437 research outputs found
Quantifying the Coexistence of Massive Black Holes and Dense Nuclear Star Clusters
In large spheroidal stellar systems, such as elliptical galaxies, one
invariably finds a 10^6-10^9 M_Sun supermassive black hole at their centre. In
contrast, within dwarf elliptical galaxies one predominantly observes a
10^5-10^7 M_Sun nuclear star cluster. To date, few galaxies have been found
with both type of nuclei coexisting and even less have had the masses
determined for both central components. Here we identify one dozen galaxies
housing nuclear star clusters and supermassive black holes whose masses have
been measured. This doubles the known number of such hermaphrodite nuclei -
which are expected to be fruitful sources of gravitational radiation. Over the
host spheroid (stellar) mass range from 10^8 to 10^11 M_Sun, we find that a
galaxy's nucleus-to-spheroid (baryon) mass ratio is not a constant value but
decreases from a few percent to ~0.3 percent such that log[(M_BH+M_NC)/M_sph] =
-(0.39+/-0.07)log[M_sph/10^10 M_Sun] -(2.18+/-0.07). Once dry merging has
commenced by M_sph ~ 10^11 M_Sun and the nuclear star clusters have
disappeared, this ratio is expected to become a constant value.
As a byproduct of our investigation, we have found that the projected flux
from resolved nuclear star clusters can be well approximated with Sersic
functions having a range of indices from ~0.5 to ~3, the latter index
describing the Milky Way's nuclear star cluster.Comment: To appear in MNRA
A New Method for Estimating Dark Matter Halo Masses using Globular Cluster Systems
All galaxies are thought to reside within large halos of dark matter, whose
properties can only be determined from indirect observations. The formation and
assembly of galaxies is determined from the interplay between these dark matter
halos and the baryonic matter they host. Although statistical relations can be
used to approximate how massive a galaxy's halo is, very few individual
galaxies have direct measurements of their halo masses. We present a method to
directly estimate the total mass of a galaxy's dark halo using its system of
globular clusters. The link between globular cluster systems and halo masses is
independent of a galaxy's type and environment, in contrast to the relationship
between galaxy halo and stellar masses. This trend is expected in models where
globular clusters form in early, rare density peaks in the cold dark matter
density field and the epoch of reionisation was roughly coeval throughout the
Universe. We illustrate the general utility of this relation by demonstrating
that a galaxy's supermassive black hole mass and global X-ray luminosity are
directly proportional to their host dark halo masses, as inferred from our new
method.Comment: 6 pages, 4 colour figures. Accepted by MNRAS Letters. Data catalogue
available from the first autho
Extending the baseline: Spitzer Mid-Infrared Photometry of Globular Cluster Systems in the Centaurus A and Sombrero Galaxies
Spitzer IRAC mid-infrared photometry is presented for the globular cluster
(GC) systems of the NGC 5128 ("Centaurus A") and NGC 4594 ("Sombrero")
galaxies. Existing optical photometric and spectroscopic are combined with this
new data in a comprehensive optical to mid-IR colour catalogue of 260 GCs.
Empirical colour-metallicity relationships are derived for all optical to
mid-IR colour combinations.
These colours prove to be very effective quantities to test the photometric
predictions of simple stellar population (SSP) models. In general, four SSP
models show larger discrepancies between each other and the data at bluer
wavelengths, especially at high metallicities. Such differences become very
important when attempting to use colour-colour model predictions to constrain
the ages of stellar populations. Furthermore, the age-substructure determined
from colour-colour diagrams and 91 NGC 5128 GCs with spectroscopic ages from
Beasley et al. (2008) are inconsistent, suggesting any apparent GC system
age-substructure implied by a colour-colour analysis must be verified
independently.
