85 research outputs found
Globular Clusters as Tracers of Stellar Bimodality in Elliptical Galaxies: The Case of NGC 1399
Globular cluster systems (GCS) frequently show a bi-modal distribution of the
cluster integrated colours. This work explores the arguments to support the
idea that the same feature is shared by the diffuse stellar population of the
galaxy they are associated with. In the particular case of NGC 1399 the results
show that the galaxy brightness profile and colour gradient as well as the
behaviour of the cumulative globular cluster specific frequency, are compatible
with the presence of two dominant stellar populations, associated with the so
called "blue" and "red" globular cluster families. These globular families are
characterized by different intrinsic specific frequencies (defined in terms of
each stellar population): Sn=3.3 +/- 0.3 in the case of the red globulars and
Sn=14.3 +/- 2.5 for the blue ones. We stress that this result is not
necessarily conflicting with recent works that point out a clear difference
between the metallicity distribution of (resolved) halo stars and globulars
when comparing their number statistics. The inferred specific frequencies imply
that, in terms of their associated stellar populations, the formation of the
blue globulars took place with an efficiency about 6 times higher than that
corresponding to their red counterparts. The similarity of the spatial
distribution of the blue globulars with that inferred for dark matter, as well
as with that of the X ray emiting hot gas associated with NGC 1399, is
emphasized. The impact of a relatively unconspicuous low metallicity
population, that shares the properties of the blue globulars, as a possible
source of chemical enrichment early in the formation history of the galaxy is
also briefly discussed.Comment: 15 pages; MNRAS (accepted; October 2004
Mapping the galaxy NGC 4486 (M87) through its Globular Cluster System
As shown in previous works, globular clusters can be used to trace the
overall structure of the diffuse stellar populations in early type galaxies if
the number of clusters per unit stellar mass depends on metallicity. In this
paper we further test this assumption in the galaxy NGC 4486 (M 87), by
combining several data sources. The results show that globular clusters allow
the mapping of the galaxy in terms of the surface brightness profile,
integrated colour gradient, chemical abundance, and mass to luminosity ratios
up to 1000 arcsec (or 80.4 kpc) from its centre (i.e. some 10 effective radii).
The analysis indicates the presence of a dominant high metallicity bulge
associated with the red globulars, whose ellipticity increases outwards, and of
a more flattened low metallicity halo connected with the blue globulars. The
chemical abundance gradient of the composite stellar population is remarkably
similar to that inferred from X ray observations of hot gas. The
mass-metallicity spectrum of the stellar population can, in principle, be
understood in terms of inhomogeneous enrichment models. In turn, the
distribution of the bluest GCs, and lowest metallicity halo stars, has an
intriguing similarity with that of dark matter, a feature shared with NGC 1399.
Also, in these two galaxies, the number of blue GCs per dark mass unit is
identical within the errors, \approx 1.0(\pm0.3) \times 10^{-9} . The total
stellar mass derived for NGC 4486 is 6.8(\pm1.1) \times 10^{11} M\odot with a
baryonic mass fraction fb = 0.08(\pm 0.01).Comment: 18 pages, 23 figure
The complex star cluster system of NGC 1316 (Fornax A)
This paper presents Gemini- high quality photometry for cluster
candidates in the field of NGC 1316 (Fornax A) as part of a study that also
includes GMOS spectroscopy. A preliminary discussion of the photometric data
indicates the presence of four stellar cluster populations with distinctive
features in terms of age, chemical abundance and spatial distribution. Two of
them seem to be the usually old (metal poor and metal rich) populations
typically found in elliptical galaxies. In turn, an intermediate-age (5 Gyr)
globular cluster population is the dominant component of the sample (as
reported by previous papers). We also find a younger cluster population with a
tentative age of 1 Gyr
The Connection between Globular Cluster Systems and the Host Galaxies
A large number of early type galaxies are now known to possess blue and red
subpopulations of globular clusters. We have compiled a database of 28 such
galaxies exhibiting bimodal globular cluster colour distributions. After
converting to a common V-I colour system, we investigate correlations between
the mean colour of the blue and red subpopulations with galaxy velocity
dispersion. We support previous claims that the mean colour of the blue
globular clusters are unrelated to their host galaxy. They must have formed
rather independently of the galaxy potential they now inhabit. The mean blue
