329 research outputs found
Structure of Stationary Photodissociation Fronts
The structure of stationary photodissociation fronts is revisited. H_2 self-
shielding is discussed, including the effects of line overlap. We find that
line overlap is important for N(H_2) > 10^{20} cm^{-2}. We compute multiline UV
pumping models, and compare these with simple analytic approximations for the
effects of self-shielding.
The overall fluorescent efficiency of the photodissociation front is obtained
for different ratios of chi/n_H (where chi characterizes the intensity of the
incident UV) and different dust extinction laws. The dust optical depth
tau_{pdr} to the point where 50% of the H is molecular is found to be a simple
function of a dimensionless quantity phi_0 depending on chi/n_H, the rate
coefficient for H_2 formation on grains, and the UV dust opacity. The
fluorescent efficiency of the PDR also depends primarily on phi_0 for chi<3000
and n_H<10^4 cm^{-3}; for stronger radiation fields and higher densities
radiative and collisional depopulation of vibrationally-excited levels
interferes with the radiative cascade. The emission spectrum from the PDR is
essentially independent of the color temperature of the incident UV
for T_{color}>10^4K, but shows some sensitivity to the v-J distribution of
newly-formed H_2. The 1-0S(1)/2-1S(1) and 2-1S(1)/6-4Q(1) intensity ratios, the
ortho/para ratio, and the rotational temperature in the =1 and =2 levels
are computed as functions of the temperature and density, for different values
of chi and n_H.
We apply our models to the reflection nebula NGC 2023. We are best able to
reproduce the observations with models having chi=5000, n_H=10^5 cm^{-3}.Comment: 50 pages, 24 eps figures, uses aaspp4.sty . To appear in Ap.
Millimeter observations of radio-loud active galaxies
In order to study the nature of the far-infrared emission observed in
radio-loud active galaxies, we have obtained 1.2 mm observations with the IRAM
30 m telescope for a sample of eight radio-loud active galaxies. In all objects
we find that the 1.2 mm emission is dominated by non-thermal emission. An
extrapolation of the non-thermal radio spectrum indicates that the contribution
of synchrotron emission to the far-infrared is less than 10% in quasars, and
negligible in the radio galaxies. The quasars in the sample show signs of
relativistic beaming at millimeter wavelengths, and the quasar 3C334 shows
evidence for strong variability.Comment: 6 pages, 1 figure, accepted by A&
Discovery of a Proto-cluster Associated with a Ly- Blob Pair at z=2.3
Bright Ly- blobs (LABs) --- extended nebulae with sizes of
100kpc and Ly- luminosities of 10erg s ---
often reside in overdensities of compact Ly- emitters (LAEs) that may
be galaxy protoclusters. The number density, variance, and internal kinematics
of LABs suggest that they themselves trace group-like halos. Here we test this
hierarchical picture, presenting deep, wide-field Ly- narrowband
imaging of a 1 0.5 region around a LAB pair at =
2.3 discovered previously by a blind survey. We find 183 Ly- emitters,
including the original LAB pair and three new LABs with Ly-
luminosities of (0.9--1.3)10erg s and isophotal areas of
16--24 arcsec. Using the LAEs as tracers and a new kernel density
estimation method, we discover a large-scale overdensity (Bo{\"o}tes
J1430+3522) with a surface density contrast of = 2.7, a
volume density contrast of 10.4, and a projected diameter of
20 comoving Mpc. Comparing with cosmological simulations, we conclude
that this LAE overdensity will evolve into a present-day Coma-like cluster with
. In this and three other wide-field LAE
surveys re-analyzed here, the extents and peak amplitudes of the largest LAE
overdensities are similar, not increasing with survey size, implying that they
were indeed the largest structures then and do evolve into rich clusters today.
