38 research outputs found
X-Ray Detection of an Obscured Active Galactic Nucleus in a z=3.09 Radio-quiet Ly-alpha Nebula
We present evidence for a highly obscured X-ray source in one of two giant
Ly-alpha emission nebulae in the z=3.09 proto-cluster region SSA22. Neither
Ly-alpha nebula is associated with significant radio emission. While one has a
significant submillimeter detection and is undetected in the X-ray, the other
is a factor of 2-10 times less submillimeter bright and appears to contain a
hard-band X-ray source. We discuss our analysis and techniques for assessing
the X-ray properties of this source and suggest that we have detected an
embedded AGN source in one of these nebulae which may be at least partially
responsible for exciting the Ly-alpha emission through a mechanism that is
essentially decoupled from the radio, submillimeter, or optical luminosities.
We also present an upper limit on the mean X-ray emission from 10 other
extended Ly-alpha objects in the SSA22 region.Comment: 12 pages, 3 figures, accepted to appear in ApJL, Vol. 615, 2004
November 1
Lyman Break Analogs: Constraints on the Formation of Extreme Starbursts at Low and High Redshift
Lyman Break Analogs (LBAs), characterized by high far-UV luminosities and
surface brightnesses as detected by GALEX, are intensely star-forming galaxies
in the low-redshift universe (), with star formation rates reaching
up to 50 times that of the Milky Way. These objects present metallicities,
morphologies and other physical properties similar to higher redshift Lyman
Break Galaxies (LBGs), motivating the detailed study of LBAs as local
laboratories of this high-redshift galaxy population. We present results from
our recent integral-field spectroscopy survey of LBAs with Keck/OSIRIS, which
shows that these galaxies have the same nebular gas kinematic properties as
high-redshift LBGs. We argue that such kinematic studies alone are not an
appropriate diagnostic to rule out merger events as the trigger for the
observed starburst. Comparison between the kinematic analysis and morphological
indices from HST imaging illustrates the difficulties of properly identifying
(minor or major) merger events, with no clear correlation between the results
using either of the two methods. Artificial redshifting of our data indicates
that this problem becomes even worse at high redshift due to surface brightness
dimming and resolution loss. Whether mergers could generate the observed
kinematic properties is strongly dependent on gas fractions in these galaxies.
We present preliminary results of a CARMA survey for LBAs and discuss the
implications of the inferred molecular gas masses for formation models.Comment: To appear in the proceedings of IAU Symposium 277, "Tracing the
Ancestry of Galaxies on the Land of our Ancestors", eds. C. Carignan, K.C.
Freeman, and F. Combe
Energy Feedback from X-ray Binaries in the Early Universe
X-ray photons, because of their long mean-free paths, can easily escape the
galactic environments where they are produced, and interact at long distances
with the inter-galactic medium, potentially having a significant contribution
to the heating and reionization of the early Universe. The two most important
sources of X-ray photons in the Universe are active galactic nuclei (AGN) and
X-ray binaries (XRBs). In this Letter we use results from detailed, large scale
population synthesis simulations to study the energy feedback of XRBs, from the
first galaxies (z~ 20) until today. We estimate that X-ray emission from XRBs
dominates over AGN at z>6-8. The shape of the spectral energy distribution of
the emission from XRBs shows little change with redshift, in contrast to its
normalization which evolves by ~4 orders of magnitude, primarily due to the
evolution of the cosmic star-formation rate. However, the metallicity and the
mean stellar age of a given XRB population affect significantly its X-ray
output. Specifically, the X-ray luminosity from high-mass XRBs per unit of
star-formation rate varies an order of magnitude going from solar metallicity
to less than 10% solar, and the X-ray luminosity from low-mass XRBs per unit of
stellar mass peaks at an age of ~300 Myr and then decreases gradually at later
times, showing little variation for mean stellar ages > 3 Gyr. Finally, we
provide analytical and tabulated prescriptions for the energy output of XRBs,
that can be directly incorporated in cosmological simulations.Comment: Accepted for publication to ApJ Letters, 6 pages, 2 figures, 2 table.
Significant changes to figure 2
The Coincidence of Nuclear Star Clusters and Active Galactic Nuclei
We study galaxies that host both nuclear star clusters and active galactic
nuclei (AGN) implying the presence of a massive black hole. We select a sample
of 176 galaxies with previously detected nuclear star clusters that range from
ellipticals to late-type spirals. We search for AGN in this sample using
optical spectroscopy and archival radio and X-ray data. We find galaxies of all
Hubble types and with a wide range of masses (10^9-11 solar masses) hosting
both AGN and nuclear star clusters. From the optical spectra, we classify 10%
of the galaxies as AGN and an additional 15% as composite, indicating a mix of
AGN and star-formation spectra. The fraction of nucleated galaxies with AGN
increases strongly as a function of galaxy and nuclear star cluster mass. For
galaxies with both a NC and a black hole, we find that the masses of these two
objects are quite similar. However, non-detections of black holes in Local
Group nuclear star clusters show that not all clusters host black holes of
similar masses. We discuss the implications of our results for the formation of
nuclear star clusters and massive black holes.Comment: Accepted for publication in ApJ. Version with high resolution figures
available at http://www.cfa.harvard.edu/~aseth/nsc_agn_paper.pd
Elevated hot gas and high-mass X-ray binary emission in low-metallicity galaxies: implications for nebular ionization and intergalactic medium heating in the early universe
High-energy emission associated with star formation has been proposed as a significant source of interstellar
medium (ISM) ionization in low-metallicity starbursts and an important contributor to the heating of the
intergalactic medium (IGM) in the high-redshift (z 8) universe. Using Chandra observations of a sample of 30
galaxies at D â 200â450 Mpc that have high specific star formation rates of 3â9 Gyrâ1 and metallicities near
Z â 0.3Ze, we provide new measurements of the average 0.5â8 keV spectral shape and normalization per unit star
formation rate (SFR). We model the sample-combined X-ray spectrum as a combination of hot gas and high-mass
X-ray binary (HMXB) populations and constrain their relative contributions. We derive scaling relations of
Llog 0.5 8 keV
HMXB
â /SFR = 40.19 ± 0.06 and Llog 0.5 2 keV
gas
â /SFR 39.58 ;0.28
0.17
= -
+ significantly elevated compared to local
relations. The HMXB scaling is also somewhat higher than L0.5 8 keV
HMXB
â âSFR-Z relations presented in the literature,
potentially due to our galaxies having relatively low HMXB obscuration and young and X-ray luminous stellar
populations. The elevation of the hot gas scaling relation is at the level expected for diminished attenuation due to a
reduction of metals; however, we cannot conclude that an L0.5 2 keV
gas
â âSFR-Z relation is driven solely by changes in
ISM metal content. Finally, we present SFR-scaled spectral models (both emergent and intrinsic) that span the
X-ray-to-IR band, providing new benchmarks for studies of the impact of ISM ionization and IGM heating in the
early universe
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