arXiv subjects

Jay Strader

Publications and source records attributed to Jay Strader.

At least 217 records · Page 12Linked to original sources

arXiv2014

The Most Distant Stars in the Milky Way

We report on the discovery of the most distant Milky Way (MW) stars known to date: ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0. These stars were selected as M giant candidates based on their infrared and optical colors and lack of proper motions. We spectroscopically confirmed them as outer halo giants using the MMT/Red Channel spectrograph. Both stars have large estimated distances, with ULAS J001535.72$+$015549.6 at $274 \pm 74$ kpc and ULAS J074417.48$+$253233.0 at 238 $\pm$ 64 kpc, making them the first MW stars discovered beyond 200 kpc. ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0 are both moving away from the Galactic center at $52 \pm 10$ km s$^{-1}$ and $24 \pm 10$ km s$^{-1}$, respectively. Using their distances and kinematics, we considered possible origins such as: tidal stripping from a dwarf galaxy, ejection from the MW's disk, or membership in an undetected dwarf galaxy. These M giants, along with two inner halo giants that were also confirmed during this campaign, are the first to map largely unexplored regions of our Galaxy's outer halo.

astro-ph.SR↗

arXiv2014

Kinematics and simulations of the stellar stream in the halo of the Umbrella Galaxy

We study the dynamics of faint stellar substructures around the Umbrella Galaxy, NGC 4651, which hosts a dramatic system of streams and shells formed through the tidal disruption of a nucleated dwarf elliptical galaxy. We elucidate the basic characteristics of the system (colours, luminosities, stellar masses) using multi-band Subaru/Suprime-Cam images. The implied stellar mass-ratio of the ongoing merger event is about 1:50. We identify candidate kinematic tracers (globular clusters, planetary nebulae, H ii regions), and follow up a subset with Keck/DEIMOS spectroscopy to obtain velocities. We find that 15 of the tracers are likely associated with halo substructures, including the probable stream progenitor nucleus. These objects delineate a kinematically cold feature in position-velocity phase space. We model the stream using single test-particle orbits, plus a rescaled pre-existing N-body simulation. We infer a very eccentric orbit with a period of roughly 0.35 Gyr and turning points at approximately 2-4 and 40 kpc, implying a recent passage of the satellite through the disc, which may have provoked the visible disturbances in the host galaxy. This work confirms that the kinematics of low surface brightness substructures can be recovered and modeled using discrete tracers - a breakthrough that opens up a fresh avenue for unraveling the detailed physics of minor merging.

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arXiv2014

The AIMSS Project I: Bridging the Star Cluster - Galaxy Divide

We describe the structural and kinematic properties of the first compact stellar systems discovered by the AIMSS project. These spectroscopically confirmed objects have sizes ($\sim$6 $<$ R$_{\rm e}$ [pc] $<$ 500) and masses ($\sim$2$\times$10$^{6}$ $<$ M$_*$/M$_\odot$ $<$ 6$\times$10$^{9}$) spanning the range of massive globular clusters (GCs), ultra compact dwarfs (UCDs) and compact elliptical galaxies (cEs), completely filling the gap between star clusters and galaxies. Several objects are close analogues to the prototypical cE, M32. These objects, which are more massive than previously discovered UCDs of the same size, further call into question the existence of a tight mass-size trend for compact stellar systems, while simultaneously strengthening the case for a universal "zone of avoidance" for dynamically hot stellar systems in the mass-size plane. Overall, we argue that there are two classes of compact stellar systems: 1) massive star clusters and 2) a population closely related to galaxies. Our data provide indications for a further division of the galaxy-type UCD/cE population into two groups, one population that we associate with objects formed by the stripping of nucleated dwarf galaxies, and a second population that formed through the stripping of bulged galaxies or are lower-mass analogues of classical ellipticals. We find compact stellar systems around galaxies in low to high density environments, demonstrating that the physical processes responsible for forming them do not only operate in the densest clusters.

