243 research outputs found
Reconciling the observed star-forming sequence with the observed stellar mass function
We examine the connection between the observed star-forming sequence (SFR
) and the observed evolution of the stellar mass function
between . We find the star-forming sequence cannot have a slope
0.9 at all masses and redshifts, as this would result in a
much higher number density at by
than is observed. We show that a transition in the slope of the star-forming
sequence, such that at and
({Whitaker} {et~al.} 2012) at
, greatly improves agreement with the
evolution of the stellar mass function. We then derive a star-forming sequence
which reproduces the evolution of the mass function by design. This
star-forming sequence is also well-described by a broken-power law, with a
shallow slope at high masses and a steep slope at low masses. At , it is
offset by 0.3 dex from the observed star-forming sequence, consistent
with the mild disagreement between the cosmic SFR and recent observations of
the growth of the stellar mass density. It is unclear whether this problem
stems from errors in stellar mass estimates, errors in SFRs, or other effects.
We show that a mass-dependent slope is also seen in other self-consistent
models of galaxy evolution, including semi-analytical, hydrodynamical, and
abundance-matching models. As part of the analysis, we demonstrate that neither
mergers nor hidden low-mass quiescent galaxies are likely to reconcile the
evolution of the mass function and the star-forming sequence. These results are
supported by observations from {Whitaker} {et~al.} (2014).Comment: 17 pages, 13 figures, accepted to ApJ Oct 31st 201
An XMM-Newton and Chandra Study of the Starburst Galaxy IC 10
We present an X-ray study of our nearest starburst galaxy IC 10, based on
XMM-Newton and Chandra observations. A list of 73 XMM-Newton and 28 Chandra
detections of point-like X-ray sources in the field is provided; a substantial
fraction of them are likely stellar objects in the Milky Way due to the low
Galactic latitude location of IC 10. The brightest source in the IC 10 field,
X-1, shows a large variation by a factor of up to on time scales
during the XMM-Newton observation. The X-ray spectra of the source indicate the
presence of a multi-color blackbody accretion disk with an inner disk
temperature T_{in} \approx 1.1 keV. These results are consistent with the
interpretation of the source as a stellar mass black hole, probably accreting
from a Wolf-Rayet star companion. We infer the mass of this black hole to be
about 4 Msun if it is not spinning, or a factor of up to about 6 higher if
there is significant spinning. We also detect an apparent diffuse soft X-ray
emission component of IC 10. An effective method is devised to remove the X-ray
CCD-readout streaks of X-1 that strongly affect the study of the diffuse
component in the XMM-Newton and Chandra observations. We find that the diffuse
X-ray morphology is oriented along the optical body of the galaxy and is
chiefly associated with starburst regions. The diffuse component can be
characterized by an optically thin thermal plasma with a mean temperature of
K and a 0.5-2 keV luminosity of , representing only a small fraction of the expected
mechanical energy inputs from massive stars in the galaxy. There is evidence
that the hot gas is driving outflows from the starburst regions; therefore, the
bulk of the energy inputs may be released in a galactic wind.Comment: 30 pages, accepted for publication in MNRA
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UNCOVER: The Growth of the First Massive Black Holes from JWST/NIRSpec—Spectroscopic Redshift Confirmation of an X-Ray Luminous AGN at z = 10.1
The James Webb Space Telescope is now detecting early black holes (BHs) as they transition from seeds to supermassive BHs. Recently, Bogdan et al. reported the detection of an X-ray luminous supermassive BH, UHZ-1, with a photometric redshift at z \u3e 10. Such an extreme source at this very high redshift provides new insights on seeding and growth models for BHs given the short time available for formation and growth. Harnessing the exquisite sensitivity of JWST/NIRSpec, here we report the spectroscopic confirmation of UHZ-1 at z = 10.073 ± 0.002. We find that the NIRSpec/Prism spectrum is typical of recently discovered z ≈ 10 galaxies, characterized primarily by star formation features. We see no clear evidence of the powerful X-ray source in the rest-frame UV/optical spectrum, which may suggest heavy obscuration of the central BH, in line with the Compton-thick column density measured in the X-rays. We perform a stellar population fit simultaneously to the new NIRSpec spectroscopy and previously available photometry. The fit yields a stellar-mass estimate for the host galaxy that is significantly better constrained than prior photometric estimates (�⋆∼1.4−0.4+0.3×108M⊙). Given the predicted BH mass (MBH ∼ 107–108M⊙), the resulting ratio of MBH/M⋆ remains 2 to 3 orders of magnitude higher than local values, thus lending support to the heavy seeding channel for the formation of supermassive BHs within the first billion years of cosmic evolution
