sub-mJy Levels Research Articles

Radio emission as a stellar activity indicator

Radio observations of stars trace the plasma conditions and magnetic field properties of stellar magnetospheres and coronae. Depending on the plasma conditions at the emitter site, radio emission in the metre- and decimetre-wave bands is generated via different mechanisms, such as gyrosynchrotron, electron cyclotron maser instability, and plasma radiation processes. The ongoing LOFAR Two-metre Sky Survey (LoTSS) and VLA Sky Survey (VLASS) are currently the most sensitive wide-field radio sky surveys ever conducted. Because these surveys are untargeted, they provide an opportunity to study the statistical properties of the radio-emitting stellar population in an unbiased manner. Here we perform an untargeted search for stellar radio sources down to sub-mJy level using these radio surveys. We find that the population of radio-emitting stellar systems is mainly composed of two distinct categories: chromospherically active stellar (CAS) systems and M dwarfs. We also seek to identify signatures of a gradual transition within the M-dwarf population, from chromospheric or coronal acceleration close to the stellar surface similar to that observed on the Sun to magnetospheric acceleration occurring far from the stellar surface similar to that observed on Jupiter. We determine that radio detectability evolves with spectral type, and we identify a transition in radio detectability around spectral type M4, where stars become fully convective. Furthermore, we compare the radio detectability versus spectra type with X-ray and optical flare (observed by TESS) incidence statistics. We find that the radio efficiency of X-ray and optical flares, which is the fraction of flare energy channelled into radio-emitting charges, increases with spectral type. These results motivate us to conjecture that the emergence of large-scale magnetic fields in CAS systems and later M dwarfs leads to an increase in radio efficiency.

IGR J18249−3243: a new GeV-emitting FR II and the emerging population of high-energy radio galaxies

ABSTRACT The advent of new all-sky radio surveys such as the VLA Sky Survey and the Rapid ASKAP Continuum Survey, performed with the latest generation radio telescopes, is opening new possibilities on the classification and study of extragalactic γ-ray sources, specially the underrepresented ones like radio galaxies. In particular, the enhanced sensitivity (sub-mJy level) and resolution (a few arcsec) provides a better morphological and spectral classification. In this work, we present the reclassification of a Fermi/Large Area Telescope (LAT) source as a new Fanaroff–Riley II radio galaxy from the International Gamma-Ray Astrophysics Laboratory sample found to emit at GeV energies. Through a broad-band spectral fitting from radio to γ-ray, we find that the commonly invoked jet contribution is not sufficient to account for the observed γ-ray flux. Our modelling suggests that the observed emission could mainly originate in the lobes (rather than in the radio core) by inverse Compton scattering of radio-emitting electrons off the ambient photon fields. In addition, we cross-correlated the latest generation radio surveys with a list of Fermi/LAT candidate misaligned AGN from the literature, finding four new radio galaxies with a double-lobed morphology. Additional four objects could be classified as such thanks to previous studies in the literature, for a total of nine new radio galaxies with GeV emission presented in this work. We foresee that further objects of this class might be found in the near future with the advent of the Square Kilometer Array, populating the GeV sky.

The thermal emission of Centaurs and trans-Neptunian objects at millimeter wavelengths from ALMA observations

