520 research outputs found
KMOS view of the Galactic Centre I. Young stars are centrally concentrated
The Galactic centre hosts a crowded, dense nuclear star cluster with a
half-light radius of 4 pc. Most of the stars in the Galactic centre are cool
late-type stars, but there are also >100 hot early-type stars in the central
parsec of the Milky Way. These stars are only 3-8 Myr old. Our knowledge of the
number and distribution of early-type stars in the Galactic centre is
incomplete. Only a few spectroscopic observations have been made beyond a
projected distance of 0.5 pc of the Galactic centre. The distribution and
kinematics of early-type stars are essential to understand the formation and
growth of the nuclear star cluster. We cover the central >4pc^2 of the Galactic
centre using the integral-field spectrograph KMOS. We extracted more than 1,000
spectra from individual stars and identified early-type stars based on their
spectra. Our data set contains 114 bright early-type stars: 6 have narrow
emission lines, 23 are Wolf-Rayet stars, 9 stars have featureless spectra, and
76 are O/B type stars. Our wide-field spectroscopic data confirm that the
distribution of young stars is compact, with 90% of the young stars identified
within 0.5 pc of the nucleus. We identify 24 new O/B stars primarily at large
radii. We estimate photometric masses of the O/B stars and show that the total
mass in the young population is >12,000M_sun. The O/B stars all appear to be
bound to the Milky Way nuclear star cluster, while less than 30% belong to the
clockwise rotating disk. The central concentration of the early-type stars is a
strong argument that they have formed in situ. A large part of the young O/B
stars is not on the disk, which either means that the early-type stars did not
all form on the same disk or that the disk is dissolving rapidly. [abridged]Comment: 27 pages, 17 figures, matches journal version: Corrected typos,
corrected Notes in Table B.
The Rotation of Sub-Populations in omega Centauri
We present the first result of the Ital-FLAMES survey of red giant branch
(RGB) stars in omega Cen. Radial velocities with a precision of ~0.5 km/s are
presented for 650 members of omega Cen observed with FLAMES-Giraffe at the Very
Large Telescope. We found that stars belonging to the metal -poor (RGB-MP),
metal-intemediate (RGB-MInt) and metal-rich (RGB-a) sub -populations of Omega
Cen are all compatible with having the same rotational pattern. Our results
appear to contradict past findings by Norris et al., who could not detect any
rotational signature for metal -rich stars. The slightly higher precision of
the present measurements and the much larger sample size, especially for the
metal-richer stars, appear as the most likely explanation for this discrepancy.
The result presented here weakens the body of evidence in favour of a merger
event in the past history of omega Cen.Comment: 5 pages, 3 fiures, electronic table can be obtained from E. Pancino.
Accepted for publication in ApJ Letter
Evidence for multiple populations in intermediate age cluster Lindsay 1 in the SMC
Lindsay 1 is an intermediate age (≈8 Gyr) massive cluster in the Small Magellanic Cloud (SMC). Using VLT FORS2 spectra of 16 probable cluster members on the lower RGB of the cluster, we measure CN and CH band strengths (at ≃ 3883 and 4300 Å respectively), along with carbon and nitrogen abundances and find that a sub-population of stars has significant nitrogen enrichment. A lack of spread in carbon abundances excludes evolutionary mixing as the source of this enrichment, so we conclude that this is evidence of multiple populations. Therefore, L1 is the youngest cluster to show such variations, implying that the process triggering the onset of multiple populations must operate until at least redshift ∼1
Discovery of two M32 twins in Abell 1689
The M31 satellite galaxy M32 has long been considered an object of unique
properties, being the most extreme example of the very rare compact elliptical
galaxy class. Here we present the spectroscopic discovery of two M32 twins in
the massive galaxy cluster Abell 1689. As these objects are so rare, this is an
important step towards a better understanding of the nature of compact
galaxies. The two M32 twins had first been detected within our photometric
search for ultra compact dwarf galaxy (UCDs) candidates in A1689 (Mieske et al.
