Abstract
Magnetic fields likely play an important role in star formation, but the number of directly measured magnetic field strengths remains scarce. We observed the 38.3 and 38.5 GHz Class II ...methanol (CH
3
OH) maser lines toward the high-mass star-forming region NGC 6334 F for the Zeeman effect. The observed spectral profiles have two prominent velocity features that can be further decomposed through Gaussian component fitting. In several of these fitted Gaussian components we find significant Zeeman detections, with
zB
los
in the range from 8 to 46 Hz. If the Zeeman splitting factor
z
for the 38 GHz transitions is of the order of ∼1 Hz mG
−1
, similar to that for several other CH
3
OH maser lines, then magnetic fields in the regions traced by these masers would be in the range of 8–46 mG. Such magnetic field values in high-mass star-forming regions agree with those detected in the better-known 6.7 GHz Class II CH
3
OH maser line. Since Class II CH
3
OH masers are radiatively pumped close to the protostar and likely occur in the accretion disk or the interface between the disk and outflow regions, such fields likely have significant impact on the dynamics of these disks.
Two 5 deg2 regions around the NGC 7332/9 galaxy pair and the isolated galaxy NGC 1156 have been mapped in the 21 cm line of neutral hydrogen (H I) with the Arecibo L-band Feed Array out to a redshift ...of ~0.065 (~20,000 km s--1) as part of the Arecibo Galaxy Environment Survey. One of the aims of this survey is to investigate the environment of galaxies by identifying dwarf companions and interaction remnants; both of these areas provide the potential for such discoveries. The neutral hydrogen observations were complemented by optical and radio follow-up observations with a number of telescopes. A total of 87 galaxies were found, of which 39 (45%) were previously cataloged and 15 (17%) have prior redshifts. Two dwarf galaxies have been discovered in the NGC 7332 group and a single dwarf galaxy in the vicinity of NGC 1156. A parallel optical search of the area revealed one further possible dwarf galaxy near NGC 7332.
We present 3, 15, and 33 GHz imaging toward galaxy nuclei and extranuclear star-forming regions using the Karl G. Jansky Very Large Array as part of the Star Formation in Radio Survey. With 3-33 GHz ...radio spectra, we measured the spectral indices and corresponding thermal (free-free) emission fractions for a sample of 335 discrete regions having significant detections in at least two radio bands. After removing 14 likely background galaxies, we find that the median thermal fraction at 33 GHz is 92% 0.8% with a median absolute deviation of 11%, when a two-component power-law model is adopted to fit the radio spectrum. Limiting the sample to 238 sources that are confidently identified as star-forming regions and not affected by potential AGN contamination (i.e., having galactocentric radii rG ≥ 250 pc) results in a median thermal fraction of 93% 0.8% with a median absolute deviation of 10%. We further measure the thermal fraction at 33 GHz for 163 regions identified at 7″ resolution to be 94% 0.8% with a median absolute deviation of 8%. Together, these results confirm that free-free emission dominates the radio spectra of star-forming regions on scales up to ∼500 pc in normal star-forming galaxies. We additionally find a factor of ∼1.6 increase in the scatter of the measured spectral index and thermal fraction distributions as a function of decreasing galactocentric radius. This trend is likely reflective of the continuous star formation activity occurring in the galaxy centers, resulting in a larger contribution of diffuse nonthermal emission relative to star-forming regions in the disk.