Unlike blue wavebands, certain optical to mid-IR colours are insensitive to
the flux from hot horizontal branch stars and thus provide an excellent
metallicity proxy. The NGC 5128 GC system shows strong bimodality in the
optical R-band to mid-IR colour distributions, hence proving it is bimodal in
metallicity. In this new colour space, a colour-magnitude trend, a "blue tilt",
is found in the NGC 5128 metal-poor GC data. The NGC 5128 young GCs do not
contribute to this trend. [abridged]Comment: 16 pages, 12 colour figures. To be published in MNRAS. Catalogue
available from the first author. Full resolution copy available here
http://lee.spitler.googlepages.com/spitzer_spitler.pd
A selection of H{\alpha} emitters at z = 2.1-2.5 using the Ks-band photometry of ZFOURGE
Large and less-biased samples of star-forming galaxies are essential to
investigate galaxy evolution. H{\alpha} emission line is one of the most
reliable tracers of star-forming galaxies because its strength is directly
related to recent star formation. However, it is observationally expensive to
construct large samples of H{\alpha} emitters by spectroscopic or narrow-band
imaging survey at high-redshifts. In this work, we demonstrate a method to
extract H{\alpha} fluxes of galaxies at z = 2.1-2.5 from Ks broad-band
photometry of ZFOURGE catalog. Combined with 25-39 other filters, we estimate
the emission line fluxes by SED fitting with stellar population models that
incorporate emission-line strengths. 2005 galaxies are selected as H{\alpha}
emitters by our method and their fluxes show good agreement with previous
measurements in the literature. On the other hand, there are more H{\alpha}
luminous galaxies than previously reported. The discrepancy can be explained by
extended H{\alpha} profiles of massive galaxies and a luminosity dependence of
dust attenuation, which are not taken into account in the previous work. We
also find that there are a large number of low-mass galaxies with much higher
specific star formation rate (sSFR) than expected from the extrapolated star
formation main sequence. Such low-mass galaxies exhibit larger ratios between
H{\alpha} and UV fluxes compared to more massive high sSFR galaxies. This
result implies that a "starburst" mode may differ among galaxies: low-mass
galaxies appear to assemble their stellar mass via short-duration bursts while
more massive galaxies tend to experience longer-duration (> 10 Myr) bursts.Comment: 18 pages, 19 figures, Resubmitted to ApJ after addressing reviewer's
comment
Infrared-Faint Radio Sources are at high redshifts
Context: Infrared-Faint Radio Sources (IFRS) are characterised by relatively
high radio flux densities and associated faint or even absent infrared and
optical counterparts. The resulting extremely high radio-to-infrared flux
density ratios up to several thousands were previously known only for
High-redshift Radio Galaxies (HzRGs), suggesting a link between the two classes
of object. Prior to this work, no redshift was known for any IFRS in the
Australia Telescope Large Area Survey (ATLAS) fields which would help to put
IFRS in the context of other classes of object, especially of HzRGs. Aims: This
work aims at measuring the first redshifts of IFRS in the ATLAS fields.
Further, we test the hypothesis that IFRS are similar to HzRGs, as
higher-redshift or dust-obscured versions of these massive galaxies. Methods: A
sample of IFRS was spectroscopically observed using the Focal Reducer and Low
Dispersion Spectrograph 2 (FORS2) at the Very Large Telescope (VLT). The data
were calibrated based on the Image Reduction and Analysis Facility (IRAF) and
redshifts extracted. This information was then used to calculate rest-frame
luminosities, and to perform the first spectral energy distribution modelling
of IFRS based on redshifts. Results: We found redshifts of 1.84, 2.13, and
2.76, for three IFRS, confirming the suggested high-redshift character of this
class of object. These redshifts as well as the resulting luminosities show
IFRS to be similar to HzRGs. We found further evidence that fainter IFRS are at
even higher redshifts. Conclusions: Considering the similarities between IFRS
and HzRGs substantiated in this work, the detection of IFRS, which have a
significantly higher sky density than HzRGs, increases the number of Active
Galactic Nuclei in the early universe and adds to the problems of explaining
the formation of supermassive black holes shortly after the Big Bang.Comment: 7 pages, 4 figures; version in prin
Surface Brightness Evolution of Galaxies in the CANDELS GOODS Fields up to : High-z Galaxies are Unique or Remain Undetected
We investigate the rest-frame Ultraviolet (UV, \AA) surface
brightness (SB) evolution of galaxies up to using a variety of deep
Hubble Space Telescope imaging. UV SB is a measure of the density of emission
