697 research outputs found
Galaxies into the Dark Ages
We consider the capabilities of current and future large facilities operating
at 2\,mm to 3\,mm wavelength to detect and image the [CII] 158\,m line
from galaxies into the cosmic "dark ages" ( to 20). The [CII] line
may prove to be a powerful tool in determining spectroscopic redshifts, and
galaxy dynamics, for the first galaxies. We emphasize that the nature, and even
existence, of such extreme redshift galaxies, remains at the frontier of open
questions in galaxy formation. In 40\,hr, ALMA has the sensitivity to detect
the integrated [CII] line emission from a moderate metallicity, active
star-forming galaxy [; star formation rate (SFR) =
5\,\,yr], at at a significance of 6. The
next-generation Very Large Array (ngVLA) will detect the integrated [CII] line
emission from a Milky-Way like star formation rate galaxy (, SFR = 1\,\,yr), at at a significance
of 6. Imaging simulations show that the ngVLA can determine rotation
dynamics for active star-forming galaxies at , if they exist. Based
on our very limited knowledge of the extreme redshift Universe, we calculate
the count rate in blind, volumetric surveys for [CII] emission at
to 20. The detection rates in blind surveys will be slow (of order unity per
40\,hr pointing). However, the observations are well suited to commensal
searches. We compare [CII] with the [OIII] 88m line, and other ancillary
information in high galaxies that would aid these studies.Comment: 11pages, 8 figures, Accepted for the Astrophysical Journa
Expanded Very Large Arrays Observations of a Proto-Cluster of Molecular Gas-Rich Galaxies at z = 4.05
We present observations of the molecular gas in the GN20 proto-cluster of galaxies at z = 4.05 using the Expanded Very Large Array (EVLA). This group of galaxies is the ideal laboratory for studying the formation of massive galaxies via luminous, gas-rich starbursts within 1.6 Gyr of the big bang. We detect three galaxies in the proto-cluster in CO 2-1 emission, with gas masses (H_2) between 10^(10) and 10^(11) × (α/0.8) M_⊙. The emission from the brightest source, GN20, is resolved with a size ~2'' and has a clear north-south velocity gradient, possibly indicating ordered rotation. The gas mass in GN20 is comparable to the stellar mass (1.3 × 10^(11) × (α/0.8) M_⊙ and 2.3 × 10^(11) M_⊙, respectively), and the sum of gas plus stellar mass is comparable to the dynamical mass of the system (~3.4 × 10^(11)[sin (i)/sin (45°)]^(–2) M_⊙), within a 5 kpc radius. There is also evidence for a tidal tail extending another 2'' north of the galaxy with a narrow velocity dispersion. GN20 may be a massive, gas-rich disk that is gravitationally disturbed, but not completely disrupted. There is one Lyman-break galaxy (BD29079) in the GN20 proto-cluster with an optical spectroscopic redshift within our search volume, and we set a 3σ limit to the molecular gas mass of this galaxy of 1.1 × 10^(10) × (α/0.8) M_⊙
Radio line and continuum observations of quasar-galaxy pairs and the origin of low reshift quasar absorption line systems
There are a number of known quasars for which our line of sight to the high redshift quasar passes within a few Holmberg radii of a low redshift galaxy. In a few of these cases, spectra of the quasar reveal absorption by gas associated with the low redshift galaxy. A number of these pairs imply absorption by gas which lies well outside the optical disk of the associated galaxy, leading to models of galaxies with 'halos' or 'disks' of gas extending to large radii. The authors present observations of 4 such pairs. In three of the four cases, they find that the associated galaxy is highly disturbed, typically due to a gravitational interaction with a companion galaxy, while in the fourth case the absorption can be explained by clouds in the optical disk of the associated galaxy. They are led to an alternative hypothesis concerning the origin of the low redshift absorption line systems: the absorption is by gas clouds which have been gravitationally stripped from the associated galaxy. These galaxies are rapidly evolving, and should not be used as examples of absorption by clouds in halos of field spirals. The authors conclude by considering the role extended gas in interacting systems plays in the origin of higher redshift quasar absorption line systems
A Chandra study of X-ray sources in the field of the z=2.16 radio galaxy MRC 1138-262
We present results from a Chandra X-ray Observatory study of the field X-ray
source population in the vicinity of the radio galaxy MRC 1138-262. Many
serendipitous X-ray sources are detected in an area of 8'x8' around the radio
source and 90% are identified in our deep VLT images. The space density of such
sources is higher than expected on the basis of the statistics of ROSAT and
Chandra deep surveys. The most likely explanation is in terms of a
concentration of AGN associated with the protocluster at z=2.16 which was found
around the radio galaxy in previous studies. Two sources have a confirmed
spectroscopic redshift close to that of the radio galaxy, and for three more
sources other observations suggest that they are associated with the
protocluster. Four of these five X-ray sources form, together with the radio
galaxy, a filament in the plane of the sky. The direction of the filament is
similar to that of the radio source axis, the large scale distribution of the
other protocluster members, the 150 kpc-sized emission-line halo and the
extended X-ray emission associated with the radio galaxy.
