Spectroscopic Study Of Globular Clusters Research Articles

Gemini/GMOS Spectroscopy of Globular Clusters in the Merger Remnant Galaxy M85

Abstract M85 is a peculiar S0 galaxy in Virgo and a well-known merger remnant. We present the first spectroscopic study of globular clusters (GCs) in M85. We obtain spectra for 21 GC candidates and the nucleus of M85 using the Gemini Multi-Object Spectrograph on the Gemini North 8.1 m telescope. From their radial velocities, 20 of the GCs are found to be members of M85. We find a strong rotation signal of the M85 GC system with a rotation amplitude of 235 km s−1. The rotation axis of the GC system has a position angle of about 161°, which is 51.°5 larger than that of the stellar light. The rotation-corrected radial velocity dispersion of the GC system is estimated to be km s−1. The rotation parameter of the GC system is derived to be , which is one of the largest among known early-type galaxies. The ages and metallicities of the GCs, which show the same trend as the results based on Lick indices, are derived from full spectrum fitting (ULySS). About half of the GCs are an intermediate-age population whose mean age is ∼3.7 ± 1.9 Gyr, having a mean [Fe/H] value of −0.26. The other half are old and metal-poor. These results suggest that M85 experienced a wet merging event about 4 Gyr ago, forming a significant population of star clusters. The strong rotational feature of the GC system can be explained by an off-center major merging.

Spectroscopic study of globular clusters in the halo of M31 with the Xinglong 2.16 m telescope II: dynamics, metallicity and age

In Paper I, we performed spectroscopic observations on 11 confirmed globular clusters (GCs) in M31 with the Xinglong 2.16 m telescope. We mainly focused on the fitting method and the metallicity gradient for the M31 GC sample. Here, we analyze and further discuss the dynamics, metallicity and age, and their distributions, as well as the relationships between these parameters. In our work, eight more confirmed GCs in the halo of M31 were observed, most of which lack previous spectroscopic information. These star clusters are located far from the galactic center at a projected radius of ∼ 14 to ∼ 117 kpc, which is more spatially extended than that in the previous work. Firstly, we measured the Lick absorption-line indices and the radial velocities. Then the ages and metallicity values of [Fe/H] and [α/Fe] were fitted by comparing the observed spectral feature indices and the Single Stellar Population model of Thomas et al. in the Cassisi and Padova stellar evolutionary tracks, respectively. Our results show that most of the star clusters in our sample are older than 10 Gyr except B290, which is ∼ 5.5 Gyr, and most of them are metal-poor with metallicity [Fe/H] < −1, suggesting that these clusters were born at the early stage of the galaxy's formation. We find that the metallicity gradient for the outer halo clusters with rp > 25 kpc may have an insignificant slope of −0.005 ± 0.005 dex kpc−1 and if the outliers G001 and H11 are excluded, the slope does not change significantly, with a value of −0.002 ± 0.003 dex kpc−1. We also find that the metallicity is not a function of age for the GCs with age < 7 Gyr, but for the old GCs with age > 7 Gyr, there seems to be a trend that the older ones have lower metallicity. Additionally, we plot metallicity distributions with the largest sample of M31 GCs so far and show the bimodality is not significant, and the number of metal-poor and metal-rich groups becomes comparable. The spatial distributions show that the metal-rich group is more centrally concentrated but the metal-poor group occupies a more extended halo. In addition, the young population is centrally concentrated but the old population is more spatially extended towards the outer halo.