The sensitivity of ALMA makes it possible to detect thermal mm/submm emission from small and/or distant solar system bodies at the sub-mJy level. While the measured fluxes are primarily sensitive to the objects’ diameters, deriving precise sizes is somewhat hampered by the uncertain effective emissivity at these wavelengths. Following recent work presenting ALMA data for four trans-Neptunian objects (TNOs) with satellites, we report on ALMA 233 GHz (1.29 mm) flux measurements of four Centaurs (2002 GZ32, Bienor, Chiron, Chariklo) and two other TNOs (Huya and Makemake), sampling a range of sizes, albedos, and compositions. These thermal fluxes are combined with previously published fluxes in the mid/far infrared in order to derive their relative emissivity at radio (mm/submm) wavelengths, using the Near Earth Asteroid Standard Model (NEATM) and thermophysical models. We reassess earlier thermal measurements of these and other objects – including Pluto/Charon and Varuna – exploring, in particular, effects due to non-spherical shape and varying apparent pole orientation whenever information is available, and show that these effects can be key for reconciling previous diameter determinations and correctly estimating the spectral emissivities. We also evaluate the possible contribution to thermal fluxes of established (Chariklo) or claimed (Chiron) ring systems. For Chariklo, the rings do not impact the diameter determinations by more than ~5%; for Chiron, invoking a ring system does not help in improving the consistency between the numerous past size measurements. As a general conclusion, all the objects, except Makemake, have radio emissivities significantly lower than unity. Although the emissivity values show diversity, we do not find any significant trend with physical parameters such as diameter, composition, beaming factor, albedo, or color, but we suggest that the emissivity could be correlated with grain size. The mean relative radio emissivity is found to be 0.70 ± 0.13, a value that we recommend for the analysis of further mm/submm data.

SENSITIVE SEARCH FOR RADIO VARIABLES AND TRANSIENTS IN THE EXTENDED CHANDRA DEEP FIELD SOUTH

We report on an analysis of the Extended Chandra Deep Field South (E-CDFS) region using archival data from the Very Large Array, with the goal of studying radio variability and transients at the sub-mJy level. The 49 epochs of E-CDFS observations at 1.4 GHz sample timescales from one day to 3 months. We find that only a fraction (1%) of unresolved radio sources above 40 uJy are variable at the 4-sigma level. There is no evidence that the fractional variability changes along with the known transition of radio source populations below one milliJansky. Optical identifications of the sources show that the variable radio emission is associated with the central regions of an active galactic nucleus or a star-forming galaxy. After a detailed comparison of the efficacy of various source-finding algorithms, we use the best to carry out a transient search. No transients were found. This implies that the areal density of transients with peak flux density greater than 0.21 mJy is less than 0.37 deg^-2 (at a confidence level of 95%). This result is approximately an order of magnitude below the transient rate measured at 5 GHz by Bower et al. (2007) but it is consistent with more recent upper limits from Frail et al. (2012). Our findings suggest that the radio sky at 1.4 GHz is relatively quiet. For multi-wavelength transient searches, such as the electromagnetic counterparts to gravitational waves, this frequency may be optimal for reducing the high background of false positives.

ULTRA STEEP SPECTRUM RADIO SOURCES IN THE LOCKMAN HOLE:SERVSIDENTIFICATIONS AND REDSHIFT DISTRIBUTION AT THE FAINTEST RADIO FLUXES

Ultra Steep Spectrum (USS) radio sources have been successfully used to select powerful radio sources at high redshifts (z>~2). Typically restricted to large-sky surveys and relatively bright radio flux densities, it has gradually become possible to extend the USS search to sub-mJy levels, thanks to the recent appearance of sensitive low-frequency radio facilities. Here a first detailed analysis of the nature of the faintest USS sources is presented. By using Giant Metrewave Radio Telescope and Very Large Array radio observations of the Lockman Hole at 610 MHz and 1.4 GHz, a sample of 58 USS sources, with 610 MHz integrated fluxes above 100 microJy, is assembled. Deep infrared data at 3.6 and 4.5 micron from the Spitzer Extragalactic Representative Volume Survey (SERVS) is used to reliably identify counterparts for 48 (83%) of these sources, showing an average total magnitude of [3.6](AB)=19.8 mag. Spectroscopic redshifts for 14 USS sources, together with photometric redshift estimates, improved by the use of the deep SERVS data, for a further 19 objects, show redshifts ranging from z=0.1 to z=2.8, peaking at z~0.6 and tailing off at high redshifts. The remaining 25 USS sources, with no redshift estimate, include the faintest [3.6] magnitudes, with 10 sources undetected at 3.6 and 4.5 micron (typically [3.6]>22-23 mag, from local measurements), which suggests the likely existence of higher redshifts among the sub-mJy USS population. The comparison with the Square Kilometre Array Design Studies Simulated Skies models indicate that Fanaroff-Riley type I radio sources and radio-quiet Active Galactic Nuclei may constitute the bulk of the faintest USS population, and raises the possibility that the high efficiency of the USS technique for the selection of high redshift sources remains even at the sub-mJy level.