Intriguingly, LABs favor the outskirts of the densest LAE concentrations, i.e.,
intermediate LAE overdensities of . We speculate that
these LABs mark infalling proto-groups being accreted by the more massive
protocluster
Extended Cold Molecular Gas Reservoirs in z~3.4 Submillimeter Galaxies
We report the detection of spatially resolved CO(1-0) emission in the z~3.4
submillimeter galaxies (SMGs) SMM J09431+4700 and SMM J13120+4242, using the
Expanded Very Large Array (EVLA). SMM J09431+4700 is resolved into the two
previously reported millimeter sources H6 and H7, separated by ~30kpc in
projection. We derive CO(1-0) line luminosities of L'(CO 1-0) = (2.49+/-0.86)
and (5.82+/-1.22) x 10^10 K km/s pc^2 for H6 and H7, and L'(CO 1-0) =
(23.4+/-4.1) x 10^10 K km/s pc^2 for SMM J13120+4242. These are ~1.5-4.5x
higher than what is expected from simple excitation modeling of higher-J CO
lines, suggesting the presence of copious amounts of low-excitation gas. This
is supported by the finding that the CO(1-0) line in SMM J13120+4242, the
system with lowest CO excitation, appears to have a broader profile and more
extended spatial structure than seen in higher-J CO lines (which is less
prominently seen in SMM J09431+4700). Based on L'(CO 1-0) and excitation
modeling, we find M_gas = 2.0-4.3 and 4.7-12.7 x 10^10 Msun for H6 and H7, and
M_gas = 18.7-69.4 x 10^10 Msun for SMM J13120+4242. The observed CO(1-0)
properties are consistent with the picture that SMM J09431+4700 represents an
early-stage, gas-rich major merger, and that SMM J13120+4242 represents such a
system in an advanced stage. This study thus highlights the importance of
spatially and dynamically resolved CO(1-0) observations of SMGs to further
understand the gas physics that drive star formation in these distant galaxies,
which becomes possible only now that the EVLA rises to its full capabilities.Comment: 6 pages, 4 figures, to appear in ApJL (EVLA Special Issue; accepted
May 19, 2011
Observations of Dense Molecular Gas in a Quasar Host Galaxy at z=6.42: Further Evidence for a Non-Linear Dense Gas - Star Formation Relation at Early Cosmic Times
We report a sensitive search for the HCN(J=2-1) emission line towards SDSS
J1148+5251 at z=6.42 with the VLA. HCN emission is a star formation indicator,
tracing dense molecular hydrogen gas (n(H2) >= 10^4 cm^-3) within star-forming
molecular clouds. No emission was detected in the deep interferometer maps of
J1148+5251. We derive a limit for the HCN line luminosity of L'(HCN) < 3.3 x
10^9 K km/s pc^2, corresponding to a HCN/CO luminosity ratio of L'(HCN)/L'(CO)
< 0.13. This limit is consistent with a fraction of dense molecular gas in
J1148+5251 within the range of nearby ultraluminous infrared galaxies (ULIRGs;
median value: L'(HCN)/L'(CO) = 0.17 {+0.05/-0.08}) and HCN-detected z>2
galaxies (0.17 {+0.09/-0.08}). The relationship between L'(HCN) and L(FIR) is
considered to be a measure for the efficiency at which stars form out of dense
gas. In the nearby universe, these quantities show a linear correlation, and
thus, a practically constant average ratio. In J1148+5251, we find
L(FIR)/L'(HCN) > 6600. This is significantly higher than the average ratios for
normal nearby spiral galaxies (L(FIR)/L'(HCN) = 580 {+510/-270}) and ULIRGs
(740 {+505/-50}), but consistent with a rising trend as indicated by other z>2
galaxies (predominantly quasars; 1525 {+1300/-475}). It is unlikely that this
rising trend can be accounted for by a contribution of AGN heating to L(FIR)
alone, and may hint at a higher median gas density and/or elevated
star-formation efficiency toward the more luminous high-redshift systems. There
is marginal evidence that the L(FIR)/L'(HCN) ratio in J1148+5251 may even
exceed the rising trend set by other z>2 galaxies; however, only future
facilities with very large collecting areas such as the SKA will offer the