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arXiv2014

1FGL J0523.5-2529: A New Probable Gamma-ray Pulsar Binary

We report optical photometric and SOAR spectroscopic observations of an X-ray source found within the localization error of the Fermi-LAT unidentified gamma-ray source J0523.5-2529. The optical data show periodic flux modulation and radial velocity variations indicative of a binary with a 16.5-hr period. The data suggest a massive non-degenerate secondary (~> 0.8 M_sun), and we argue the source is likely a pulsar binary. The radial velocities have good phase coverage and show evidence for a measurable eccentricity (e=0.04). There is no clear sign of irradiation of the secondary in either photometry or spectroscopy. The spatial location out of the Galactic plane and gamma-ray luminosity of the source are more consistent with classification as a recycled millisecond pulsar than as a young pulsar. Future radio timing observations can confirm the identity of the primary and further characterize this interesting system.

astro-ph.HE↗

arXiv2014

The SLUGGS survey: Exploring the metallicity gradients of nearby early-type galaxies to large radii

Stellar metallicity gradients in the outer regions of galaxies are a critical tool for disentangling the contributions of in-situ and ex-situ formed stars. In the two-phase galaxy formation scenario, the initial gas collapse creates steep metallicity gradients, while the accretion of stars formed in satellites tends to flatten these gradients in the outskirts, particularly for massive galaxies. This work presents the first compilation of extended metallicity profiles over a wide range of galaxy mass. We use the DEIMOS spectrograph on the Keck telescope in multi-slit mode to obtain radial stellar metallicity profiles for 22 nearby early-type galaxies. From the calcium triplet lines in the near-infrared we measure the metallicity of the starlight up to 3 effective radii. We find a relation between the outer metallicity gradient and galaxy mass, in the sense that lower mass systems show steeper metallicity gradients than more massive galaxies. This result is consistent with a picture in which the ratio of ex-situ to in-situ formed stars is lower in less massive galaxies as a consequence of the smaller contribution by accretion. In addition, we infer a correlation between the strength of the calcium triplet feature in the near-infrared and the stellar initial mass function slope that is consistent with recent models in the literature.

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arXiv2014

The SLUGGS Survey: HST/ACS Mosaic Imaging of the NGC 3115 Globular Cluster System

We present HST/ACS $g$ and $z$ photometry and half-light radii $R_{\rm h}$ measurements of 360 globular cluster (GC) candidates around the nearby S0 galaxy NGC 3115. We also include Subaru/Suprime-Cam $g$, $r$, and $i$ photometry of 421 additional candidates. The well-established color bimodality of the GC system is obvious in the HST/ACS photometry. We find evidence for a "blue tilt" in the blue GCs, wherein the blue GCs get redder as luminosity increases, indicative of a mass-metallicity relationship. We find a color gradient in both the red and blue subpopulations, with each group of clusters becoming bluer at larger distances from NGC 3115. The gradient is of similar strength in both subpopulations, but is monotonic and more significant for the blue clusters. On average, the blue clusters have ~10% larger $R_{\rm h}$ than the red clusters. This average difference is less than is typically observed for early-type galaxies but does match that measured in the literature for M104, suggesting that morphology and inclination may affect the measured size difference between the red and blue clusters. However, the scatter on the $R_{\rm h}$ measurements is large. We also identify 31 clusters more extended than typical GCs, which we consider ultra-compact dwarf (UCD) candidates. Many of these objects are fainter than typical UCDs. While it is likely that a significant number will be background contaminants, six of these UCD candidates are spectroscopically confirmed. To explore low-mass X-ray binaries in the GC system, we match our ACS and Suprime-Cam detections to corresponding Chandra X-ray sources. We identify 45 X-ray - GC matches, 16 among the blue subpopulation and 29 among the red subpopulation. These X-ray/GC coincidence fractions are larger than is typical for most GC systems, probably due to the increased depth of the X-ray data compared to previous studies of GC systems.