Star Formation at z=2.481 in the Lensed Galaxy SDSS J1110+6459, I: Lens Modeling and Source Reconstruction
Using the combined resolving power of the Hubble Space Telescope and
gravitational lensing, we resolve star-forming structures in a z~2.5 galaxy on
scales much smaller than the usual kiloparsec diffraction limit of HST. SGAS
J111020.0+645950.8 is a clumpy, star forming galaxy lensed by the galaxy
cluster SDSS J1110+6459 at z = 0.659, with a total magnification ~30x across
the entire arc. We use a hybrid parametric/non-parametric strong lensing mass
model to compute the deflection and magnification of this giant arc,
reconstruct the light distribution of the lensed galaxy in the source plane,
and resolve the star formation into two dozen clumps. We develop a
forward-modeling technique to model each clump in the source plane. We ray
trace the model to the image plane, convolve with the instrumental point spread
function (PSF), and compare with the GALFIT model of the clumps in the image
plane, which decomposes clump structure from more extended emission. This
technique has the advantage, over ray tracing, by accounting for the asymmetric
lensing shear of the galaxy in the image plane and the instrument PSF. At this
resolution, we can begin to study star formation on a clump-by-clump basis,
toward the goal of understanding feedback mechanisms and the buildup of
exponential disks at high redshift.Comment: 19 pages, 12 figures, accepted to Ap
X-ray properties of K-selected galaxies at 0.5<z<2.0: Investigating trends with stellar mass, redshift and spectral type
We examine how the total X-ray luminosity correlates with stellar mass,
stellar population, and redshift for a K-band limited sample of ~3500 galaxies
at 0.5<z<2.0 from the NEWFIRM Medium Band Survey in the COSMOS field. The
galaxy sample is divided into 32 different galaxy types, based on similarities
between the spectral energy distributions. For each galaxy type, we further
divide the sample into bins of redshift and stellar mass, and perform an X-ray
stacking analysis using the Chandra COSMOS (C-COSMOS) data. We find that full
band X-ray luminosity is primarily increasing with stellar mass, and at similar
mass and spectral type is higher at larger redshifts. When comparing at the
same stellar mass, we find that the X-ray luminosity is slightly higher for
younger galaxies (i.e., weaker 4000\AA breaks), but the scatter in this
relation is large. We compare the observed X-ray luminosities to those expected
from low and high mass X-ray binaries (XRBs). For blue galaxies, XRBs can
almost fully account for the observed emission, while for older galaxies with
larger 4000\AA breaks, active galactic nuclei (AGN) or hot gas dominate the
measured X-ray flux. After correcting for XRBs, the X-ray luminosity is still
slightly higher in younger galaxies, although this correlation is not
significant. AGN appear to be a larger component of galaxy X-ray luminosity at
earlier times, as the hardness ratio increases with redshift. Together with the
slight increase in X-ray luminosity this may indicate more obscured AGNs or
higher accretion rates at earlier times.Comment: 9 pages, 9 figures, ApJ accepte
High Redshift Massive Quiescent Galaxies are as Flat as Star Forming Galaxies: The Flattening of Galaxies and the Correlation with Structural Properties in CANDELS/3D-HST
We investigate the median flattening of galaxies at in all five
CANDELS/3D-HST fields via the apparent axis ratio . We separate the sample
into bins of redshift, stellar-mass, s\'ersic index, size, and UVJ determined
star-forming state to discover the most important drivers of the median
(). Quiescent galaxies at are
rounder than those at lower masses, consistent with the hypothesis that they
have grown significantly through dry merging. The massive quiescent galaxies at
higher redshift become flatter, and are as flat as star forming massive
galaxies at , consistent with formation through direct
transformations or wet mergers. We find that in quiescent galaxies,
correlations with and , and are driven by the
evolution in the s\'ersic index (), consistent with the growing accumulation
of minor mergers at lower redshift. Interestingly, does not drive these
trends fully in star-forming galaxies. Instead, the strongest predictor of
in star-forming galaxies is the effective radius, where larger galaxies are
flatter. Our findings suggest that is tracing bulge-to-total ()
galaxy ratio which would explain why smaller/more massive star-forming galaxies
are rounder than their extended/less massive analogues, although it is unclear
why s\'ersic index correlates more weakly with flattening for star forming
galaxies than for quiescent galaxies.Comment: 13 pages, 11 figures, accepted to Ap
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