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Register a new userDetection of Voigt Spectral Line Profiles of Hydrogen Radio Recombination Lines toward Sagittarius B2(N)
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Added by
Azrael von Proch\\'azka
Publication date
2010
fields
Physics
and research's language is
English
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We report the detection of Voigt spectral line profiles of radio recombination lines (RRLs) toward Sagittarius B2(N) with the 100-m Green Bank Telescope (GBT). At radio wavelengths, astronomical spectra are highly populated with RRLs, which serve as ideal probes of the physical conditions in molecular cloud complexes. An analysis of the Hn(alpha) lines presented herein shows that RRLs of higher principal quantum number (n>90) are generally divergent from their expected Gaussian profiles and, moreover, are well described by their respective Voigt profiles. This is in agreement with the theory that spectral lines experience pressure broadening as a result of electron collisions at lower radio frequencies. Given the inherent technical difficulties regarding the detection and profiling of true RRL wing spans and shapes, it is crucial that the observing instrumentation produce flat baselines as well as high sensitivity, high resolution data. The GBT has demonstrated its capabilities regarding all of these aspects, and we believe that future observations of RRL emission via the GBT will be crucial towards advancing our knowledge of the larger-scale extended structures of ionized gas in the interstellar medium (ISM).
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The Galactic Center lobe is a degree-tall shell seen in radio continuum images of the Galactic center (GC) region. If it is actually located in the GC region, formation models would require massive energy input (e.g., starburst or jet) to create it. At present, observations have not strongly constrained the location or physical conditions of the GC lobe. This paper describes the analysis of new and archival single-dish observations of radio recombination lines toward this enigmatic object. The observations find that the ionized gas has a morphology similar to the radio continuum emission, suggesting that they are associated. We study averages of several transitions from H106alpha to H191epsilon and find that the line ratios are most consistent with gas in local thermodynamic equilibrium. The radio recombination line widths are remarkably narrow, constraining the typical electron temperature to be less than about 4000 K. These observations also find evidence of pressure broadening in the higher electronic states, implying a gas density of n_e=910^{+310}_{-450} cm^{-3}. The electron temperature, gas pressure, and morphology are all consistent with the idea that the GC lobe is located in the GC region. If so, the ionized gas appears to form a shell surrounding the central 100 parsecs of the galaxy with a mass of roughly 10^5 Msun, similar to ionized outflows seen in dwarf starbursts.
We have undertaken a spectral-line imaging survey of a 6 x 6 arcmin^2 area around Sgr B2 near the centre of the Galaxy, in the range from 30 to 50 GHz, using the Mopra telescope. The spatial resolution varies from 1.0 to 1.4 arcmin and the spectral resolution from 1.6 to 2.7 km s^-1 over the frequency range. We present velocity-integrated emission images for 47 lines: 38 molecular lines and 9 radio recombination lines. There are significant differences between the distributions of different molecules, in part due to spatial differences in chemical abundance across the complex. For example, HNCO and HOCO^+ are found preferentially in the north cloud, and CH_2NH near Sgr B2 (N). Some of the differences between lines are due to excitation differences, as shown by the 36.17 and 44.07 GHz lines of CH_3OH, which have maser emission, compared to the 48.37 GHz line of CH_3OH. Other major differences in integrated molecular line distribution are due to absorption of the 7-mm free-free continuum emission (spatially traced by the radio recombination line emission) by cool intervening molecular material, causing a central dip in the molecular line distributions. These line distribution similarities and differences have been statistically described by principal component analysis (PCA), and interpreted in terms of simple Sgr B2 physical components of the cooler, lower density envelope, and dense, hot cores Sgr B2 (N), (M) and (S).
Recombination lines involving high principal quantum numbers populate the radio spectrum in large numbers. Low-frequency (<1 GHz) observations of radio recombination lines (RRLs) primarily from carbon and hydrogen offer a new, if not unique, way to probe cold, largely atomic gas and warm, ionised gas in other galaxies. Furthermore, RRLs can be used to determine the physical state of the emitting regions, such as temperature and density. These properties make RRLs, potentially, a powerful tool of extragalactic ISM physics. At low radio frequencies, its conceivable to detect RRLs out to cosmological distances when illuminated by a strong radio continuum. However, they are extremely faint (tau ~ 1e-3 -- 1e-4) and have so far eluded detection outside of the local universe. With LOFAR observations of the radio quasar 3C 190 (z=1.1946), we aim to demonstrate that the ISM can be explored out to great distances through low-frequency RRLs. We report the detection of RRLs in the frequency range 112--163 MHz in the spectrum of 3C 190. Stacking 13 a-transitions with principal quantum numbers n=266-301, a peak 6sigma feature of optical depth, tau(peak) = (1.0 +- 0.2) x 1e-3 and FWHM = 31.2 +/- 8.3 km/s was found at z=1.124. This corresponds to a velocity offset of -9965 km/s with respect to the systemic redshift of 3C 190. We consider three interpretations of the origin of the RRL emission: an intervening dwarf-like galaxy, an AGN-driven outflow, and the inter-galactic medium. We argue that the RRLs most likely originate in a dwarf-like galaxy (M ~ 1e9 Msun) along the line of sight, although we cannot rule out an AGN-driven outflow. We do find the RRLs to be inconsistent with an inter-galactic medium origin. With this detection, we have opened up a new way to study the physical properties of cool, diffuse gas out to cosmological distances.
We have initiated a spectral line survey, at a wavelength of 3 millimeters, toward the hot molecular core Sagittarius B2(N-LMH). This is the first spectral line survey of the Sgr B2(N) region utilizing data from both an interferometer (BIMA Array) and a single-element radio telescope (NRAO 12 meter). In this survey, covering 3.6 GHz in bandwidth, we detected 218 lines (97 identified molecular transitions, 1 recombination line, and 120 unidentified transitions). This yields a spectral line density (lines per 100 MHz) of 6.06, which is much larger than any previous 3 mm line survey. We also present maps from the BIMA Array that indicate that most highly saturated species (3 or more H atoms) are products of grain chemistry or warm gas phase chemistry. Due to the nature of this survey we are able to probe each spectral line on multiple spatial scales, yielding information that could not be obtained by either instrument alone.
The detection E-cyanomethanimine (E-HNCHCN) towards Sagittarius B2(N) is made by comparing the publicly available Green Bank Telescope (GBT) PRIMOS survey spectra (Hollis et al.) to laboratory rotational spectra from a reaction product screening experiment. The experiment uses broadband molecular rotational spectroscopy to monitor the reaction products produced in an electric discharge source using a gas mixture of NH3 and CH3CN. Several transition frequency coincidences between the reaction product screening spectra and previously unassigned interstellar rotational transitions in the PRIMOS survey have been assigned to E cyanomethanimine. A total of 8 molecular rotational transitions of this molecule between 9 and 50 GHz are observed with the GBT. E-cyanomethanimine, often called the HCN dimer, is an important molecule in prebiotic chemistry because it is a chemical intermediate in proposed synthetic routes of adenine, one of the two purine nucleobases found in DNA and RNA. New analyses of the rotational spectra of both E-cyanomethanimine and Z-cyanomethanimine that incorporate previous mm-wave measurements are also reported.
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