David A. Hanes

Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). II. Analysis

We present a dynamical analysis of the globular cluster system associated with M87 (=NGC 4486), the cD galaxy near the dynamical center of the Virgo Cluster. The analysis utilizes a new spectroscopic and photometric database, which is described in a companion paper. Using a sample of 278 globular clusters with measured radial velocities and metallicities and new surface density profiles based on wide-field Washington photometry, we study the dynamics of the M87 globular cluster system both globally (for the entire cluster sample) and separately (for the metal-rich and metal-poor globular cluster samples). This constitutes the largest sample of radial velocities for pure Population II tracers yet assembled for any external galaxy. Our principal findings are summarized as follows:

Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters

We present new (B, I) photometry for the globular cluster systems in eight brightest cluster galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope. In the very rich cluster systems that reside within these giant galaxies, we find that all have strongly bimodal color distributions that are clearly resolved by the metallicity-sensitive (B - I) index. Furthermore, the mean colors and internal color range of the blue subpopulation are remarkably similar from one galaxy to the next, to well within the ±0.02-0.03 mag uncertainties in the foreground reddenings and photometric zero points. By contrast, the mean color and internal color range for the red subpopulation differ from one galaxy to the next by twice as much as the blue population. All the BCGs show population gradients, with much higher relative numbers of red clusters within 5 kpc of their centers, consistent with their having formed at later times than the blue, metal-poor population. A striking new feature of the color distributions emerging from our data is that for the brightest clusters (MI < -10.5) the color distribution becomes broad and less obviously bimodal. This effect was first noticed by Ostrov et al. and Dirsch et al. for the Fornax giant NGC 1399; our data suggest that it may be a characteristic of many BCGs and perhaps other large galaxies. Our data indicate that the blue (metal-poor) clusters brighter than MI -10 become progressively redder with increasing luminosity, following a mass/metallicity scaling relation Z ~ M0.55. A basically similar relation has been found for M87 by Strader et al. (2005). We argue that these GCS characteristics are consistent with a hierarchical-merging galaxy formation picture in which the metal-poor clusters formed in protogalactic clouds or dense starburst complexes with gas masses in the range 107-1010 M☉, but where the more massive clusters on average formed in bigger clouds with deeper potential wells where more preenrichment could occur.

The Kinematics and Metallicity of the M31 Globular Cluster System

With the ultimate aim of distinguishing between various models describing the formation of galaxy halos (e.g., radial or multiphase collapse and random mergers), we have completed a spectroscopic study of the globular cluster system of M31. We present the results of deep intermediate-resolution fiber-optic spectroscopy of several hundred of the M31 globular clusters using the Wide Field Fibre Optic Spectrograph at the William Herschel Telescope in La Palma, Canary Islands. These observations have yielded precise radial velocities (±12 km s-1) and metallicities (±0.26 dex) for over 200 members of the M31 globular cluster population out to a radius of 15 from the galaxy center. Many of these clusters have no previous published radial velocity or [Fe/H] estimates, and the remainder typically represent significant improvements over earlier determinations. We present analyses of the spatial, kinematic, and metal abundance properties of the M31 globular clusters. We find that the abundance distribution of the cluster system is consistent with a bimodal distribution with peaks at [Fe/H] ~ -1.4 and -0.5. The metal-rich clusters demonstrate a centrally concentrated spatial distribution with a high rotation amplitude, although this population does not appear significantly flattened and is consistent with a bulge population. The metal-poor clusters tend to be less spatially concentrated and are also found to have a strong rotation signature.

Stokes IQUV magnetic Doppler imaging of Ap stars – I. ESPaDOnS and NARVAL observations