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arXiv2014

AGN Feedback in the Hot Halo of NGC 4649

Using the deepest available $\textit{Chandra}$ observations of NGC 4649 we find strong evidences of cavities, ripples and ring like structures in the hot interstellar medium (ISM) that appear to be morphologically related with the central radio emission. These structures show no significant temperature variations in correspondence with higher pressure regions ($0.5\mbox{kpc}<r<3\mbox{kpc}$). On the same spatial scale, a discrepancy between the mass profiles obtained from stellar dynamic and $\textit{Chandra}$ data represents the telltale evidence of a significant non-thermal pressure component in this hot gas, which is related to the radio jet and lobes. On larger scale we find agreement between the mass profile obtained form $\textit{Chandra}$ data and planetary nebulae and globular cluster dynamics. The nucleus of NGC 4649 appears to be extremely radiatively inefficient, with highly sub-Bondi accretion flow. Consistently with this finding, the jet power evaluated from the observed X-ray cavities implies that a small fraction of the accretion power calculated for the Bondi mass accretion rate emerges as kinetic energy. Comparing the jet power to radio and nuclear X-ray luminosity the observed cavities show similar behavior to those of other giant elliptical galaxies.

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arXiv2014

A Globular Cluster Toward M87 with a Radial Velocity < -1000 km/s: The First Hypervelocity Cluster

We report the discovery of an object near M87 in the Virgo Cluster with an extraordinary blueshift of -1025 km/s, offset from the systemic velocity by >2300 km/s. Evaluation of photometric and spectroscopic data provides strong evidence that this object is a distant massive globular cluster, which we call HVGC-1 in analogy to Galactic hypervelocity stars. We consider but disfavor more exotic interpretations, such as a system of stars bound to a recoiling black hole. The odds of observing an outlier as extreme as HVGC-1 in a virialized distribution of intracluster objects are small; it appears more likely that the cluster was (or is being) ejected from Virgo following a three-body interaction. The nature of the interaction is unclear, and could involve either a subhalo or a binary supermassive black hole at the center of M87.

astro-ph.GA↗

arXiv2014

Ultra compact dwarfs in the Perseus Cluster: UCD formation via tidal stripping

We present the results of a Keck/DEIMOS survey of Ultra Compact Dwarfs (UCDs) in the Perseus Cluster core. We confirm cluster membership for 14 UCDs, with radial velocities ~5300 km s$^{-1}$. Two of these confirmed Perseus UCDs have extremely blue colours ($B-R < 0.6$ mag), reside in star forming filaments surrounding NGC 1275, and have likely formed as massive star clusters in the last ~100 Myr. We also measure a central velocity dispersion of a third, UCD13 ($σ_0 = 38 \pm 8$ km s$^{-1}$), the most extended UCD in our sample. We determine it to have radius $R_{e} = 85 \pm 1.1$ pc, a dynamical mass of ($2.3 \pm 0.8)\times10^{8}$ M$_{\odot}$, and a metallicity [Z/H]$= -0.52^{+0.33}_{-0.29}$ dex. UCD13 and the cluster's central galaxy, NGC 1275, have a projected separation of 30 kpc and a radial velocity difference of ~20 km s$^{-1}$. Based on its size, red colour, internal velocity dispersion, dynamical mass, metallicity and proximity to NGC 1275, we argue that UCD13 is likely the remnant nucleus of a tidally stripped dE, with this progenitor dE having $M_{B} \approx -16$ mag and mass $\sim10^{9}$ M$_{\odot}$.

