dSph Galaxies Research Articles

VLT Spectroscopy of RR Lyrae Stars in the Sagittarius Tidal Stream

Sixteen RR Lyrae variables from the QUEST survey that lie in the leading arm of the tidal stream from the Sagittarius dSph galaxy have been observed spectroscopically to measure their radial velocities and metal abundances. The systemic velocities of 14 stars, which were determined by fitting a standard velocity curve to the individual measurements, have a sharply peaked distribution with a mean of 33 km s-1 and a standard deviation of only 25 km s-1. The [Fe/H] distribution of these stars has a mean of -1.76 and a standard deviation of 0.22. These measurements are in good agreement with previous ones from smaller samples of stars. The mean metallicity is consistent with the age-metallicity relation that is observed in the main body of the Sgr dSph galaxy. The radial velocities and the distances from the Sun of these stars are compared with recent numerical simulations of the Sgr streams that assume different shapes for the dark matter halo. Models that assume a oblate halo do not fit the data as well as ones that assume a spherical or a prolate distribution. However, none of the fits are completely satisfactory. Every model fails to reproduce the long extent of the stream in right ascension (36°) that is seen in the region covered by the QUEST survey. Further modeling is required to see if this and the other mismatches between theory and observation can be removed by judicial choices for the model parameters or instead rule out a class of models.

A Second Shell in the Fornax dSph Galaxy

AbstractIn the search for tidal structure in Galactic satellite systems, we have conducted a photometric survey over a 10 deg2 area centred on the Fornax dSph galaxy. The survey was made in two colours, and the resulting colour–magnitude data were used as a mask to select candidate Fornax RGB stars, thereby increasing the contrast of Fornax stars to background sources in the outer regions. Previously, we reported the presence of a shell (age 2 Gyr) located towards the centre of Fornax. In this contribution we reveal a second shell, significantly larger than the first, located 1.3° north-west from the centre of Fornax, outside the nominal tidal radius. Moreover, the distribution of Fornax RGB stars reveals two lobes extending to the spatial limit of our survey, and aligned with the minor axis and with the two shells. These results support the hypothesis of a merger between Fornax and a gas-rich companion approximately 2 Gyr ago.

Subaru/HDS Abundances in Three Giant Stars in the Ursa Minor Dwarf Spheroidal Galaxy

Abstract With the HDS (High Dispersion Spectrograph) on the Subaru Telescope, we obtained high-resolution optical region spectra of three red giant stars (COS 4, COS 82, and COS 347) in the Ursa Minor dwarf spheroidal galaxy. The chemical abundances in these stars were analyzed for 26 elements, including $\alpha$-, iron-peak, and neutron-capture elements. All three stars show low abundances of $\alpha$-elements (Mg, Si, and Ca), and two stars (COS 82 and COS 347) show high abundances of Mn compared to Galactic halo stars of similar metallicity. One star (COS 4) has been confirmed to be very metal deficient ($\mathrm{[Fe/H]} =-2.7$) and found to show anomalously low abundances of Mn, Cu, and Ba. In another star, COS 82 ($\mathrm{[Fe/H]} =-1.5$), we have found a large excess of heavy neutron-capture elements with a general abundance pattern similar to the scaled solar system $r$-process abundance curve. These observational results are rather puzzling: low abundances of $\alpha$-elements and high abundance of Mn seem to suggest a significant contribution of SNe Ia at low metallicity, while there is no hint of an $s$-process (i.e., AGB stars) contribution, even at $\mathrm{[Fe/H]} =-1.5$, suggesting a peculiar nucleosynthetic history of the UMi dSph galaxy.

