The properties of the brightest resolved stars in an arc that was originally identified by Sun et al. (2005) and is located in the extraplanar regions of M82 are discussed. The stars form an elongated structure that is traced over a projected area of 3.0 x 0.8 kpc. The integrated brightness is M_V ~ -11, while the total stellar mass is between 3 x 10^5 and 2 x 10^6 solar masses. If there is only foreground extinction then the youngest stars have a metallicity Z > 0.008 and an age log(tyears) ~ 7.75; thus, the youngest stars formed at roughly the same time as stars in tidal features that are associated with other M81 Group galaxies. If the arc is dispersing then it will deposit young, chemically enriched stars into the M82 halo.
We report on $BVI$ CCD photometry of a field centered in the region of the Galactic star cluster Whiting~1 down to $V=23.0$. This cluster has never been studied insofar, and we provide for the first time estimates of its fundamental parameters, namely, radial extent, age, distance and reddening. Whiting~1 turns out to be a compact star cluster with a diameter of about 1$^{\prime}$. We find that the cluster is about 5 Gyr old and has a probable metal abundance around [Fe/H]= -1.20. Its position at $b=-60^{o}.64$ and at a heliocentric distance of about 45 kpc makes the cluster a rather strange o…
Nine presently known "young" Galactic globular clusters, which have ages / 3 Gyr smaller than the oldest clusters of similar metallicity, all have RGC > 15 kpc. Furthermore these young globulars are found to have below-average luminosities. These results suggest that (1) globular cluster formation extended over a considerable period of time in the outermost part of the Galactic halo, and (2) that typical luminosities (masses) of such outer halo clusters decreased with time.
We report here on the discovery of a red quasar, J004929.4+351025.7 at a redshift of z = 2.48, situated within a large Lya emission-line halo. The radio spectral energy distribution implies that the radio jets were triggered < 10^4 years prior to the time at which the object is observed, suggesting that the jet triggering of the active galactic nucleus is recent. The loosely biconical structure of the emission-line halo suggests that it is ionised by photons emitted by the central quasar nucleus and that the central nucleus is obscured by a dusty torus with Av ~ 3.0. The large spatial extent o…
Most of the globular clusters in the main body of the Galactic halo were formed almost simultaneously. However, globular cluster formation in dwarf spheroidal galaxies appears to have extended over a significant fraction of a Hubble time. This suggests that the factors which suppressed late-time formation of globulars in the main body of the Galactic halo were not operative in dwarf spheroidal galaxies. Possibly the presence of significant numbers of ``young'' globulars at R_{GC} > 15 kpc can be accounted for by the assumption that many of these objects were formed in Sagittarius-like (but not…
We perform an extensive analysis of nonlinear and stochastic biasing of galaxies and dark halos in spatially flat low-density CDM universe using cosmological hydrodynamic simulations. We compare their biasing properties with the predictions of an analytic halo biasing model. Dark halos in our simulations exhibit reasonable agreement with the predictions only on scales larger than 10h^{-1}Mpc, and on smaller scales the volume exclusion effect of halos due to their finite size becomes substantial. Interestingly the biasing properties of galaxies are better described by extrapolating the halo bia…
Deep V and I CCD images of the loosely populated Galactic globular cluster Pal 1 and the surrounding field have been obtained with the INT. A color magnitude diagram down to V(lim) ~ 24, and a luminosity function complete down to V(lim) ~ 23.25 have been constructed. Adopting a reddening E(V-I)=0.20+/-0.04 and comparing the CMD of Pal 1 with 47 Tucanae, we obtain a distance modulus (m-M)o=15.25+/-0.25, indicating that Pal 1 is located 3.7+/-0.4 kpc above the Galactic disc and 17.3+/-1.6 kpc from the Galactic center. Comparison with 47 Tuc and M71 shows that Pal 1 must be significantly younger.…
A detailed analysis of halo white dwarf candidates is presented, which is based on model atmosphere fits to observed energy distributions built from photoelectric or photographic magnitudes. Most of the candidates identified in reduced proper motion diagrams are shown to be too warm (Teff > 5000 K) and most likely too young to be members of the galactic halo, while the tangential velocities of the cooler and thus older white dwarfs are shown to be entirely consistent with the disk population. The results suggest that some white dwarf stars born in the young disk may have high velocities with r…
