Big fish, small fish: Two New Ultra-Faint Satellites of the Milky Way

We report the discovery of two new Milky Way satellites in the neighboring constellations of Pisces and Pegasus identified in data from the Sloan Digital Sky Survey. Pisces II, an ultra-faint dwarf galaxy lies at the distance of ~180 kpc, some 15 degrees away from the recently detected Pisces I. Segue 3, an ultra-faint star cluster lies at the distance of 16 kpc. We use deep follow-up imaging obtained with the 4-m Mayall telescope at Kitt Peak National Observatory to derive their structural parameters. Pisces II has a half-light radius of ~60 pc, while Segue 3 is twenty times smaller at only 3pc.Comment: Accepted for publication in ApJ

Was the Progenitor of the Sagittarius Stream a Disc Galaxy?

We use N-body simulations to explore the possibility that the Sagittarius (Sgr) dwarf galaxy was originally a late-type, rotating disc galaxy, rather than a non-rotating, pressure-supported dwarf spheroidal galaxy, as previously thought. We find that bifurcations in the leading tail of the Sgr stream, similar to those detected by the SDSS survey, naturally arise in models where the Sgr disc is misaligned with respect to the orbital plane. Moreover, we show that the internal rotation of the progenitor may strongly alter the location of the leading tail projected on the sky, and thus affect the constraints on the shape of the Milky Way dark matter halo that may be derived from modelling the Sgr stream. Our models provide a clear, easily-tested prediction: although tidal mass stripping removes a large fraction of the original angular momentum in the progenitor dwarf galaxy, the remnant core should still rotate with a velocity amplitude ~20 km/s that could be readily detected in future, wide-field kinematic surveys of the Sgr dwarf.Comment: Letter accepted by MNRAS. N-body model animations can be downloaded from http://www.ast.cam.ac.uk/~jorpega/files/sgr

A Quasar Without Broad Lyman-alpha Emission

The z=3.02 quasar SDSS J095253.83+011421.9 exhibits broad metal-line emission (CIV FWHM=9000 km/s), but broad Ly-alpha emission is not present. Instead, only a narrow Ly-alpha line is observed (FWHM=1140 km/s). The large CIV/Ly-alpha ratio in the broad-line region (BLR) emission from this object can be matched most closely by a BLR dominated by gas at very high densities (10^15 cm^-3), which suppresses the Ly-alpha emission, and illuminated by an incident power-law extending to ~200 micron, which yields increased emission from purely collisionally excited coolant lines (such as CIV, NV and OVI) but not from recombination lines like Ly-alpha. However, the strong CIII emission predicted by this model is not observed, and the observed broad CIII] emission must come from a lower-density BLR component and should be accompanied by broad Ly-alpha emission which is not observed. The least unlikely explanation for this spectrum seems to be that any intrinsic broad Ly-alpha emission is removed by smooth NV absorption in the red wing of the Ly-alpha emission line and by smooth Ly-alpha absorption in the blue wing of the Ly-alpha emission line. This postulated smooth absorption would be in addition to the strong, associated, narrow absorption seen in numerous ions. Smooth absorption in Ly-alpha, NV and OVI but not in CIV would be unusual, but not impossible, although it is suspicious that the postulated absorption must almost exactly cancel the postulated intrinsic broad emission. We conclude that the spectrum of SDSS J0952+0114 appears unique (among ~3600 SDSS spectra of quasars at z>2.12) because of some_combination_ of unusual parameters, and we discuss possible observations to determine the combination of circumstances responsible for the spectrum.Comment: 9 pages, 4 figures, accepted by AJ for the August 2004 issu

Swift observation of Segue 1: constraints on sterile neutrino parameters in the darkest galaxy

Some extensions of standard particle physics postulate that dark matter may be partially composed of weakly interacting sterile neutrino particles that have so far eluded detection. We use a short (~5 ks) archival X-ray observation of Segue 1 obtained with the X-ray Telescope (XRT) onboard the Swift satellite to exclude the presence of sterile neutrinos in the 1.6 - 14 keV mass range down to a flux limit of 6 x 10^{-12} erg cm-2 s-1 within 67 pc of its centre. With an estimated mass-to-light ratio of ~3400 Msun/Lsun, Segue 1 is the darkest ultrafaint dwarf galaxy currently measured. Spectral analysis of the Swift XRT data fails to find any non-instrumental spectral feature possibly connected with the radiative decay of a dark matter particle. Accordingly, we establish upper bounds on the sterile neutrino parameter space based on the non-detection of emission lines in the spectrum. The present work provides the most sensitive X-ray search for sterile neutrinos in a region with the highest dark matter density yet measured.Comment: 5 pages, 3 figures. Accepted in MNRAS Letter