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arXiv2014

Hunting The Most Distant Stars in the Milky Way: Methods and Initial Results

We present a new catalog of 404 M giant candidates found in the UKIRT Infrared Deep Sky Survey (UKIDSS). The 2,400 deg$^2$ available in the UKIDSS Large Area Survey Data Release 8 resolve M giants through a volume four times larger than that of the entire Two Micron All Sky Survey. Combining near-infrared photometry with optical photometry and proper motions from the Sloan Digital Sky Survey yields an M giant candidate catalog with less M dwarf and quasar contamination than previous searches for similarly distant M giants. Extensive follow-up spectroscopy of this sample will yield the first map of our Galaxy's outermost reaches over a large area of sky. Our initial spectroscopic follow-up of $\sim$ 30 bright candidates yielded the positive identification of five M giants at distances $\sim 20-90$ kpc. Each of these confirmed M giants have positions and velocities consistent with the Sagittarius stream. The fainter M giant candidates in our sample have estimated photometric distances $\sim 200$ kpc (assuming $[Fe/H]$ = 0.0), but require further spectroscopic verification. The photometric distance estimates extend beyond the Milky Way's virial radius, and increase by $\sim 50\%$ for each 0.5 dex decrease in assumed $[Fe/H]$. Given the number of M giant candidates, initial selection efficiency, and volume surveyed, we loosely estimate that at least one additional Sagittarius-like accretion event could have contributed to the hierarchical build-up of the Milky Way's outer halo.

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arXiv2013

A Radio-Selected Black Hole X-ray Binary Candidate in the Milky Way Globular Cluster M62

We report the discovery of a candidate stellar-mass black hole in the Milky Way globular cluster M62. We detected the black hole candidate, which we term M62-VLA1, in the core of the cluster using deep radio continuum imaging from the Karl G. Jansky Very Large Array. M62-VLA1 is a faint source, with a flux density of 18.7 +/- 1.9 microJy at 6.2 GHz and a flat radio spectrum (alpha=-0.24 +/- 0.42, for S_nu = nu^alpha). M62 is the second Milky Way cluster with a candidate stellar-mass black hole; unlike the two candidate black holes previously found in the cluster M22, M62-VLA1 is associated with a Chandra X-ray source, supporting its identification as a black hole X-ray binary. Measurements of its radio and X-ray luminosity, while not simultaneous, place M62-VLA1 squarely on the well-established radio--X-ray correlation for stellar-mass black holes. In archival Hubble Space Telescope imaging, M62-VLA1 is coincident with a star near the lower red giant branch. This possible optical counterpart shows a blue excess, H alpha emission, and optical variability. The radio, X-ray, and optical properties of M62-VLA1 are very similar to those for V404 Cyg, one of the best-studied quiescent stellar-mass black holes. We cannot yet rule out alternative scenarios for the radio source, such as a flaring neutron star or background galaxy; future observations are necessary to determine whether M62-VLA1 is indeed an accreting stellar-mass black hole.

astro-ph.HE↗

arXiv2013

Star Clusters in M31. V. Evidence for Self-Enrichment in Old M31 Clusters from Integrated Spectroscopy

In the past decade, the notion that globular clusters (GCs) are composed of coeval stars with homogeneous initial chemical compositions has been challenged by growing evidence that they host an intricate stellar population mix, likely indicative of a complex history of star formation and chemical enrichment. Several models have been proposed to explain the existence of multiple stellar populations in GCs, but no single model provides a fully satisfactory match to existing data. Correlations between chemistry and global parameters such as cluster mass or luminosity are fundamental clues to the physics of GC formation. In this Letter, we present an analysis of the mean abundances of Fe, Mg, C, N, and Ca for 72 old GCs from the Andromeda galaxy. We show for the first time that there is a correlation between the masses of GCs and the mean stellar abundances of nitrogen, spanning almost two decades in mass. This result sheds new light on the formation of GCs, providing important constraints on their internal chemical evolution and mass loss history.

