Katherine L. Rhode

The SLUGGS Survey : kinematics for over 2500 globular clusters in 12 early-type galaxies

We present a spectro-photometric survey of 2522 extragalactic globular clusters (GCs) around twelve early-type galaxies, nine of which have not been published previously. Combining space‐based and multi‐colour wide field ground‐based imagi ng, with spectra from the Keck DEIMOS instrument, we obtain an average of 160 GC radial velocities per galaxy, with a high velocity precision of � 15 km s 1 per GC. After studying the photometric properties of the GC systems, such as their spatial and colour distribut ions, we focus on the kinematics of metal-poor (blue) and metal-rich (red) GC subpopulations to an average distance of � 8 effective radii from the galaxy centre. Our results show that for some systems the bimodality in GC colour is also present in GC kinematics. The kinematics of the red GC subpopulations are strongly coupled with the host galaxy stellar kinematics. The blue GC subpopulations are more dominated by random motions, especially in the outer regions, and decoupled fro m the red GCs. Peculiar GC kinematic profiles are seen in some galaxies: the blue GCs in NGC 821 rotate along the galaxy minor axis, whereas the GC system of the lenticular galaxy NGC 7457 appears to be strongly rotation supported in the outer region. We supplement our galaxy sample with data from the literature and carry out a number of tests to study the kinematic differences between the two GC subpopulations. We confirm that the GC kinematics are coupled with the host galaxy properties and find that the velocity kurtosis and the slope of their velocity dispersion profiles is different between the two GC subpopulations in more massive galaxies.

A black hole in a globular cluster

Globular star clusters contain thousands to millions of old stars packed within a region only tens of light years across. Their high stellar densities make it very probable that their member stars will interact or collide. There has accordingly been considerable debate about whether black holes should exist in these star clusters. Some theoretical work suggests that dynamical processes in the densest inner regions of globular clusters may lead to the formation of black holes of ∼1,000 solar masses. Other numerical simulations instead predict that stellar interactions will eject most or all of the black holes that form in globular clusters. Here we report the X-ray signature of an accreting black hole in a globular cluster associated with the giant elliptical galaxy NGC 4472 (in the Virgo cluster). This object has an X-ray luminosity of about 4u2009×u20091039u2009ergu2009s-1, which rules out any object other than a black hole in such an old stellar population. The X-ray luminosity varies by a factor of seven in a few hours, which excludes the possibility that the object is several neutron stars superposed.

THE GLOBULAR CLUSTER SYSTEMS OF THE EARLY-TYPE GALAXIES NGC 3379, NGC 4406, AND NGC 4594 AND IMPLICATIONS FOR GALAXY FORMATION

We have investigated the global properties of the globular cluster (GC) systems of three early-type galaxies: the Virgo Cluster elliptical NGC 4406, the field elliptical NGC 3379, and the field S0 galaxy NGC 4594. These galaxies were observed as part of a wide-field CCD survey of the GC populations of a large sample of normal galaxies beyond the Local Group. Images obtained with the Mosaic detector on the Kitt Peak 4 m telescope provide radial coverage to at least 24, or ~70–100 kpc. We use BVR photometry and image classification to select GC candidates and thereby reduce contamination from non-GCs, as well as Hubble Space Telescope WFPC2 data to help quantify the contamination that remains. The GC systems of all three galaxies have color distributions with at least two peaks and show modest negative color gradients. The proportions of blue GCs range from 60% to 70% of the total populations. The GC specific frequency (SN) of NGC 4406 is 3.5 ± 0.5, ~20% lower than past estimates and nearly identical to SN for the other Virgo Cluster elliptical included in our survey, NGC 4472. SN for NGC 3379 and NGC 4594 are 1.2 ± 0.3 and 2.1 ± 0.3, respectively; these are similar to past values, but the errors have been reduced by a factor of 2–3. We compare our results for the early-type sample (including NGC 4472) to models for the formation of massive galaxies and their GC systems. Of the scenarios we consider, a hierarchical merging picture, in which metal-poor GCs form at high redshift in protogalactic building blocks and metal-rich GC populations are built up over time during subsequent gas-rich mergers, appears most consistent with the data.

