Christopher Palma

On the Distribution of Orbital Poles of Milky Way Satellites

In numerous studies of the outer Galactic halo some evidence for accretion has been found. If the outer halo did form in part or wholly through merger events, we might expect to find coherent streams of stars and globular clusters following orbits similar to those of their parent objects, which are assumed to be present or former Milky Way dwarf satellite galaxies. We present a study of this phenomenon by assessing the likelihood of potential descendant dynamical families in the outer halo. We conduct two analyses: one that involves a statistical analysis of the spatial distribution of all known Galactic dwarf satellite galaxies (DSGs) and globular clusters, and a second, more specific analysis of those globular clusters and DSGs for which full phase space dynamical data exist. In both cases our methodology is appropriate only to members of descendant dynamical families that retain nearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) is considered a paradigm for the type of merger/tidal interaction event for which we are searching, we also undertake a case study of the Sgr system and identify several globular clusters that may be members of its extended dynamical family. In our first analysis, the distribution of possible orbital poles for the entire sample of outer (Rgc > 8 kpc) halo globular clusters is tested for statistically significant associations among globular clusters and DSGs. Our methodology for identifying possible associations is similar to that used by Lynden-Bell & Lynden-Bell, but we put the associations on a more statistical foundation. Moreover, we study the degree of possible dynamical clustering among various interesting ensembles of globular clusters and satellite galaxies. Among the ensembles studied, we find the globular cluster subpopulation with the highest statistical likelihood of association with one or more of the Galactic DSGs to be the distant, outer halo (Rgc > 25 kpc), second-parameter globular clusters. The results of our orbital pole analysis are supported by the great circle cell count methodology of Johnston, Hernquist, & Bolte. The space motions of the clusters Pal 4, NGC 6229, NGC 7006, and Pyxis are predicted to be among those most likely to show the clusters to be following stream orbits, since these clusters are responsible for the majority of the statistical significance of the association between outer halo, second-parameter globular clusters and the Milky Way DSGs. In our second analysis, we study the orbits of the 41 globular clusters and six Milky Way-bound DSGs having measured proper motions to look for objects with both coplanar orbits and similar angular momenta. Unfortunately, the majority of globular clusters with measured proper motions are inner halo clusters that are less likely to retain memory of their original orbit. Although four potential globular cluster/DSG associations are found, we believe three of these associations involving inner halo clusters to be coincidental. While the present sample of objects with complete dynamical data is small and does not include many of the globular clusters that are more likely to have been captured by the Milky Way, the methodology we adopt will become increasingly powerful as more proper motions are measured for distant Galactic satellites and globular clusters, and especially as results from the Space Interferometry Mission (SIM) become available.

Starcounts Redivivus. III. A Possible Detection of the Sagittarius Dwarf Spheroidal Galaxy at b = ?40?

As part of the Selected Areas Starcounts Survey, a CCD survey to V > 21, we have obtained VI photometry of two fields at b = ?40? aligned roughly with an extrapolation of the major axis of the Sagittarius dwarf spheroidal galaxy. Comparison of the color-magnitude diagram (CMD) for some of these fields with the CMDs of fields reflected about the Galactic l = 0? meridian reveals an excess of stars at V0 = 17.85 and 0.9 < (V-I)0 < 1.1 in the (l, b) = (11?, -40?) field. The excess stars have colors consistent with the Sgr red clump, and deeper CMD imaging in these locations shows evidence of a main-sequence turnoff (MSTO) at V = 21, with the main sequence extending to the limit of our data (V = 24). The surface brightnesses we derive from either the potential excess of red clump stars or the apparent excess of MSTO stars are consistent with each other and with the results of other surveys at this latitude. No similar excess appears in our northern Galactic hemisphere fields near the (l, b) = (353?, +41?) field. We have obtained spectroscopy of all 30 candidate red clump stars in the range 0.9 < (V-I)0 < 1.1and 17.75 < V0 < 17.95. The radial velocity distribution of the stars, while dissimilar from expectations of Galactic structure models, does not show a contribution by stars near the Galactocentric radial velocity seen in other studies near the Sgr core. It is difficult to reconcile a photometric result that is consistent with other explorations of the Sagittarius stream with a radial velocity distribution that is apparently inconsistent. In a companion paper, we discuss how some of the discrepancies are resolved if our potential Sgr detection corresponds to a different Sgr tidal streamer than that detected by most other surveys.

