237 research outputs found
The globular cluster system of NGC 1316 IV. Nature of the star cluster complex SH2
The light of the merger remnant NGC 1316 is dominated by old and
intermediate-age stars. The only sign of current star formation in this big
galaxy is the HII region SH2, an isolated star cluster complex with a ring-like
morphology and an estimated age of 0.1 Gyr at a galactocentric distance of
about 35 kpc. A nearby intermediate-age globular cluster, surrounded by weak
line emission and a few more young star clusters, is kinematically associated.
The origin of this complex is enigmatic. The nebular emission lines permit a
metallicity determination which can discriminate between a dwarf galaxy or
other possible precursors. We used the Integrated Field Unit of the VIMOS
instrument at the Very Large Telescope of the European Southern Observatory to
study the morphology, kinematics, and metallicity employing line maps, velocity
maps, and line diagnostics of a few characteristic spectra. The line ratios of
different spectra vary, indicating highly structured HII regions, but define a
locus of uniform metallicity. The strong-line diagnostic diagrams and empirical
calibrations point to a nearly solar or even super-solar oxygen abundance. The
velocity dispersion of the gas is highest in the region offset from the bright
clusters. Star formation may be active on a low level. There is evidence for a
large-scale disk-like structure in the region of SH2, which would make the
similar radial velocity of the nearby globular cluster easier to understand.
The high metallicity does not fit to a dwarf galaxy as progenitor. We favour
the scenario of a free-floating gaseous complex having its origin in the merger
2 Gyr ago. Over a long period the densities increased secularly until finally
the threshold for star formation was reached. SH2 illustrates how massive star
clusters can form outside starbursts and without a considerable field
population.Comment: 10 pages, 5 figures, accepted for Astronomy & Astrophysic
The curious case of the companion: evidence for cold accretion onto a dwarf satellite near the isolated elliptical NGC 7796
The isolated elliptical (IE) NGC 7796 is accompanied by an interesting
early-type dwarf galaxy, named NGC7796-DW1. It exhibits a tidal tail, very boxy
isophotes, and multiple nuclei or regions (A, B, and C) that are bluer than the
bulk population of the galaxy, indicating a younger age. These properties are
suggestive of a dwarf-dwarf merger remnant. We use the Multi-Unit Spectroscopic
Explorer (MUSE) at the VLT to investigate NGC 7796-DW1. We extract
characteristic spectra to which we apply the STARLIGHT population synthesis
software to obtain ages and metallicities of the various population components
of the galaxy. The galaxy's main body is old and metal-poor. A surprising
result is the extended line emission in the galaxy, forming a ring-like
structure with a projected diameter of 2.2 kpc. The line ratios fall into the
regime of HII-regions, although OB-stellar populations cannot be identified by
spectral signatures. Nucleus A is a relatively old (7 Gyr or older) and
metal-poor super star cluster, most probably the nucleus of the dwarf, now
displaced. The star-forming regions B and C show younger and distinctly more
metal-rich components. The emission line ratios of regions B and C indicate an
almost solar oxygen abundance, if compared with radiation models of HII
regions. NGC7796-DW1 occupies a particular role in the group of transition-type
galaxies with respect to its origin and current evolutionary state, being the
companion of an IE. The dwarf-dwarf merger scenario is excluded because of the
missing metal-rich merger component. A viable alternative is gas accretion from
a reservoir of cold, metal-rich gas. NGC7796 has to provide this gas within its
X-ray bright halo. As illustrated by NGC7796-DW1, cold accretion may be a
general solution to the problem of extended star formation histories in
transition dwarf galaxies. (abridged)Comment: comments: 13 pages, 8 figures, accepted for publication in Astronomy
& Astrophysic
Extragalactic Globular Clusters in the Near-Infrared II. The Globular Clusters Systems of NGC 3115 and NGC 4365
We combine near-infrared photometry obtained with the VLT/ISAAC instrument
and archival HST/WFPC2 optical images to determine VIK magnitudes and colours
of globular clusters in two early-type galaxies, NGC 3115 and NGC 4365. The
combination of near-IR and optical photometry provides a way to lift the
age-metallicity degeneracy. For NGC 3115, the globular cluster colours reveal
two major sub-populations, consistent with previous studies. By comparing the
V-I, V-K colours of the NGC 3115 globular clusters with SSP models, we find
that the colour difference between the two >10 Gyr old major sub-populations is
primarily due to a difference in metallicity. We find \Delta[Fe/H] = 1.0 +/-
0.3 dex and the blue and red globular cluster sub-populations being coeval
within 3 Gyr. In contrast to the NGC 3115 globular clusters, the globular
cluster system in NGC 4365 exhibits a more complex age and metallicity
structure. We find a significant population of intermediate-age very metal-rich
globular clusters along with an old population of both metal-rich and
metal-poor clusters. Specifically, we observe a large population of globular
clusters with V-K and V-I colours, for which all current SSP models give ages
and metallicities in the range ~2-8 Gyr and ~0.5-3 Z_solar, respectively. After
10 Gyr of passive evolution, the intermediate-age globular clusters in NGC 4365
will have colours which are consistent with the very metal-rich population of
globular clusters in giant elliptical galaxies, such as M87. Our results for
both globular cluster systems are consistent with previous age and metallicity
studies of the diffuse galactic light. In addition to the major globular