A detailed analysis is presented of the dominant ionised knot in the halo of the planetary nebula NGC 6543. Observations were made at high spectral and spatial resolution of the [OIII] 5007 line using the Manchester echelle spectrometer combined with the 2.1-m San Pedro Martir Telescope. A 20-element multislit was stepped across the field to give almost complete spatial coverage of the knot and surrounding halo. The spectra reveal, for the first time, gas flows around the kinematically inert knot. The gas flows are found to have velocities comparable to the sound speed as gas is photo-evaporat…
The initial radial density profiles of dark matter halos are laid down by gravitational collapse in hierarchical structure formation scenarios and are subject to further compression as baryons cool and settle to the halo centers. We here describe an explicit implementation of the algorithm, originally developed by Young, to calculate changes to the density profile as the result of adiabatic infall in a spherical halo model. Halos with random motion are more resistant to compression than are those in which random motions are neglected, which is a key weakness of the simple method widely employe…
We have studied the fate of initial magnetic fields in the hot halo gas out of which the visible parts of galaxies form, using three-dimensional numerical MHD-experiments. The halo gas undergoes compression by several orders of magnitude in the subsonic cooling flow that forms the cold disk. The magnetic field is carried along and is amplified considerably in the process, reaching muG levels for reasonable values of the initial ratio of magnetic to thermal energy density.
We used the direct CCD camera at the Magellan I telescope at Las Campanas Observatory and FORS1 at Antu VLT at ESO Paranal Observatory to image fields centered on the inner and outer optical filaments in the halo of NGC 5128. In the V vs. U-V color-magnitude diagrams we have identified young blue supergiants associated with these line-emitting filaments located between the inner radio lobe and the northern middle lobe. Around the outer filament stars as young as 10 Myr were detected. They are principally aligned with the direction of the radio jet, but a vertical north-east alignment along the…
We have retrieved multicolor WFPC2/HST data from the STScI archive for 27 nearby Massive (>= 3x10^4 M_Sun) Young (< 20 Myr) star Clusters (MYCs). The data represents the most-complete-to-date sample of clearly resolved MYCs. We have analyzed their structural properties and have found that they can be classified as either Super Star Clusters (SSCs) or as Scaled OB Associations (SOBAs). SSCs have a compact core possibly surrounded by a halo while SOBAs have no core. A morphological sequence can be established from SSCs with weak halos to SSCs with strong halos to SOBAs and we propose that this i…
Lyman break galaxies (LBGs) at $z \sim 3$ exhibit number densities and clustering similar to local $L_*$ galaxies; however, their star formation rates (SFRs) are much higher. We explore the scenario in which LBGs are starburst galaxies triggered by collisions, and thus provide an explanation for these key properties. The relative paucity of starburst galaxies at low redshift can be attributed to a much higher collision rate in the past. We use high-resolution cosmological $N$-body simulations and a hierarchical halo finder to estimate the galaxy collision rate as a function of time in the popu…
We have measured the Li abundance of 18 stars with -2 <~ [Fe/H] ~< -1 and 6000 K <~ Teff <~ 6400 K, a parameter range that was poorly represented in previous studies. We examine the Galactic chemical evolution (GCE) of this element, combining these data with previous samples of turnoff stars over the full range of halo metallicities. We find that A(Li) increases from a level of \~2.10 at [Fe/H] = -3.5, to ~2.40 at [Fe/H] = -1.0, where A(Li) = log_10 (n(Li)/n(H)) + 12.00. We compare the observations with several GCE calculations, including existing one-zone models, and a new model developed in …
We have searched the IRAS Point Source Catalog and HIRES maps for young stellar objects (YSOs) in the direction of five \HI high-velocity clouds (HVCs). In agreement with optical searches in the halo, no evidence was found for extensive star-forming activity inside the high-latitude HVCs. Specifically, we have found no signs of star formation or YSOs in the direction of the A IV cloud or in the very-high-velocity clouds HVC~110-7-465 and HVC~114-10-440. We have identified only one young star in the direction of the M~I.1 cloud, which shows almost perfect alignment with a knot of \HI emission. …
