325 research outputs found
The Arecibo HII Region Discovery Survey
We report the detection of radio recombination line emission (RRL) using the
Arecibo Observatory at X-band (9GHz, 3cm) from 37 previously unknown HII
regions in the Galactic zone 66 deg. > l > 31 deg. and |b| < 1 deg. This
Arecibo HII Region Discovery Survey (Arecibo HRDS) is a continuation of the
Green Bank Telescope (GBT) HRDS. The targets for the Arecibo HRDS have
spatially coincident 24 micron and 20 cm emission of a similar angular
morphology and extent. To take advantage of Arecibo's sensitivity and small
beam size, sources in this sample are fainter, smaller in angle, or in more
crowded fields compared to those of the GBT HRDS. These Arecibo nebulae are
some of the faintest HII regions ever detected in RRL emission. Our detection
rate is 58%, which is low compared to the 95% detection rate for GBT HRDS
targets. We derive kinematic distances to 23 of the Arecibo HRDS detections.
Four nebulae have negative LSR velocities and are thus unambiguously in the
outer Galaxy. The remaining sources are at the tangent point distance or
farther. We identify a large, diffuse HII region complex that has an associated
HI and 13CO shell. The ~90 pc diameter of the G52L nebula in this complex may
be the largest Galactic HII region known, and yet it has escaped previous
detection.Comment: Accepted to ApJ Data can be found here: http://go.nrao.edu/hrd
The Green Bank Telescope H II Region Discovery Survey: IV. Helium and Carbon Recombination Lines
The Green Bank Telescope H II Region Discovery Survey (GBT HRDS) found
hundreds of previously unknown Galactic regions of massive star formation by
detecting hydrogen radio recombination line (RRL) emission from candidate H II
region targets. Since the HRDS nebulae lie at large distances from the Sun,
they are located in previously unprobed zones of the Galactic disk. Here we
derive the properties of helium and carbon RRL emission from HRDS nebulae. Our
target sample is the subset of the HRDS that has visible helium or carbon RRLs.
This criterion gives a total of 84 velocity components (14% of the HRDS) with
helium emission and 52 (9%) with carbon emission. For our highest quality
sources, the average ionic He-4+/H+ abundance ratio by number, , is 0.068
+/- 0.023 (1-sigma). This is the same ratio as that measured for the sample of
previously known Galactic H II regions. Nebulae without detected helium
emission give robust y+ upper limits. There are 5 RRL emission components with
y+ less than 0.04 and another 12 with upper limits below this value. These H II
regions must have either a very low He-4 abundance or contain a significant
amount of neutral helium. The HRDS has 20 nebulae with carbon RRL emission but
no helium emission at its sensitivity level. There is no correlation between
the carbon RRL parameters and the 8 microns mid-infrared morphology of these
nebulae.Comment: Accepted to ApJ. The survey website can be found here:
http://go.nrao.edu/hrd
Radio continuum observations of local star-forming galaxies using the Caltech Continuum Backend on the Green Bank Telescope
We observed radio continuum emission in 27 local (D < 70 Mpc) star-forming
galaxies with the Robert C. Byrd Green Bank Telescope between 26 GHz and 40 GHz
using the Caltech Continuum Backend. We obtained detections for 22 of these
galaxies at all four sub-bands and four more marginal detections by taking the
average flux across the entire bandwidth. This is the first detection (full or
marginal) at these frequencies for 22 of these galaxies. We fit spectral energy
distributions (SEDs) for all of the four-sub-band detections. For 14 of the
galaxies, SEDs were best fit by a combination of thermal free-free and
nonthermal synchrotron components. Eight galaxies with four-sub-band detections
had steep spectra that were only fit by a single nonthermal component. Using
these fits, we calculated supernova rates, total number of equivalent O stars,
and star formation rates within each ~23 arcsecond beam. For unresolved
galaxies, these physical properties characterize the galaxies' recent star
formation on a global scale. We confirm that the radio-far-infrared correlation
holds for the unresolved galaxies' total 33 GHz flux regardless of their
thermal fractions, though the scatter on this correlation is larger than that
at 1.4 GHz. In addition, we found that for the unresolved galaxies, there is an
inverse relationship between the ratio of 33 GHz flux to total far-infrared
flux and the steepness of the galaxy's spectral index between 1.4 GHz and 33
GHz. This relationship could be an indicator of the timescale of the observed
episode of star formation.Comment: 36 pages, 9 figures; accepted for publication in ApJ. First and
second author affiliation updated to reflect departmental name chang
The Electron Temperature Gradient in the Galactic Disk
We derive the electron temperature gradient in the Galactic disk using a
sample of HII regions that spans Galactocentric distances 0--17 kpc. The
electron temperature was calculated using high precision radio recombination
line and continuum observations for more than 100 HII regions. Nebular
Galactocentric distances were calculated in a consistent manner using the
radial velocities measured by our radio recombination line survey. The large
number of nebulae widely distributed over the Galactic disk together with the
uniformity of our data provide a secure estimate of the present electron
temperature gradient in the Milky Way. Because metals are the main coolants in
the photoionized gas, the electron temperature along the Galactic disk should
be directly related to the distribution of heavy elements in the Milky Way. Our
best estimate of the electron temperature gradient is derived from a sample of
76 sources for which we have the highest quality data. The present gradient in
electron temperature has a minimum at the Galactic Center and rises at a rate
of 287 +/- 46 K/kpc. There are no significant variations in the value of the
gradient as a function of Galactocentric radius or azimuth. The scatter we find
in the HII region electron temperatures at a given Galactocentric radius is not
due to observational error, but rather to intrinsic fluctuations in these
temperatures which are almost certainly due to fluctuations in the nebular
heavy element abundances. Comparing the HII region gradient with the much
steeper gradient found for planetary nebulae suggests that the electron
temperature gradient evolves with time, becoming flatter as a consequence of
the chemical evolution of the Milky Way's disk.Comment: 43 pages, 9 figures (accepted for publication in the ApJ
HII Region Ionization of the Interstellar Medium: A Case Study of NGC 7538
Using data from the Green Bank Telescope, we analyze the radio continuum
(free-free) and radio recombination line (RRL) emission of the compact HII
region NGC 7538 (Sharpless 158). We detect extended radio continuum and
hydrogen RRL emission beyond the photodissociation region (PDR) toward the
north and east, but a sharp decrease in emission toward the south and west.