sensitivity required to further investigate this question.Comment: 5 pages, 2 figures, 2 tables, to appear in ApJL (accepted October 24,
2007
ALMA-SZ Detection of a Galaxy Cluster Merger Shock at Half the Age of the Universe
We present ALMA measurements of a merger shock using the thermal
Sunyaev-Zel'dovich (SZ) effect signal, at the location of a radio relic in the
famous El Gordo galaxy cluster at . Multi-wavelength analysis in
combination with the archival Chandra data and a high-resolution radio image
provides a consistent picture of the thermal and non-thermal signal variation
across the shock front and helps to put robust constraints on the shock Mach
number as well as the relic magnetic field. We employ a Bayesian analysis
technique for modeling the SZ and X-ray data self-consistently, illustrating
respective parameter degeneracies. Combined results indicate a shock with Mach
number , which in turn suggests a high value of
the magnetic field (of the order of G) to account for the observed
relic width at 2 GHz. At roughly half the current age of the universe, this is
the highest-redshift direct detection of a cluster shock to date, and one of
the first instances of an ALMA-SZ observation in a galaxy cluster. It shows the
tremendous potential for future ALMA-SZ observations to detect merger shocks
and other cluster substructures out to the highest redshifts.Comment: Matched to the ApJL published version (2016 September 22), minor
grammar and typo fixe
A Sensitive Search for [N II]205 μm Emission in a z = 6.4 Quasar Host Galaxy
We present a sensitive search for the 3P1 → 3P0 ground-state fine structure line at 205 μm of ionized nitrogen ([N II]205μm) in one of the highest-redshift quasars (J1148+5251 at z = 6.42) using the IRAM 30 m telescope. The line is not detected at a (3σ) depth of 0.47 Jy km s^−1, corresponding to a [N II]205μm luminosity limit of L[N II] 7) using the Atacama Large Millimeter/submillimeter Array, for which the highly excited rotational transitions of CO will be shifted outside the accessible (sub-)millimeter bands
A Molecular Einstein Ring at z=4.12: Imaging the Dynamics of a Quasar Host Galaxy Through a Cosmic Lens
We present high-resolution (0.3") Very Large Array (VLA) imaging of the
molecular gas in the host galaxy of the high redshift quasar PSS J2322+1944
(z=4.12). These observations confirm that the molecular gas (CO) in the host
galaxy of this quasar is lensed into a full Einstein ring, and reveal the
internal dynamics of the molecular gas in this system. The ring has a diameter
of ~1.5", and thus is sampled over ~20 resolution elements by our observations.
Through a model-based lens inversion, we recover the velocity gradient of the
molecular reservoir in the quasar host galaxy of PSS J2322+1944. The Einstein
ring lens configuration enables us to zoom in on the emission and to resolve
scales down to ~1 kpc. From the model-reconstructed source, we find that the
molecular gas is distributed on a scale of 5 kpc, and has a total mass of
M(H2)=1.7 x 10^10 M_sun. A basic estimate of the dynamical mass gives M_dyn =
4.4 x 10^10 (sin i)^-2 M_sun, that is, only ~2.5 times the molecular gas mass,
and ~30 times the black hole mass (assuming that the dynamical structure is
highly inclined). The lens configuration also allows us to tie the optical
emission to the molecular gas emission, which suggests that the active galactic
nucleus (AGN) does reside within, but not close to the center of the molecular
reservoir. Together with the (at least partially) disturbed structure of the
CO, this suggests that the system is interacting. Such an interaction, possibly
caused by a major `wet' merger, may be responsible for both feeding the quasar
and fueling the massive starburst of 680 M_sun/yr in this system, in agreement
with recently suggested scenarios of quasar activity and galaxy assembly in the
early universe.Comment: 9 pages, 7 figures, to appear in ApJ (accepted June 27, 2008
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