Spectroscopic study of globular clusters in the halo of M31 with the Xinglong 2.16 m telescope

We present spectroscopic observations for 11 confirmed globular clusters (GCs) of M31 with the OMR spectrograph on the 2.16 m telescope at the Xinglong site of National Astronomical Observatories, Chinese Academy of Sciences. Nine of our sample clusters are located in the halo of M31 and the most remote one is out to a projected radius of 78.75 kpc from the galactic center. For all our sample clusters, we measured the Lick absorption-line indices and radial velocities. It is noted that most GCs in our sample are distinct from the HI rotation curve of galaxy M31, especially for B514, MCGC5, H12 and B517, suggesting that most of our sample clusters do not have a kinematic association with the star-forming young disk of the galaxy. We separately fitted the absorption line indices from the updated stellar population model of Thomas et al. with two different tracks of Cassisi and Padova, by applying the χ2—minimization method. The fitting results show that all our sample clusters are older than 10Gyr, and metal-poor (−2.38 ≤ [Fe/H] ≤ −0.91dex). After merging the spectroscopic metallicity of our work with the previously published ones, we extended the cluster sample out to a projected radius of 117 kpc from the galaxy's center. We found the metallicity gradient exists for all the confirmed clusters with a slope of −0.028 ± 0.001 dex kpc−1. However, the slope turns out to be −0.018 ± 0.001 dex kpc−1 for all the halo clusters, which is much shallower. If we only consider the outer halo clusters with rp > 25 kpc, the slope becomes −0.010 ± 0.002 dex kpc−1 and if one cluster G001 is excluded from the outer halo sample, the slope is −0.004 ± 0.002 dex kpc−1. Thus, we conclude that the metallicity gradient for M31's outer halo clusters is not significant, which agrees well with previous findings.

A 2dF spectroscopic study of globular clusters in NGC 5128: probing the formation history of the nearest giant elliptical

We have performed a spectroscopic study of globular clusters (GCs) in the giant elliptical NGC 5128 using the 2dF facility at the Anglo-Australian telescope. We obtained integrated optical spectra for a total of 254 GCs, 79 of which are newly confirmed on the basis of their radial velocities and spectra. In addition, we obtained an integrated spectrum of the galaxy starlight along the southern major axis. We derive an empirical metallicity distribution function (MDF) for 207 GCs (~14 of the estimated total GC system) based upon Milky Way GCs. This MDF is multimodal at high statistical significance with peaks at [Z/H]~-1.3 and -0.5. A comparison between the GC MDF and that of the stellar halo at 20 kpc (~4 Reff) reveals close coincidence at the metal-rich ends of the distributions. However, an inner 8 kpc stellar MDF shows a clear excess of metal-rich stars when compared to the GCs. We compare a higher S/N subsample (147 GCs) with two stellar population models which include non-solar abundance ratio corrections. The vast majority of our sample (~90%) appears old, with ages similar to the Milky Way GC system. There is evidence for a population of intermediate-age (~4-8 Gy) GCs (<15% of the sample) which are on average more metal-rich than the old GCs. We also identify at least one younger cluster (~1-2 Gy) in the central regions of the galaxy. Our observations are consistent with a picture where NGC 5128 has undergone at least two mergers and/or interactions involving star formation and limited GC formation since z=1, however the effect of non-canonical hot stellar populations on the integrated spectra of GCs remains an outstanding uncertainty in our GC age estimates.

Keck Spectroscopy of Globular Clusters in the Virgo Cluster Dwarf Elliptical Galaxy VCC 1386

We present the results of a Keck spectroscopic study of globular clusters associated with the Virgo Cluster dwarf elliptical galaxy (dE) VCC 1386. We analyze blue spectroscopic absorption lines from 3500 to 5500 A for 13 globular cluster candidates and confirm that five are associated with VCC 1386. By comparing metal and Balmer line indices of these globular clusters with α-enhanced single stellar population models, we find that these systems are metal poor, with [Fe/H] > -1.35, and old, with ages >5 Gyr at 3 σ confidence, placing their formation at z 1. This is one of the first spectroscopic studies of globular clusters surrounding dEs in a cluster, revealing that some low-mass galaxies in rich environments form at least part of their stellar mass early in the history of the universe. We further find that the luminosity-weighted stellar population of VCC 1386 itself is younger and more metal rich than its globular clusters, consistent with (V - I)0 colors from Hubble Space Telescope imaging. This implies that VCC 1386, like the Local Group dEs, has had multiple episodes of star formation. Globular clusters associated with low-luminosity systems, however, appear to be roughly as old as those associated with giant galaxies, contrary to the downsizing formation of their bulk stellar populations.