arXiv subjects

Jean P. Brodie

Publications and source records attributed to Jean P. Brodie.

At least 163 records · Page 9Linked to original sources

arXiv2004

A Principal Components Analysis of the Lick Indices of Galactic Globular Clusters

We present a principal components analysis (PCA) of high-quality Lick/IDS absorption-line measurements for 11 indices in the wavelength range 4100-5400 Ang for 39 Galactic globular clusters (GCs). Only the first principal component appears to be physically significant. We find that there is a tight linear relationship between this first component (PC1) and GC metallicity over a wide range in [m/H] (-1.8 <= [m/H] <= 0), suggesting that PC1 may be used to accurately estimate metallicities for old extragalactic GCs from their integrated spectra. The classic GC ``second parameter effect'' is noticeable in the Balmer indices, though it does not appear in our PCA. We find little evidence for substantial differences in broad abundance patterns among Galactic GCs. One implication is that the metal-poor and metal-rich GC subpopulations formed from very similar physical processes.

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arXiv2004

Keck Spectroscopy of Two Young Globular Clusters in the Merger Remnant NGC 3921

Low-resolution UV-to-visual spectra of two candidate globular clusters in the merger remnant NGC 3921 are presented. These two clusters of apparent magnitude V = 22.2 (Mv = -12.5) lie at projected distances of ~5 kpc from the center and move with halo-type radial velocities relative to the local galaxy background. Their spectra show strong Balmer absorption lines indicative of main-sequence turnoffs dominated by A-type stars. Comparisons with model-cluster spectra computed by Bruzual & Charlot and others yield cluster ages in the range of 200-530 Myr, and metallicities about solar to within a factor of three. Given their small half-light radii (Reff < 5 pc) and ages corresponding to ~100 core- crossing times, these clusters are gravitationally bound and, hence, indeed young globulars. Assuming that they had Chabrier-type initial mass functions, their estimated current masses are 2.3(+-0.1)x10^6 Msun and 1.5(+-0.1)x10^6 Msun, respectively, or roughly half the mass of omegaCen. Since NGC 3921 itself shows many signs of being a 0.7(+-0.3) Gyr old protoelliptical, these two young globulars of roughly solar metallicity and their many counterparts observed with the Hubble Space Telescope provide supporting evidence that, in the process of forming elliptical-like remnants, major mergers of gas-rich disks can also increase the number of metal-rich globular clusters. (Abridged)

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arXiv2004

Metal-Poor Globular Clusters and Galaxy Formation

We demonstrate a significant (> 5-sigma) correlation between the mean color of metal-poor globular cluster (GC) systems and parent galaxy luminosity. A Bayesian Markov Chain Monte Carlo method is introduced to find the mean color, and is easily generalizable to quantify multimodality in other astronomical datasets. We derive a GC color--galaxy luminosity relation of the form Z ~ L^ (0.15 +/- 0.03). When combined with evidence against a single primordial GC metallicity--galaxy luminosity relation for protogalactic fragments, the existence of such a correlation is evidence against both accretion and major merger scenarios as an explanation of the entire metal-poor GC systems of luminous galaxies. However, our relation arises naturally in an in situ picture of GC formation, and is consistent with the truncation of metal-poor GC formation by reionization. A further implication is that the ages of metal-poor GCs in dwarf galaxies constrain the main epoch of galaxy formation in hierarchical models. If the ages of old metal-poor GCs in Local Group dwarfs (> 11 Gyr) are typical of those in dwarfs elsewhere, then the bulk of galaxy assembly (at least in clusters and groups) must have occurred at z > 2.5, contrary to the predictions of some structure formation models.