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arXiv2013

The SLUGGS Survey: Wide Field Imaging of the Globular Cluster System of NGC 4278

We use multi-pointing HST ACS and wide field Subaru Suprime-Cam imaging to study the globular cluster system of the L* elliptical galaxy NGC 4278. We have also obtained a handful of new globular cluster spectra with Keck/DEIMOS. We determine the globular cluster surface density profile and use it to calculate the total number of globular clusters, finding the system to be slightly more populous than average for galaxies of its luminosity. We find clear evidence for bimodality in the globular cluster colour distribution and for a colour-magnitude relation in the blue subpopulation (a 'blue tilt'). We also find negative radial colour gradients in both colour subpopulations of equal strength which are similar in strength to those reported in other galaxies. The sizes of NGC 4278's globular clusters decrease with redder colours and increase with galactocentric radius. The ratio of the sizes of blue to red globular clusters is independent of galactocentric radius demonstrating that internal effects are responsible for the size difference with colour.

astro-ph.CO↗

arXiv2013

The SLUGGS Survey: outer triaxiality of the Fast Rotator elliptical NGC 4473

Systematic surveys of nearby early type galaxies (ETGs) using integral field unit spectrograph (IFU) data have revealed that galaxies can hide interesting structures only visible through kinematic studies. As part of their pioneering work, the ATLAS3D team have shown that most morphologically elliptical galaxies are centrally kinematically disk-like. Hence, while global morphology suggests that ellipticals are ellipsoidal/triaxial in shape, their central kinematics may be consistent with (inclined) oblate systems. Here we study the Fast Rotator elliptical galaxy: NGC 4473. Using slitlets, we obtain galaxy light kinematics out to unprecedentedly large galactocentric radii (2.5 effective radii). While we confirm the IFU results in the central regions, we find that at large galactocentric radii NGC 4473 exhibits a kinematic transition. In the outskirts, we observe clear minor and major axis rotation, a tell-tale sign of triaxiality, which agrees well with the galaxy's Hubble type. This outer "kinematically distinct halo" (KDH) may be expected from simulations of galaxy formation, and in this system contains around one-third of the stellar light. While this galaxy may be a special case, it suggests further investigation of the outskirts of galaxies is needed to confirm the new paradigm of galaxy classification.

astro-ph.CO↗

arXiv2013

The Densest Galaxy

We report the discovery of a remarkable ultra-compact dwarf galaxy around the massive Virgo elliptical galaxy NGC 4649 (M60), which we term M60-UCD1. With a dynamical mass of 2.0 x 10^8 M_sun but a half-light radius of only ~ 24 pc, M60-UCD1 is more massive than any ultra-compact dwarfs of comparable size, and is arguably the densest galaxy known in the local universe. It has a two-component structure well-fit by a sum of Sersic functions, with an elliptical, compact (r_h=14 pc; n ~ 3.3) inner component and a round, exponential, extended (r_h=49 pc) outer component. Chandra data reveal a variable central X-ray source with L_X ~ 10^38 erg/s that could be an active galactic nucleus associated with a massive black hole or a low-mass X-ray binary. Analysis of optical spectroscopy shows the object to be old (~> 10 Gyr) and of solar metallicity, with elevated [Mg/Fe] and strongly enhanced [N/Fe] that indicates light element self-enrichment; such self-enrichment may be generically present in dense stellar systems. The velocity dispersion (~ 70 km/s) and resulting dynamical mass-to-light ratio (M/L_V=4.9 +/- 0.7) are consistent with---but slightly higher than---expectations for an old, metal-rich stellar population with a Kroupa initial mass function. The presence of a massive black hole or a mild increase in low-mass stars or stellar remnants is therefore also consistent with this M/L_V. The stellar density of the galaxy is so high that no dynamical signature of dark matter is expected. However, the properties of M60-UCD1 suggest an origin in the tidal stripping of a nucleated galaxy with M_B ~ -18 to -19.

