way satellite. The minimum required mass of the Milky Way for Phoenix to be bound is $M_{MW}(<450 {\rm kpc}) \ge 1.2 \times 10^{12}$ M$_{\odot}$ which comfortably fits within most current estimates.Comment: 17 pages, 8 figures, accepted for publication in Astronomical Journa

A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion onto the Milky Way

We present an analysis of the asymmetries in the population of Galactic M-giant stars present in the 2MASS All Sky catalogue. Several large-scale asymmetries are detected, the most significant of which is a strong elliptical-shaped stellar over-density, close to the Galactic plane at (l=240, b=-8), in the constellation of Canis Major. A small grouping of globular clusters (NGC 1851, NGC 1904, NGC 2298, and NGC 2808), coincident in position and radial velocity, surround this structure, as do a number of open clusters. The population of M-giant stars in this over-density is similar in number to that in the core of the Sagittarius dwarf galaxy. We argue that this object is the likely dwarf galaxy progenitor of the ring-like structure that has recently been found at the edge of the Galactic disk. A numerical study of the tidal disruption of an accreted dwarf galaxy is presented. The simulated debris fits well the extant position, distance and velocity information on the ``Galactic Ring'', as well as that of the M-giant over-densities, suggesting that all these structures are the consequence of a single accretion event. The disrupted dwarf galaxy stream orbits close to the Galactic Plane, with a pericentre at approximately the Solar circle, an orbital eccentricity similar to that of stars in the Galactic thick disk, as well as a vertical scale height similar to that of the thick disk. This finding strongly suggests that the Canis Major dwarf galaxy is a building block of the Galactic thick disk, that the thick disk is continually growing, even up to the present time, and that thick disk globular clusters were accreted onto the Milky Way from dwarf galaxies in co-planar orbits.Comment: 13 pages, 18 figures (2 in colour), accepted for publication in MNRA

Structure in the Velocity Space of Globular Clusters

We present an analysis of the velocity space of a sample of globular clusters (GC) with absolute proper motions. The vertical component of the velocity is found to be correlated with luminosity and galactocentric radius. We divided the sample into two luminosity groups above and below the peak of the luminosity function (LF), MV=-7.5, for Galactic GCs. The two groups display different kinematic behaviour according to the first and second statitical moments of the velocity distribution as well as distinct velocity ellipsoids. The velocity ellipsoid of the high luminosity clusters is aligned with the symmetry axes of the Galaxy, whereas the minor axis of the Low Luminosity group is strongly inclined relative to the Galactic rotation axis.Comment: 4 pages, 2 Poscript figures, uses new aa.cls. Astronomy and Astrophysics (Letters) accepte