from mostly young stars and correlates with an unknown combination of star
formation rate, initial mass function, cold gas mass density, dust attenuation,
and the size evolution of galaxies. In addition to physical effects, the SB is,
unlike magnitude, a more direct way in which a galaxy's detectability is
determined. We find a very strong evolution in the intrinsic SB distribution
which declines as , decreasing by 4-5 mag arcsec between
to . This change is much larger than expected in terms of the
evolution in UV luminosity, sizes or dust extinction and we demonstrate that
this evolution is 'unnatural' and due to selection biases. We also find no
strong correlation between mass and UV SB. Thus, deep HST imaging is unable to
discover all of the most massive galaxies in the distant universe. Through
simulations we show that only \% of galaxies that we can detect at
would be detected at high-. We furthermore explore possible origins of
high SB galaxies at high- by investigating the relationship between
intrinsic SB and star formation rates. We conclude that ultra-high SB galaxies
are produced by very gas rich dense galaxies which are in a unique phase of
evolution, possibly produced by mergers. Analogues of such galaxies do not
exist in the relatively nearby universe.Comment: Accepted for publication in AAS Journals (ApJ). 25 pages, 14 figure
A Blue Tilt in the Globular Cluster System of the Milky Way-like Galaxy NGC 5170
Here we present HST/ACS imaging, in the B and I bands, of the edge-on Sb/Sc
galaxy NGC 5170. Excluding the central disk region region, we detect a 142
objects with colours and sizes typical of globular clusters (GCs). Our main
result is the discovery of a `blue tilt' (a mass-metallicity relation), at the
3sigma level, in the metal-poor GC subpopulation of this Milky Way like galaxy.
The tilt is consistent with that seen in massive elliptical galaxies and with
the self enrichment model of Bailin & Harris. For a linear mass-metallicity
relation, the tilt has the form Z ~ L^{0.42 +/- 0.13}. We derive a total GC
system population of 600 +/- 100, making it much richer than the Milky Way.
However when this number is normalised by the host galaxy luminosity or stellar
mass it is similar to that of M31. Finally, we report the presence of a
potential Ultra Compact Dwarf of size ~ 6 pc and luminosity M_I ~ -12.5,
assuming it is physically associated with NGC 5170.Comment: Accepted for publication in MNRAS, 11 pages, 10 figure
Spectra of globular clusters in the Sombrero galaxy: evidence for spectroscopic metallicity bimodality
We present a large sample of over 200 integrated-light spectra of confirmed
globular clusters (GCs) associated with the Sombrero (M104) galaxy taken with
the DEIMOS instrument on the Keck telescope. A significant fraction of the
spectra have signal-to-noise levels high enough to allow measurements of GC
metallicities using the method of Brodie & Huchra (1990). We find a
distribution of spectroscopic metallicities ranging from -2.2 < [Fe/H] < +0.1
that is bimodal, with peaks at [Fe/H] ~ -1.4 and -0.6. Thus the GC system of
the Sombrero galaxy, like a few other galaxies now studied in detail, reveals a
bimodal spectroscopic metallicity distribution supporting the long-held belief
that colour bimodality reflects two metallicity subpopulations. This further
suggests that the transformation from optical colour to metallicity for old
stellar populations, such as GCs, is not strongly non-linear. We also explore
the radial and magnitude distribution with metallicity for GC subpopulations
but small number statistics prevent any clear trends in these distributions.Comment: 18 pages, 10 figures, 3 tables, MNRAS accepte
The Baryonic Halos of Elliptical Galaxies: Radial Distribution of Globular Clusters and Diffuse Hot Gas
For a sample of 9 well-studied giant ellipticals we compare the projected
radial distribution of their red and blue globular cluster (GC) subpopulations
with their host galaxy stellar and X-ray surface brightness profiles. We
support previous findings that the surface density distribution of red
(metal-rich) GCs follows that of the host galaxy starlight. We find good
agreement between the outer slope of the blue GC surface density and that of
the galaxy X-ray emission. This coincidence of projected radial profiles is
likely due to the fact that both blue GCs and X-ray emitting hot gas share the
same gravitational potential in equilibrium. When deprojected the X-ray
emitting hot gas has a radial density dependence that is the square root of
that for the GC density. We further show that the energy per unit mass for blue
GCs is roughly half that of the hot gas.Comment: 11 pages, 5 figures, accepted for publication in MNRA
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