The Multiphase Absorption Systems toward PG 1206+459

A high-resolution (R = 30,000) ultraviolet spectrum is presented, which covers Lyα and many low-, intermediate-, and high-ionization transitions in the three Mg II-selected absorption systems toward the quasar PG 1206+459. Three systems (A, B, and C), which are clustered within 1500 km s-1 at z ~ 0.93, were originally identified in a spectrum obtained with the High Resolution Spectrograph (HIRES) on the Keck I Telescope. A WIYN (Wisconsin-Indiana-Yale-NOAO) Gunn i-band image of the quasar field and spectroscopy of two galaxy candidates are presented. A multiphase medium is seen in all three systems, consistent with smaller, denser clouds producing low-ionization transitions (Mg II, Fe II, and Si II) and larger, diffuse, photoionized clouds giving rise to higher ionization transitions (C IV, N V, and/or O VI). (1) System A, a multicloud, weak Mg II absorber at z = 0.9254, requires a supersolar metallicity in both low- and high-ionization phases, unless an α-group enhancement is included. The low-ionization absorption is produced in clouds with sizes of 10-70 pc, which are surrounded in velocity space by broader, high-ionization components. With the unusually complex velocity structure resolved in the N V profiles, this system is unlikely to represent a traditional galaxy disk/corona. The most likely candidate host galaxy is a ~2L*, apparently warped, spiral at an impact parameter of 43 h-1 kpc. (2) System B, at z = 0.9276, has the strongest Mg II absorption and has an approximately solar metallicity in the low-ionization phase. The smooth, broad high-ionization profiles may indicate a coronal structure similar to that of the Milky Way. The redshift of an L* galaxy (z = 0.9289), at an impact parameter of 38 h-1 kpc, is consistent with the redshift of this system. (3) System C, at z = 0.9342, has a single component in Mg II, separated from the other two systems by ~+1000 km s-1. The Lyα profile is not aligned with the Mg II, requiring an additional velocity component offset by -40 km s-1. System C lacks the small, low-ionization cloud characteristic of an isolated single-cloud, weak Mg II absorber. Its absorption properties are similar to the satellite clouds of classic strong Mg II absorbers, so this could be a high-velocity cloud in the galaxy group responsible for the systems, possibly related to a 0.2L* galaxy at an impact parameter of 43 h-1 kpc.

An H i threshold for star cluster formation in tidal debris

Superstar clusters are young, compact star clusters found in the central regions of interacting galaxies. Recently, they have also been reported to preferentially form in certain tidal tails, but not in others. In this paper, we have used 21-cm H I maps and the Hubble Space Telescope Wide Field Planetary Camera 2 images of eight tidal tail regions of four merging galaxy pairs to compare the kiloparsec scale H I distribution with the location of superstar clusters found from the optical images. For most of the tails, we find that there is an increase in superstar cluster density with increasing projected H I column density, such that the star cluster density is highest when log N HI ≥ 20.6 cm -2 , but equal to the background count rate at lower H I column density. However, for two tails (NGC 4038/39 Pos A and 3921), there is no significant star cluster population despite the presence of gas at high column density. This implies that the N HI threshold is a necessary but not sufficient condition for cluster formation. Gas volume density is likely to provide a more direct criterion for cluster formation, and other factors such as gas pressure or strength of encounter may also have an influence. Comparison of H I thresholds needed for formation of different types of stellar structures await higher resolution H I and optical observations of larger numbers of interacting galaxies.

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155–304 in 1994 May. I. The Ground-based Campaign

Optical, near-infrared, and radio observations of the BL Lac object PKS 2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were gathered throughout most of 1994. The radio, millimeter, and near-infrared data show no strong correlations with the higher energies. The optical light curves exhibit flickering of 0.2-0.3 mag on timescales of 1-2 days, superposed on longer timescale variations. Rapid variations of ~0.01 mag minute-1, if real, are the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases in the V and R bands occur simultaneously with a flare seen at higher energies. All optical wave bands (UBVRI) track each other well over the period of observation, with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for the colors (in particular, B-V) to vary more when the source fades. In polarized light, PKS 2155-304 showed strong color dependence (polarization increases toward the blue, PU/PI = 1.31) and the highest optical polarization (U = 14.3%) ever observed for this source. The polarization variations trace the flares seen in the UV flux. For the fastest variability timescale observed, we estimate a central black hole mass of 1.5 × 109(δ/10) M☉, consistent with UV and X-ray constraints and smaller than previously calculated for this object.

EXPLORING HALO SUBSTRUCTURE WITH GIANT STARS III: FIRST RESULTS FROM THE GRID GIANT STAR SURVEY AND DISCOVERY OF A POSSIBLE NEARBY SAGITTARIUS TIDAL STRUCTURE IN VIRGO

We describe first results of a spectroscopic probe of selected fields from the Grid Giant Star Survey. Multifiber spectroscopy of several hundred stars in a strip of 11 fields along δ ≈ -17°, in the range 12 hr α 17 hr, reveals a group of eight giants that have kinematical characteristics differing from the main field population but that as a group maintain coherent, smoothly varying distances and radial velocities with position across the fields. Moreover, these stars have roughly the same abundance, according to their MgH+Mgb absorption line strengths. Photometric parallaxes place these stars in a semiloop structure, arcing in a contiguous distribution between 5.7 and 7.9 kpc from the Galactic center. The spatial, kinematical, and abundance coherence of these stars suggests that they are part of a diffuse stream of tidal debris, and one roughly consistent with a wrapped, leading tidal arm of the Sagittarius dwarf spheroidal galaxy.