ALFALFA DISCOVERY OF THE NEARBY GAS-RICH DWARF GALAXY LEO P. I. H I OBSERVATIONS

The discovery of a previously unknown 21 cm H I line source identified as an ultra-compact high velocity cloud in the ALFALFA survey is reported. The H I detection is barely resolved by the Arecibo 305 m telescope {approx}4 beam and has a narrow H I linewidth (half-power full width of 24 km s{sup -1}). Further H I observations at Arecibo and with the Very Large Array corroborate the ALFALFA H I detection, provide an estimate of the H I radius, {approx}1 at the 5 Multiplication-Sign 10{sup 19} cm{sup -2} isophote, and show the cloud to exhibit a velocity field which, if interpreted as disk rotation, has an amplitude of {approx_equal}9.0 {+-} 1.5 km s{sup -1}. In other papers, Rhode et al. show the H I source to have a resolved stellar counterpart and ongoing star forming activity, while Skillman et al. reveal it as having extremely low metallicity: 12 + log (O/H) = 7.16 {+-} 0.04. The H I mass to stellar mass ratio of the object is found to be 2.6. We use the Tully-Fisher template relation in its baryonic form to obtain a distance estimate D{sub Mpc}=1.3{sup +0.9}{sub -0.5}. Additional constraints on the distance are also providedmorexa0» by the optical data of Rhode et al. and McQuinn et al., both indicating a distance in the range of 1.5 to 2.0 Mpc. The three estimates are compatible within their errors. The object appears to be located beyond the dynamical boundaries of, but still in close proximity to the Local Group. Its pristine properties are consistent with the sedate environment of its location. At a nominal distance of 1.75 Mpc, it would have an H I mass of {approx_equal} 1.0 Multiplication-Sign 10{sup 6} M {sub Sun }, a stellar mass of {approx_equal} 3.6 Multiplication-Sign 10{sup 5} M {sub Sun }, and a dynamical mass within the H I radius of {approx_equal} 1.5 Multiplication-Sign 10{sup 7} M {sub Sun }. This discovery supports the idea that optically faint-or altogether dark-low mass halos may be detectable through their non-stellar baryons.«xa0less

A Test of the Standard Hypothesis for the Origin of the H I Holes in Holmberg II

The nearby irregular galaxy Holmberg II (Ho II, DDO 50) has been extensively mapped in H I using the Very Large Array, revealing intricate structure in its interstellar gas component, as reported by Puche et al. in 1992. An analysis of these structures shows the neutral gas to contain a number of expanding H I holes. The formation of the H I holes has been attributed to multiple supernova (SN) events occurring within wind-blown shells around young, massive star clusters, with as many as 10–200 SNe required to produce many of the holes. From the sizes and expansion velocities of the holes, Puche et al. assigned ages of ~107 to 108 years. If the SN scenario for the formation of the H I holes is correct, it implies the existence of star clusters with a substantial population of late B, A, and F main-sequence stars at the centers of the holes. Many of these clusters should be detectable in deep ground-based CCD images of the galaxy. To test the SN hypothesis for the formation of the H I holes, we have obtained and analyzed deep broadband BVR and narrowband Hα images of Ho II. We compare the optical and H I data and search for evidence of the expected star clusters in and around the H I holes. We also use the H I data to constrain models of the expected remnant stellar population. Assuming that the H I holes are created by multiple SNe, that the number of SNe required can be determined from the observed energetics of the holes, and that the SNe represent the high-mass population of a cluster with a normal initial mass function, we show that in several of the holes the observed upper limits for the remnant cluster brightness are strongly inconsistent with the SN hypothesis described by Puche et al. Moreover, many of the H I holes are located in regions of very low optical surface brightness that show no indication of recent star formation. Here we present our findings, discuss their implications, and explore possible alternative explanations for the existence of the H I holes in Ho II, including the recent suggestion that some of the holes could be produced by gamma-ray burst events.

The Globular Cluster System in the Outer Regions of NGC 4472

We have undertaken a study of the globular cluster (GC) systems of a large sample of elliptical and spiral galaxies in order to test specific predictions of galaxy formation models. Here we present results for the first elliptical in the study, the giant Virgo cluster galaxy NGC 4472 (M49). The galaxy was observed in three filters (BVR) using the wide-field Mosaic Imager on the Kitt Peak 4 m telescope. The Mosaic images roughly double the radial coverage of previous CCD observations of NGC 4472. We have combined the Mosaic data with published spectroscopic data and archival HST observations in order to study NGC 4472s GC system in detail, and to fully characterize the amount of contamination in our sample of GC candidates. We find that the radial profile of the GC system is fit fairly well by a de Vaucouleurs law of the form log σGC = (3.38 ± 0.08) - (1.56 ± 0.05) r1/4 out to 17 (~80 kpc), but that the observed profile falls slightly below the de Vaucouleurs law between 17 and 23, the limit of the data. The bimodal color distribution observed in previous studies is apparent in our data. We find a small metallicity gradient in the inner 8 of the total GC system due to the increasing ratio of blue to red clusters, consistent with results from past studies. The gradient vanishes, however, when the entire radial extent of the data is taken into account. We estimate a total of ~5900 GCs in NGC 4472 out to 23, yielding a specific frequency of 3.6 ± 0.6 within this radius. This specific frequency value is smaller than that found by previous studies of NGC 4472. We examine the implications of these results with regard to predictions made by four different galaxy formation models, and we find that all four models have at least one inconsistency with the data.