Stellar Chemical Signatures and Hierarchical Galaxy Formation

To compare the chemistries of stars in the Milky Way dwarf spheroidal (dSph) satellite galaxies with stars in the Galaxy, we have compiled a large sample of Galactic stellar abundances from the literature. When kinematic information is available, we have assigned the stars to standard Galactic components through Bayesian classification based on Gaussian velocity ellipsoids. As found in previous studies, the [α/Fe] ratios of most stars in the dSph galaxies are generally lower than similar metallicity Galactic stars in this extended sample. Our kinematically selected stars confirm this for the Galactic halo, thin-disk, and thick-disk components. There is marginal overlap in the low [α/Fe] ratios between dSph stars and Galactic halo stars on extreme retrograde orbits (V < -420 km s-1), but this is not supported by other element ratios. Other element ratios compared in this paper include r- and s-process abundances, where we find a significant offset in the [Y/Fe] ratios, which results in a large overabundance in [Ba/Y] in most dSph stars compared with Galactic stars. Thus, the chemical signatures of most of the dSph stars are distinct from the stars in each of the kinematic components of the Galaxy. This result rules out continuous merging of low-mass galaxies similar to these dSph satellites during the formation of the Galaxy. However, we do not rule out very early merging of low-mass dwarf galaxies, since up to one-half of the most metal-poor stars ([Fe/H] ≤ -1.8) have chemistries that are in fair agreement with Galactic halo stars. We also do not rule out merging with higher mass galaxies, although we note that the LMC and the remnants of the Sgr dwarf galaxy are also chemically distinct from the majority of the Galactic halo stars. Formation of the Galaxy's thick disk by heating of an old thin disk during a merger is also not ruled out; however, the Galaxy's thick disk itself cannot be comprised of the remnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarf galaxy like the LMC or Sgr, because of differences in chemistry.

The predicted metallicity distribution of stars in dwarf spheroidal galaxies

We predict the metallicity distribution of stars and the age-metallicity relation for six dwarf spheroidal (dSph) galaxies of the Local Group by means of a chemical evolution model that is able to reproduce several observed abundance ratios, and the present-day total mass and gas content of these galaxies. The model adopts up-to-date nucleosynthesis and takes into account the role played by supernovae of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca and Fe). Each galaxy model is specified by the prescriptions of the star formation rate and by the galactic wind efficiency chosen to reproduce the main features of these galaxies. These quantities are constrained by the star formation histories of the galaxies as inferred by the observed colour-magnitude diagrams (CMD). The main conclusions are: (i) five of the six dSph galaxies are characterized by very low star formation efficiencies (v = 0.005-0.5 Gyr -1 ) with only Sagittarius having a higher one (ν = 1.0-5.0 Gyr -1 ); (ii) the wind rate is proportional to the star formation rate and the wind efficiency is high for all galaxies, in the range w i = 6-15; (iii) a high wind efficiency is required in order to reproduce the abundance ratios and the present-day gas mass of the galaxies; (iv) the predicted age-metallicity relation implies that the stars of the dSphs reach solar metallicities in a time-scale of the order of 2-6 Gyr, depending on the particular galaxy; (v) the metallicity distributions of stars in dSphs exhibit a peak around [Fe/H] ∼ -1.8 to -1.5 dex, with the exception of Sagittarius, which shows a peak around [Fe/H] ∼ -0.8 dex; (iv) the predicted metallicity distributions of stars suggest that the majority of stars in dSphs are formed in a range of metallicity in agreement with the one of the observed stars.

The predicted metallicity distribution of stars in dwarf spheroidal galaxies

We predict the metallicity distribution of stars for 6 Dwarf Spheroidal (dSph) galaxies of the Local Group by means of a chemical evolution model which is able to reproduce several observed abundance ratios and the present day total mass and gas content of these galaxies. The model adopts up to date nucleosynthesis and takes into account the role played by supernovae of different types (II, Ia). Each galaxy model is specified by the prescriptions of the star formation rate and by the galactic wind efficiency chosen to reproduce the main features of these galaxies. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Shell Structure in the Fornax Dwarf Spheroidal Galaxy

We present initial results from a wide-field photometric survey of the Fornax dwarf spheroidal (dSph) galaxy. The focus here is on a small overdensity of stars located approximately 17' (670 pc in projection) from the center of Fornax. Based on imaging data in both V and I bands down to V ≈ 21, the dimensions of the feature are ~17 × 32 (68 × 128 pc) with an average surface brightness of 25.8 mag arcsec-2 in V. A follow-up analysis using the deep B- and R-band data obtained by Stetson, Hesser, & Smecker-Hane indicates that the overdensity is apparently dominated by a relatively young stellar population with an age of ≈2 Gyr, though the current data do not rule out the presence of older stars. Our preferred interpretation is that the overdensity represents shell structure in the Fornax dSph galaxy, a phenomenon previously unseen in dwarf galaxies. The shell may be the remnant of a merger with Fornax of a smaller, gas-rich system that occurred approximately 2 Gyr ago. The comparatively recent star formation seen in some dSph and dE galaxies may then also be the result of infall of small relatively gas-rich systems.