colour is similar to that for halo globular clusters in our Galaxy and M31. The
red globular clusters, on the other hand, reveal a strong correlation with
galaxy velocity dispersion. Futhermore, in well-studied galaxies the red
subpopulation has similar, and possibly identical, colours to the galaxy halo
stars. Our results indicate an intimate link between the red globular clusters
and the host galaxy; they share a common formation history. A natural
explanation for these trends would be the formation of the red globular
clusters during galaxy collapse.Comment: 6 pages, Latex, 3 figures, 1 table, submitted to MNRA
Multicolour-metallicity Relations from Globular Clusters in NGC 4486 (M87)
We present Gemini griz photometry for 521 globular cluster (GC) candidates in
a 5.5 x 5.5 arcmin field centered 3.8 arcmin to the south and 0.9 arcmin to the
west of the center of the giant elliptical galaxy NGC 4486. All these objects
have previously published (C-T1) photometry. We also present new (C-T1)
photometry for 338 globulars, within 1.7 arcmin in galactocentric radius, which
have (g-z) colors in the photometric system adopted by the Virgo Cluster Survey
of the Advanced Camera for Surveys of the Hubble Space Telescope. These
photometric data are used to define a self-consistent multicolor grid (avoiding
polynomial fits) and preliminary calibrated in terms of two chemical abundance
scales. The resulting multicolor color-chemical abundance relations are used to
test GC chemical abundance distributions. This is accomplished by modelling the
ten GC color histograms that can be defined in terms of the Cgriz bands. Our
results suggest that the best fit to the GC observed color histograms is
consistent with a genuinely bimodal chemical abundance distribution NGC(Z). On
the other side, each (blue and red) GC subpopulation follows a distinct
color-color relation.Comment: 12 pages, 21 figures, 8 tables. Accepted to be published in MNRA
The globular clusters-stellar haloes connection in early type galaxies
This paper explores if, and to what an extent, the stellar populations of
early type galaxies can be traced through the colour distribution of their
globular cluster systems. The analysis, based on a galaxy sample from the Virgo
ACS data, is an extension of a previous approach that has been successful in
the cases of the giant ellipticals NGC 1399 and NGC 4486, and assumes that the
two dominant GC populations form along diffuse stellar populations sharing the
cluster chemical abundances and spatial distributions. The results show that a)
Integrated galaxy colours can be matched to within the photometric
uncertainties and are consistent with a narrow range of ages; b) The inferred
mass to luminosity ratios and stellar masses are within the range of values
available in the literature; c) Most globular cluster systems occupy a thick
plane in the volume space defined by the cluster formation efficiency, total
stellar mass and projected surface mass density. The formation efficiency
parameter of the red clusters shows a dependency with projected stellar mass
density that is absent for the blue globulars. In turn, the brightest galaxies
appear clearly detached from that plane as a possible consequence of major past
mergers; d) The stellar mass-metallicity relation is relatively shallow but
shows a slope change at . Galaxies with smaller
stellar masses show predominantly unimodal globular cluster colour
distributions. This result may indicate that less massive galaxies are not able
to retain chemically enriched intestellar matter.Comment: 19 pages, 10 figure
Field/Isolated lenticular galaxies with high SN values: the case of NGC 4546 and its globular cluster system
Abstract We present a photometric study of the field lenticular galaxy NGC 4546 using Gemini/GMOS imaging in g′r′i′z′. We perform a 2D image decomposition of the surface brightness distribution of the galaxy using galfit, finding that four components adequately describe it. The subtraction of this model from our images and the construction of a colour map allow us to examine in great detail the asymmetric dust structures around the galactic centre. In addition, we perform a detailed analysis of the globular cluster (GC) system of NGC 4546. Using a Gaussian Mixture Model algorithm in the colour-colour plane we detected hints of multiple groups of GC candidates: the classic blue and red subpopulations, a group with intermediate colours that present a concentrated spatial distribution towards the galaxy, and an additional group towards the red end of the colour distribution. We estimate a total GC population for NGC 4546 of 390 ± 60 members and specific frequency SN = 3.3 ± 0.7, which is relatively high compared to the typical value for galaxies of similar masses and environment. We suggest that the unusual GC population substructures were possibly formed during the interaction that led to the formation of the young ultra-compact dwarf (NGC 4546-UCD1) found in this system. Finally, we estimate the distance modulus of NGC 4546 by analyzing its luminosity function, resulting in (m − M) = 30.75 ± 0.12 mag (14.1 Mpc)