Dark matter dominated dwarf disc galaxy Segue 1

Several observations reveal that dwarf galaxy Segue 1 has a dark matter (DM) halo at least ~ 200 times more massive than its visible baryon mass of only ~ 103 solar masses. The baryon mass is dominated by stars with perhaps an interstellar gas mass of < 13 solar masses. Regarding Segue 1 as a dwarf disc galaxy by its morphological appearance of long stretch, we invoke the dynamic model of Xiang-Gruess, Lou & Duschl (XLD) to estimate its physical parameters for possible equilibria with and without an isopedically magnetized gas disc. We estimate the range of DM mass and compare it with available observational inferences. Due to the relatively high stellar velocity dispersion compared to the stellar surface mass density, we find that a massive DM halo would be necessary to sustain disc equilibria. The required DM halo mass agrees grossly with observational inferences so far. For an isopedic magnetic field in a gas disc, the ratio f between the DM and baryon potentials depends strongly on the magnetic field strength. Therefore, a massive DM halo is needed to counteract either the strong stellar velocity dispersion and rotation of the stellar disc or the magnetic Lorentz force in the gas disc. By the radial force balances, the DM halo mass increases for faster disc rotation.Comment: 5 pages, 2 figures, accepted for publication in MNRA

Segue 2: A Prototype of the Population of Satellites of Satellites

We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope. From this, we derive a luminosity of M_v = -2.5, a half-light radius of 34 pc and a systemic velocity of -40$ km/s. Our MMT data also provides evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighboring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km/s and the possible stream as about 7 km/s. This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites -- survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z > 10 and are good candidates for fossils of the reionization epoch.Comment: MNRAS, Submitte

Multi-Element Abundance Measurements from Medium-Resolution Spectra. III. Metallicity Distributions of Milky Way Dwarf Satellite Galaxies

We present metallicity distribution functions (MDFs) for the central regions of eight dwarf satellite galaxies of the Milky Way: Fornax, Leo I and II, Sculptor, Sextans, Draco, Canes Venatici I, and Ursa Minor. We use the published catalog of abundance measurements from the previous paper in this series. The measurements are based on spectral synthesis of iron absorption lines. For each MDF, we determine maximum likelihood fits for Leaky Box, Pre-Enriched, and Extra Gas (wherein the gas supply available for star formation increases before it decreases to zero) analytic models of chemical evolution. Although the models are too simplistic to describe any MDF in detail, a Leaky Box starting from zero metallicity gas fits none of the galaxies except Canes Venatici I well. The MDFs of some galaxies, particularly the more luminous ones, strongly prefer the Extra Gas Model to the other models. Only for Canes Venatici I does the Pre-Enriched Model fit significantly better than the Extra Gas Model. The best-fit effective yields of the less luminous half of our galaxy sample do not exceed 0.02 Z_sun, indicating that gas outflow is important in the chemical evolution of the less luminous galaxies. We surmise that the ratio of the importance of gas infall to gas outflow increases with galaxy luminosity. Strong correlations of average [Fe/H] and metallicity spread with luminosity support this hypothesis.Comment: 17 pages, 5 figures; accepted for publication in ApJ; minor corrections in v3; corrected typographical errors in Tables 1 and 3 in v

High Resolution HDS/SUBARU chemical abundances of the young stellar cluster Palomar 1

Context. Palomar\,1 is a peculiar globular cluster (GC). It is the youngest Galactic GC and it has been tentatively associated to several of the substructures recently discovered in the Milky Way (MW), including the Canis Major (CMa) overdensity and the Galactic Anticenter Stellar Structure (GASS). Aims. In order to provide further insights into its origin, we present the first high resolution chemical abundance analysis for one red giant in Pal\,1. Methods. We obtained high resolution (R=30000) spectra for one red giant star in Pal\,1 using the High Dispersion Spectrograph (HDS) mounted at the SUBARU telescope. We used ATLAS-9 model atmospheres coupled with the SYNTHE and WIDTH calculation codes to derive chemical abundances from the measured line equivalent widths of 18 among $\alpha$, Iron-peak, light and heavy elements. Results. The Palomar~1 chemical pattern is broadly compatible to that of the MW open clusters population and similar to disk stars. It is, instead, remarkably different from that of the Sagittarius (Sgr) dwarf spheroidal galaxy. Conclusions. If Pal\,1 association with either CMa or GASS will be confirmed, this will imply that these systems had a chemical evolution similar to that of the Galactic disk.Comment: 13 pages, 9 figures, 5 tables. Accepted for publication in Astronomy and Astrophysics. Minor changes in the tex

A Lyman-alpha-only AGN from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has discovered a z=2.4917 radio-loud active galactic nucleus (AGN) with a luminous, variable, low-polarization UV continuum, H I two-photon emission, and a moderately broad Lyman-alpha line (FWHM = 1430 km/s) but without obvious metal-line emission. SDSS J113658.36+024220.1 does have associated metal-line absorption in three distinct, narrow systems spanning a velocity range of 2710 km/s. Despite certain spectral similarities, SDSS J1136+0242 is not a Lyman-break galaxy. Instead, the Ly-alpha and two-photon emission can be attributed to an extended, low-metallicity narrow-line region. The unpolarized continuum argues that we see SDSS J1136+0242 very close to the axis of any ionization cone present. We can conceive of two plausible explanations for why we see a strong UV continuum but no broad-line emission in this `face-on radio galaxy' model for SDSS J1136+0242: the continuum could be relativistically beamed synchrotron emission which swamps the broad-line emission; or, more likely, SDSS J1136+0242 could be similar to PG 1407+265, a quasar in which for some unknown reason the high-ionization emission lines are very broad, very weak, and highly blueshifted.Comment: AJ, in press, 10 pages emulateapj forma