CCD photometry in the region of NGC 6994: the remains of an old open cluster

We present the results of BV(RI)_KC CCD photometry down to V=21 mag in the region of NGC 6994. To our knowledge, no photometry has previously been reported for this object and we find evidences that it is a poor and sparse old open cluster, with a minimum angular diameter of 9 arcmin, i.e. larger than the 3 arcmin originally assigned to it. We obtain a color excess E(B-V) = 0.07 +/- 0.02 mag by means of the BVI_(C) technique. Based on the theoretical isochrones from VandenBergh (1985) that are in better agreement with our data, we estimate for this cluster a distance from the Sun of 620 pc (Vo-Mv = 9 +/- 0.25 mag) and an age lying within the range of 2 - 3 Gyr, adopting solar metallicity. Thus, the corresponding cluster's Galactocentric distance is 8.1 kpc and is placed at about 350 pc below the Galactic plane. According to this results, NGC 6994 belongs to the old open cluster population located in the outer disk and at large distances from the Galactic plane, and must have suffered significant individual dynamical evolution, resulting in mass segregation and evaporation of low mass stars.Comment: 10 pages including 11 figures and 1 table. Accepted for publication in Astronomy & Astrophysic

Compact Stellar Systems around NGC 1399

We have obtained spectroscopic redshifts of colour-selected point sources in four wide area VLT-FLAMES fields around the Fornax Cluster giant elliptical galaxy NGC 1399, identifying as cluster members 30 previously unknown faint (-10.5<M_g'<-8.8) compact stellar systems (CSS), and improving redshift accuracy for 23 previously catalogued CSS. By amalgamating our results with CSS from previous 2dF observations and excluding CSS dynamically associated with prominent (non-dwarf) galaxies surrounding NGC 1399, we have isolated 80 `unbound' systems that are either part of NGC 1399's globular cluster (GC) system or intracluster GCs. For these unbound systems, we find (i) they are mostly located off the main stellar locus in colour-colour space; (ii) their projected distribution about NGC 1399 is anisotropic, following the Fornax Cluster galaxy distribution, and there is weak evidence for group rotation about NGC 1399; (iii) their completeness-adjusted radial surface density profile has a slope similar to that of NGC 1399's inner GC system; (iv) their mean heliocentric recessional velocity is between that of NGC 1399's inner GCs and that of the surrounding dwarf galaxies, but their velocity dispersion is significantly lower; (v) bright CSS (M_V<-11) are slightly redder than the fainter systems, suggesting they have higher metallicity; (vi) CSS show no significant trend in $g' - i'$ colour index with radial distance from NGC 1399.Comment: 13 pages (including supplementary table), 13 figures, 5 tables. Accepted for publication in MNRA

Multicolour-metallicity Relations from Globular Clusters in NGC 4486 (M87)

We present Gemini griz photometry for 521 globular cluster (GC) candidates in a 5.5 x 5.5 arcmin field centered 3.8 arcmin to the south and 0.9 arcmin to the west of the center of the giant elliptical galaxy NGC 4486. All these objects have previously published (C-T1) photometry. We also present new (C-T1) photometry for 338 globulars, within 1.7 arcmin in galactocentric radius, which have (g-z) colors in the photometric system adopted by the Virgo Cluster Survey of the Advanced Camera for Surveys of the Hubble Space Telescope. These photometric data are used to define a self-consistent multicolor grid (avoiding polynomial fits) and preliminary calibrated in terms of two chemical abundance scales. The resulting multicolor color-chemical abundance relations are used to test GC chemical abundance distributions. This is accomplished by modelling the ten GC color histograms that can be defined in terms of the Cgriz bands. Our results suggest that the best fit to the GC observed color histograms is consistent with a genuinely bimodal chemical abundance distribution NGC(Z). On the other side, each (blue and red) GC subpopulation follows a distinct color-color relation.Comment: 12 pages, 21 figures, 8 tables. Accepted to be published in MNRA

Dissolving star cluster candidates

We present a list of 34 neglected entries from star cluster catalogues located at relatively high galactic latitudes ($|b| >$ 15$^{\circ}$) which appear to be candidate late stages of star cluster dynamical evolution. Although underpopulated with respect to usual open clusters, they still present a high number density contrast as compared to the galactic field. This was verified by means of (i) predicted model counts from different galactic subsystems in the same direction, and (ii) Guide Star Catalog equal solid angle counts for the object and surrounding fields. This suggests that the objects are physical systems, possibly star clusters in the process of disruption or their fossil remains. The sample will be useful for followup studies in view of verifying their physi cal nature.Comment: manuscript in LATEX with 7 pages, 7 figures .ps Accepted for Astronomy and Astrophysics main journa