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arXiv2003

Keck Spectroscopy and Imaging of Globular Clusters in the Lenticular Galaxy NGC 524

We have obtained Keck LRIS imaging and spectra for 29 globular clusters associated with the lenticular galaxy NGC 524. Using the empirical calibration of Brodie & Huchra we find that our spectroscopic sample spans a metallicity range of --2.0 < [Fe/H] < 0. We have compared the composite spectrum of the metal-poor ([Fe/H] < --1) and metal-rich clusters with stellar population models and conclude that the clusters are generally old and coeval at the 2 sigma confidence level. To determine the mean [alpha/Fe] ratios of the globular clusters, we have employed the Milone et al. 'alpha-enhanced' stellar population models. We verified the reliability of these models by comparing them with high S/N Galactic globular cluster data. We observe a weak trend of decreasing [alpha/Fe] with increasing metallicity in the NGC 524 clusters. Analysis of the cluster system kinematics reveals that the full sample exhibits a rotation of 114+/-60 km/s around a position angle of 22+/-27 deg, and a velocity dispersion of 186+/-29 km/s at a mean radius of 89 arcsec from the galaxy centre. Subdividing the clusters into metal-poor and metal-rich subcomponents we find that the metal-poor (17) clusters and metal-rich (11) clusters have similar velocity dispersions (197+/-40 km/s and 169+/-47 km/s respectively). The metal-poor clusters dominate the rotation in our sample with 147+/-75 km/s, whilst the metal-rich clusters show no significant rotation (68+/-84 km/s). We derive a virial and projected mass estimation for NGC 524 of between 4 and 13 x 10^11 Msun (depending on the assumed orbital distribution) interior to 2 effective radii of this galaxy.

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arXiv2003

Revisiting the Globular Cluster System of the Merger Remnant Elliptical NGC 3610

We have obtained Keck spectra of six candidate globular clusters (GCs) in the dynamically young elliptical galaxy NGC 3610, supplementing our previous Keck spectra of eight confirmed GCs (Strader et al. 2003). Five of our new candidates are confirmed to be GCs. Of the thirteen GCs, eleven are located within the K band effective radius of 7 kpc. Two of these thirteen clusters are found to be young (~ 2 Gyr) and very metal-rich ([Z/H] ~ +0.5), three are old and metal-poor, and the remaining eight clusters are old and metal-rich. The ages of the young clusters are consistent with a recent spectroscopic age estimate of 1.6+/-0.5 Gyr for the galaxy itself (Denicolo et al. 2003) and suggest that these clusters formed in the disk-disk merger which likely created NGC 3610. Intriguingly, both young GCs have [alpha/Fe] ~ +0.3, while the majority of the old clusters are not alpha-enhanced, in contrast to Galactic and M31 GCs, and contrary to predictions of nucleosynthetic calculations. The two old subpopulations of GCs can be attributed to the merger progenitors. The relative numbers of old and new metal-rich GCs are poorly constrained because of the expected differences in radial distributions of the two subpopulations. However, based on our spectroscopic results and a comparison of the Hubble Space Telescope color magnitude diagram (Whitmore et al. 2002) with stellar population models, we argue that more than half of the metal-rich GCs are likely to be old.

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arXiv2003

On the Size Difference between Red and Blue Globular Clusters

Several recent studies have reported a mean size difference of about 20% between the metal-rich and metal-poor subpopulations of globular clusters (GCs) in a variety of galaxies. In this paper we investigate the possibility that the size difference might be a projection effect, resulting from a correlation between cluster size and galactocentric distance, combined with different radial distributions of the GC subpopulations. We find that projection effects may indeed account for a size difference similar to the observed one, provided that there is a steep relation between GC size and galactocentric distance in the central parts of the GC system and that the density of GCs flattens off near the center in a manner similar to a King profile. For more centrally peaked distributions, such as a de Vaucouleurs law, or for shallower size-radius relations, projection effects are unable to produce the observed differences in the size distributions.