Mass Stripping in Dwarf Spheroidal Galaxies and ω Cen

The stellar abundance pattern of neutron-capture elements such as Ba is used as a powerful tool to infer how star formation proceeded in dwarf spheroidal (dSph) galaxies. It is found that the abundance correlation of Ba with Fe in stars belonging to dSph galaxies have a feature similar to the Ba-Fe correlation in Galactic metal-poor stars. The common feature of these two correlations implies that dSph stars formed from gas with a velocity dispersion of ~26 km s−1. This velocity dispersion together with the stellar luminosities strongly suggests that dark matter dominated dSph galaxies. The tidal force of the Milky Way links this velocity dispersion with the currently observed value ≤ 10 km s−1 by stripping the dark matter in dSph galaxies. We also investigate the ram pressure exerted on the gas in ω Cen. It is found that the ram pressure is not strong enough to strip the gas but is expected to induce a bulk motion of the gas during the star formation epoch which is compatible with the recent observed finding and then the remaining gas after the star formation is likely to be stripped due to the gradual increase in the gas density in the forming Galactic disk.

Scaling Laws for Dark Matter Halos in Late-Type and Dwarf Spheroidal Galaxies

Published mass models fitted to galaxy rotation curves are used to study the systematic properties of dark matter (DM) halos in late-type and dwarf spheroidal (dSph) galaxies. Halo parameters are derived by fitting non-singular isothermals to (V2 – V2vis)1/2, where V(r) is the observed rotation curve and Vvis is the rotation curve of the visible matter. the latter is calculated from the surface brightness assuming that the mass-to-light ratio M/L is constant with radius. “Maximum disk” values of M/L are adjusted to fit as much of the inner rotation curve as possible without making the halo have a hollow core. Rotation curve decomposition becomes impossible fainter than absolute magnitude Mb ≃ −14, where V becomes comparable to the velocity dispersion of the gas. To increase the luminosity range further, we include dSph galaxies, which are physically related to spiral and irregular galaxies. Combining the data, we find that DM halos satisfy well defined scaling laws analogous to the “fundamental plane” relations for elliptical galaxies. Halos in less luminous galaxies have smaller core radii rc, higher central densities ρ0, and smaller central velocity dispersions σ. Scaling laws provide new and detailed constraints on the nature of DM and on galaxy formation and evolution. Some simple implications include:1 – A single, continuous physical sequence of increasing mass extends from dSph galaxies with Mb ≃ −7.6 to Sc I galaxies with Mb ≃ −22.4.2 – the high DM densities in dSph galaxies are normal for such tiny galaxies. Since virialised density depends on collapse redshift zcoll, ρ0 ∝ (1 + zcoll)3, the smallest dwarfs formed at least Δzcoll ≃ 7 earlier than the biggest spirals.3 – the high DM densities of dSphs implies that they are real galaxies formed from primordial density fluctuations. They are not tidal fragments. Tidal dwarfs cannot retain even the low DM densities of their giant-galaxy progenitors. in contrast, dSphs have higher DM densities than do giant-galaxy progenitors.4 – the fact that, as luminosity decreases, dwarf galaxies become much more numerous and also more nearly dominated by DM raises the possibility that there exists a large population of objects that are completely dark. Such objects are a canonical prediction of cold DM theory. If they exist, “empty halos” are likely to be small and dense -that is, darker versions of Draco and UMi.5 – the slopes of the DM parameter correlations provide a measure on galactic mass scales of the slope n of the power spectrum |δk|2 ∝ kn of primordial density fluctuations. Our preliminary results, not yet corrected for baryonic compression of DM, give n ≃ –1.9 ± 0.2. This is consistent with cold DM theory.