A Quantitative Link Between Globular Clusters and the Stellar Halos in Elliptical Galaxies
This paper explores the quantitative connection between globular clusters and
the diffuse stellar population of the galaxies they are associated with. Both
NGC 1399 and NGC 4486 (M87) are well suited for this kind of analysis due to
their large globular cluster populations. The main assumption of our Monte
Carlo based models is that each globular cluster is formed along with a given
diffuse stellar mass that shares the same spatial distribution, chemical
composition and age. The main globular clusters subpopulations, that determine
the observed bimodal colour distribution, are decomposed avoiding a priori
parametric (e.g. Gaussian) fits and using a new colour (C-T1)-metallicity
relation. The eventual detectability of a blue tilt in the colour magnitude
diagrams of the blue globulars subpopulation is also addressed. A successful
link between globular clusters and the stellar galaxy halo is established by
assuming that the number of globular clusters per associated diffuse stellar
mass t is a function of total abundance [Z/H] and behaves as
t=gamma*exp(delta[Z/H]) (i.e. increases when abundance decreases).Comment: 19 pages, 24 figures and 4 tables. Accepted for publication in MNRA
An extended star formation history in an ultra-compact dwarf
There has been significant controversy over the mechanisms responsible for forming compact stellar systems like ultra-compact dwarfs (UCDs), with suggestions that UCDs are simply the high-mass extension of the globular cluster population, or alternatively, the liberated nuclei of galaxies tidally stripped by larger companions. Definitive examples of UCDs formed by either route have been difficult to find, with only a handful of persuasive examples of stripped-nucleus-type UCDs being known. In this paper, we present very deep Gemini/GMOS spectroscopic observations of the suspected stripped-nucleus UCD NGC 4546-UCD1 taken in good seeing conditions (<0.7 arcsec). With these data we examine the spatially resolved kinematics and star formation history of this unusual object. We find no evidence of a rise in the central velocity dispersion of the UCD, suggesting that this UCD lacks a massive central black hole like those found in some other compact stellar systems, a conclusion confirmed by detailed dynamical modelling. Finally, we are able to use our extremely high signal-to-noise spectrum to detect a temporally extended star formation history for this UCD. We find that the UCD was forming stars since the earliest epochs until at least 1-2 Gyr ago. Taken together these observations confirm that NGC 4546-UCD1 is the remnant nucleus of a nucleated dwarf galaxy that was tidally destroyed by NGC 4546 within the last 1-2 Gyr.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
First confirmed ultra-compact dwarf galaxy in the NGC 5044 group
Context. Ultra-compact dwarfs (UCDs) are stellar systems displaying colours and metallicities between those of globular clusters (GCs) and early-type dwarf galaxies, as well as sizes of Reff 100 pc and luminosities in the range-13.5 < MV<-11 mag. Although their origin is still subject of debate, the most popular scenarios suggest that they are massive star clusters or the nuclei of tidally stripped dwarf galaxies. Aims. NGC 5044 is the central massive elliptical galaxy of the NGC 5044 group. Its GC/UCD system is completely unexplored. Methods. In Gemini+GMOS deep images of several fields around NGC 5044 and in spectroscopic multi-object data of one of these fields, we detected an unresolved source with g′ ∼ 20.6 mag, compatible with being an UCD. Its radial velocity was obtained with FXCOR and the penalized pixel-fitting (pPXF) code. To study its stellar population content, we measured the Lick/IDS indices and compared them with predictions of single stellar population models, and we used the full spectral fitting technique. Results. The spectroscopic analysis of the UCD revealed a radial velocity that agrees with the velocity of the elliptical galaxy NGC 5044. From the Lick/IDS indices, we have obtained a luminosity-weighted age and metallicity of 11.7+ 1.4 -1.2 Gyr and [Z/H] =-0.79 ± 0.04 dex, respectively, as well as [α/Fe] = 0.30 ± 0.06. From the full spectral fitting technique, we measured a lower age (8.52 Gyr) and a similar total metallicity ([Z/H] =-0.86 dex). Conclusions. Our results indicate that NGC 5044-UCD1 is most likely an extreme GC (MV ∼-12.5 mag) belonging to the GC system of the elliptical galaxy NGC 5044.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
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