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arXiv2002

Spectroscopy of Globular Clusters in the Fornax dwarf galaxy

We present low resolution, integrated Keck spectra of the five globular clusters (GCs) of the Fornax dwarf spheroidal galaxy. We find a tentative age spread among the clusters, with the GC H5 younger by 2-3 Gyr than the others. The clusters generally appear to be very metal-poor ([Fe/H] ~ -1.8), with the cluster H4 slightly more metal-rich at [Fe/H] = -1.5. We speculate that cluster H4 is similar to the Galactic GC Ruprecht 106, which lacks the [alpha/Fe] enhancement typical among metal-poor GCs in the Milky Way. High-resolution spectroscopy of individual cluster and field stars will be needed to sort out the surprisingly complex history of GC formation and evolution in this galaxy.

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arXiv2002

Keck Spectroscopy of Globular Clusters in the Elliptical Galaxy NGC 3610

We present moderate-resolution Keck spectra of nine candidate globular clusters in the possible merger-remnant elliptical galaxy NGC 3610. Eight of the objects appear to be bona fide globular clusters of NGC 3610. We find that two of the clusters belong to an old metal-poor population, five to an old metal-rich population, and only one to an intermediate-age metal-rich population. The estimated age of the intermediate-age cluster is 1-5 Gyr, which is in agreement with earlier estimates of the merger age, and suggests that this cluster was formed during the merger. However, the presence of five old metal-rich clusters indicates that a substantial number of the metal-rich clusters in NGC 3610 likely came from the progenitor galaxies, although the global ratio of old to intermediate-age metal-rich clusters remains very uncertain.

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arXiv2002

Spectroscopy of a Globular Cluster in the Local Group dIrr NGC 6822

We present low-resolution Keck spectroscopy for the globular cluster H VIII in the Local Group dIrr galaxy NGC 6822. We find the metallicity of the cluster to be [Fe/H]= -1.58 +/- 0.28 and the age of the cluster to be 3-4 Gyr, slightly older than but consistent with previous age estimates. H VIII seems to be more metal-poor than most intermediate-age globular clusters in the Local Group, and appears most similar to the anomalous Small Magellanic Cloud clusters Lindsay 113 and NGC 339.

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arXiv2002

Extragalactic Globular Clusters in the Near Infrared I: A comparison between M87 and NGC 4478

We compare optical and near infrared colours of globular clusters in M87, the central giant elliptical in Virgo, and NGC 4478, an intermediate luminosity galaxy in Virgo, close in projection to M87. Combining V and I photometry obtained with the WFPC2 on HST and Ks photometry obtained with the NIRC on Keck 1, we find the broad range in colour and previously detected bi-modality in M87. We confirm that NGC 4478 only hosts a blue sub-population of globular clusters and now show that these clusters' V-I and V-K colours are very similar to those of the halo globular clusters in Milky Way and M31. Most likely, a metal-rich sub-population never formed around this galaxy (rather than having formed and been destroyed later), probably because its metal-rich gas was stripped during its passage through the centre of the Virgo cluster. The V-I, V-K colours are close to the predicted colours from SSP models for old populations. However, M87 hosts a few red clusters that are best explained by intermediate ages (a few Gyr). Generally, there is evidence that the red, metal-rich sub-population has a complex colour structure and is itself composed of clusters spanning a large metallicity and, potentially, age range. This contrasts with the blue, metal-poor population which appears very homogeneous in all galaxies observed so far.

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arXiv2002

New Members of the Cluster Family in Nearby Lenticulars

Using spectra obtained with the Keck I telescope we have demonstrated conclusively that the faint (23 7--8 Gyr), moderately metal-rich ([Fe/H]=-0.58+/-0.24) and, having a system rotation curve which is very similar to that of the host galaxy, are associated with the lenticular disk. Alpha-element to iron abundance ratios are highly supersolar with [alpha/Fe] between +0.3 and +0.6. With moderately high metallicities and luminosities, and effective radii in the range 7--15 pc (compared to the 2--3 pc sizes typical of normal globular and open clusters), this population of clusters has no known analog in the Milky Way or elsewhere in the Local Group.