A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. I. Morphology of the Sagittarius Core and Tidal Arms

We present the first all-sky view of the Sagittarius (Sgr) dwarf galaxy mapped by M-giant star tracers detected in the complete Two Micron All Sky Survey (2MASS). Near-infrared photometry of Sgr's prominent M-giant population permits an unprecedentedly clear view of the center of Sgr. The main body is fitted with a King profile of limiting major-axis radius 30°—substantially larger than previously found or assumed—beyond which is a prominent break in the density profile from stars in the Sgr tidal tails; thus the Sgr radial profile resembles that of Galactic dwarf speroidal (dSph) satellites. Adopting traditional methods for analyzing dSph light profiles, we determine the brightness of the main body of Sgr to be MV = -13.27 (the brightest of the known Galactic dSph galaxies) and the total Sgr mass-to-light ratio to be 25 in solar units. However, we regard the latter result with suspicion and argue that much of the observed structure beyond the King-fit core radius (224') may be outside the actual Sgr tidal radius as the former dwarf spiral/irregular satellite undergoes catastrophic disruption during its last orbits. The M-giant distribution of Sgr exhibits a central density cusp at the same location as, but not due to, the old stars constituting the globular cluster M54. A striking trailing tidal tail is found to extend from the Sgr center and arc across the south Galactic hemisphere with approximately constant density and mean distance varying from ~20 to 40 kpc. A prominent leading debris arm extends from the Sgr center northward of the Galactic plane to an apogalacticon ~45 kpc from the Sun and then turns toward the north Galactic cap (NGC), from where it descends back toward the Galactic plane, becomes foreshortened, and, at brighter magnitudes, covers the NGC. The leading and trailing Sgr tails lie along a well-defined orbital plane about the Galactic center. The Sun lies within a kiloparsec of that plane and near the path of leading Sgr debris; thus, it is possible that former Sgr stars are near or in the solar neighborhood. We discuss the implications of this new view of the Sgr galaxy and its entrails for the character of the Sgr orbit, mass, mass-loss rate, and contribution of stars to the Milky Way halo. The minimal precession displayed by the Sgr tidal debris along its inclined orbit supports the notion of a nearly spherical Galactic potential. The number of M giants in the Sgr tails is at least 15% that contained within the King limiting radius of the main Sgr body. The fact that M giants, presumably formed within the past few gigayears in the Sgr nucleus, are nevertheless so widespread along the Sgr tidal arms not only places limits on the dynamical age of these arms but also poses a timing problem that bears on the recent binding energy of the Sgr core and that is most naturally explained by recent and catastrophic mass loss. Sgr appears to contribute more than 75% of the high-latitude, halo M giants, despite substantial reservoirs of M giants in the Magellanic Clouds. No evidence of extended M-giant tidal debris from the Magellanic Clouds is found. Generally good correspondence is found between the M-giant, all-sky map of the Sgr system and all previously published detections of potential Sgr debris, with the exception of Sgr carbon stars, which must be subluminous compared with counterparts in other Galactic satellites in order to resolve the discrepancy.

Palomar 12 as a Part of the Sagittarius Stream: The Evidence from Abundance Ratios

We present a detailed abundance analysis for 21 elements based on high-dispersion, high spectral resolution Keck spectra for four members of the outer halo "young" Galactic globular cluster Palomar 12. All four stars show identical abundance distributions with no credible indication of any star-to-star scatter. However, the abundance ratios of the Pal 12 stars are very peculiar. There is no detected enhancement of the α-elements; the mean of [Si/Fe], [Ca/Fe], and [Ti/Fe] is -0.07 ± 0.05 dex, O/Fe is also solar, while Na is very deficient. The distribution among the heavy elements shows anomalies as well. These are inconsistent with those of almost all Galactic globular clusters or of field stars in the Galaxy. The peculiarities shown by the Pal 12 stars are, however, in good general agreement with the trends established by Smecker-Hane & McWilliam and by Bonifacio et al. for stars in the Sagittarius dwarf spheroidal (dSph) galaxy evaluated at the [Fe/H] of Pal 12. This reinforces earlier suggestions that Pal 12 originally was a cluster in the Sgr dSph galaxy that during the process of accretion of this galaxy by our own was tidally stripped from the Sgr galaxy to become part of the extended Sgr stream.