2004) with the Advanced Camera for Surveys (ACS). Their luminosities (M_V ~ -17
mag) are very similar to M32 and their surface brightness profiles are
consistent with that of M32 projected to A1689's distance. From our ACS imaging
we detect several fainter compact galaxy candidates with luminosities
intermediate between M32 and the Fornax UCDs. If spectroscopically confirmed as
cluster members, this would almost close the gap in the magnitude-surface
brightness plane between the region of UCDs and the compact ellipticals,
implying a sequence of faint compact galaxies well separated from that of dwarf
ellipticals.Comment: 5 pages, 5 figures, accepted for publication in A&A letter
The Fornax Deep Survey with VST. I. The extended and diffuse stellar halo of NGC~1399 out to 192 kpc
[Abrigded] We have started a new deep, multi-imaging survey of the Fornax
cluster, dubbed Fornax Deep Survey (FDS), at the VLT Survey Telescope. In this
paper we present the deep photometry inside two square degrees around the
bright galaxy NGC1399 in the core of the cluster. We found a very extended and
diffuse envelope surrounding the luminous galaxy NGC1399: we map the surface
brightness out to 33 arcmin (~ 192 kpc) from the galaxy center and down to
about 31 mag/arcsec^2 in the g band. The deep photometry allows us to detect a
faint stellar bridge in the intracluster region between NGC1399 and NGC1387. By
analyzing the integrated colors of this feature, we argue that it could be due
to the ongoing interaction between the two galaxies, where the outer envelope
of NGC1387 on its east side is stripped away. By fitting the light profile, we
found that it exists a physical break radius in the total light distribution at
R=10 arcmin (~58 kpc) that sets the transition region between the bright
central galaxy and the outer exponential stellar halo. We discuss the main
implications of this work on the build-up of the stellar halo at the center of
the Fornax cluster. By comparing with the numerical simulations of the stellar
halo formation for the most massive BCGs, we find that the observed stellar
halo mass fraction is consistent with a halo formed through the multiple
accretion of progenitors with a stellar mass in the range 10^8 - 10^11 M_sun.
This might suggest that the halo of NGC1399 has also gone through a major
merging event. The absence of a significant number of luminous stellar streams
and tidal tails out to 192 kpc suggests that the epoch of this strong
interaction goes back to an early formation epoch. Therefore, differently from
the Virgo cluster, the extended stellar halo around NGC1399 is characterised by
a more diffuse and well-mixed component, including the ICL.Comment: Accepted for publication in ApJ; 25 pages and 14 figures. An higher
resolution file is available at the following link
https://www.dropbox.com/s/fvltppduysdn6pb/NGC1399_fin_2c.pdf?dl=
The Fornax Deep Survey with VST. VIII. Connecting the accretion history with the cluster density
This work is based on deep multi-band (g, r, i) data from the Fornax Deep
Survey with VST. We analyse the surface brightness profiles of the 19 bright
ETGs inside the virial radius of the Fornax cluster. The main aim of this work
is to identify signatures of accretion onto galaxies by studying the presence
of outer stellar halos, and understand their nature and occurrence. Our
analysis also provides a new and accurate estimate of the intra-cluster light
inside the virial radius of Fornax. We performed multi-component fits to the
azimuthally averaged surface brightness profiles available for all sample
galaxies. This allows to quantify the relative weight of all components in the
galaxy structure that contribute to the total light. In addition, we derived
the average g-i colours in each component identified by the fit, as well as the
azimuthally averaged g-i colour profiles, to correlate them with the stellar
mass of each galaxy and the location inside the cluster. We find that in the
most massive and reddest ETGs the fraction of light in, probably accreted,
halos is much larger than in the other galaxies. Less-massive galaxies have an
accreted mass fraction lower than 30%, bluer colours and reside in the
low-density regions of the cluster. Inside the virial radius of the cluster,
the total luminosity of the intra-cluster light, compared with the total
luminosity of all cluster members, is about 34%. Inside the Fornax cluster
there is a clear correlation between the amount of accreted material in the
stellar halos of galaxies and the density of the environment in which those
galaxies reside. By comparing this quantity with theoretical predictions and
previous observational estimates, there is a clear indication that the driving
factor for the accretion process is the total stellar mass of the galaxy, in
agreement with the hierarchical accretion scenario.Comment: 18 pages, 10 figures. Accepted for publication in A&
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