10C survey of radio sources at 15.7 GHz - II. First results★

The first results from the Tenth Cambridge (10C) Survey of Radio Sources, carried out using the AMI Large Array (LA) at an observing frequency of 15.7 GHz, are presented. The survey fields cover an area of approximately 27 sq. degrees to a flux-density completeness of 1 mJy. Results for some deeper areas, covering approximately 12 sq. degrees, wholly contained within the total areas and complete to 0.5 mJy, are also presented. The completeness for both areas is estimated to be at least 93 per cent. The source catalogue contains 1897 entries and is available at www.mrao.cam.ac.uk/surveys/10C. It has been combined with that of the 9C Survey to calculate the 15.7-GHz source counts. A broken power law is found to provide a good parameterisation of the differential count between 0.5 mJy and 1 Jy. The measured count has been compared to that predicted by de Zotti et al. (2005). The model displays good agreement with the data at the highest flux densities but under-predicts the integrated count between 0.5 mJy and 1 Jy by about 30 per cent. Entries from the source catalogue have been matched to those contained in the catalogues of NVSS and FIRST (both of which have observing frequencies of 1.4 GHz). This matching provides evidence for a shift in the typical 1.4-to-15.7-GHz spectral index of the 15.7-GHz-selected source population with decreasing flux density towards sub-mJy levels - the spectra tend to become less steep. Automated methods for detecting extended sources have been applied to the data; approximately 5 per cent of the sources are found to be extended relative to the LA synthesised beam of approximately 30 arcsec. Investigations using higher-resolution data showed that most of the genuinely extended sources at 16 GHz are classical doubles, although some nearby galaxies and twin-jet sources were also identified.

LONG-DURATION RADIO TRANSIENTS LACKING OPTICAL COUNTERPARTS ARE POSSIBLY GALACTIC NEUTRON STARS

(abridged) Recently, a new class of radio transients in the 5-GHz band was detected by Bower et al. We present new deep near-Infrared (IR) observations of the field containing these transients, and find no counterparts down to a limiting magnitude of K=20.4 mag. We argue that the bright (>1 Jy) radio transients recently reported by Kida et al. are consistent with being additional examples of the Bower et al. transients. We refer to these groups of events as "long-duration radio transients". The main characteristics of this population are: time scales longer than 30 minute but shorter than several days; rate, ~10^3 deg^-2 yr^-1; progenitors sky surface density of >60 deg^-2 (95% C.L.) at Galactic latitude ~40 deg; 1.4-5 GHz spectral slopes, f_\nu ~ \nu^alpha, with alpha>0; and most notably the lack of any counterparts in quiescence in any wavelength. We rule out an association with many types of objects. Galactic brown-dwarfs or some sort of exotic explosions remain plausible options. We argue that an attractive progenitor candidate for these radio transients is the class of Galactic isolated old neutron stars (NS). We confront this hypothesis with Monte-Carlo simulations of the space distribution of old NSs, and find satisfactory agreement for the large areal density. Furthermore, the lack of quiescent counterparts is explained quite naturally. In this framework we find: the mean distance to events in the Bower et al. sample is of order kpc; the typical distance to the Kida et al. transients are constrained to be between 30 pc and 900 pc (95% C.L.); these events should repeat with a time scale of order several months; and sub-mJy level bursts should exhibit Galactic latitude dependence. We discuss possible mechanisms giving rise to the observed radio emission.