Star formation in starbursts appears to be biased toward compact clusters, with up to 20% of all stars formed in them. Observations with HST show that many of these clusters have luminosities (-9 > Mv > -16), UBVI colors, and half-light radii (R_eff = ~3 pc) consistent with their being young globular clusters. Although we know little about the long-term stability of the youngest clusters (<20 Myr), compact clusters older than ~10 core-crossing times (20 - 40 Myr) are bound gravitationally and very likely young globulars. The present review concentrates on recent progress achieved in dating suc…
We present the results of quantitative spectral analyses of ten apparently normal B-type stars. These stars were found to be young massive B-type stars at distances of z=2.6 to 7.6 kpc from the galactic plane based on their positions in the (Teff, log g) diagram, normal abundance patterns and/or large projected rotational velocities. We discuss formation scenarios (runaway star scenarios versus a scenario for star formation in the halo) by comparing times-of-flight and evolutionary time scales. For all stars (except SB 357 and HS 1914+7139) both the scales are similar, indicating that the star…
We study the arc statistics of gravitational lensing generated by dark matter halos in order to probe their density profile. We characterize the halo profile by two parameters, the inner slope of the central cusp $α$, and the median amplitude of the concentration parameter, $c_{\rm norm}$, for a halo of mass $10^{14}h^{-1}M_\odot$ at $z=0$, and compute the numbers of tangential and radial arcs produced by gravitational lensing of galaxy clusters. We find that the number of arcs divided by the number of halos is a good statistic which is sensitive to both $c_{\rm norm}$ and $α$ with very weak d…
We report the discovery of PM J13420-3415, a faint (V=17) white dwarf with a very high proper motion mu = 2.55 arcsec/yr. The star was found in the southern sky extension of the SUPERBLINK proper motion survey. A red spectrum shows the classical signature of a DA white dwarf, with a weak H-alpha line in absorption as the only prominent feature. The star is also found to have a large radial velocity V_rad = +212+/-15 km/s. At the adopted distance of 18 pc, the star has a very large space motion of 313 km/s relative to the Sun. An integration of the space motion shows that the star is on a nearl…
I assess the similarities and differences between the star-formation modes in quiescent spiral galaxies versus those in violent starburst regions, including galactic nuclei. As opposed to the quiescent star-formation mode, current empirical evidence on the star-formation processes in the extreme, high-pressure environments induced by galaxy encounters strongly suggests that star cluster formation is an important and perhaps even the dominant mode of star formation in such starburst events. This implies that by using star clusters as unique diagnostic probes, we can trace a galaxy's most violen…
Stellar streams in galaxy halos are the natural consequence of a history of merging and accretion. We present evidence for a blue tidal stream of young stars in the nearest giant elliptical galaxy, NGC 5128 (Centaurus A). Using optical UBVR color maps, unsharp masking, and adaptive histogram equalization, we detect a blue arc in the northwest portion of the galaxy that traces a partial ellipse with an apocenter of 8 kpc. We also report the discovery of numerous young star clusters that are associated with the arc. The brightest of these clusters is spectroscopically confirmed, has an age of 35…
We use a series of high-resolution N-body simulations of a `Milky-Way' halo, coupled to semi-analytic techniques, to study the formation of our own Galaxy and of its stellar halo. Our model Milky Way galaxy is a relatively young system whose physical properties are in quite good agreement with observational determinations. In our model, the stellar halo is mainly formed from a few massive satellites accreted early on during the galaxy's lifetime. The stars in the halo do not exhibit any metallicity gradient, but higher metallicity stars are more centrally concentrated than stars with lower abu…
In this contribution we outline plans for identifying and characterising numerous young, low-mass stars within 150 pc of the Sun using the new SkyMapper telescope and Southern Sky Survey. We aim to learn more about the star formation history of the solar neighbourhood over the past 5-50 Myr, the dispersal processes involved, as well as testing pre-main sequence evolutionary models and the universality of the stellar Inital Mass Function. Searching for the dispersed halo of low-mass objects predicted to surround the Eta Chamaeleontis cluster will be one of the first goals of the project.