astro-ph.CO↗

arXiv2013

A Search for RR Lyrae Stars in Segue 2 and Segue 3

We present an extensive search for RR Lyrae stars in and around the ultra-faint Milky Way companions Segue 2 and Segue 3. The former (M_V = -2.5, Belokurov et al. 2009) appears to be an extremely faint dwarf galaxy companion of the Milky Way. The latter (M_V = 0.0, Fadely et al. 2011) is among the faintest star clusters known. We use B and V band time-series imaging obtained at the WIYN 0.9 meter telescope at Kitt Peak National Observatory to search for RR Lyrae in these objects. In our Segue 2 observations, we present a previously unknown fundamental mode (RRab) RR Lyrae star with a period of P_ab = 0.748 days. With this measurement, we revisit the inverse correlation between and <[Fe/H]> established in the literature for Milky Way dwarf galaxies and their RR Lyrae. In this context, the long period of Segue 2's RRab star as well as the known significant spread in metallicity in this dwarf galaxy are consistent with the observed trend in and <[Fe/H]>. We derive the first robust distance to Segue 2, using both its RRab star and spectroscopically confirmed blue horizontal branch stars. Using [Fe/H] = -2.16 and -2.44 dex, we find d_RRL = 36.6 +2.5/-2.4 and 37.7 +/- 2.7 kpc; assuming [Fe/H] = -2.257 dex, we find d_BHB = 34.4 +/- 2.6 kpc. Although no RR Lyrae were present in the Segue 3 field, we found a candidate eclipsing binary star system.

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arXiv2013

Filling the gap: a new class of old star cluster?

It is not understood whether long-lived star clusters possess a continuous range of sizes and masses (and hence densities), or if rather, they should be considered as distinct types with different origins. Utilizing the Hubble Space Telescope (HST) to measure sizes, and long exposures on the Keck 10m telescope to obtain distances, we have discovered the first confirmed star clusters that lie within a previously claimed size-luminosity gap dubbed the `avoidance zone' by Hwang et al (2011). The existence of these star clusters extends the range of sizes, masses and densities for star clusters, and argues against current formation models that predict well-defined size-mass relationships (such as stripped nuclei, giant globular clusters or merged star clusters). The red colours of these gap objects suggests that they are not a new class of object but are related to Faint Fuzzies observed in nearby lenticular galaxies. We also report a number of low luminosity UCDs with sizes of up to 50 pc. Future, statistically complete, studies will be encouraged now that it is known that star clusters possess a continuous range of structural properties.

astro-ph.CO↗

arXiv2013

The SLUGGS survey: Probing the supermassive black hole connection with bulges and haloes using red and blue globular cluster systems

Understanding whether the bulge or the halo provides the primary link to the growth of supermassive black holes has strong implications for galaxy evolution and supermassive black hole formation itself. In this paper, we approach this issue by investigating extragalactic globular cluster (GC) systems, which can be used to probe the physics of both the bulge and the halo of the host galaxy. We study the relation between the supermassive black hole masses M_BH and the globular cluster system velocity dispersions sigma_GC using an updated and improved sample of 21 galaxies. We exploit the dichotomy of globular cluster system colours, to test if the blue and red globular clusters correlate differently with black hole mass. This may be expected if they trace the potentially different formation history of the halo and of the bulge of the host galaxy respectively. We find that M_BH correlates with the total GC system velocity dispersion, although not as strongly as claimed by recent work of Sadoun & Colin. We also examine the M_BH - sigma_GC relation for barred/bar-less and core/non-core galaxies, finding no significant difference, and for the first time we quantify the impact of radial gradients in the GC system velocity dispersion profile on the M_BH - sigma_GC relation. We additionally predict M_BH in 13 galaxies, including dwarf elliptical galaxies and the cD galaxy NGC 3311. We conclude that our current results cannot discriminate between the bulge/halo scenario. Although there is a hint that the red GC velocity dispersion might correlate better with M_BH than the blue GC velocity dispersion, the number statistics are still too low to be certain.

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