230 research outputs found
Molecular gas at intermediate redshifts
We present Giant Metrewave Radio Telescope (GMRT) observations of OH
absorption in B3~1504+377 () and PKS 1413+135 ().
OH has now been detected in absorption towards four intermediate redshift
systems, viz. the lensing galaxies towards B~0218+357 (; Kanekar
et al. 2001) and 1830-211 (; Chengalur et al. 1999), in addition
to the two systems listed above. All four systems also give rise to well
studied millimetre wavelength molecular line absorption from a host of
molecules, including HCO. Comparing our OH data with these millimetre line
transitions, we find that the linear correlation between and
found in molecular clouds in the Milky Way (Liszt & Lucas 1996)
persists out to . It has been suggested (Liszt & Lucas 1999) that OH
is a good tracer of , with
under a variety of physical conditions. We use this relationship to estimate
in these absorbers. The estimated is \ga 10^{22}
in all four cases and substantially different from estimates based on CO
observations.Comment: 4 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysics Letter
A Laboratory for Constraining Cosmic Evolution of the Fine Structure Constant: Conjugate 18 cm OH Lines Toward PKS 1413+135 at z=0.2467
We report the detection of the satellite 18 cm OH lines at 1612 and 1720 MHz
in the z=0.2467 molecular absorption system toward the radio source PKS
1413+135. The two OH lines are conjugate; the 1612 MHz line is seen in
absorption while the 1720 MHz line is seen in weak maser emission of equal, but
negative, optical depth. We do not detect the main 18 cm OH lines at 1667 and
1665 MHz down to 1.1 mJy rms in 4.0 km/s channels. The detected and undetected
18 cm OH lines support a scenario of radiatively pumped stimulated absorption
and emission with pumping dominated by the intraladder 119 micron line of OH,
suggesting a column density N(OH) ~= 10^15 to 10^16 cm^-2. Combined with
simultaneous HI 21 cm observations and published CO data, we apply the OH
redshifts to measurements of cosmic evolution of the fine structure constant
alpha = e^2/(hbar c). We obtain highly significant (~25 sigma) velocity offsets
between the OH and HI lines and the OH and CO lines, but measurements of
alpha-independent systematics demonstrate that the observed velocity
differences are entirely attributable to physical velocity offsets between
species rather than a change in alpha. The OH alone, in which conjugate line
profiles guarantee that both lines originate in the same molecular gas,
provides a weak constraint of Delta alpha/alpha_o = (+0.51 +/- 1.26) x 10^-5 at
z=0.2467. Higher frequency OH line detections can provide a larger lever arm on
Delta alpha and can increase precision by an order of magnitude. The OH
molecule can thus provide precise measurements of the cosmic evolution of alpha
that include quantitative constraints on systematic errors. Application of this
technique is limited only by the detectability of |tau|~0.01 OH lines toward
radio continuum sources and may be possible to z~5.Comment: AASTeX, 11 pages, 2 figures, accepted by Astrophysical Journa
Constraining the variation of fundamental constants using 18cm OH lines
We describe a new technique to estimate variations in the fundamental
constants using 18cm OH absorption lines. This has the advantage that all lines
arise in the same species, allowing a clean comparison between the measured
redshifts. In conjunction with one additional transition (for example, an
HCO line), it is possible to simultaneously measure changes in ,
and . At present, only the 1665 MHz and 1667 MHz lines
have been detected at cosmological distances; we use these line redshifts in
conjunction with those of HI 21cm and mm-wave molecular absorption in a
gravitational lens at to constrain changes in the above three
parameters over the redshift range . While the constraints
are relatively weak ( 1 part in ), this is the first
simultaneous constraint on the variation of all three parameters. We also
demonstrate that either one (or more) of , and must vary with
cosmological time or there must be systematic velocity offsets between the OH,
HCO and HI absorbing clouds.Comment: 5 pages, no figures. Accepted for publication in Phys. Rev. Let
The temperature of the WNM in the Milky Way
We report high spectral resolution Australia Telescope Compact Array HI 21 cm
observations resulting in the detection of the warm neutral medium (WNM) of the
Galaxy in absorption against two extragalactic radio sources, PKS 1814-637 and
PKS 0407-658. The two lines of sight were selected on the basis of the
simplicity of their absorption profiles and the strength of the background
sources; the high velocity resolution of the spectra then enabled us to
estimate the kinetic temperatures of the absorbing gas by fitting multiple
Gaussians to the absorption profiles. Four separate WNM components were
detected toward the two sources, with peak optical depths , , and and kinetic temperatures
K, K, K and K respectively. All four components were thus found to have temperatures
in the thermally unstable range K; this suggests that
thermal equilibrium has not been reached throughout the WNM.Comment: 5 pages, 6 figures. Accepted for publication in MNRAS (Letters).
Minor typos removed to match version in pres
Constraints on changes in fundamental constants from a cosmologically distant OH absorber/emitter
We have detected the four 18cm OH lines from the gravitational
lens toward PMN J0134-0931. The 1612 and 1720 MHz lines are in conjugate
absorption and emission, providing a laboratory to test the evolution of
fundamental constants over a large lookback time. We compare the HI and OH main
line absorption redshifts of the different components in the
absorber and the lens toward B0218+357 to place stringent
constraints on changes in . We obtain
,
consistent with no evolution over the redshift range . The
measurements have a sensitivity of or to fractional
changes in and over a period of Gyr, half the age of
the Universe. These are among the most sensitive current constraints on changes
in .Comment: 4 pages, 3 figures. Final version, with minor changes to match the
version in print in Phys. Rev. Let
Dense Ionized and Neutral Gas Surrounding Sgr A*
We present high resolution H41a hydrogen recombination line observations of
the 1.2' (3 pc) region surrounding Sgr A* at 92 GHz using the OVRO Millimeter
Array with an angular resolution of 7" x 3" and velocity resolution of 13 km/s.