Statistics of lensed arcs in clusters of galaxies serve as a powerful probe of both the non-sphericity and the inner slope of dark matter halos. We develop a semi-analytic method to compute the number of arcs in triaxial dark matter halos. This combines the lensing cross section from the Monte Carlo ray-tracing simulations, and the probability distribution function (PDF) of the axis ratios evaluated from cosmological N-body simulations. This approach enables one to incorporate both asymmetries in the projected mass density and elongations along the line-of-sight analytically, for the first tim…
Young stars observed in the distant Galactic halo are usually thought to have formed elsewhere, either in the Galactic disk or perhaps the Galactic center, and subsequently ejected at high velocities to their current position. However, some of these stars have apparent lifetimes shorter the required flight time from the Galactic disk/center. We suggest that such stars have evolved in close runaway or hypervelocity binaries. Stellar evolution of such binaries can drive them into mass transfer configurations and even mergers. Such evolution could then rejuvenate them (e.g. blue stragglers) and e…
Evolutionary synthesis models for star clusters of various metallicities including gaseous emission during the lifetime of the ionising stars are used to model star cluster systems comprising two populations: an old metal-poor globular clusters (GC) population similar to that of the Milky Way halo and a second GC population of arbitrary metallicity. We investigate the time evolution of color distributions and luminosity functions for the two GC populations in comparison with observations on E/S0 galaxies. We show that multi-passband data for GC populations give clues to the relative ages and m…
The age distribution, and chemical elemental abundances, of stars in the halo of the Milky Way provide constraints on theories of galaxy formation. As one specific example, the accretion of satellite galaxies similar to the present retinue of dwarf spheroidals (dSphs) would provide an observable metal-poor, intermediate-age population. This paper presents a quantitative assessment of any contribution made by such stars to the stellar halo. The bulk of the stellar populations in the halo show a well-defined turn-off, at B-V ~ 0.4, implying that the vast majority of the stars are old. The fracti…
The Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey has revealed a loop of H II reaching 1300 pc from the Galactic midplane above the Cas OB6 association in the Perseus sprial arm. This enormous feature surrounds and extends far above the "W4 Chimney" identified by Normandeau et al. and appears to be associated with the star formation activity near the W3/W4/W5 H II region complex. The existence of this ionized structure suggests that past episodes of massive star formation have cleared the H I from an enormous volume above the Perseus arm, allowing Lyman continuum photons from O stars nea…
We present a method for finding clusters of young stars in M31 using broadband WFPC2 data from the HST data archive. Applying our identification method to 13 WFPC2 fields, covering an area of ~60 arcmin^2, has revealed 79 new candidate young star clusters in these portions of the M31 disk. Most of these clusters are small (~<5 pc) young (~10-200 Myr) star groups located within large OB associations. We have estimated the reddening values and the ages of each candidate individually by fitting isochrones to the stellar photometry. We provide a catalog of the candidates including rough approximat…
There has been much debate in recent decades as to what fraction of ionizing photons from star forming regions in the Galactic disk escape into the halo. The recent detection of the Magellanic Stream in optical line emission at the CTIO 4m and the AAT 3.9m telescopes may now provide the strongest evidence that at least some of the radiation escapes the disk completely. We present a simple model to demonstrate that, while the distance to the Magellanic Stream is uncertain, the observed emission measures (${\cal E}_m \approx 0.5 - 1$ cm$^{-6}$ pc) are most plausibly explained by photoionization …
We have used the ACS camera on HST to obtain (V,I) photometry for 57,000 red-giant stars in the halo of the Leo elliptical NGC 3377. We use this sample of stars to derive the metallicity distribution function (MDF) for its halo field stars, and comment on its chemical evolution history compared with both larger and smaller E galaxies. Our ACS/WFC field spans a radial range extending from 4 to 18 kpc projected distance from the center of NGC 3377 and thus covers a significant portion of this galaxy's halo. We find that the MDF is broad, reaching a peak at [m/H] ~ -0.6$, but containing virtually…
We use 2MASS and MSX infrared observations, along with new molecular line (CO) observations, to examine the distribution of young stellar objects (YSOs) in the molecular cloud surrounding the halo HII region KR 140 in order to determine if the ongoing star-formation activity in this region is dominated by sequential star formation within the photodissociation region (PDR) surrounding the HII region. We find that KR 140 has an extensive population of YSOs that have spontaneously formed due to processes not related to the expansion of the HII region. Much of the YSO population in the molecular c…