In this paper we describe and evaluate new spectral line polarization observations obtained with the goal of mapping the surfaces of magnetic Ap stars in great detail. One hundred complete or partial Stokes IQUV sequences, corresponding to 297 individual polarized spectra, have been obtained for seven bright Ap stars using the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) and NARVAL high-resolution spectropolarimeters. The targets span a range of masses from approximately 1.8 to 3.4 M circle dot, a range of rotation periods from 2.56 to 6.80 ?d and a range of maximum longitudinal magnetic field strengths from 0.3 to over 4 kG. For three of the seven stars, we have obtained dense phase coverage sampling the entire rotational cycle. These data sets are suitable for immediate magnetic and chemical abundance surface mapping using magnetic Doppler imaging. For the remaining four stars, partial phase coverage has been obtained, and additional observations will be required in order to map the surfaces of these stars. The median signal-to-noise ratio of the reduced observations is over 700 per 1.8 km?s-1 pixel. Spectra of all stars show Stokes V Zeeman signatures in essentially all individual lines, and most stars show clear Stokes QU signatures in many individual spectral lines. The observations provide a vastly improved data set compared to previous generations of observations in terms of signal-to-noise ratio, resolving power and measurement uncertainties. Measurement of the longitudinal magnetic field demonstrates that the data are internally consistent within computed uncertainties typically at the 50-100 sigma level. Data are also shown to be in excellent agreement with published observations and in qualitative agreement with the predictions of published surface structure models. In addition to providing the foundation for the next generation of surface maps of Ap stars, this study establishes the performance and stability of the ESPaDOnS and NARVAL high-resolution spectropolarimeters during the period 2006-2010.

Gemini/GMOS imaging of globular cluster systems in five early-type galaxies★

This paper presents deep high quality photometry of globular cluster (GC) systems belonging to five early-type galaxies covering a range of mass and en vironment. Photometric data were obtained with the Gemini North and Gemini South telescopes in the filter passbands g ′ , r ′ , and i ′ . The combination of these filters with good seeing condition s allows an excellent separation between GC candidates and unresolved field o bjects. In fact, our previously published spectroscopic data indicates a contamination level of only �10 percent in our sample of GC candidates. Bimodal GC colour distributions are found in all five galaxies. Most of the GC systems appear bimodal even in the (g ′ -r ′ ) vs (r ′ -i ′ ) plane. A population of resolved/marginally resolved GC and Ultra Compact Dwarf candidates was found in all the galaxies. A search for the so-called “blue tilt” in the colou r-magnitude diagrams reveals that NGC 4649 clearly shows that phenomenon although no conclusive evidence was found for the other galaxies in the sample. This “blue tilt” translate s into a mass-metallicity relation given by Z / M 0.28±0.03 . This dependence was found using a new empirical (g ′ -i ′ ) vs [Z/H] relation which relies on an homogeneous sample of GC colours and metallicities. This paper also explores the radial trends in both colour and surface density for the blue (metal-poor) and red (metal-rich) GC subpopulations. As usual, the red GCs show a steeper radial distribution than the blue ones. Evidence of galactocentric colour gradients is found in some of the GC systems, being more significant for the two S0 galaxies in the sample. Red GC subpopulations show similar colours and gradients to the galaxy halo stars in their inner region. A GC mean colour-galaxy luminosity relation, consistent with [Z/H]/ LB 0.26±0.08 , is present for the red GCs. An estimate of the total GC populations and specific freq uency SN values is presented for NGC 3115, NGC 3923 and NGC 4649.

Gemini/GMOS Spectroscopy of the Spheroid and Globular Cluster System of NGC 3923

We present a technique to extract ultra-deep diuse-light spectra from the standard multi-object spectroscopic observations used to investigate extragalactic globular cluster (GC) systems. This technique allows a clean extraction of the spectrum of the host galaxy diuse light from the same slitlets as the GC targets. We show the utility of the method for investigating the kinematics and stellar populations of galaxies at radii much greater than usually probed in longslit studies, at no additional expense in terms of telescope time. To demonstrate this technique we present Gemini/GMOS spectroscopy of 29 GCs associated with the elliptical galaxy NGC 3923. We compare the measured stellar population parameters of the GC system with those of the spheroid of NGC 3923 at the same projected radii, and nd the GCs to have old ages > 10 Gyr, [ /Fe] 0.3 and a range of metallicities running from [Z/H] = -1.8 to +0.35. The diuse light of the galaxy is found to have ages, metallicities and [ /Fe] abundance ratios indistinguishable from those of the red GCs.

