arXiv subjects

C. Palma

Publications and source records attributed to C. Palma.

16 recordsLinked to original sources

arXiv2016

Effect of disorder on superconductivity and Rashba spin-orbit coupling in LaAlO3/SrTiO3 interfaces

A rather unique feature of the two-dimensional electron gas (2-DEG) formed at the interface between the two insulators LaAlO3 and SrTiO3 is to host both gate-tunable superconductivity and strong spin-orbit coupling. In the present work, we use the disorder generated by Cr substitution of Al atoms in LaAlO3 as a tool to explore the nature of superconductivity and spin-orbit coupling in these interfaces. A reduction of the superconducting Tc is observed with Cr doping consistent with an increase of electron-electron interaction in presence of disorder. In addition, the evolution of spin-orbit coupling with gate voltage and Cr doping suggests a D'Yakonov-Perel mechanism of spin relaxation in the presence of a Rashba-type spin-orbit interaction.

cond-mat.supr-con↗

arXiv2011

Star Clusters in the Tidal Tails of Interacting Galaxies: Cluster Populations Across a Variety of Tail Environments

We have searched for compact stellar structures within 17 tidal tails in 13 different interacting galaxies using F606W- and F814W- band images from the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). The sample of tidal tails includes a diverse population of optical properties, merging galaxy mass ratios, HI content, and ages. Combining our tail sample with Knierman et al. (2003), we find evidence of star clusters formed in situ with Mv < -8.5 and V-I < 2.0 in 10 of 23 tidal tails; we are able to identify cluster candidates to Mv = -6.5 in the closest tails. Three tails offer clear examples of "beads on a string" star formation morphology in V-I color maps. Two tails present both tidal dwarf galaxy (TDG) candidates and cluster candidates. Statistical diagnostics indicate that clusters in tidal tails may be drawn from the same power-law luminosity functions (with logarithmic slopes ~ -2 - -2.5) found in quiescent spiral galaxies and the interiors of interacting systems. We find that the tail regions with the largest number of observable clusters are relatively young (< 250 Myr old) and bright (V < 24 mag arcsec^(-2)), probably attributed to the strong bursts of star formation in interacting systems soon after periapse. Otherwise, we find no statistical difference between cluster-rich and cluster-poor tails in terms of many observable characteristics, though this analysis suffers from complex, unresolved gas dynamics and projection effects.

astro-ph.CO↗

arXiv2009

Tidal Tails in Interacting Galaxies: Formation of Compact Stellar Structures

We have used V- and I- band images from the Hubble Space Telescope (HST) to identify compact stellar clusters within the tidal tails of twelve different interacting galaxies. The seventeen tails within our sample span a physical parameter space of HI/stellar masses, tail pressure and density through their diversity of tail lengths, optical brightnesses, mass ratios, HI column densities, stage on the Toomre sequence, and tail kinematics. Our preliminary findings in this study indicate that star cluster demographics of the tidal tail environment are compatible with the current understanding of star cluster formation in quiescent systems, possibly only needing changes in certain parameters or normalization of the Schechter cluster initial mass function (CIMF) to replicate what we observe in color-magnitude diagrams and a brightest absolute magnitude -- log N plot.

astro-ph.CO↗

arXiv2007

An HI Threshold for Star Cluster Formation in Tidal Debris

Super star 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 HI 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 HI distribution with the location of super star clusters found from the optical images. For most of the tails, we find that there is an increase in super star cluster density with increasing projected HI 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 HI column density. However, for two tails (NGC 4038/39 Pos A and NGC 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 HI thresholds needed for formation of different types of stellar structures await higher resolution HI and optical observations of larger numbers of interacting galaxies.

astro-ph↗

arXiv2007

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.

astro-ph↗

arXiv2005

Exploring Halo Substructure with Giant Stars. VI. Extended Distributions of Giant Stars Around the Carina Dwarf Spheroidal Galaxy -- How Reliable Are They?

