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Register a new userRadio Recombination Lines at Decametre Wavelengths: Prospects for the Future
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This paper considers the suitability of a number of emerging and future instruments for the study of radio recombination lines (RRLs) at frequencies below 200 MHz. These lines arise only in low-density regions of the ionized interstellar medium, and they may represent a frequency-dependent foreground for next-generation experiments trying to detect H I signals from the Epoch of Reionization and Dark Ages (21-cm cosmology). We summarize existing decametre-wavelength observations of RRLs, which have detected only carbon RRLs. We then show that, for an interferometric array, the primary instrumental factor limiting detection and study of the RRLs is the areal filling factor of the array. We consider the Long Wavelength Array (LWA-1), the LOw Frequency ARray (LOFAR), the low-frequency component of the Square Kilometre Array (SKA-lo), and a future Lunar Radio Array (LRA), all of which will operate at decametre wavelengths. These arrays offer digital signal processing, which should produce more stable and better defined spectral bandpasses; larger frequency tuning ranges; and better angular resolution than that of the previous generation of instruments that have been used in the past for RRL observations. Detecting Galactic carbon RRLs, with optical depths at the level of 10^-3, appears feasible for all of these arrays, with integration times of no more than 100 hr. The SKA-lo and LRA, and the LWA-1 and LOFAR at the lowest frequencies, should have a high enough filling factor to detect lines with much lower optical depths, of order 10^-4 in a few hundred hours. The amount of RRL-hosting gas present in the Galaxy at the high Galactic latitudes likely to be targeted in 21-cm cosmology studies is currently unknown. If present, however, the spectral fluctuations from RRLs could be comparable to or exceed the anticipated H I signals.
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The Arcminute Microkelvin Imager (AMI) is a telescope specifically designed for high sensitivity measurements of low-surface-brightness features at cm-wavelength and has unique, important capabilities. It consists of two interferometer arrays operating over 13.5-18 GHz that image structures on scales of 0.5-10 arcmin with very low systematics. The Small Array (AMI-SA; ten 3.7-m antennas) couples very well to Sunyaev-Zeldovich features from galaxy clusters and to many Galactic features. The Large Array (AMI-LA; eight 13-m antennas) has a collecting area ten times that of the AMI-SA and longer baselines, crucially allowing the removal of the effects of confusing radio point sources from regions of low surface-brightness, extended emission. Moreover AMI provides fast, deep object surveying and allows monitoring of large numbers of objects. In this White Paper we review the new science - both Galactic and extragalactic - already achieved with AMI and outline the prospects for much more.
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.
DM-Ice is a program towards the first direct detection search for dark matter in the Southern Hemisphere with a 250 kg-scale NaI(Tl) crystal array. It will provide a definitive understanding of the modulation signal reported by DAMA by running an array at both Northern and Southern Hemisphere sites. A 17 kg predecessor, DM-Ice17, was deployed in December 2010 at a depth of 2457 m under the ice at the geographic South Pole and has concluded its 3.5 yr data run. An active R&D program is underway to investigate detectors with lower backgrounds and improved readout electronics; two crystals with 37 kg combined mass are currently operating at the Boulby Underground Laboratory. We report on the final analyses of the DM-Ice17 data and describe progress towards a 250 kg DM-Ice experiment.
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.
The intensities of the three widely observed radio-wavelength hyperfine structure (HFS) lines between the {Lambda}-doublet components of the rotational ground state of CH are inconsistent with LTE and indicate ubiquitous population inversion. While this can be qualitatively understood assuming a pumping cycle that involves collisional excitation processes, the relative intensities of the lines and in particular the dominance of the lowest frequency satellite line has not been well understood. This has limited the use of CH radio emission as a tracer of the molecular interstellar medium. We present the first interferometric observations, with the Karl G. Jansky Very Large Array, of the CH 9 cm ground state HFS transitions at 3.264 GHz, 3.335 GHz, and 3.349 GHz toward four high mass star-forming regions (SFRs) Sgr B2 (M), G34.26+0.15, W49 (N), and W51. We investigate the nature of the (generally) weak CH ground state masers by employing synergies between the ground state HFS transitions themselves and with the far-infrared lines, near 149 {mu}m (2 THz), that connect these levels to an also HFS split rotationally excited level. Employing recently calculated collisional rate coefficients, we perform statistical equilibrium calculations with the non-LTE radiative transfer code MOLPOP-CEP in order to model the excitation conditions traced by the ground state HFS lines of CH and to infer the physical conditions in the emitting regions while also accounting for the effects of far-infrared line overlap.
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