The Beginning of the End: Hubble Space Telescope Images of Seyfert’s Sextet

Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images of Hickson Compact Group (HCG) 79, Seyferts Sextet, are presented. Both point sources and extended sources detected on the three wide-field chips were photometered in four filters: F336W, F439W, F555W, and F814W. Unlike other HCGs that have been imaged with HST, there do not appear to be any candidate young star clusters among the detected point sources. The majority of the point sources that may be star clusters associated with the Sextet have red colors consistent with stellar populations older than 1 Gyr. A similar conclusion is drawn with regard to the extended sources. The majority of these appear to be background galaxies, but a few candidate dwarf galaxies are identified as potentially associated with Seyferts Sextet. However, no blue, star-forming objects similar to the tidal dwarf galaxy candidates identified in other HCGs are found among the extended objects identified in this study. A redshift for one dwarf galaxy candidate was measured from a spectrum obtained with the Hobby-Eberly Telescope, and this object was found to have a redshift similar to NGC 6027e, the discordant spiral formerly identified as a member of this compact group. The HST observations presented here and previous radio observations of the neutral gas content of this group suggest that the interactions that have taken place in the Sextet only redistributed the stars from the member galaxies within the group. We speculate that future interactions may be strong enough to strip the gas from NGC 6027d and trigger star-cluster formation.

Spectroscopy of Giant Stars in the Pyxis Globular Cluster

The Pyxis globular cluster is a recently discovered globular cluster that lies in the outer halo (Rgc ~ 40 kpc) of the Milky Way. Pyxis lies along one of the proposed orbital planes of the Large Magellanic Cloud (LMC), and it has been proposed to be a detached LMC globular cluster captured by the Milky Way. We present the first measurement of the radial velocity of the Pyxis globular cluster based on spectra of six Pyxis giant stars. The mean heliocentric radial velocity is ~36 km s–1, and the corresponding velocity of Pyxis with respect to a stationary observer at the position of the Sun is ~–191 km s–1. This radial velocity is a large enough fraction of the clusters expected total space velocity—assuming that it is bound to the Milky Way—that it allows strict limits to be placed on the range of permissible transverse velocities that Pyxis could have in the case that it still shares or nearly shares an orbital pole with the LMC. We can rule out that Pyxis is on a near circular orbit if it is Magellanic debris, but we cannot rule out an eccentric orbit associated with the LMC. We have calculated the range of allowed proper motions for the Pyxis globular cluster that result in the cluster having an orbital pole within 15° of the present orbital pole of the LMC and that are consistent with our measured radial velocity, but verification of the tidal capture hypothesis must await proper motion measurement from the Space Interferometry Mission or Hubble Space Telescope. A spectroscopic metallicity estimate of [Fe/H] = -1.4±0.1 is determined for Pyxis from several spectra of its brightest giant; this is consistent with photometric determinations of the cluster metallicity from isochrone fitting.

A Catalog of Absorption Lines in Eight HST/STIS E230M 1.0 < z < 1.7 Quasar Spectra

We have produced a catalog of line identifications and equivalent width measurements for all absorption features in eight ultraviolet echelle quasar spectra. These spectra were selected as having the highest signal-to-noise among the HST/STIS spectra obtained with the E230M grating. We identify 56 metal-line systems toward the eight quasars, and present plots of detected transitions, aligned in velocity-space. We found that about 1/4 - 1/3 of the features in the Lya forest region, redward of the incidence of the Lyb forest, are metal lines. High ionization transitions are common. We see both O VI and C IV in 88 - 90% of the metal-line systems for which the spectra cover the expected wavelength. Si III is seen in 58%, while low ionization absorption in C II, Si II, and/or Al II is detected in 50% of the systems for which they are covered. This catalog will facilitate future studies of the Lya forest and of metal-line systems of various types.

A catalogue of absorption lines in eight Hubble Space Telescope/STIS E230M 1.0 < z < 1.7 quasar spectra

We have produced a catalog of line identifications and equivalent width measurements for all absorption features in eight ultraviolet echelle quasar spectra. These spectra were selected as having the highest signal-to-noise among the HST/STIS spectra obtained with the E230M grating. We identify 56 metal-line systems toward the eight quasars, and present plots of detected transitions, aligned in velocity-space. We found that about 1/4 - 1/3 of the features in the Lya forest region, redward of the incidence of the Lyb forest, are metal lines. High ionization transitions are common. We see both O VI and C IV in 88 - 90% of the metal-line systems for which the spectra cover the expected wavelength. Si III is seen in 58%, while low ionization absorption in C II, Si II, and/or Al II is detected in 50% of the systems for which they are covered. This catalog will facilitate future studies of the Lya forest and of metal-line systems of various types.