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arXiv2001

Keck Spectroscopy of Young and Old Star Clusters in NGC 1023

We present Keck/LRIS spectra for 11 old globular clusters in the S0-type galaxy NGC 1023 and 2 young blue clusters in the nearby companion NGC 1023A. Analysis of the spectra of 7 old clusters with sufficient S/N shows generally good agreement between spectroscopic and previous photometric metallicity estimates, but the integrated colors of two clusters are too blue for their spectroscopic metallicities. Although the cluster ages are not well constrained, they are most likely similar to those of Milky Way globular clusters and certainly older than about 5 Gyr. The brightest GC in the sample shows enhanced cyanogen, possibly indicating an abundance anomaly similar to that observed in some M31 globular clusters. For the two blue clusters in NGC 1023A we estimate ages between 125 and 500 Myr, based on their strong Balmer lines. The total masses are about 80000 Msun and 60000 Msun for a Miller-Scalo IMF and Z=0.004, making these objects similar to the young populous clusters in the LMC. It is suggested that the two young clusters might have formed during a period of enhanced star formation activity in NGC 1023A, stimulated by a close encounter with NGC 1023.

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arXiv2001

Age Estimates for Globular Clusters in NGC 1399

We present high signal-to-noise Keck spectra for 10 globular clusters associated with the giant Fornax elliptical NGC 1399, and compare measured line indices with current stellar population models. Our data convincingly demonstrate, for the first time, that at least some of the clusters in a giant elliptical galaxy have super-solar abundance ratios, similar to the host galaxy. From Hbeta line-strengths the majority of clusters have ages of ~11 Gyrs (within 2sigma), which is similar to the luminosity-weighted stellar age of NGC 1399. Two of the clusters (which also reveal enhanced abundance ratios) show significantly higher Hbeta values than the others. It remains unclear whether this is due to young (~2 Gyr) ages, or extremely old (> 15 Gyr) ages with a warm blue horizontal branch. However a conflict with current cosmological parameters is avoided if the young age is favored. Either alternative indicates a complicated age distribution among the metal-rich clusters and sets interesting constraints on their chemical enrichment at late epochs.

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arXiv2001

Spectroscopy of Globular Clusters in M81

We present moderate-resolution spectroscopy of globular clusters (GCs) around the Sa/Sb spiral galaxy M81 (NGC 3031). Sixteen candidate clusters were observed with the Low Resolution Imaging Spectrograph on the Keck I telescope. All are confirmed as bona fide GCs, although one of the clusters appears to have been undergoing a transient event during our observations. In general, the M81 globular cluster system (GCS) is found to be very similar to the Milky Way (MW) and M31 systems, both chemically and kinematically. A kinematic analysis of the velocities of 44 M81 GCS, (the 16 presented here and 28 from previous work) strongly suggests that the red, metal-rich clusters are rotating in the same sense as the gas in the disk of M81. The blue, metal-poor clusters have halo-like kinematics, showing no evidence for rotation. The kinematics of clusters whose projected galactocentric radii lie between 4 and 8 kpc suggest that they are rotating much more than those which lie outside these bounds. We suggest that these rotating, intermediate-distance clusters are analogous to the kinematic sub-population in the metal-rich, disk GCs observed in the MW and we present evidence for the existence of a similar sub-population in the metal-rich clusters of M31. With one exception, all of the M81 clusters in our sample have ages that are consistent with MW and M31 GCs. One cluster may be as young as a few Gyrs. The correlations between absorption-line indices established for MW and M31 GCs also hold in the M81 cluster system, at least at the upper end of the metallicity distribution (which our sample probes). On the whole, the mean metallicity of the M81 GCS is similar to the metallicity of the MW and M31 GCSs. The projected mass of M81 is similar to the masses of the MW and M31. Its mass profile indicates the presence of a dark matter halo.