A cellular Potts model analyzing differentiated cell behavior during in vivo vascularization of a hypoxic tissue

Angiogenesis, the formation of new blood vessel networks from existing capillary or post-capillary venules, is an intrinsically multiscale process occurring in several physio-pathological conditions. In particular, hypoxic tissue cells activate downstream cascades culminating in the secretion of a wide range of angiogenic factors, including VEGF isoforms. Such diffusive chemicals activate the endothelial cells (ECs) forming the external walls of the nearby vessels that chemotactically migrate toward the hypoxic areas of the tissue as multicellular sprouts. A functional network eventually emerges by further branching and anastomosis processes. We here propose a CPM-based approach reproducing selected features of the angiogenic progression necessary for the reoxygenation of a hypoxic tissue. Our model is able to span the different scale involved in the angiogenic progression as it incorporates reaction-diffusion equations for the description of the evolution of microenvironmental variables in a discrete mesoscopic cellular Potts model (CPM) that reproduces the dynamics of the vascular cells. A key feature of this work is the explicit phenotypic differentiation of the ECs themselves, distinguished in quiescent, stalk and tip. The simulation results allow identifying a set of key mechanisms underlying tissue vascularization. Further, we provide evidence that the nascent pattern is characterized by precise topological properties. Finally, we link abnormal sprouting angiogenesis with alteration in selected cell behavior

Gemini/GMOS Imaging of Globular Cluster Systems in Five Early-type Galaxies

This paper presents deep high quality photometry of globular cluster (GC) systems belonging to five early-type galaxies covering a range of mass and environment. Photometric data were obtained with the Gemini North and Gemini South telescopes in the filter passbands g', r', and i'. The combination of these filters with good seeing conditions allows an excellent separation between GC candidates and unresolved field objects. Bimodal GC colour distributions are found in all five galaxies. Most of the GC systems appear bimodal even in the (g' -r') vs (r' -i') plane. A population of resolved/marginally resolved GC and Ultra Compact Dwarf candidates was found in all the galaxies. A search for the so-called "blue tilt" in the colour-magnitude diagrams reveals that NGC 4649 clearly shows that phenomenon although no conclusive evidence was found for the other galaxies in the sample. This "blue tilt" translates into a mass-metallicity relation given by Z \propto M^0.28\pm0.03 . This dependence was found using a new empirical (g' -i') vs [Z/H] relation which relies on an homogeneous sample of GC colours and metallicities. This paper also explores the radial trends in both colour and surface density for the blue (metal-poor) and red (metal-rich) GC subpopulations. As usual, the red GCs show a steeper radial distribution than the blue ones. Evidence of galactocentric colour gradients is found in some of the GC systems, being more significant for the two S0 galaxies in the sample. Red GC subpopulations show similar colours and gradients to the galaxy halo stars in their inner region. A GC mean colour-galaxy luminosity relation, consistent with [Z/H] \propto L_B ^0.26\pm0.08, is present for the red GCs. An estimate of the total GC populations and specific frequency SN values is presented for NGC 3115, NGC 3379, NGC 3923 and NGC 4649.Comment: 23 pages, 13 figures and 9 tables. Tables A1 and A2 will be published in full online only. Accepted for publication in MNRA

CCD photometry in the region of NGC 6994: the remains of an old open cluster

We present the results of BV(RI)KC CCD photometry down to V=21 mag in the region of NGC 6994. To our knowledge, no photometry has previously been reported for this object and we find evidences that it is a poor and sparse old open cluster, with a minimum angular diameter of 9 arcmin, i.e. larger than the 3 arcmin originally assigned to it. We obtain a color excess EB-V = 0.07 ± 0.02 mag by means of the BVIC technique. Based on the theoretical isochrones from VandenBerg (1985) that are in better agreement with our data, we estimate for this cluster a distance from the Sun of 620 pc (V0 - MV = 9 ± 0.25 mag) and an age lying within the range of 2 - 3 Gyr, adopting solar metallicity. Thus, the corresponding cluster's Galactocentric distance is 8.1 kpc and is placed at about 350 pc below the Galactic plane. According to these results, NGC 6994 belongs to the old open cluster population located in the outer disk and at large distances from the Galactic plane, and must have suffered significant individual dynamical evolution, resulting in mass segregation and evaporation of low mass stars.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