RR Lyrae stars in four globular clusters in the Fornax dwarf galaxy

(Abridged) We have surveyed four globular clusters in the Fornax dwarf galaxy for RR Lyrae stars, using archival HST observations. We identify 197 new RR Lyrae stars in these four clusters. Despite the short observational baseline, we derive periods, light-curves, and photometric parameters for each. The Fornax clusters have exceptionally large RR Lyrae specific frequencies compared with the Galactic globular clusters. Furthermore, the Fornax cluster RR Lyrae stars are unusual in that their characteristics are intermediate between the two Galactic Oosterhoff groups. In this respect the Fornax clusters are similar to the field populations in several dwarf galaxies. We revise previous measurements of the HB morphology in each cluster. The Fornax clusters closely resemble the ``young'' Galactic halo population defined by Zinn. The existence of the second parameter effect among the Fornax clusters is also confirmed. Finally, we determine foreground reddening and distance estimates for each cluster. We find a mean distance modulus to Fornax of (m-M)_0 = 20.66 +/- 0.03 (random) +/- 0.15 (systematic). Our measurements are consistent with a line of sight depth of 8-10 kpc for this galaxy, matching its projected dimensions, and incompatible with tidal model explanations for the observed high velocity dispersions in many dSph galaxies. Dark matter dominance is suggested.

The Carina Project. I. Bright Variable Stars

We present new BV time series data of the Carina dwarf spheroidal galaxy. Current data cover an area of about 0.3 square degrees around the center of the galaxy and allowed us to identify 92 variables. Among them 75 are RR Lyrae stars, 15 are bona fide Anomalous Cepheids, one might be a Galactic field RR Lyrae, and one is located along the Carina Red Giant Branch. Expanding upon the seminal photographic investigation by Saha, Monet & Seitzer (1986) we supply for the first time accurate estimates of their pulsation parameters (periods, amplitudes, mean magnitude and colors) on the basis of CCD photometry. Approximately 50% of both RR Lyrae and Anomalous Cepheids are new identifications. Among the RR Lyrae sample 6 objects are new candidate double-mode variables. On the basis of their pulsation properties we estimated that two variables (V152, V182) are about 50% more massive than typical RR Lyrae stars, while the bulk of the Anomalous Cepheids are roughly a factor of two more massive than fundamental mode RR Lyrae stars. This finding supports the evidence that these objects are intermediate-mass stars during central He burning phases. The data for Carina, together with data available in the literature, strongly support the conclusion that dSph galaxies can barely be classified into the classical Oosterhoff dichotomy. The mean period of fundamental mode RR Lyrae in Carina resembles that found for Oosterhoff type II clusters, whereas the ratio between first overtones and total number of RR Lyrae is quite similar to that found in Oosterhoff type I clusters.

Chemical evolution of dwarf spheroidal and blue compact galaxies

We studied the star formation and chemical evolution in a sample of eight dwarf spheroidal (dSph) galaxies of the Local Group and in blue compact galaxies (BCGs) by means of comparison between the predictions of chemical evolution models and several observed abundance ratios. Detailed models with up-to-date nucleosynthesis taking into account the role played by supernovae of different types (II, Ia) were developed for both types of galaxies, allowing us to follow the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca and Fe). The models are specified by the prescriptions of the star formation and galactic wind efficiencies chosen to reproduce the main features of these galaxies. The BCGs are characterized by a star formation proceeding in several short bursts separated by long quiescent periods and by a low wind efficiency, whereas one or two long bursts and a very efficient wind well describe the dSph galaxies. We also investigated a possible connection in the evolution of dSph and BCGs and compared the predictions of the models to the abundance ratios observed in damped Lyman α systems (DLAs). The main conclusions are: (i) the observed distribution of [α/Fe] versus [Fe/H] in dSph galaxies is mainly a result of the star formation rate coupled with the wind efficiency; (ii) a low star formation efficiency (ν= 0.01–1 Gyr−1) and a high wind efficiency (wi∼ 5–15) are required to reproduce the observational data for dSph galaxies; (iii) the low gas content of these galaxies is the result of the combined action of gas consumption by star formation and gas removal by galactic winds; (iv) the abundance ratios of the BCGs are reproduced by models with two to seven bursts of star formation and an efficiency in the range ν= 0.1–0.9 Gyr−1; (v) the low values of N/O observed in BCGs are the natural result of a bursting star formation; (vi) a connection between dSph and BCGs in a unified evolutionary scenario is unlikely; (vii) the models for both the dSph galaxies and BCGs reproduce the abundance ratios observed in DLAs, but imply different formation scenarios for these objects; (viii) a suitable amount of primary N produced in massive stars can be perhaps an explanation for the low plateau in the [N/α] distribution observed in DLAs, if real.

