898 research outputs found
Evidence for Elevated X-ray Emission in Local Lyman Break Galaxy Analogs
In this paper, we study the relationship between the 2-10 keV X-ray
luminosity (L_X), assumed to originate from X-ray binaries (XRBs), and star
formation rate (SFR) in UV-selected z<0.1 Lyman break analogs (LBAs). We
present Chandra observations for four new GALEX-selected LBAs. Including
previously studied LBAs, Haro 11 and VV 114, we find that LBAs demonstrate
L_X/SFR ratios that are elevated by ~1.5sigma compared to local galaxies,
similar to the ratios found for stacked LBGs in the early Universe (z>2). We
show that these LBAs are unlikely to harbor AGN, based on their optical and
X-ray spectra and the spatial distribution of the X-rays in three spatially
extended cases. We expect that high-mass X-ray binaries (HMXBs) dominate the
X-ray emission in these galaxies, based on their high specific SFRs
(sSFRs=SFR/M* > 10^{-9}/yr), which suggest the prevalence of young stellar
populations. Since both LBAs and LBGs have lower dust attenuations and
metallicities compared to similar samples of more typical local galaxies, we
investigate the effects of dust extinction and metallicity on the L_X/SFR for
the broader population of galaxies with high sSFRs (>10^{-10}/yr). The
estimated dust extinctions (corresponding to column densities of
N_H<10^{22}/cm^2) are expected to have insignificant effects on observed
L_X/SFR ratio for the majority of galaxy samples. We find that the observed
relationship between L_X/SFR and metallicity appears consistent with
theoretical expectations from X-ray binary population synthesis models.
Therefore, we conclude that lower metallicities, related to more luminous HMXBs
such as ultraluminous X-ray sources (ULXs), drive the elevated L_X/SFR observed
in our sample of z<0.1 LBAs. The relatively metal-poor, active mode of star
formation in LBAs and distant z>2 LBGs may yield higher total HMXB luminosity
than found in typical galaxies in the local Universe.Comment: 11 pages, 7 figures, Submitted to ApJ (references updated in v2
A First Estimate Of The X-Ray Binary Frequency As A Function Of Star Cluster Mass In A Single Galactic System
We use the previously-identified 15 infrared star-cluster counterparts to
X-ray point sources in the interacting galaxies NGC 4038/4039 (the Antennae) to
study the relationship between total cluster mass and X-ray binary number. This
significant population of X-Ray/IR associations allows us to perform, for the
first time, a statistical study of X-ray point sources and their environments.
We define a quantity, \eta, relating the fraction of X-ray sources per unit
mass as a function of cluster mass in the Antennae. We compute cluster mass by
fitting spectral evolutionary models to K_s luminosity. Considering that this
method depends on cluster age, we use four different age distributions to
explore the effects of cluster age on the value of \eta and find it varies by
less than a factor of four. We find a mean value of \eta for these different
distributions of \eta = 1.7 x 10^-8 M_\sun^-1 with \sigma_\eta = 1.2 x 10^-8
M_\sun^-1. Performing a \chi^2 test, we demonstrate \eta could exhibit a
positive slope, but that it depends on the assumed distribution in cluster
ages. While the estimated uncertainties in \eta are factors of a few, we
believe this is the first estimate made of this quantity to ``order of
magnitude'' accuracy. We also compare our findings to theoretical models of
open and globular cluster evolution, incorporating the X-ray binary fraction
per cluster.Comment: 20 pages, 6 figures, accepted by Ap
Feedback in the local LBG Analog Haro 11 as probed by far-UV and X-ray observations
We have re-analyzed FUSE data and obtained new Chandra observations of Haro
11, a local (D_L=88 Mpc) UV luminous galaxy. Haro 11 has a similar far-UV
luminosity (10^10.3 L_\odot), UV surface brightness (10^9.4 L_\odot kpc^-2),
SFR, and metallicity to that observed in Lyman Break Galaxies (LBGs). We show
that Haro 11 has extended, soft thermal (kT~0.68 keV) X-ray emission with a
luminosity and size which scales with the physical properties (e.g. SFR,
stellar mass) of the host galaxy. An enhanced alpha/Fe, ratio of ~4 relative to
solar abundance suggests significant supernovae enrichment. These results are
consistent with the X-ray emission being produced in a shock between a
supernovae driven outflow and the ambient material. The FUV spectra show strong
absorption lines similar to those observed in LBG spectra. A blueshifted
absorption component is identified as a wind outflowing at ~200-280 km/s.
OVI\lambda\lambda1032,1038 emission, the dominant cooling mechanism for coronal
gas at T~10^5.5 K is also observed. If associated with the outflow, the
luminosity of the OVI emission suggests that <20% of the total mechanical
energy from the supernovae and solar winds is being radiated away. This implies
that radiative cooling through OVI is not significantly inhibiting the growth
of the outflowing gas. In contradiction to the findings of Bergvall et al 2006,
we find no convincing evidence of Lyman continuum leakage in Haro 11. We
conclude that the wind has not created a `tunnel' allowing the escape of a
significant fraction of Lyman continuum photons and place a limit on the escape
fraction of f_{esc}<2%. Overall, both Haro 11 and a previously observed LBG
analogue VV 114, provide an invaluable insight into the X-ray and FUV
properties of high redshift LBGs.Comment: Accepted for publication in ApJ, 40 pages, 17 figure
Heterometallic perovskite-type metal-organic framework with an ammonium cation: structure, phonons, and optical response of [NH4]Na0.5CrxAl0.5-x(HCOO)3 (x = 0, 0.025 and 0.5).
We report the synthesis, crystal structure, vibrational and luminescence properties of two heterometallic perovskite-type metal-organic frameworks (MOFs) containing the ammonium cation (NH4+, Am+): [NH4][Na0.5Cr0.5(HCOO)3] (AmNaCr) and [NH4][Na0.5Al0.475Cr0.025(HCOO)3] (AmNaAlCr) in comparison to the previously reported [NH4][Na0.5Al0.5(HCOO)3] (AmNaAl). The room-temperature crystal structure of AmNaCr and AmNaAlCr was determined to be R3[combining macron]. The hydrogen bonding (HB) energy calculated using density functional theory (DFT) agrees well with experimental data, and confirms the existence of almost identical H-bonding in AmNaCr and AmNaAl, with three short hydrogen bonds and a longer trifurcated H-bond. Temperature-dependent Raman measurements supported by differential scanning calorimetry show that AmNaCr does not undergo any structural phase transitions in the 80-400 K temperature range. The high-pressure Raman spectra of AmNaCr show the onset of two structural instabilities near 0.5 and 1.5 GPa. The first instability involves weak distortion of the framework, while the second leads to irreversible amorphization of the sample. High-pressure DFT simulations show that the unit cell of the AmNaCr compound contracts along the c axis, which leads to a shortening of the trifurcated H-bond. The optical properties show that both studied crystals exhibit Cr3+-based emission characteristic of intermediate ligand field strength
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