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arXiv2001

HST photometry of Globular Clusters in the Sombrero Galaxy

We explore the rich globular cluster (GC) system of the nearby Sa galaxy M104, the "Sombrero" (NGC 4594), using archive WFPC2 data. The GC colour distribution is found to be bimodal at the > 99% level, with peaks at V-I = 0.96+/-0.03 and 1.21+/-0.03 (corrected for foreground reddening). The inferred metallicities are very similar to those of globular clusters in our Galaxy and M31. However the Sombrero reveals a much enhanced number of red (metal-rich) GCs compared to other well-studied spirals. Because the Sombrero is dominated by a huge bulge and only has a modest disk, we associate the two sub-populations with the halo and bulge components respectively. Thus our analysis supports the view that the metal-rich GCs in spirals are associated with the bulge rather than with the disk. The Sombrero GCs have typical effective (half-light) radii of about 2 pc with the red ones being about 30% smaller than the blue ones. We identify many similarities between the Sombrero's GC system and those of both late type spirals and early-type galaxies. Thus both the GC system and the Hubble type of the Sombrero galaxy appear to be intermediate in their nature.

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arXiv2001

Globular Cluster Subpopulations in Early-Type Galaxies: Insights into Galaxy Formation

New and archival HST images of the globular cluster systems of 17 relatively nearby galaxies and 6 more distant galaxies have been used to establish the characteristics of GC subpopulations over a range of host galaxy morphological type from E5 to Sa. GC color/metallicity, size and luminosity distributions have been examined in the nearby galaxies and color distributions have been determined for the more distant sample. Correlations with parent galaxy properties and trends with galactocentric radius have been explored. Supplemented with Keck spectroscopy, our results are best explained by an in situ formation scenario in which both GC subpopulations formed at early times within the potential well of the protogalaxy, in multiple episodes of star formation. We have also discovered a third population of clusters, fundamentally distinct from the compact red and blue clusters common in early type galaxies.

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arXiv2001

Bulge Globular Clusters in Spiral Galaxies

There is now strong evidence that the metal-rich globular clusters (GC) near the center of our Galaxy are associated with the Galactic bulge rather than the disk as previously thought. Here we extend the concept of bulge GCs to the GC systems of nearby spiral galaxies. In particular, the kinematic and metallicity properties of the GC systems favor a bulge rather than a disk origin. The number of metal-rich GCs normalized by the bulge luminosity is roughly constant (i.e. bulge S_N ~ 1) in nearby spirals, and this value is similar to that for field ellipticals when only the red (metal--rich) GCs are considered. We argue that the metallicity distributions of GCs in spiral and elliptical galaxies are remarkably similar, and that they obey the same correlation of mean GC metallicity with host galaxy mass. We further suggest that the metal-rich GCs in spirals are the direct analogs of the red GCs seen in ellipticals. The formation of a bulge/spheroidal stellar system is accompanied by the formation of metal-rich GCs. The similarities between GC systems in spiral and elliptical galaxies appear to be greater than the differences.

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arXiv2001

Keck Imaging of the Globular Cluster Systems in the Early--type Galaxies NGC 1052 and NGC 7332

The presence of two globular cluster subpopulations in early-type galaxies is now the norm rather than the exception. Here we present two more examples for which the host galaxy appears to have undergone a recent merger. Using multi-colour Keck imaging of NGC 1052 and NGC 7332 we find evidence for a bimodal globular cluster colour distribution in both galaxies, with roughly equal numbers of blue and red globular clusters. The blue ones have similar colours to those in the Milky Way halo and are thus probably very old and metal-poor. If the red GC subpopulations are at least solar metallicity, then stellar population models indicate young ages. We discuss the origin of globular clusters within the framework of formation models. We conclude that recent merger events in these two galaxies have had little effect on their overall GC systems. We also derive globular cluster density profiles, global specific frequencies and in the case of NGC 1052, radial colour gradients and azimuthal distribution. In general these globular cluster properties are normal for early-type galaxies.

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