Tracing the Sgr Stream with 2MASS

We use infrared Color Magnitude Diagrams from the 2-Micron All-Sky Survey (2MASS) to search for stars belonging to the tidal stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) around selected Galactic globular clusters. Statistically significant detections are presented for the cases of Pal 12 and NGC 4147, strongly supporting the idea that these clusters are associated with the Sgr Stream and that they were previous members of the Sgr dSph galaxy.

Draco: A Failure of the Tidal Model

We test whether the structural properties of the nearby dwarf spheroidal (dSph) galaxy Draco, a well-studied Milky Way companion, can be reconciled with the suggestion that dSphs are unbound tidal remnants with a large depth extent along the line of sight. In order to apply the observational test of this hypothesis, suggested by Klessen & Zhao, we use public photometric data from the Sloan Digital Sky Survey (SDSS) to explore the width of Draco's blue horizontal branch over a range of areas covering 0.06-6.25 deg2 centered on Draco. The SDSS database is the only currently existing database with sufficient depth and area coverage to permit a stringent test of the tidal models. Blue horizontal branch stars were chosen as tracers of Draco's spatial extent and depth because of their low contamination by Galactic foreground stars and since they have a spatially more extended distribution than the more centrally concentrated red horizontal branch stars. Indeed, we show that blue horizontal branch stars extend beyond the previously inferred limiting radii of Draco, consistent with the observed absence of a truncated stellar surface density profile of this dSph (Odenkirchen et al.). Following the method of Klessen & Kroupa, we calculate new models for a galaxy without dark matter, using Draco's morphological properties as constraints. The resulting models are unable to reproduce the narrow observed horizontal branch width of Draco, which stays roughly constant regardless of the field of view. We conclude that Draco cannot be the remnant of a tidally disrupted satellite but is probably strongly dark matter-dominated, as suggested independently by the structural analysis conducted by Odenkirchen et al. and by the kinematic analysis of Kleyna et al.

The Progenitors of Dwarf Spheroidal Galaxies

Dwarf spheroidal (dSph) galaxies present an evolutionary puzzle that we explore in 40 early- and late-type dwarfs in the Local Group and nearby field. Although dSphs formed stars over extended periods, today all but one are free of detectable interstellar matter (ISM), even in the Fornax dSph, where stars still formed 100 Myr ago. Combining metallicities for red giants with HI data from the literature, we show that the well-known offset in luminosity-metallicity (L-Z) relations for dSphs and dwarf irregular (dIrr) galaxies exists also when comparing only their old stellar populations: dSphs have higher mean stellar metallicities for a fixed luminosity. Evidently younger dSphs experienced more efficient enrichment than young dIrrs. Dwarf galaxies, whose locus in the L-Z diagram is consistent with that of dSphs even for baryonic luminosities, are the ``transition-type dwarfs'' Phoenix, DDO210, LGS3, Antlia, and KKR25. They have mixed dIrr/dSph morphologies, low stellar masses, low angular momentum, and HI contents of less than a few 10^6 solar masses. Unlike dIrrs, many transition-type dwarfs would closely resemble dSphs if their gas were removed; they are likely dSph progenitors. As gas removal is key, we consider the empirical evidence for various gas removal processes. We suggest that internal gas removal mechanisms are inadequate and favor ram pressure stripping to make dSphs. A combination of initial conditions and environment seems to support the formation of dSphs, which appear to form from small galaxies with active early star formation, whose evolution halts due to externally induced gas loss. Transition-type dwarfs then are dSphs that kept their ISM, and therefore should replace dSphs in isolated locations where stripping is ineffective. (Abridged)