A model for the cosmological evolution of low-frequency radio sources

We present a new evolutionary model that describes the population properties of radio sources at frequencies <5 GHz, thus complementing the De Zotti et al. (2005) model, holding at higher frequencies. We find that simple analytic luminosity evolution is still sufficient to fit the wealth of available data on local luminosity functions, multi-frequency source counts, and redshift distributions. However, the fit requires a luminosity-dependent decline of source luminosities at high redshifts, at least for steep-spectrum sources, thus confirming earlier indications of a "downsizing" also for radio sources. The upturn of source counts at sub-mJy levels is accounted for by a straightforward extrapolation, using the empirical far-IR/radio correlation, of evolutionary models matching the far-IR counts and redshift distributions of star-forming galaxies. We also discuss the implications of the new model for the interpretation of data on large-scale clustering of radio sources and on the Integrated Sachs-Wolfe (ISW) effect, and for the investigation of the contribution of discrete sources to the extragalactic background. As for the ISW effect, a new analysis exploiting a very clean CMB map, yields at a substantially higher significance than reported before.

Radio and millimeter continuum surveys and their astrophysical implications

We review the statistical properties of the main populations of radio sources, as emerging from radio and millimeter sky surveys. Recent determinations of local luminosity functions are presented and compared with earlier estimates still in widespread use. A number of unresolved issues are discussed. These include: the (possibly luminosity-dependent) decline of source space densities at high redshifts; the possible dichotomies between evolutionary properties of low- versus high-luminosity and of flat- versus steep-spectrum AGN-powered radio sources; and the nature of sources accounting for the upturn of source counts at sub-mJy levels. It is shown that straightforward extrapolations of evolutionary models, accounting for both the far-IR counts and redshift distributions of star-forming galaxies, match the radio source counts at flux-density levels of tens of microJy remarkably well. We consider the statistical properties of rare but physically very interesting classes of sources, such as GHz Peak Spectrum and ADAF/ADIOS sources, and radio afterglows of gamma-ray bursts. We also discuss the exploitation of large-area radio surveys to investigate large scale structure through studies of clustering and the Integrated Sachs-Wolfe effect. Finally we briefly describe the potential of the new and forthcoming generations of radio telescopes. A compendium of source counts at different frequencies is given in an appendix.

The radio-mid-infrared correlation and the contribution of 15-μm galaxies to the 1.4-GHz source counts

The radio counterparts to the 15-µm sources in the European Large Area ISO Survey southern fields are identified in 1.4-GHz maps down to ∼80 µJy. The radio‐mid-infrared correlation is investigated and derived for the first time at these flux densities for a sample of this size. Our results show that radio and mid-infrared (MIR) luminosities correlate almost as well as radio and far-infrared (FIR), at least up to z � 0.6. Using the derived relation and its spread together with the observed 15-µm counts, we have estimated the expected contribution of the 15-µm extragalactic populations to the radio source counts and the role of MIR starburst galaxies in the well-known 1.4-GHz source excess observed at sub-mJy levels. Our analysis demonstrates that IR emitting starburst galaxies do not contribute significantly to the 1.4-GHz counts for strong sources, but start to become a significant fraction of the radio source population at flux densities 0.5‐0.8 mJy. They are expected to be responsible for more than 60 per cent of the observed radio counts at 0.05 mJy. These results are in agreement with the existing results on optical identifications of faint radio

On the possibility of observing H2 emission from primordial molecular cloud kernels

We study the prospects for observing H$_2$ emission during the assembly of primordial molecular cloud kernels. The primordial molecular cloud cores, which resemble those at the present epoch, can emerge around $1+z=20$ according to recent numerical simulations. The kernels form inside the cores, and the first stars will appear inside the kernels. A kernel typically contracts to form one of the first generation stars with an accretion rate that is as large as $\sim 0.01 M_\odot$ year$^{-1}$. This occurs due to the primordial abundances that result in a kernel temperature of order 1000K, and the collapsing kernel emits H$_2$ line radiation at a rate $\sim 10^{35}$ erg sec$^{-1}$. Principally $J=5-3$ (v=0) rotational emission of H$_2$ is expected. At redshift $1+z=20$, the expected flux is $\sim 0.01~\mu$Jy for a single kernel. While an individual object is not observable by any facilities available in the near future, the expected assembly of primordial star clusters on sub-galactic scales can result in fluxes at the sub-mJy level. This is marginally observable with ASTRO-F. We also examine the rotational $J=2-0$ (v=0) and vibrational $\delta v = 1$ emission lines. The former may possibly be detectable with ALMA.

