91 research outputs found
The Initial Mass Functions in the Super-Star-Clusters NGC 1569A and NGC 1705-1
I use recent photometric and stellar velocity dispersion measurements of the
super-star-clusters (SSCs) NGC 1569A and NGC 1705-1 to determine their
present-day luminosity/mass (L_V/M) ratios. I then use the inferred L_V/M
ratios, together with population synthesis models of evolving star-clusters, to
constrain the initial-mass-functions (IMFs) in these objects.
I find that (L_V/M)_solar=28.9 in 1569A, and (L_V/M)_solar=126 in 1705-1. It
follows that in 1569A the IMF is steep with alpha~2.5 for m**(-alpha)dm IMFs
which extend to 0.1 M_sun. This implies that most of the stellar mass in 1569A
is contained in low-mass (< 1 M_sun) stars. However, in 1705-1 the IMF is
either flat, with alpha<2$, or it is truncated at a lower mass-limit between 1
and 3 M_sun.
I compare the inferred IMFs with the mass functions (MFs) of Galactic
globular clusters. It appears that 1569A has a sufficient reservoir of low-mass
stars for it to plausibly evolve into an object similar to Galactic globular
clusters. However, the apparent deficiency of low-mass stars in 1705-1 may make
it difficult for this SSC to become a globular cluster. If low-mass stars do
dominate the cluster mass in 1705-1, the large L_V/M ratio in this SSC may be
evidence that the most massive stars have formed close to the cluster cores.Comment: ApJ, in press. 19 Pages, Latex; [email protected]
Globular Cluster Formation in M82
We present high resolution mid-infrared (mid-IR; 11.7 and 17.65 micron) maps
of the central 400 pc region of the starburst galaxy M82. Seven star forming
clusters are identified which together provide ~ 15% of the total mid-IR
luminosity of the galaxy. Combining the mid-IR data with thermal radio
measurements and near- and mid-IR line emission, we find that these young
stellar clusters have inferred masses and sizes comparable to globular
clusters. At least 20% of the star formation in M82 is found to occur in
super-star clusters.Comment: 12 pages including three color figures; accepted for publication in
Ap
Hierarchical Structure Formation and Chemical Evolution of Galaxies
We present an analytical and phenomenological model for metal enrichment in
halos based on hierarchical structure formation. This model assumes that
astration of normal stellar populations along with SNe II already occurs at
very high redshift. For halos that are not disrupted by SN II explosions, the
chemical evolution of the gas and stars is explicitly determined by the rate of
gas infall as compared with the astration rate and the corresponding rate of
metal production by SNe II per H atom in the gas. This model provides a good
description of the data on [Fe/H] for damped Ly alpha systems over a wide range
of redshift 0.5 < z < 5. For all halos not disrupted by SN II explosions, if
there is a cessation of gas infall, the metallicities of stars follow a bimodal
distribution. This distribution is characterized by a sharp peak at the value
of [Fe/H] corresponding to the time of infall cessation and by a broad peak at
a higher value of [Fe/H] corresponding to the subsequent period of astration
during which the bulk of the remaining gas forms stars. Such a distribution may
be compared to that observed for the Galactic halo stars. If the gas in a halo
is rapidly lost upon cessation of infall, then an assemblage of stars with a
very sharply-defined [Fe/H] value will be left behind. This assemblage of stars
may be accreted by a larger system and become a globular cluster of the larger
system. We also discuss the masses and metallicities of the globular clusters
in this model. (Abridged)Comment: 24 pages, 10 figures, to appear in Ap
A Low-Mass Central Black Hole in the Bulgeless Seyfert 1 Galaxy NGC 4395
NGC 4395 is one of the least luminous and nearest known type 1 Seyfert
galaxies, and it also lacks a bulge. We present an HST I-band image of its
nuclear region and Keck high-resolution (8 km/s) echelle spectra containing the
Ca II near-infrared triplet. In addition to the unresolved point source, there
is a nuclear star cluster of size r ~ 3.9 pc; the upper limit on its velocity
dispersion is only 30 km/s. We thus derive an upper limit of 6.2x10^6 solar
masses for the mass of the compact nucleus. Based on the amount of spatially