CCD photometry in the region of NGC 6994: the remains of an old open cluster

We present the results of BV(RI)KC CCD photometry down to V=21 mag in the region of NGC 6994. To our knowledge, no photometry has previously been reported for this object and we find evidences that it is a poor and sparse old open cluster, with a minimum angular diameter of 9 arcmin, i.e. larger than the 3 arcmin originally assigned to it. We obtain a color excess EB-V = 0.07 ± 0.02 mag by means of the BVIC technique. Based on the theoretical isochrones from VandenBerg (1985) that are in better agreement with our data, we estimate for this cluster a distance from the Sun of 620 pc (V0 - MV = 9 ± 0.25 mag) and an age lying within the range of 2 - 3 Gyr, adopting solar metallicity. Thus, the corresponding cluster's Galactocentric distance is 8.1 kpc and is placed at about 350 pc below the Galactic plane. According to these results, NGC 6994 belongs to the old open cluster population located in the outer disk and at large distances from the Galactic plane, and must have suffered significant individual dynamical evolution, resulting in mass segregation and evaporation of low mass stars.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

The Nature of UCDs: Internal Dynamics from an Expanded Sample and Homogeneous Database

We have obtained high-resolution spectra of 23 ultra-compact dwarf galaxies (UCDs) in the Fornax cluster with -10.4>M_V>-13.5 mag (10^6<M/M_*<10^8), using FLAMES/Giraffe at the VLT. This is the largest homogeneous data set of UCD internal dynamics assembled to date. We derive dynamical M/L ratios for 15 UCDs covered by HST imaging. In the M_V-sigma plane, UCDs with M_V<-12 mag are consistent with the extrapolated Faber-Jackson relation for luminous ellipticals, while fainter UCDs are closer to the extrapolated globular cluster (GC) relation. At a given metallicity, Fornax UCDs have on average 30-40% lower M/L ratios than Virgo UCDs, suggesting possible differences in age or dark matter content between Fornax and Virgo UCDs. For our sample of Fornax UCDs we find no significant correlation between M/L ratio and mass. We combine our data with available M/L ratio measurements of compact stellar systems with 10^4<M/M_*<10^8, and normalise all M/L estimates to solar metallicity. We find that UCDs (M > 2*10^6 M_*) have M/L ratios twice as large as GCs (M < 2*10^6 M_*). We show that stellar population models tend to under-predict dynamical M/L ratios of UCDs and over-predict those of GCs. Considering the scaling relations of stellar spheroids, UCDs align well along the 'Fundamental Manifold', constituting the small-scale end of the galaxy sequence. The alignment for UCDs is especially clear for r_e >~ 7 pc, which corresponds to dynamical relaxation times that exceed a Hubble time. In contrast, GCs exhibit a broader scatter and do not appear to align along the manifold. We argue that UCDs are the smallest dynamically un-relaxed stellar systems, with M > 2*10^6 M_* and 7<r_e<100 pc. Future studies should aim at explaining the elevated M/L ratios of UCDs and the environmental dependence of their properties.Comment: 17 pages, 14 figures, accepted for publication in A&A. V3 taking into account proof corrections: Table 3 radial velocity entries corrected by heliocentric correction, updated sigma entries in Table 5 for a few CenA sources, updated references for G1 and omega Ce

The nucleus of the Sagittarius dSph galaxy and M54: a window on the process of galaxy nucleation

We present the results of a thorough study of the nucleus of the Sgr dwarf spheroidal galaxy and of the bright globular cluster M54 that resides within the same nucleus (Sgr,N). We have obtained accurate radial velocities and metallicity estimates for 1152 candidate Red Giant Branch stars of Sgr and M54 lying within ~ 9 arcmin from the center of the galaxy, from Keck/DEIMOS and VLT/FLAMES spectra of the infrared Calcium II triplet. Using both velocity and metallicity information we selected two samples of 425 and 321 very-likely members of M54 and of Sgr,N, respectively. The two considered systems display significantly different velocity dispersion profiles: M54 has a steeply decreasing profile from r=0, where sigma= 14.2 km/s, to r=3.5 arcmin where it reaches sigma=5.3 km/s, then it appears to rise again to sigma= 10 km/s at r=7 arcmin. In contrast Sgr,N has a uniformly flat profile at sigma=9.6 km/s over the whole 0 < r < 9 arcmin range. Using data from the literature we show that the velocity dispersion of Sgr remains constant at least out to r ~ 100 arcmin and there is no sign of the transition between the outer flat-luminosity-profile core and the inner nucleus in the velocity profile. These results - together with a re-analysis of the surface brightness profile of Sgr,N and a suite of dedicated N-body simulations - provide very strong support for the hypothesis that the nucleus of Sgr formed independently of M54, which probably plunged to its present position, coincident with Sgr,N, because of significant decay of the original orbit due to dynamical friction.Comment: Accepted for publication by the Astronomical Journal. emulateapj.cls, 26 pag., 21 low resolution figures. A full-resolution color version of the paper can be retrieved from http://www.bo.astro.it/SGR/Sgr_nucleus.ps.g