The majority of optically identified X-ray sources in this field have
properties consistent with type I AGN, a few could be soft, low luminosity
galaxies, one is probably an obscured (type II) AGN and one is a star. These
statistics are consistent with the results of deep X-ray surveys.Comment: 8 pages, 2 figures, to appear in Astronomy and Astrophysic
The kiloparsec-scale star formation law at redshift 4: wide-spread, highly efficient star formation in the dust-obscured starburst galaxy GN20
We present high-resolution observations of the 880 m (rest-frame FIR)
continuum emission in the z4.05 submillimeter galaxy GN20 from the IRAM
Plateau de Bure Interferometer (PdBI). These data resolve the obscured star
formation in this unlensed galaxy on scales of
0.30.2 (2.11.3 kpc).
The observations reveal a bright (161 mJy) dusty starburst centered on the
cold molecular gas reservoir and showing a bar-like extension along the major
axis. The striking anti-correlation with the HST/WFC3 imaging suggests that the
copious dust surrounding the starburst heavily obscures the rest-frame
UV/optical emission. A comparison with 1.2 mm PdBI continuum data reveals no
evidence for variations in the dust properties across the source within the
uncertainties, consistent with extended star formation, and the peak star
formation rate surface density (1198 M yr kpc)
implies that the star formation in GN20 remains sub-Eddington on scales down to
3 kpc. We find that the star formation efficiency is highest in the central
regions of GN20, leading to a resolved star formation law with a power law
slope of , and that
GN20 lies above the sequence of normal star-forming disks, implying that the
dispersion in the star formation law is not due solely to morphology or choice
of conversion factor. These data extend previous evidence for a fixed star
formation efficiency per free-fall time to include the star-forming medium on
kpc-scales in a galaxy 12 Gyr ago.Comment: 6 pages, 5 figures, accepted to ApJ
Jet Interactions with the Hot Halos of Clusters and Galaxies
X-ray observations of cavities and shock fronts produced by jets streaming
through hot halos have significantly advanced our understanding of the
energetics and dynamics of extragalactic radio sources. Radio sources at the
centers of clusters have dynamical ages between ten and several hundred million
years. They liberate between 1E58-1E62 erg per outburst, which is enough energy
to regulate cooling of hot halos from galaxies to the richest clusters. Jet
power scales approximately with the radio synchrotron luminosity to the one
half power. However, the synchrotron efficiency varies widely from nearly unity
to one part in 10,000, such that relatively feeble radio source can have
quasar-like mechanical power. The synchrotron ages of cluster radio sources are
decoupled from their dynamical ages, which tend to be factors of several to
orders of magnitude older. Magnetic fields and particles in the lobes tend to
be out of equipartition. The lobes may be maintained by heavy particles (e.g.,
protons), low energy electrons, a hot, diffuse thermal gas, or possibly
magnetic (Poynting) stresses. Sensitive X-ray images of shock fronts and
cavities can be used to study the dynamics of extragalactic radio sources.Comment: 10 pages, 3 figures, invited review, "Extragalactic Jets: Theory and
Observation from Radio to Gamma Ray, held in Girdwood, Alaska, U.S.A. 21-24
May, 2007, minor text changes; one added referenc
Cicrumnuclear Supernova Remnants and HII Regions in NGC 253
Archival VLA data has been used to produce arcsecond-resolution 6- and 20-cm
images of the region surrounding the nuclear 200-pc (~15") starburst in NGC
253. Twenty-two discrete sources stronger than 0.4 mJy have been detected
within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be
associated with the galaxy. None of the radio sources coincides with a detected
X-ray binary, so they appear to be due to supernova remnants and H II regions.
The region outside the central starburst has a derived radio supernova rate of
<~0.1/yr, and may account for at least 20% of the recent star formation in NGC
253. Most of the newly identified sources have steep, nonthermal radio spectra,
but several relatively strong thermal sources also exist, containing the
equivalent of tens of O5 stars. These stars are spread over tens of parsecs,
and are embedded in regions having average ionized gas densities of
20-200/cm^3, much lower than in the most active nuclear star-forming regions in
NGC 253 or in the super star clusters seen in other galaxies. The strongest
region of thermal emission coincides with a highly reddened area seen at
near-infrared wavelengths, possibly containing optically obscured H II regions.Comment: 17 pages, 3 postscript figures, AASTeX format, in press for
Astronomical Journal, July 200
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