We present an analysis of the spatial and chemical sub-structures in a remote halo field in the nearby giant elliptical galaxy Centaurus A (NGC~5128), situated at about 38 kpc from the centre of the galaxy. The observations were taken with the Advanc
ed Camera for Surveys instrument on board the Hubble Space Telescope, and reach down to the horizontal branch. In this relatively small 3.8 kpc by 3.8 kpc field, after correcting for Poisson noise, we do not find any statistically strong evidence for the presence of small-scale sub-structures in the stellar spatial distribution on scales greater than 100 pc. However, we do detect the presence of significant small spatial-scale inhomogeneities in the stellar median metallicity over the surveyed field. We argue that these localized chemical substructures could be associated with not-fully mixed debris from the disruption of low mass systems. NGC 5128 joins the ranks of the late-type spiral galaxies the Milky Way, for which the stellar halo appears to be dominated by small-scale spatial sub-structures, and NGC~891, where localized metallicity variations have been detected in the inner extra-planar regions. This suggests that the presence of small-scale sub-structures may be a generic property of stellar halos of large galaxies.
Recent panoramic observations of the dominant spiral galaxies of the Local Group have revolutionized our view of how these galaxies assemble their mass. However, it remains completely unclear whether the properties of the outer regions of the Local G
roup large spirals are typical. Here, we present the first panoramic view of a spiral galaxy beyond the Local Group, based on the largest, contiguous, ground-based imaging survey to date resolving the stellar halo of the nearest prime analogue of the Milky Way, NGC 891 (D~10 Mpc). The low surface brightness outskirts of this galaxy are populated by multiple, coherent, and vast substructures over the 90kpc * 90kpc extent of the survey. These include a giant stream, the first to be resolved into stars beyond the Local Group using ground-based facilities, that loops around the parent galaxy up to distances of ~50kpc. The bulge and the disk of the galaxy are found to be surrounded by a previously undetected large, flat and thick cocoon-like stellar structure at vertical and radial distances of up to ~15kpc and ~40kpc respectively.
We report the discovery of an extended globular cluster in a halo field in Centaurus A (NGC 5128), situated $sim 38kpc$ from the centre of that galaxy, imaged with the Advanced Camera for Surveys on board the Hubble Space Telescope. At the distance o
f the galaxy, the half-light radius of the cluster is r_h ~ 17pc, placing it among the largest globular clusters known. The faint absolute magnitude of the star cluster, M_(V,o)=-5.2, and its large size render this object somewhat different from the population of extended globular clusters previously reported, making it the first firm detection in the outskirts of a giant galaxy of an analogue of the faint, diffuse globular clusters present in the outer halo of the Milky Way. The colour-magnitude diagram of the cluster, covering approximately the brightest four magnitudes of the red giant branch, is consistent with an ancient, i.e., older than ~8 Gyr, intermediate-metallicity, i.e., [M/H] ~-1.0 dex, stellar population. We also report the detection of a second, even fainter cluster candidate which would have r_h ~ 9pc, and M_(V,o)=-3.4 if it is at the distance of NGC 5128. The properties of the extended globular cluster and the diffuse stellar populations in its close vicinity suggest that they are part of a low mass accretion in the outer regions of NGC 5128.
Using the MegaCam imager on the Canada-France-Hawaii Telescope, we have resolved individual stars in the outskirts of the nearby large spiral galaxy M81 (NGC 3031) well below the tip of the red giant branch of metal-poor stellar populations over 60 k
pc * 58 kpc. In this paper, we report the discovery of new young stellar systems in the outskirts of M81. The most prominent feature is a chain of clumps of young stars distributed along the extended southern HI tidal arm connecting M 81 and NGC 3077. The colour-magnitude diagrams of these stellar systems show plumes of bright main sequence stars and red supergiant stars, indicating extended events of star formation. The main sequence turn-offs of the youngest stars in the systems are consistent with ages of ~40 Myr. The newly reported stellar systems show strong similarities with other known young stellar systems in the debris field around M81, with their properties best explained by these systems being of tidal origin.
We present detailed observations of a z~1.99 cluster of submillimeter galaxies (SMGs), discovered as the strongest redshift spike in our entire survey of ~100 SMGs across 800 square arcmin. It is the largest blank-field SMG concentration currently kn
own and has <0.01% chance of being drawn from the underlying selection function for SMGs. We have compared UV observations of galaxies at this redshift, where we find a much less dramatic overdensity, having an 11% chance of being drawn from its selection function. We use this z~1.99 overdensity to compare the biasing of UV- and submm-selected galaxies, and test whether SMGs could reside in less overdense environments, with their apparent clustering signal being dominated by highly active merger periods in modest mass structures. This impressively active formation phase in a low mass cluster is not something seen in simulations, although we propose a toy model using merger bias which could account for the bias seen in the SMGs. While enhanced buildup of stellar mass appears characteristic of other high-z galaxy clusters, neither the UV- nor submm-galaxies in this structure exhibit larger stellar masses than their field galaxy counterparts (although the excess of SMGs in the structure represents a larger volume-averaged stellar mass than the field). Our findings have strong implications for future surveys for high-z galaxies at long wavelengths such as SCUBA2 and Herschel. We suggest that since these surveys will select galaxies during their episodes of peak starbursts, they could probe a much wider range of environments than just the progenitors of rich clusters, revealing more completely the key events and stages in galaxy formation and assembly.
(abridged) We present a spectroscopic analysis of five stellar streams (`A, `B, `Cr, `Cp and `D) as well as the extended star cluster, EC4, which lies within streamC, all discovered in the halo of M31 from our CFHT/MegaCam survey. These spectroscopic
results were initially serendipitous, making use of our existing observations from the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, and thereby emphasizing the ubiquity of tidal streams that account for ~70% of the M31 halo stars in the targeted fields. Subsequent spectroscopy was then procured in streamCr/p and streamD to trace the velocity gradient along the streams. For the cluster EC4, candidate member stars with average [Fe/H]~-1.4 (Fe/H_spec=-1.6), are found at v_{hel}=-285 km/s suggesting it could be related to streamCp. No similarly obvious cold kinematic candidate is found for streamD, although candidates are proposed in both of two spectroscopic pointings along the stream (both at -400 km/s). Spectroscopy near the edge of streamB suggests a likely kinematic detection, while a candidate kinematic detection of streamA is found (plausibly associated to M33 rather than M31). The low dispersion of the streams in kinematics, physical thickness, and metallicity makes it hard to reconcile with a scenario whereby these stream structures as an ensemble are related to the giant southern stream. We conclude that the M31 stellar halo is largely made up of multiple kinematically cold streams.
We present spectroscopic observations of the AndXII dwarf spheroidal galaxy using DEIMOS/Keck-II, showing it to be moving rapidly through the Local Group (-556 km/s heliocentric velocity, -281 km/s relative to Andromeda from the MW), falling into the
Local Group from ~115 kpc beyond Andromedas nucleus. AndXII therefore represents a dwarf galaxy plausibly falling into the Local Group for the first time, and never having experienced a dense galactic environment. From Green Bank Telescope observations, a limit on the H{I} gas mass of <3000 Msun suggests that AndXIIs gas could have been removed prior to experiencing the tides of the Local Group galaxies. Orbit models suggest the dwarf is close to the escape velocity of M31 for published mass models. AndXII is our best direct evidence for the late infall of satellite galaxies, a prediction of cosmological simulations.