Context. Quantifying the fraction of active galactic nuclei (AGN) in the faint radio population and understanding their relation with star-forming activity are fundamental to studies of galaxy ...evolution. Very long baseline interferometry (VLBI) observations are able to identify AGN above relatively low redshifts (z> 0.1) since they provide milli-arcsecond resolution. Aims. We have created an AGN catalogue from 2865 known radio sources observed in the Cosmic Evolution Survey (COSMOS) field, which has exceptional multi-wavelength coverage. With this catalogue we intend to study the faint radio sky with statistically relevant numbers and to analyse the AGN – host galaxy co-evolution, making use of the large amount of ancillary data available in the field. Methods. Wide-field VLBI observations were made of all known radio sources in the COSMOS field at 1.4 GHz to measure the AGN fraction, in particular in the faint radio population. We describe in detail the observations, data calibration, source detection and flux density measurements, parts of which we have developed for this survey. The combination of number of sources, sensitivity, and area covered with this project are unprecedented. Results. We have detected 468 radio sources, expected to be AGN, with the Very Long Baseline Array (VLBA). This is, to date, the largest sample assembled of VLBI detected sources in the sub-mJy regime. The input sample was taken from previous observations with the Very Large Array (VLA). We present the catalogue with additional optical, infrared and X-ray information. Conclusions. We find a detection fraction of 20 ± 1%, considering only those sources from the input catalogue which were in principle detectable with the VLBA (2361). As a function of the VLA flux density, the detection fraction is higher for higher flux densities, since at high flux densities a source could be detected even if the VLBI core accounts for a small percentage of the total flux density. As a function of redshift, we see no evolution of the detection fraction over the redshift range 0.5 <z< 3. In addition, we find that faint radio sources typically have a greater fraction of their radio luminosity in a compact core – ~70% of the sub-mJy sources detected with the VLBA have more than half of their total radio luminosity in a VLBI-scale component, whereas this is true for only ~30% of the sources that are brighter than 10 mJy. This suggests that fainter radio sources differ intrinsically from brighter ones. Across our entire sample, we find the predominant morphological classification of the host galaxies of the VLBA detected sources to be early type (57%), although this varies with redshift and at z> 1.5 we find that spiral galaxies become the most prevalent (48%). The number of detections is high enough to study the faint radio population with statistically significant numbers. We demonstrate that wide-field VLBI observations, together with new calibration methods such as multi-source self-calibration and mosaicing, result in information which is difficult or impossible to obtain otherwise.
ABSTRACT We report detection of the Zeeman effect in the 44 GHz Class I methanol maser line, toward the star-forming region DR21(OH). In a 219 Jy beam−1 maser centered at an LSR velocity of 0.83 km ...s−1, we find a 20- detection of zBlos = 53.5 2.7 Hz. If 44 GHz methanol masers are excited at n ∼ 107-8 cm−3, then the B versus n1/2 relation would imply, from comparison with Zeeman effect detections in the CN(1 − 0) line toward DR21(OH), that magnetic fields traced by 44 GHz methanol masers in DR21(OH) should be ∼10 mG. Combined with our detected zBlos = 53.5 Hz, this would imply that the value of the 44 GHz methanol Zeeman splitting factor z is ∼5 Hz mG−1. Such small values of z would not be a surprise, as the methanol molecule is non-paramagnetic, like H2O. Empirical attempts to determine z, as demonstrated, are important because there currently are no laboratory measurements or theoretically calculated values of z for the 44 GHz CH3OH transition. Data from observations of a larger number of sources are needed to make such empirical determinations robust.
We report the detection of the Zeeman effect in the 44 GHz Class I methanol maser line toward the high-mass star-forming region DR21W. There are two prominent maser spots in DR21W at the ends of a ...northwest-southeast linear arrangement. For the maser at the northwestern end (maser A), we fit three Gaussian components. In the strongest component, we obtain a significant Zeeman detection, with zBlos = −23.4 3.2 Hz. If we use z = −0.920 Hz mG−1 for the F = 5 → 4 hyperfine transition, this corresponds to a magnetic field, ; Blos would be higher if a different hyperfine were responsible for the 44 GHz maser, but our results also rule out some hyperfines, as fields in these regions cannot be hundreds of mG. Class I methanol masers form in outflows where shocks compress magnetic fields in proportion to gas density. Designating our detected Blos = 25 mG as the magnetic field in the postshock gas, we find that Blos in the preshock gas should be 0.1-0.8 mG. Although there are no thermal line Zeeman detections toward DR21W, such values are in good agreement with Zeeman measurements in the CN thermal line of 0.36 and 0.71 mG about 3 5 away in DR21(OH) in gas of comparable density to the preshock gas density in DR21W. Comparison of our derived magnetic energy density with the kinetic energy density in DR21W indicates that magnetic fields likely play a significant role in shaping the dynamics of the postshocked gas in DR21W.