The question of the existence of active tidal disruption around various dSph galaxies remains controversial. That debate often centers on the nature (bound vs. unbound) of extended populations of stars. However, the more fundamental issue of the very existence of the extended populations is still contentious. We present an evaluation of the debate centering on one particular dSph, Carina, for which claims both for and against the existence of stars beyond the King radius have been made. Our review includes an examination of all previous studies bearing on the Carina radial profile and shows that the survey method which achieves the highest detected dSph signal-to-background in the outer parts of the galaxy is the Washington M, T2 + DDO51 (MTD) filter approach from Paper II in this series. We then address statistical methods used to evaluate the reliability of MTD surveys in the presence of photometric errors and for which a new, a posteriori statistical analysis methodology is provided. Finally, these statistical methods are tested by new spectroscopy of stars in the MTD-selected Carina candidate sample. Of 74 candidate giants with follow-up spectroscopy, the MTD technique identified 61 new Carina members, including 8 stars outside the King radius. From a sample of 29 stars not initially identified as candidate Carina giants but that lie just outside of our selection criteria, 12 have radial velocities consistent with membership, including 5 extratidal stars. Carina is shown to have an extended population of giant stars extending to a major axis radius of 40' (1.44x the nominal King radius).

astro-ph↗

arXiv2003

Tidal Streams around External Galaxies

We have the unique opportunity to observe and model nearby streams around the two large Local Group spirals Milky Way and M31 in great detail. However, the detection of streams around other external galaxies is required to verify the general application of the derived results. We give a short summary of streams around other galaxies known in the literature, measuring for the first time the surface brightness of Malin's M83 stream with modern CCD imaging. In addition, we present four new detections of possible stellar streams around disk galaxies.

astro-ph↗

arXiv2003

The Multi-phase Absorption Systems Toward PG 1206+459

A high-resolution HST/STIS spectrum (R=30,000) is presented of the three MgII-selected absorption systems (A, B, and C, clustered within 1500km/s) at z~0.93 toward the quasar PG 1206+459. A WIYN Gunn i-band image of the quasar field and spectroscopy of two galaxy candidates are also presented. A multi-phase medium is seen in all three systems, consistent with smaller, denser clouds producing low-ionization transitions (MgII, FeII, and SiII) and larger, diffuse clouds giving rise to high-ionization transitions (CIV, NV, and/or OVI). (1) System A, a multi-cloud, weak MgII absorber at z=0.9254, requires a super-solar metallicity in both low- and high-ionization phases, unless an alpha-group enhancement is included. With the unusually complex velocity structure resolved in the NV 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 43h^-1 kpc. (2) System B, at z=0.9276, has the strongest MgII 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 38h^-1 kpc is consistent with the redshift of this system. (3) System C, at z=0.9342, has a single component in MgII, separated from the other two systems by ~1000km/s. It lacks the small, low-ionization cloud characteristic of an isolated single-cloud, weak MgII absorber. Its absorption properties are similar to the ``satellite clouds'' of classic strong MgII 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 43h^-1 kpc.

astro-ph↗

arXiv2002

Exploring Halo Substructure with Giant Stars IV: The Extended Structure of the Ursa Minor Dwarf Spheroidal

We present a large area photometric survey of the Ursa Minor dSph. We identify UMi giant star candidates extending to ~3 deg from the center of the dSph. Comparison to previous catalogues of stars within the tidal radius of UMi suggests that our photometric luminosity classification is 100% accurate. Over a large fraction of the survey area, blue horizontal branch stars associated with UMi can also be identified. The spatial distribution of both the UMi giant stars and the BHB stars are remarkably similar, and a large fraction of both samples of stars are found outside the tidal radius of UMi. An isodensity contour map of the stars within the tidal radius of UMi reveals two morphological peculiarities: (1) The highest density of dSph stars is offset from the center of symmetry of the outer isodensity contours. (2) The overall shape of the outer contours appear S-shaped. We find that previously determined King profiles with ~50' tidal radii do not fit well the distribution of our UMi stars. A King profile with a larger tidal radius produces a reasonable fit, however a power law with index -3 provides a better fit for radii > 20'. The existence of UMi stars at large distances from the core of the galaxy, the peculiar morphology of the dSph within its tidal radius, and the shape of its surface density profile all suggest that UMi is evolving significantly due to the tidal influence of the Milky Way. However, the photometric data on UMi stars alone does not allow us to determine if the candidate extratidal stars are now unbound or if they remain bound to the dSph within an extended dark matter halo. (Abridged)

astro-ph↗

arXiv2002

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 eleven fields along delta approximately -17^{circ}, in the range 12 <~ alpha <~ 17 hours, reveals a group of 8 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 semi-loop 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.