The Phoenix Survey: optical and near-infrared observations of faint radio sources

Using a deep Australia Telescope Compact Array (ATCA) radio survey covering an area of ≈3 deg2 to a 4σ sensitivity of ≥ 100 μJy at 1.4 GHz, we study the nature of faint radio galaxies. About 50 per cent of the detected radio sources are identified with an optical counterpart revealed by CCD photometry to mR=22.5 mag. Near-infrared (K-band) data are also available for a selected sample of the radio sources, while spectroscopic observations have been carried out for about 40 per cent of the optically identified sample. These provide redshifts and information on the stellar content. Emission-line ratios imply that most of the emission-line sources are star-forming galaxies, with a small contribution (≈ 10 per cent) from Sy1/Sy2 type objects. We also find a significant number of absorption-line systems, likely to be ellipticals. These dominate at high flux densities (>1 mJy) but are also found at sub-mJy levels. Using the Balmer decrement we find a visual extinction AV=1.0 for the star-forming faint radio sources. This moderate reddening is consistent with the V-R and R-K colours of the optically identified sources. For emission-line galaxies, there is a correlation between the radio power and the Hα luminosity, in agreement with the result of Benn et al. This suggests that the radio emission of starburst radio galaxies is a good indicator of star formation activity.

Optical identifications and spectroscopy of a faint radio source sample: the nature of the sub-mJy population

Deep imaging and spectroscopy have been carried out for optical counterparts of a sample of 68 faint radio sources (S > 0.2 mJy) in the ``Marano Field''. About 60% of the sources have been optically identified on deep CCD exposures (limit R ~ 24.0) or ESO 3.6-m plates (limit bJ ~ 22.5). Thirty-four spectra (50% of the total radio sample) were obtained with the ESO 3.6-m telescope and 30 redshifts were determined. In addition to a few broad line active galactic nuclei, three main spectroscopic classes have been found to dominate the faint radio galaxy population: (1) Early-type galaxies with 0.1 0.8) radio galaxies with moderate-to-strong [OII] emission. They have spectra, colours and absolute magnitudes similar to those of the classical bright elliptical radio galaxies found in surveys carried out at higher radio fluxes. Star-forming galaxies do not constitute the main population of our radio sources identified with galaxies. In fact, even at sub-mJy level the majority of our radio sources are identified with early-type galaxies. This apparent discrepancy with previous results is due to the fainter magnitude limit reached in our spectroscopic identifications. Moreover, using mainly the large radio-to-optical ratio and the information from the available limits on the optical magnitudes of the unidentified radio sources, we conclude that the great majority of them are likely to be early-type galaxies, at z > 1. If correct, it would suggest that the evolution of the radio luminosity function of spiral galaxies, including starbursts, might not be as strong as suggested in previous evolutionary models.

Radio Polarisation Studies of Galaxies at z &gt; 2

We present some interesting results from a radio study of 17 radio galaxies at z &gt;2, selected from a complete sample of Molonglo sources with S408 &gt; 0.95 Jy for which optical identifications and spectroscopic redshifts are being obtained in a major observational programme (Kapahi et al. 1995).Observations and results : The 17 galaxies were mapped using the VLA at 1.4, 4.8 and 8.3 GHz with the highest resolution of ≃ 0.3″ at 8.3 GHz. The integrated spectra are generally quite steep and convex in many sources, steepening to α ≥ 1.2 (S ∝ v–α) at the highest frequency. Most of the sources have a double structure with sizes ranging up to 22″. Unresolved radio cores, coincident with the optical galaxies, are detected at the mJy or sub-mJy level in most cases. Several sources also show one-sided jetlike features. Surprisingly, most of the cores appear to have steep spectra between 5 and 8.3 GHz!