Galaxy flow in the Canes Venatici I cloud

We present an analysis of Hubble Space Telescope/WFPC2 images of eighteen galaxies in the Canes Venatici I cloud. We derive their distances from the luminosity of the tip of the red giant branch stars with a typical accuracy of ~12 %. The resulting distances are 3.9 Mpc (UGC 6541), 4.9 Mpc (NGC 3738), 3.0 Mpc (NGC 3741), 4.5 Mpc (KK 109), >6.3 Mpc (NGC 4150), 4.2 Mpc (UGC 7298), 4.5 Mpc (NGC 4244), 4.6 Mpc (NGC 4395), 4.9 Mpc (UGC 7559), 4.2 Mpc (NGC 4449), 4.4 Mpc (UGC 7605), 4.6 Mpc (IC 3687), 4.7 Mpc (KK 166), 4.7 Mpc (NGC 4736), 4.2 Mpc (UGC 8308), 4.3 Mpc (UGC 8320), 4.6 Mpc (NGC 5204), and 3.2 Mpc (UGC 8833). The CVn I cloud has a mean radial velocity of 286 +- 9 km/s, a mean distance of 4.1 +- 0.2 Mpc, a radial velocity dispersion of 50 km/s, a mean projected radius of 760 kpc, and a total blue luminosity of 2.2 * 10^{10} L_{\sun}. Assuming virial or closed orbital motions for the galaxies, we estimated their virial and their orbital mass-to-luminosity ratio to be 176 and 88 M_{\sun}/L_{\sun}, respectively. However, the CVn I cloud is characterized by a crossing time of 15 Gyr, and is thus far from a state of dynamical equilibrium. The large crossing time for the cloud, its low content of dSph galaxies ($ < 6$ %), and the almost ``primordial'' shape of its luminosity function show that the CVn I complex is in a transient dynamical state, driven rather by the free Hubble expansion than by galaxy interactions.

Inhomogeneous Chemical Evolution of Dwarf Spheroidal Galaxies

Stellar abundance pattern of n-capture elements such as barium is used as a powerful tool to infer how the star formation proceeded in dwarf spheroidal (dSph) galaxies. It is found that the abundance correlation of barium with iron in stars belonging to dSph galaxies orbiting the Milky Way, i.e., Draco, Sextans, and Ursa Minor have a feature similar to that in Galactic metal-poor stars. The common feature of these two correlations can be realized by our in homogeneous chemical evolution model based on the supernova-driven star formation scenario if dSph stars formed from gas with a velocity dispersion of ∼ 26 km s-1. This velocity dispersion together with the stellar luminosities strongly suggest that dark matter dominated dSph galaxies. The tidal force of the Milky Way links this velocity dispersion with the currently observed value ≲ 10 km s-1 by stripping the dark matter in dSph galaxies. As a result, the total mass of each dSph galaxy is found to have been originally ∼ 25 times larger than at present. In this model, supernovae immediately after the end of the star formation can expel the remaining gas over the gravitational potential of the dSph galaxy.

Second parameter globulars and dwarf spheroidals around the Local Group massive galaxies: What can they evidence?

We suggest that the majority of the young, so-called second parameter globular clusters (SPGCs) have origi- nated in the outer Galactic halo due to a process other than a tidal disruption of the dwarf spheroidal (dSph) galaxies. Basic observational evidence regarding both the dSphs and the SPGCs, coupled with the latest data about a rather large relative num- ber of such clusters among globulars in M 33 and their low portion in M 31, seems to be consistent with the suspected process. It might have taken place within the system of the most massive galaxies of the Local Group (LG) at the earliest stages of their formation and evolution. We argue that the origin and basic characteristics of the SPGCs can naturally be explained as a result of mass outflow from M 31, during and due to formation of its Pop. II stars, and subsequent accretion of gas onto its massive companions, the Galaxy and M 33. An amount of the gas accreted onto the Milky Way is expected to have been quite enough for the formation in the outer Galactic halo not only of the clusters under consideration but also a number of those dSph galaxies which are as young as the SPGCs. A less significant, but notable mass transfer from the starbursting protoGalaxy to the massive members of the LG might have occurred, too.