Gemini/GMOS spectra of globular clusters in the Leo group elliptical NGC 3379

The Leo group elliptical NGC 3379 is one of the few normal elliptical galaxies close enough to make possible observations of resolved stellar populations, deep globular cluster (GC) photometry and high signal-to-noise ratio GC spectra. We have obtained Gemini/GMOS spectra for 22 GCs associated with NGC 3379. We derive ages, metallicities and α-element abundance ratios from simple stellar population models using the recent multi-index χ 2 minimization method of Proctor & Sansom. All of these GCs are found to be consistent with old ages, i.e. 10 Gyr, with a wide range of metallicities. This is comparable to the ages and metallicities that Gregg et al. found a couple of years ago for resolved stellar populations in the outer regions of this elliptical. A trend of decreasing α-element abundance ratio with increasing metallicity is indicated. The projected velocity dispersion of the GC system is consistent with being constant with radius. Non-parametric, isotropic models require a significant increase in the mass-to-light ratio at large radii. This result is in contrast to that of Romanowsky et al., who recently found a decrease in the velocity dispersion profile as determined from planetary nebulae (PN). Our constant dispersion requires a normal-sized dark halo, although without anisotropic models we cannot rigorously determine the dark halo mass. A two-sided χ 2 test over all radii gives a 2σ difference between the mass profile derived from our GCs compared to the PN-derived mass model of Romanowsky et al. However, if we restrict our analysis to radii beyond one effective radius and test if the GC velocity dispersion is consistently higher, we determine a > 3σ difference between the mass models, and hence we favour the conclusion that NGC 3379 does indeed have dark matter at large radii in its halo.

Dynamical Constraints on the Formation of NGC 4472 and Its Globular Clusters

We present new radial velocities for 87 globular clusters around the elliptical galaxy NGC 4472 and combine these with our previously published data to create a data set of velocities for 144 globular clusters around NGC 4472. We utilize this data set to analyze the kinematics of the NGC 4472 globular cluster system. The new data confirm our previous discovery that the metal-poor clusters have significantly higher velocity dispersion than the metal-rich clusters in NGC 4472. We also find very little or no rotation in the more spatially concentrated metal-rich population, with a resulting upper limit for this population of (v/σ)proj < 0.34 at a 99% confidence level. The very small angular momentum in the metal-rich population requires efficient angular momentum transport during the formation of this population, which is spatially concentrated and chemically enriched. Such angular momentum transfer can be provided by galaxy mergers, but it has not been achieved in other extant models of elliptical galaxy formation that include dark matter halos. We also calculate the velocity dispersion as a function of radius and show that it is consistent with roughly isotropic orbits for the clusters and the mass distribution of NGC 4472 inferred from X-ray observations of the hot gas around the galaxy.

Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). I. New CFHT MOS Spectroscopy and the Composite Database

We present a comprehensive database of kinematic, photometric, and positional information for 352 objects in the field of M87 (NGC 4486), the central giant elliptical galaxy in the Virgo Cluster; the majority of the tracers are globular clusters associated with that galaxy. New kinematic information comes from multislit observations with the Multi-Object Spectrograph (MOS) of the Canada-France-Hawaii Telescope (CFHT), an investigation that has added 96 new velocities to and confirmed many of the earlier values in a preexisting data set of 256 velocities published elsewhere. The photometry, consisting of magnitudes and colors in the Washington (T1, C-T1) system, is based on CCD observations made at the Cerro Tololo Inter-American Observatory (CTIO) and the Kitt Peak National Observatory (KPNO). The composite database represents the largest compilation of pure Population II dynamical tracers yet identified in any external galaxy; moreover, it extends to larger spatial scales than have earlier investigations. The inclusion of photometric information allows independent study of the distinct red and blue subpopulations of the bimodal globular cluster system of M87. In a companion paper we use this powerful data set to analyze the present dynamical state of the M87 globular cluster system and consider the question of its interaction and formation history.

The globular cluster system of NGC 1399

Profile-fitting photometry of objects of stellar appearance in B and V CCD frames of the Fornax cluster cD galaxy NGC 1399, as well as in background frames awsay from the galaxy, have been performed. The radial surface density profile of the NGC 1399 globular cluster system (GCS) obeys a power law between 0.5 and 3 arcmin, with a slope of -1.5 + or - 0.2. The radial profile of the GCS is a good match to the halo light over this limited range in radius. The globular clusters are bluer than the halo light near the center of NGC 1399, and there is evidence for a radial color gradient in the NGC 1399 CGS. There is no evidence for such a gradient in the halo light. The globular cluster luminosity functions (GCLFs) in B and V are consistent with being Gaussian in shape. A turnover in the GCLF at V about 23.8 or B about 24.6 is inferred. NGC 1399 has a GCS of specific frequency S = 16 + or - 4; S is only moderately sensitive to the adopted distance scale and assumed dispersion of the GCLF. 90 refs.