astro-ph↗

arXiv2002

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

Hubble Space Telescope Wide Field Planetary Camera 2 images of Hickson Compact Group 79, Seyfert's Sextet, are presented. Both point sources and extended sources detected on the three WF 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 Seyfert's 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 NGC6027e, 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 NGC6027d and trigger star cluster formation.

astro-ph↗

arXiv2001

The Extended Shapes of Galactic Satellites

We are exploring the extended stellar distributions of Galactic satellite galaxies and globular clusters. For seven objects studied thus far, the observed profile departs from a King function at large r, revealing a ``break population'' of stars. In our sample, the relative density of the ``break'' correlates to the inferred M/L of these objects. We discuss opposing hypotheses for this trend: (1) Higher M/L objects harbor more extended dark matter halos that support secondary, bound, stellar ``halos''. (2) The extended populations around dwarf spheroidals (and some clusters) consist of unbound, extratidal debris from their parent objects, which are undergoing various degrees of tidal disruption. In this scenario, higher M/L ratios reflect higher degrees of virial non-equilibrium in the parent objects, thus invalidating a precept underlying the use of core radial velocities to obtain masses.

astro-ph↗

arXiv2000

Stable oscillating nonlinear beams in square-wave-biased-photorefractives

We demonstrate experimentally that in a centrosymmetric paraelectric non-stationary boundary conditions can dynamically halt the intrinsic instability of quasi-steady-state photorefractive self-trapping, driving beam evolution into a stable oscillating two-soliton-state configuration.

physics.optics↗

arXiv1999

Omega Centauri: Nucleus of a Milky Way Dwarf Spheroidal?

We focus on two aspects of Omega Cen that are rather uncharacteristic of globular clusters: its metallicity distribution and its orbit. We use (1) a Washington (M-T_2,M)_o color-magnitude diagram containing 130,000 stars, (2) DDO51 photometry to separate out giants from field dwarfs, and (3) a mapping of (M-T_2,M-DDO51)_o colors to [Fe/H] to derive a metallicity distribution function (MDF) for Omega Cen. The MDF is corrected for field giant contamination by a radial velocity membership study of a subsample of stars spanning all abundances. As with previous studies, the MDF is very wide with a peak near [Fe/H]=-1.7 and extending to both higher and lower abundance, but with few stars having [Fe/H] >~ -1.2. The orbit of Omega Cen is strongly retrograde, has small apogalacticon and is almost coplanar with the Milky Way disk -- i.e., an orbit unlike any known for almost all Galactic globular clusters, and unique among clusters with its horizontal branch type and [Fe/H]. We propose that the MDF and orbit of Omega Cen, when considered in light of other unusual Omega Cen properties are consistent with a picture wherein this object is the remains (nucleus) of a once larger satellite dwarf galaxy that has been substantially reduced by tidal stripping. A model wherein Omega Cen faced considerable stripping by the Galactic disk can account for the cluster's current state. A present-day counterpart of this process may be the Sagittarius dSph system, for which the globular cluster M54 (which has a mass similar to Omega Cen) has been purported to be the nucleus. A larger proto-Omega Cen on a retrograde orbit plowing through the Milky Way disk may have aided thick disk formation both through the contribution of tidally stripped stars and through substantial thin disk heating.

astro-ph↗

arXiv1999

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

As part of the Selected Areas Starcount Survey (SASS), 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 (Sgr) dwarf spheroidal galaxy. Comparison of the color-magnitude diagram (CMD) for some of these fields to the CMDs of fields reflected about the Galactic l=0 meridian reveals an excess of stars at V_o=17.85 and 0.9 < (V-I)_o < 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 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 l=353, b=+41 field. We have obtained spectroscopy of all 30 candidate red clump stars in the range 0.9 < (V-I)_o < 1.1 and 17.75 < V_o < 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 (Johnston et al. 1999), we discuss how some of the discrepancies are resolved if ur potential Sgr detection corresponds to a different Sgr tidal streamer than that detected by most other surveys.

astro-ph↗

arXiv1997

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

Optical, near-infrared, and radio observations of the BL Lac object PKS2155-304 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, superimposed on longer timescale variations. Rapid variations of ~0.01 mag/min, which, 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 wavebands (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 and the highest optical polarization (U = 14.3%) ever observed for this source. The polarization variations trace the flares seen in the ultraviolet flux.

astro-ph↗