An HI census of Loose Groups of Galaxies

We present results from our Parkes Multibeam HI survey of 3 loose groups of galaxies that are analogous to the Local Group. This is a survey of groups containing only spiral galaxies with mean separations of a few hundred kpc, and total areas of approximately 1 sq. Mpc; groups similar to our own Local Group. We present a census of the HI-rich objects in these groups down to an M(HI), 1-sigma sensitivity ~7x10^5 M(sun), as well as the detailed properties of these detections from follow-up Compact Array observations. We found 7 new HI-rich members in the 3 groups, all of which have stellar counterparts and are, therefore, typical dwarf galaxies. The ratio of low-mass to high-mass gas-rich galaxies in these groups is less than in the Local Group meaning that the ``missing satellite'' problem is not unique. No high-velocity cloud analogs were found in any of the groups. If HVCs in these groups are the same as in the Local Group, this implies that HVCs must be located within ~300-400 kpc of the Milky Way.Comment: 6 pages, to appear in the ASP proceedings of IAU Symposium 217, "Recycling intergalactic and interstellar matter", eds. Pierre-Alain Duc, Jonathan Braine, Elias Brink

The Parent Populations of 6 groups identified from Chemical Tagging in the Solar neighborhood

We estimate the size and distribution of the parent populations for the 6 largest (at least 20 stars in the Solar neighborhood) chemical groups identified in the Chemical Tagging experiment by Mitschang et al.~2014. Stars in the abundance groups tend to lie near a boundary in angular momentum versus eccentricity space where the probability is highest for a star to be found in the Solar neighborhood and where orbits have apocenter approximately equal to the Sun's galactocentric radius. Assuming that the parent populations are uniformly distributed at all azimuthal angles in the Galaxy, we estimate that the parent populations of these abundance groups contain at least 200,000 members. The spread in angular momentum of the groups implies that the assumption of a uniform azimuthal distribution only fails for the two youngest groups and only for the highest angular momentum stars in them. The parent populations of three thin disk groups have narrow angular momentum distributions, but tails in the eccentricity and angular momentum distributions suggest that only a small fraction of stars have migrated and increased in eccentricity. In contrast, the parent populations of the thick disk groups exhibit both wide angular momentum and eccentricity distributions implying that both heating and radial migration has taken place.Comment: accepted for publication in MNRA

Large stellar disks in small elliptical galaxies

We present the rotation velocities V and velocity dispersions sigma along the principal axes of seven elliptical galaxies less luminous than M_B= -19.5. These kinematics extend beyond the half-light radii for all systems in this photometrically selected sample. At large radii the kinematics not only confirm that rotation and "diskiness" are important in faint ellipticals, as was previously known, but also demonstrate that in most sample galaxies the stars at large galactocentric distances have (V/sigma)_max of about 2, similar to the disks in bona-fide S0 galaxies. Comparing this high degree of ordered stellar motion in all sample galaxies with numerical simulations of dissipationless mergers argues against mergers with mass ratios <=3:1 as an important mechanism in the final shaping of low-luminosity ellipticals, and favors instead the dissipative formation of a disk.Comment: 11 pages LaTex with 4 Postscript figure

Beyond the Brim of the Hat: Kinematics of Globular Clusters out to Large Radius in the Sombrero Galaxy

We have obtained radial velocity measurements for 51 new globular clusters around the Sombrero galaxy. These measurements were obtained using spectroscopic observations from the AAOmega spectrograph on the Anglo-Australian Telescope and the Hydra spectrograph at WIYN. Combined with our own past measurements and velocity measurements obtained from the literature we have constructed a large database of radial velocities that contains a total of 360 confirmed globular clusters. Previous studies' analyses of the kinematics and mass profile of the Sombrero globular cluster system have been constrained to the inner ~9' (~24 kpc or ~5 effective radii), but our new measurements have increased the radial coverage of the data, allowing us to determine the kinematic properties of M104 out to ~15' (~41 kpc or ~9 effective radii). We use our set of radial velocities to study the GC system kinematics and to determine the mass profile and V-band mass-to-light profile of the galaxy. We find that the V-band mass-to-light ratio increases from 4.5 at the center to a value of 20.9 at 41 kpc (~9 effective radii or 15'), which implies that the dark matter halo extends to the edge of our available data set. We compare our mass profile at 20 kpc (~4 effective radii or ~7.4') to the mass computed from x-ray data and find good agreement. We also use our data to look for rotation in the globular cluster system as a whole, as well as in the red and blue subpopulations. We find no evidence for significant rotation in any of these samples.Comment: Accepted for publication in the Astronomical Journal; 23 pages, 14 figures, and 2 table

The Arcturus Moving Group: Its Place in the Galaxy

The Arcturus moving group is a well-populated example of phase space substructure within the thick disk of our Galaxy. Traditionally regarded as the remains of a dissolved open cluster, it has recently been suggested to be a remnant of a satellite accreted by our Galaxy. To investigate the origin of the group we undertook a high-resolution spectroscopic abundance study of Arcturus group members and candidates. Examining abundance of Fe, Mg, Ca, Ti, Cr, Ni, Zn, Ce, Nd, Sm and Gd for 134 stars we found that the group is chemically similar to disk stars and does not exhibit a clear chemical homogeneity. Furthermore, we confirm the existence of the Arcturus group using the Nordstroem et al. (2004), Schuster et al. (2006) and RAdial Velocity Experiment (RAVE) surveys (Steinmetz et al. 2006). The origin of the group still remains unresolved: the chemical results are consistent with a dynamical origin but do not entirely rule out a merger one. Certainly, the Arcturus group provides a challenge to our understanding of the nature and origin of the Galaxy's thick disk.Comment: 6 pages, 2 figures, to appear in the Proceedings of the IAU Symposium 254: The Galaxy Disk in Cosmological Contex