cluster populations in NGC 3115 and NGC 4365 we report on the detection of
objects with extremely red colours.Comment: accepted for publication in A&A, 19 pages, incl. 9 figure
Probing the Low Mass X-ray Binaries/Globular Cluster connection in NGC1399
We present a wide field study of the Globular Clusters/Low Mass X-ray
Binaries connection in the cD elliptical NGC1399, combining HST/ACS and Chandra
high resolution data. We find evidence that LMXB formation likelihood is
influenced by GCs structural parameters, in addition to the well known effects
of mass and metallicity, independently from galactocentric distance.Comment: in press in the Proceedings of the X-ray 2009 Conference, 7-11
September 2009, Bologna, Ital
Observations of Lick Standard Stars Using the SCORPIO Multi-Slit Unit at the SAO 6-m Telescope
We present Lick line-index measurements of standard stars from the list of
Worthey. The spectra were taken with the multi-slit unit of the SCORPIO
spectrograph at the 6-m Special Astrophysical observatory telescope. We
describe in detail our method of analysis and explain the importance of using
the Lick index system for studying extragalactic globular clusters. Our results
show that the calibration of our instrumental system to the standard Lick
system can be performed with high confidence.Comment: 12 pages, 3 figure
The Chemistry of Extragalactic Globular Clusters
We present preliminary results of VLT/FORS spectroscopy of globular clusters
in nearby early-type galaxies. Our project aims at studying the chemistry and
determine the ages of globular cluster (sub-)populations. First results
indicate that the different galaxies host from little to significant
intermediate-age populations, and that the latter have alpha-element over iron
ratios closer to solar than the old population that show an alpha-element
enhancement similar to the diffuse stellar light.Comment: 4 pages (incl 2 figures) to appear in the proceedings of
"Extragalactic Globular Cluster Systems", ed.M.Kissler-Patig, Springer; see
also related contributions by T.H.Puzia and M.Hempel et a
Ages, Metallicities, and [alpha/Fe] ratios of globular clusters in NGC 147, NGC 185, and NGC 205
We present measurements of ages, metallicities, and [alpha/Fe] ratios for 16
globular clusters (GC) in NGC 147, NGC 185, and NGC 205 and of the central
regions of the diffuse galaxy light in NGC 185, and NGC 205. Our results are
based on spectra obtained with the SCORPIO multi-slit spectrograph at the 6-m
telescope of the Russian Academy of Sciences. We include in our analysis
high-quality HST/WFPC2 photometry of individual stars in the studied GCs to
investigate the influence of their horizontal branch (HB) morphology on the
spectroscopic analysis. All our sample GCs appear to be old (T>8 Gyr) and
metal-poor ([Z/H] <~ -1.1), except for the globular clusters Hubble V in NGC
205 (T=1.2+/-0.6 Gyr, [Z/H]=-0.6+/-0.2), Hubble VI in NGC 205 (T=4+/-2 Gyr,
[Z/H]=-0.8+/-0.2), and FJJVII in NGC 185 (T=7+/-3 Gyr, [Z/H]=-0.8+/-0.2). The
majority of our GC sample has solar [alpha/Fe] enhancements in contrast to the
halo population of GCs in M31 and the Milky Way. The HB morphologies for our
sample GCs follow the same behavior with metallicity as younger halo Galactic
globular clusters. We show that it is unlikely that they bias our spectroscopic
age estimates based on Balmer absorption line indices. Spectroscopic ages and
metallicities of the central regions in NGC 205 and NGC 185 coincide with those
obtained from color-magnitude diagrams. The central field stellar populations
in these galaxies have approximately the same age as their most central GCs
(Hubble V in NGC 205 and FJJIII in NGC 185), but are more metal-rich than the
central globular clusters.Comment: 17 pages, 8 figures, accepted for publication in MNRAS, a
high-resolution version of the paper is available from
http://www.stsci.edu/~tpuzi
Globular Clusters in the dE,N galaxy NGC 3115 DW1: New Insights from Spectroscopy and HST Photometry
The properties of globular clusters in dwarf galaxies are key to
understanding the formation of globular cluster systems, and in particular in
verifying scenarios in which globular cluster systems of larger galaxies formed
(at least partly) from the accretion of dwarf galaxies. Here, we revisit the
globular cluster system of the dE,N galaxy NGC 3115 DW1 - a companion of the
nearby S0 galaxy NGC 3115 - adding Keck/LRIS spectroscopy and HST/WFPC2 imaging
to previous ground-based photometry. Spectra for seven globular clusters reveal
normal abundance ratios with respect to the Milky Way and M31 clusters, as well
as a relatively high mean metallicity ([Fe/H] = -1.0+/-0.1 dex). Crude
kinematics indicate a high velocity dispersion within 10 kpc which could either
be caused by dark matter dominated outer regions, or by the stripping of outer
globular clusters by the nearby giant galaxy NGC 3115. The total galaxy mass
out to 3 and 10 kpc lies between 10^10 and 10^11 solar masses and 2*10^10 and
4*10^11 solar masses, respectively, depending on the mass estimator used and
the assumptions on cluster orbits and systemic velocity. The HST imaging allows
measurement of sizes for two clusters, returning core radii around 2.0 pc,
similar to the sizes observed in other galaxies. Spectroscopy allows an
estimate of the degree of contamination by foreground stars or background
galaxies for the previous ground-based photometry, but does not require a
revision of previous results: NGC 3115 DW1 hosts around 60+/-20 clusters which
corresponds to a specific frequency of 4.9+/-1.9, on the high end for massive
dEs. Given its absolute magnitude (M_V=-17.7 mag) and the properties of its
cluster system, NGC 3115 DW1 appears to be a transition between a luminous dE
and low-luminosity E galaxy.Comment: 25 pages, 8 figures, accepted for publication in The Astronomical
Journal, August 2000 issu
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