resolved light in the HST image, a sizable fraction of this is likely to reside
in stars. Hence, this estimate sets a stringent upper limit on the mass of the
central black hole. We argue, from other lines of evidence, that the true mass
of the black hole is likely to be 10^4-10^5 solar masses. Although the black
hole is much less massive than those thought to exist in classical active
galactic nuclei, its accretion rate of L_bol/L_Edd ~ 2x10^-2 to 2x10^-3 is
consistent with the mass-luminosity relation obeyed by classical AGNs. This may
explain why NGC 4395 has a high-excitation (Seyfert) emission-line spectrum;
active galaxies having low-ionization spectra seem to accrete at significantly
lower rates. NGC 4395, a pure disk galaxy, demonstrates that supermassive black
holes are not associated exclusively with bulges.Comment: To appear in ApJ (Letters). 5 page
Massive perturbers and the efficient merger of binary massive black holes
We show that dynamical relaxation in the aftermath of a galactic merger and
the ensuing formation and decay of a binary massive black hole (MBH), are
dominated by massive perturbers (MPs) such as giant molecular clouds or
clusters. MPs accelerate relaxation by orders of magnitude relative to 2-body
stellar relaxation alone, and efficiently scatter stars into the binary MBH's
orbit. The 3-body star-binary MBH interactions shrink the binary MBH to the
point where energy losses from the emission of gravitational waves (GW) lead to
rapid coalescence. We model this process based on observed and simulated MP
distributions and take into account the decreased efficiency of the star-binary
MBH interaction due to acceleration in the galactic potential. We show that
mergers of gas-rich galactic nuclei lead to binary MBH coalescence well within
the Hubble time. Moreover, lower-mass binary MBHs (<10^8 Msun) require only a
few percent of the typical gas mass in a post-merger nucleus to coalesce in a
Hubble time. The fate of a binary MBH in a gas poor galactic merger is less
certain, although massive stellar structures (e.g. clusters, stellar rings)
could likewise lead to efficient coalescence. These coalescence events are
observable by their strong GW emission. MPs thus increase the cosmic rate of
such GW events, lead to a higher mass deficit in the merged galactic core and
suppress the formation of triple MBH systems and the resulting ejection of MBHs
into intergalactic space.Comment: 14 pages, 4 figures, 3 tables. More detailed explanations and changes
in structure. Section on hypervelocity stars moved to another paper (in
preparation). Results and conclusions unchanged. Accepted to Ap
Binding Energy and the Fundamental Plane of Globular Clusters
A physical description of the fundamental plane of Galactic globular clusters
is developed which explains all empirical trends and correlations in a large
number of cluster observables and provides a small but complete set of truly
independent constraints on theories of cluster formation and evolution in the
Milky Way. Within the theoretical framework of single-mass, isotropic King
models, it is shown that (1) 39 regular (non--core-collapsed) globulars with
measured core velocity dispersions share a common V-band mass-to-light ratio of
1.45 +/- 0.10, and (2) a complete sample of 109 regular globulars reveals a
very strong correlation between cluster binding energy and total luminosity,
regulated by Galactocentric position: E_b \propto (L^{2.05} r_{\rm gc}^{-0.4}).
The observational scatter about either of these two constraints can be
attributed fully to random measurement errors, making them the defining
equations of a fundamental plane for globular clusters. A third, weaker
correlation, between total luminosity and the King-model concentration
parameter, c, is then related to the (non-random) distribution of globulars on
the plane. The equations of the FP are used to derive expressions for any
cluster observable in terms of only L, r_{\rm gc}, and c. Results are obtained
for generic King models and applied specifically to the globular cluster system
of the Milky Way.Comment: 60 pages with 19 figures, submitted to Ap
HAT-P-4b: A metal-rich low-density transiting hot Jupiter
We describe the discovery of HAT-P-4b, a low-density extrasolar planet
transiting BD+36 2593, a V = 11.2 mag slightly evolved metal-rich late F star.