We present 33 GHz imaging for 112 pointings toward galaxy nuclei and extranuclear star-forming regions at 2″ resolution using the Karl G. Jansky Very Large Array (VLA) as part of the Star Formation ...in Radio Survey. A comparison with 33 GHz Robert C. Byrd Green Bank Telescope single-dish observations indicates that the interferometric VLA observations recover 78% 4% of the total flux density over 25″ regions ( kpc scales) among all fields. On these scales, the emission being resolved out is most likely diffuse non-thermal synchrotron emission. Consequently, on the 30-300 pc scales sampled by our VLA observations, the bulk of the 33 GHz emission is recovered and primarily powered by free-free emission from discrete H ii regions, making it an excellent tracer of massive star formation. Of the 225 discrete regions used for aperture photometry, 162 are extranuclear (i.e., having galactocentric radii rG ≥ 250 pc) and detected at >3 significance at 33 GHz and in H . Assuming a typical 33 GHz thermal fraction of 90%, the ratio of optically-thin 33 GHz to uncorrected H star formation rates indicates a median extinction value on 30-300 pc scales of AH 1.26 0.09 mag, with an associated median absolute deviation of 0.87 mag. We find that 10% of these sources are "highly embedded" (i.e., AH 3.3 mag), suggesting that on average, H ii regions remain embedded for 1 Myr. Finally, we find the median 33 GHz continuum-to-H line flux ratio to be statistically larger within rG < 250 pc relative to the outer disk regions by a factor of 1.82 0.39, while the ratio of 33 GHz to 24 m flux densities is lower by a factor of 0.45 0.08, which may suggest increased extinction in the central regions.
Abstract
To investigate the growth history of galaxies, we measure the rest-frame radio, ultraviolet (UV), and optical sizes of 98 radio-selected, star-forming galaxies (SFGs) distributed over 0.3 ≲
...z
≲ 3 with a median stellar mass of
log
(
M
⋆
/
M
⊙
)
≈
10.4
. We compare the size of galaxy stellar disks, traced by rest-frame optical emission, relative to the overall extent of star formation activity that is traced by radio continuum emission. Galaxies in our sample are identified in three
Hubble
Frontier Fields: MACS J0416.1−2403, MACS J0717.5+3745, and MACS J1149.5+2223. Radio continuum sizes are derived from 3 and 6 GHz radio images (≲0.″6 resolution, ≈0.9
μ
Jy beam
−1
noise level) from the Karl G. Jansky Very Large Array. Rest-frame UV and optical sizes are derived using observations from the
Hubble Space Telescope
and the Advanced Camera for Surveys and Wide Field Camera 3 instruments. We find no clear dependence between the 3 GHz radio size and stellar mass of SFGs, which contrasts with the positive correlation between the UV/optical size and stellar mass of galaxies. Focusing on SFGs with
log
(
M
⋆
/
M
⊙
)
>
10
, we find that the radio/UV/optical emission tends to be more compact in galaxies with high star formation rates (≳100
M
⊙
yr
−1
), suggesting that a central, compact starburst (and/or an active galactic nucleus) resides in the most luminous galaxies of our sample. We also find that the physical radio/UV/optical size of radio-selected SFGs with log(
M
⋆
/
M
⊙
) > 10 increases by a factor of 1.5–2 from
z
≈ 3 to
z
≈ 0.3, yet the radio emission remains two to three times more compact than that from the UV/optical. These findings indicate that these massive, radio-selected SFGs at 0.3 ≲
z
≲ 3 tend to harbor centrally enhanced star formation activity relative to their outer disks.
We present Very Large Array observations of the 33 GHz radio continuum emission from 22 local ultraluminous and luminous infrared (IR) galaxies (U/LIRGs). These observations have spatial (angular) ...resolutions of 30-720 pc (0 07-0 67) in a part of the spectrum that is likely to be optically thin. This allows us to estimate the size of the energetically dominant regions. We find half-light radii from 30 pc to 1.7 kpc. The 33 GHz flux density correlates well with the IR emission, and we take these sizes as indicative of the size of the region that produces most of the energy. Combining our 33 GHz sizes with unresolved measurements, we estimate the IR luminosity and star formation rate per area and the molecular gas surface and volume densities. These quantities span a wide range (4 dex) and include some of the highest values measured for any galaxy (e.g., ). At least 13 sources appear Compton thick ( ). Consistent with previous work, contrasting these data with observations of normal disk galaxies suggests a nonlinear and likely multivalued relation between star formation rate and molecular gas surface density, though this result depends on the adopted CO-to-H2 conversion factor and the assumption that our 33 GHz sizes apply to the gas. Eleven sources appear to exceed the luminosity surface density predicted for starbursts supported by radiation pressure and supernova feedback; however, we note the need for more detailed observations of the inner disk structure. U/LIRGs with higher surface brightness exhibit stronger C ii 158 m deficits, consistent with the suggestion that high energy densities drive this phenomenon.
PDF