New observations of H31a, H35a, H41a, and H44a lines were obtained using the
NRAO 12-m telescope, and their relative line strengths are interpreted in terms
of various emission mechanisms. These are the most extensive and most sensitive
observations of recombination line to date. Observations of HCO+ (1 - 0)
transition at 89 GHz are also obtained simultaneously with a 40% improved
angular resolution and 4-15 times improved sensitivity over previous
observations, and the distribution and kinematics of the dense molecular gas in
the circumnuclear disk (CND) are mapped and compared with those of the ionized
gas. The line brightness ratios of the hydrogen recombination lines are
consistent with purely spontaneous emission from 7000 K gas with n_e = 20,000
cm near LTE condition. A virial analysis suggests that the most
prominent molecular gas clumps in the CND have mean densities of 10^7 cm^{-3},
sufficient to withstand the tidal shear in the Galactic Center region.
Therefore, these clumps may survive over several dynamical times, and the CND
may be a dynamically stable structure. We estimate a total gas mass of 3 x 10^5
solar mass for the CND. \Comment: 34 pages including 11 figures (4 jpgs), Latex, uses aastex. The full
pdf format file including high resolution figures is available at
http://www.astro.umass.edu/~myun/papers/SgrA.pdf . To appear in the 20
November 2004 (V616) issue of the Astrophysical Journa
First Detection of HCO Absorption in the Magellanic System
We present the first detection of HCO absorption in the Magellanic
System. Using the Australia Telescope Compact Array (ATCA), we observed 9
extragalactic radio continuum sources behind the Magellanic System and detected
HCO absorption towards one source located behind the leading edge of the
Magellanic Bridge. The detection is located at LSR velocity of , with a full width at half maximum of and optical depth of .
Although there is abundant neutral hydrogen (HI) surrounding the sightline in
position-velocity space, at the exact location of the absorber the HI column
density is low, , and there is little evidence for dust
or CO emission from Planck observations. While the origin and survival of
molecules in such a diffuse environment remains unclear, dynamical events such
as HI flows and cloud collisions in this interacting system likely play an
important role.Comment: Accepted for publication in ApJ. 6 pages, 2 figures, 2 table
The use of OH "main" lines to constrain the variation of fundamental constants
We describe a new technique to measure variations in the fundamental
parameters and , using the sum of the frequencies of
cm-wave OH ``main'' lines. The technique is three orders of magnitude
more sensitive than that of Chengalur & Kanekar (2003), which utilised only the
four 18cm OH lines. The increase in sensitivity stems from the use of OH
``main'' lines arising from different rotational states, instead of the
frequency difference between lines from the same state. We also show that
redshifts of the main OH 18cm and 6cm lines can be combined with the redshift
of an HCO transition to measure any evolution in and . Both
18cm main lines and a number of HCO lines have already been detected in
absorption in four cosmologically distant systems; the detection of the main
6cm OH line in any of these systems would thus be sufficient to simultaneously
constrain changes in and between the absorption redshift and
today.Comment: 5 pages, no figures. MNRAS (Letters), in pres
Obscuration of the Parsec Scale Jets in the Compact Symmetric Object 1946+708
We present results of VLA and VLBA observations of the 1.420 GHz neutral
hydrogen absorption associated with the Compact Symmetric Object 1946+708
(z=0.101). We find significant structure in the gas on parsec scales. The peak
column density in the HI (N_HI~2.2x10^23 cm^-2 (T_s/8000K)) occurs toward the
center of activity of the source, as does the highest velocity dispersion
(FWHM~350 \kms). In addition, we find that the continuum spectra of the various
radio components associated with these jets strongly indicate free-free
absorption. This effect is particularly pronounced toward the core and inner
components of the receding jet, suggesting the presence of a screen local to
the source, perhaps part of an obscuring torus.Comment: revised version, some text added, 1 figure changed; accepted to
Astrophysical Journal, 22 page LaTeX document includes 8 postscript figure
Modelling the spinning dust emission from dense interstellar clouds
Electric dipole emission arising from PAHs is often invoked to explain the
anomalous microwave emission (AME). This assignation is based on an observed
tight correlation between the mid-IR emission of PAHs and the AME; and a good
agreement between models of spinning dust and the broadband AME spectrum. So
far often detected at large scale in the diffuse interstellar medium, the AME
has recently been studied in detail in well-known dense molecular clouds with
the help of Planck data. While much attention has been given to the physics of
spinning dust emission, the impact of varying local physical conditions has not
yet been considered in detail. Our aim is to study the emerging spinning dust
emission from interstellar clouds with realistic physical conditions and
radiative transfer. We use the DustEM code from Compiegne et al. to describe
the extinction and IR emission of all dust populations. The spinning dust
emission is obtained with SpDust, as described by Silsbee et al., that we have
coupled to DustEM. We carry out full radiative transfer simulations and
carefully estimate the local gas state as a function of position within
interstellar clouds. We show that the spinning dust emission is sensitive to
the abundances of the major ions and we propose a simple scheme to estimate
these abundances. We also investigate the effect of changing the cosmic-ray
rate. In dense media, where radiative transfer is mandatory, we show that the
relationship between the spinning and mid-IR emissivities of PAHs is no longer
linear and that the spinning dust emission may actually be strong at the centre
of clouds where the mid-IR PAH emission is weak. These results provide new ways
to trace grain growth from diffuse to dense medium and will be useful for the
analysis of AME at the scale of interstellar clouds.Comment: 7 pages, 10 figures, accepted by A&
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