The planet's orbital period is 3.056536+/-0.000057 d with a mid-transit epoch
of 2,454,245.8154 +/- 0.0003 (HJD). Based on high-precision photometric and
spectroscopic data, and by using transit light curve modeling, spectrum
analysis and evolutionary models, we derive the following planet parameters:
Mp= 0.68 +/- 0.04 MJ, Rp= 1.27 +/- 0.05 RJ, rho = 0.41 +/- 0.06 g cm-3 and a =
0.0446 +/- 0.0012 AU. Because of its relatively large radius, together with its
assumed high metallicity of that of its parent star, this planet adds to the
theoretical challenges to explain inflated extrasolar planets.Comment: 5 pages, accepted for publication in ApJ
K-band Spectroscopy of Clusters in NGC 4038/4039
Integral field spectroscopy in the K-band (1.9-2.4um) was performed on four
IR-bright star clusters and the two nuclei in NGC 4038/4039 (``The Antennae'').
Two of the clusters are located in the overlap region of the two galaxies, and
together comprise ~25% of the total 15um and ~10% of the total 4.8 GHz emission
from this pair of merging galaxies. The other two clusters, each of them
spatially resolved into two components, are located in the northern galaxy, one
in the western and one in the eastern loop of blue clusters. Comparing our
analysis of Brgamma, CO band-heads, He I (2.058um), Halpha (from archival HST
data), and V-K colors with stellar population synthesis models indicates that
the clusters are extincted (A_V ~ 0.7 - 4.3 mags) and young, displaying a
significant age spread (4-13 Myrs). The starbursts in the nuclei are much older
(65 Myrs), with the nucleus of NGC 4038 displaying a region of recent star
formation northward of its K-band peak. Using our derived age estimates and
assuming the parameters of the IMF (Salpeter slope, upper mass cut-off of 100
M_sun, Miller-Scalo between 1 M_sun and 0.1 M_sun), we find that the clusters
have masses between 0.5 and 5 * 10^6M_sun.Comment: 10 pages, 3 figures, ApJ accepte
A New Spectroscopic and Photometric Analysis of the Transiting Planet Systems TrES-3 and TrES-4
We report new spectroscopic and photometric observations of the parent stars
of the recently discovered transiting planets TrES-3 and TrES-4. A detailed
abundance analysis based on high-resolution spectra yields [Fe/H] , K, and for TrES-3,
and [Fe/H] , K, and for TrES-4. The accuracy of the effective temperatures is supported
by a number of independent consistency checks. The spectroscopic orbital
solution for TrES-3 is improved with our new radial-velocity measurements of
that system, as are the light-curve parameters for both systems based on newly
acquired photometry for TrES-3 and a reanalysis of existing photometry for
TrES-4. We have redetermined the stellar parameters taking advantage of the
strong constraint provided by the light curves in the form of the normalized
separation (related to the stellar density) in conjunction with our
new temperatures and metallicities. The masses and radii we derive are
M_\star=0.928_{-0.048}^{+0.028} M_{\sun},R_\star = 0.829_{-0.022}^{+0.015}
R_{\sun}, and M_\star = 1.404_{-0.134}^{+0.066} M_{\sun},
R_\star=1.846_{-0.087}^{+0.096} R_{\sun} for TrES-3 and TrES-4, respectively.
With these revised stellar parameters we obtain improved values for the
planetary masses and radii. We find , for TrES-3, and
, for TrES-4. We confirm TrES-4 as the planet with the largest
radius among the currently known transiting hot Jupiters.Comment: 42 pages, 10 tables, 8 figures. Accepted for publication in the
Astrophysical Journa
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