1,419 research outputs found
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.Comment: 35 pages, including 11 figures and 9 tables. Accepted for publication
in the Astronomical Journa
The Chemical Properties of Milky Way and M31 Globular Clusters: I. A Comparative Study
A comparative analysis is performed between high-quality integrated spectra
of 30 globular clusters in M31, 20 Milky Way clusters, and a sample of field
and cluster elliptical galaxies. We find that the Lick CN indices in the M31
and Galactic clusters are enhanced relative to the bulges of the Milky Way,
M31, and elliptical spheroids. Although not seen in the Lick CN indices, the
near-UV cyanogen feature (3883 A) is strongly enhanced in M31 clustesr with
respect to the Galactic globulars at metallicities, --1.5<[Fe/H]<--0.3. Carbon
shows signs of varying amongst these two groups. For [Fe/H]>--0.8, we observe
no siginificant differences in the Hdelta, Hgamma, or Hbeta indices between the
M31 and Galactic globulars. The sample of ellipticals lies offset from the loci
of all the globulars in the Cyanogen--[MgFe], and Balmer--[MgFe] planes. Six of
the M31 cluster spectra appear young, and are projected onto the M31 disk.
Population synthesis models suggest that these are metal-rich clusters with
ages 100--800 Myr, metallicities --0.20 < [Fe/H] <0.35, and masses 0.7
-7.0x10^4 Msun. Two other young clusters are Hubble V in NGC 205, and an older
(~3 Gyr) cluster ~7 kpc away from the plane of the disk. The six clusters
projected onto the disk rotate in a similar fashion to the HI gas in M31, and
three clusters exhibit thin disk kinematics (Morrison et al.). Dynamical masses
and structural parameters are required for these objects to determine whether
they are massive open clusters or globular clusters. If the latter, our
findings suggest globular clusters may trace the build up of galaxy disks. In
either case, we conclude that these clusters are part of a young, metal-rich
disk cluster system in M31, possibly as young as 1 Gyr old.Comment: 52 pages, 14 figures, 8 tables, minor revisions in response to
referee, conclusions remain the same. Scheduled to appear in the October 2004
issue of The Astronomical Journa
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 multi-phase collapse, random
mergers), we have completed a spectroscopic study of the globular cluster
system of M31. We present the results of deep, intermediate-resolution,
fibre-optic spectroscopy of several hundred of the M31 globular clusters using
the Wide Field Fibre Optic Spectrograph (WYFFOS) at the William Herschel
Telescope in La Palma, Canary Islands. These observations have yielded precise
radial velocities (+/-12 km/s) and metallicities (+/-0.26 dex) for over 200
members of the M31 globular cluster population out to a radius of 1.5 degrees
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.Comment: 33 pages, 20 figure
The outermost cluster of M31
We report on the identification of a new cluster in the far halo of the M31
galaxy. The cluster, named Bologna 514 (B514) has an integrated magnitude
M_V=-8.5 +- 0.6, and a radial velocity, as estimated from two independent
low-resolution spectra, V_r=-456 +- 23 km/s, which fully confirms its
membership to the M31 system. The observed integrated spectrum is very similar
to those of classical globular clusters. Being located at ~ 4^o (~55 kpc in
projected distance) from the center of the parent galaxy, B514 is by far the
most remote M31 cluster ever discovered. Its projected position, near the
galaxy major axis, and M31-centric velocity, similar to that observed in the
outermost regions of the HI rotation curve, may indicate that it belongs to the
subsystem of M31 clusters that has been recently proposed (Morrison et al.
2004) to be part of the dynamically-cold thin disc of the galaxy.Comment: 6 pages, 4 figures, accepted for publication in A &
The Specific Globular Cluster Frequencies of Dwarf Elliptical Galaxies from the Hubble Space Telescope
The specific globular cluster frequencies (S_N) for 24 dwarf elliptical (dE)
galaxies in the Virgo and Fornax Clusters and the Leo Group imaged with the
Hubble Space Telescope are presented. Combining all available data, we find
that for nucleated dEs --- which are spatially distributed like giant
ellipticals in galaxy clusters --- S_N(dE,N)=6.5 +- 1.2 and S_N increases with
M_V, while for non-nucleated dEs --- which are distributed like late-type
galaxies --- S_N(dE,noN)=3.1 +- 0.5 and there is little or no trend with M_V.
The S_N values for dE galaxies are thus on average significantly higher than
those for late-type galaxies, which have S_N < 1. This suggests that dE
galaxies are more akin to giant Es than to late-type galaxies. If there are
dormant or stripped irregulars hiding among the dE population, they are likely
to be among the non-nucleated dEs. Furthermore, the similarities in the
properties of the globular clusters and in the spatial distributions of dE,Ns
and giant Es suggest that neither galaxy mass or galaxy metallicity is
responsible for high values of S_N. Instead, most metal-poor GCs may have
formed in dwarf-sized fragments that merged into larger galaxies.Comment: 12 pages (uses aaspp4.sty), 2 figures, 1 table, to appear in the
Astrophysical Journa
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