Looking for imprints of the first stellar generations in metal-poor bulge field stars

© 2016 ESO. Context. Efforts to look for signatures of the first stars have concentrated on metal-poor halo objects. However, the low end of the bulge metallicity distribution has been shown to host some of the oldest objects in the Milky Way and hence this Galactic component potentially offers interesting targets to look at imprints of the first stellar generations. As a pilot project, we selected bulge field stars already identified in the ARGOS survey as having [Fe/H] 1 and oversolar [α/Fe] ratios, and we used FLAMES-UVES to obtain detailed abundances of key elements that are believed to reveal imprints of the first stellar generations. Aims. The main purpose of this study is to analyse selected ARGOS stars using new high-resolution (R ∼ 45 000) and high-signal-tonoise (S=N > 100) spectra. We aim to derive their stellar parameters and elemental ratios, in particular the abundances of C, N, the α-elements O, Mg, Si, Ca, and Ti, the odd-Z elements Na and Al, the neutron-capture s-process dominated elements Y, Zr, La, and Ba, and the r-element Eu. Methods. High-resolution spectra of five field giant stars were obtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. Spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe i and Fe ii. The abundance analysis was performed with a MARCS LTE spherical model atmosphere grid and the Turbospectrum spectrum synthesis code. Results.We confirm that the analysed stars are moderately metal-poor (-1:04≤[Fe/H]≤-0:43), non-carbon-enhanced (non-CEMP) with [C/Fe] ≤+0:2, and α-enhanced.We find that our three most metal-poor stars are nitrogen enhanced. The α-enhancement suggests that these stars were formed from a gas enriched by core-collapse supernovae, and that the values are in agreement with results in the literature for bulge stars in the same metallicity range. No abundance anomalies (Na-O, Al-O, Al-Mg anti-correlations) were detected in our sample. The heavy elements Y, Zr, Ba, La, and Eu also exhibit oversolar abundances. Three out of the five stars analysed here show slightly enhanced [Y/Ba] ratios similar to those found in other metal-poor bulge globular clusters (NGC 6522 and M 62). Conclusions. This sample shows enhancement in the first-to-second peak abundance ratios of heavy elements, as well as dominantly s-process element excesses. This can be explained by different nucleosynthesis scenarios: (a) the main r-process plus extra mechanisms, such as the weak r-process; (b) mass transfer from asymptotic giant branch stars in binary systems; (c) an early generation of fast-rotating massive stars. Larger samples of moderately metal-poor bulge stars, with detailed chemical abundances, are needed to better constrain the source of dominantly s-process elements in the early Universe

The origin of the Milky Way's halo age distribution

© 2018. The American Astronomical Society.. We present an analysis of the radial age gradients for the stellar halos of five Milky Way (MW) mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately -7 to -19 Myr kpc-1, shallower than those determined by recent observational studies of the Milky Way's stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values ranging from -8 to -32 Myr kpc-1, closer to those observed in the Galaxy. Two of the accretion-dominated simulated halos show a large concentration of old stars in the center, in agreement with the Ancient Chronographic Sphere reported observationally. The stellar halo that best reproduces the current observed characteristics of the age distributions of the Galaxy is that formed principally by the accretion of small satellite galaxies. Our findings suggest that the hierarchical clustering scenario can reproduce the MW's halo age distribution if the stellar halo was assembled from accretion and the disruption of satellite galaxies with dynamical masses less than ∼109.5M, and a minimal in situ contribution

The Baryonic Tully Fisher Relation

We validate the baryonic Tully Fisher (BTF) relation by exploring the Tully Fish er (TF) and BTF properties of optically and HI-selected disk galaxies. The data includes galaxies from: Sakai et al. (2000) calibrator sample; McGaugh et al. (2000: MC2000) I-band sample; and 18 newly acquired HI-selected field dwarf galaxies observed with the ANU 2.3m telescope and the ATNF Parkes telescope from Gurovich's thesis sample (2005). As in MC2000, we re-cast the TF and BTF relations as relationships between baryo n mass and W_{20}. First we report some numerical errors in MC2000. Then, we c alculate weighted bi-variate linear fits to the data, and finally we compare the fits of the intrinsically fainter dwarfs with the brighter galaxies of Sakai et al. (2000). With regards to the local calibrator disk galaxies of Sakai et al. (2000), our results suggest that the BTF relation is indeed tighter than the T F relation and that the slopes of the BTF relations are statistically flatter th an the equivalent TF relations. Further, for the fainter galaxies which include the I-band MCG2000 and HI-selected galaxies of Gurovich's thesis sample, we calc ulate a break from a simple power law model because of what appears to be real c osmic scatter. Not withstanding this point, the BTF models are marginally better models than the equivalent TF ones with slightly smaller reduced chi^2.Comment: 3 pages, 3 figs, refereed conference proceeding based on an invited talk at the Structure and Dynamics in the Local Universe, a Sydney workshop to honour Brent Tully's 60th birthday. To appear in Publications of the Astronomical Society of Australia, ed. J. Bland-Hawthor