This indicates that a non-uniform PDR morphology is affecting the amount of
radiation "leaking" through the PDR. The strongest carbon RRL emission is found
in the western PDR that appears to be dense. We compute a leaking fraction % of the radio continuum emission measured in the plane of the sky
which represents a lower limit when accounting for the three-dimensional
geometry of the region. We detect an average
abundance ratio by number of inside the HII region and a
decrease in this ratio with increasing distance from the region beyond the PDR.
Using Herschel Space Observatory data, we show that small dust temperature
enhancements to the north and east of NGC 7538 coincide with extended radio
emission, but that the dust temperature enhancements are mostly contained
within a second PDR to the east. Unlike the giant HII region W43, the radiation
leaking from NGC 7538 seems to only affect the local ambient medium. This
suggests that giant HII regions may have a large effect in maintaining the
ionization of the interstellar medium.Comment: Accepted for publication in ApJ (15 pages, 10 figures, 2 tables
Diffuse Ionized Gas in the Milky Way Disk
We analyze the diffuse ionized gas (DIG) in the first Galactic quadrant from
l=18deg to 40deg using radio recombination line (RRL) data from the Green Bank
Telescope. These data allow us to distinguish DIG emission from HII region
emission and thus study the diffuse gas essentially unaffected by confusion
from discrete sources. We find that the DIG has two dominant velocity
components, one centered around 100km/s associated with the luminous HII region
W43, and the other centered around 45km/s not associated with any large HII
region. Our analysis suggests that the two velocity components near W43 may be
caused by non-circular streaming motions originating near the end of the
Galactic bar. At lower Galactic longitudes, the two velocities may instead
arise from gas at two distinct distances from the Sun, with the most likely
distances being ~6kpc for the 100km/s component and ~12kpc for the 45km/s
component. We show that the intensity of diffuse Spitzer GLIMPSE 8.0um emission
caused by excitation of polyaromatic hydrocarbons (PAHs) is correlated with
both the locations of discrete HII regions and the intensity of the RRL
emission from the DIG. This implies that the soft ultra-violet photons
responsible for creating the infrared emission have a similar origin as the
harder ultra-violet photons required for the RRL emission. The 8.0um emission
increases with RRL intensity but flattens out for directions with the most
intense RRL emission, suggesting that PAHs are partially destroyed by the
energetic radiation field at these locations.Comment: Accepted for publication in ApJ (16 pages, 11 figures, 2 tables
Diffuse Ionized Gas in the Milky Way Disk
We analyze the diffuse ionized gas (DIG) in the first Galactic quadrant from
l=18deg to 40deg using radio recombination line (RRL) data from the Green Bank
Telescope. These data allow us to distinguish DIG emission from HII region
emission and thus study the diffuse gas essentially unaffected by confusion
from discrete sources. We find that the DIG has two dominant velocity
components, one centered around 100km/s associated with the luminous HII region
W43, and the other centered around 45km/s not associated with any large HII
region. Our analysis suggests that the two velocity components near W43 may be
caused by non-circular streaming motions originating near the end of the
Galactic bar. At lower Galactic longitudes, the two velocities may instead
arise from gas at two distinct distances from the Sun, with the most likely
distances being ~6kpc for the 100km/s component and ~12kpc for the 45km/s
component. We show that the intensity of diffuse Spitzer GLIMPSE 8.0um emission
caused by excitation of polyaromatic hydrocarbons (PAHs) is correlated with
both the locations of discrete HII regions and the intensity of the RRL
emission from the DIG. This implies that the soft ultra-violet photons
responsible for creating the infrared emission have a similar origin as the
harder ultra-violet photons required for the RRL emission. The 8.0um emission
increases with RRL intensity but flattens out for directions with the most
intense RRL emission, suggesting that PAHs are partially destroyed by the
energetic radiation field at these locations.Comment: Accepted for publication in ApJ (16 pages, 11 figures, 2 tables
Untangling the Recombination Line Emission from HII Regions with Multiple Velocity Components
HII regions are the ionized spheres surrounding high-mass stars. They are
ideal targets for tracing Galactic structure because they are predominantly
found in spiral arms and have high luminosities at infrared and radio
wavelengths. In the Green Bank Telescope HII Region Discovery Survey (GBT HRDS)
we found that >30% of first Galactic quadrant HII regions have multiple
hydrogen radio recombination line (RRL) velocities, which makes determining
their Galactic locations and physical properties impossible. Here we make
additional GBT RRL observations to determine the discrete HII region velocity
for all 117 multiple-velocity sources within 18deg. < l < 65deg. The
multiple-velocity sources are concentrated in the zone 22deg. < l < 32deg.,
coinciding with the largest regions of massive star formation, which implies
that the diffuse emission is caused by leaked ionizing photons. We combine our
observations with analyses of the electron temperature, molecular gas, and
carbon recombination lines to determine the source velocities for 103 discrete
H II regions (88% of the sample). With the source velocities known, we resolve
the kinematic distance ambiguity for 47 regions, and thus determine their
heliocentric distances.Comment: 44 pages, 5 figures, 16 pages of tables; Accepted by ApJ
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