Rotation in the Orion Nebula Cluster

Eighteen small (4 square) fields within the Orion Nebula cluster (ONC) have been photometrically monitored for one or more observing seasons between 1990 and 1999 with a CCD attached to the 0.6 m telescope at Van Vleck Observatory on the campus of Wesleyan University. Data were obtained exclusively in the Cousins I band on between 25 and 40 nights per season. Results from the first 3 yr of operation of this program were summarized and analyzed by Choi & Herbst. Here we provide an update based on an additional 6 yr of observation and the extensive optical and infrared study of the cluster by Hillenbrand et al. Rotation periods with false-alarm probabilities FAP 0.25 M⊙. Our results provide new observational support for the importance of disks in the early rotational evolution of low-mass stars.

ALFALFA Discovery of the nearby Gas-rich Dwarf Galaxy Leo P. III. An Extremely Metal Deficient Galaxy

We present KPNO 4 m and LBT/MODS spectroscopic observations of an H II region in the nearby dwarf irregular galaxy Leo P discovered recently in the Arecibo ALFALFA survey. In both observations, we are able to accurately measure the temperature sensitive [O III] {lambda}4363 line and determine a direct oxygen abundance of 12 + log(O/H) = 7.17 {+-} 0.04. Thus, Leo P is an extremely metal deficient (XMD) galaxy, and, indeed, one of the most metal deficient star-forming galaxies ever observed. For its estimated luminosity, Leo P is consistent with the relationship between luminosity and oxygen abundance seen in nearby dwarf galaxies. Leo P shows normal {alpha} element abundance ratios (Ne/O, S/O, and Ar/O) when compared to other XMD galaxies, but elevated N/O, consistent with the delayed release hypothesis for N/O abundances. We derive a helium mass fraction of 0.2509{sup +0.0184}{sub -0.0123}, which compares well with the WMAP + BBN prediction of 0.2483 {+-} 0.0002 for the primordial helium abundance. We suggest that surveys of very low mass galaxies compete well with emission line galaxy surveys for finding XMD galaxies. It is possible that XMD galaxies may be divided into two classes: the relatively rare XMD emission line galaxies whichmorexa0» are associated with starbursts triggered by infall of low-metallicity gas and the more common, relatively quiescent XMD galaxies like Leo P, with very low chemical abundances due to their intrinsically small masses.«xa0less

ALFALFA DISCOVERY OF THE NEARBY GAS-RICH DWARF GALAXY LEO P. II. OPTICAL IMAGING OBSERVATIONS

We present results from ground-based optical imaging of a low-mass dwarf galaxy discovered by the ALFALFA 21?cm H I survey. Broadband (BVR) data obtained with the WIYN 3.5?m telescope at Kitt Peak National Observatory (KPNO) are used to construct color-magnitude diagrams of the galaxys stellar population down to Vo ~ 25. We also use narrowband H? imaging from the KPNO 2.1?m telescope to identify a H II region in the galaxy. We use these data to constrain the distance to the galaxy to be between 1.5 and 2.0?Mpc. This places Leo P within the Local Volume but beyond the Local Group. Its properties are extreme: it is the lowest-mass system known that contains significant amounts of gas and is currently forming stars.

The Infrared Light Curve of SN 2011fe in M101 and the Distance to M101

We present near-infrared light curves of supernova (SN) 2011fe in M101, including 34 epochs in H band starting 14 days before maximum brightness in the B band. The light curve data were obtained with the WIYN High-Resolution Infrared Camera. When the data are calibrated using templates of other Type Ia SNe, we derive an apparent H-band magnitude at the epoch of B-band maximum of 10.85 ± 0.04. This implies a distance modulus for M101 that ranges from 28.86 to 29.17 mag, depending on which absolute calibration for Type Ia SNe is used.