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Register a new userIdentifying radio active galactic nuclei among radio-emitting galaxies
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Added by
Dorota Kozie{\\l}-Wierzbowska
Publication date
2020
fields
Physics
and research's language is
English
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Basing our analysis on ROGUE I, a catalog of over 32,000 radio sources associated with optical galaxies, we provide two diagnostics to select the galaxies where the radio emission is due to an active galactic nucleus (AGN). Each of these diagnostics can be applied independently. The first one, dubbed MIRAD, compares the flux $F_{W3}$ in the $W3$ mid-infrared band of the WISE telescope, with the radio flux at 1.4 GHz, $Frad$. MIRAD requires no optical spectra. The second diagnostic, dubbed DLM, relates the 4000 AA break strength, $D_{rm n}(4000)$, with the radio luminosity per unit stellar mass. The DLM diagram has already been used in the past, but not as standalone. For these two diagrams, we propose simple, empirical dividing lines that result in the same classification for the objects in common. These lines correctly classify as radio-AGN 99.5 percent of the extended radio sources in the ROGUE~I catalog, and as star-forming (SF) galaxies 98--99 percent of the galaxies identified as such by their emission line ratios. Both diagrams clearly show that radio AGNs are preferentially found among elliptical galaxies and among galaxies hosting the most massive black holes. Most of the radio sources classified as radio-AGNs in the MIRAD or DLM diagrams are either optically weak AGNs or retired galaxies.
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The X-ray source CXO J133815.6+043255 has counterparts in the UV, optical, and radio bands. Based on the multi-band investigations, it has been recently proposed by Kim et al. (2015) as a rarely-seen off-nucleus ultraluminous X-ray (ULX) source with a black hole mass of >= 10^4 solar mass in the nearby Seyfert galaxy NGC 5252. To explore its radio properties at very high angular resolution, we performed very long-baseline interferometry (VLBI) observations with the European VLBI Network (EVN) at 1.7 GHz. We find that the radio counterpart is remarkably compact among the known ULXs. It does not show a resolved structure with a resolution of a few milliarcsecond (mas), and the total recovered flux density is comparable to that measured in earlier sub-arcsecond-resolution images. The compact radio structure, the relatively flat spectrum, and the high radio luminosity are consistent with a weakly accreting supermassive black hole in a low-luminosity active galactic nucleus. The nucleus of NGC 5252 itself has similar radio properties. We argue that the system represents a relatively rare pair of active galactic nuclei, where both components emit in the radio.
For nearly seven decades astronomers have been studying active galaxies, that is to say galaxies with actively accreting central supermassive black holes, AGN. A small fraction of these are characterized by luminous, powerful radio emission: this class is known as radio-loud. A substantial fraction, the so-called radio-quiet AGN population, displays intermediate or weak radio emission. However, an appreciable fraction of strong X-rays emitting AGN are characterized by the absence of radio emission, down to an upper limit of about $10^{-7}$ times the luminosity of the most powerful radio-loud AGN. We wish to address the nature of these - seemingly radio-silent - X-ray-luminous AGN and their host galaxies: is there any radio emission, and if so, where does it originate? Focusing on the GOODS-N field, we examine the nature of these objects employing stacking techniques on ultra-deep radio data obtained with the JVLA. We combine these radio data with Spitzer far-infrared data. We establish the absence, or totally insignificant contribution of jet-driven radio-emission in roughly half of the otherwise normal population of X-ray luminous AGN, which appear to reside in normal star-forming galaxies. We conclude that AGN- or jet-driven radio emission is simply a mechanism that may be at work or may be dormant in galaxies with actively accreting black holes. The latter can be classified as radio-silent AGN.
Merger simulations predict that tidally induced gas inflows can trigger kpc-scale dual active galactic nuclei (dAGN) in heavily obscured environments. Previously with the Very Large Array, we have confirmed four dAGN with redshifts between $0.04 < z < 0.22$ and projected separations between 4.3 and 9.2 kpc in the SDSS Stripe 82 field. Here, we present $Chandra$ X-ray observations that spatially resolve these dAGN and compare their multi-wavelength properties to those of single AGN from the literature. We detect X-ray emission from six of the individual merger components and obtain upper limits for the remaining two. Combined with previous radio and optical observations, we find that our dAGN have properties similar to nearby low-luminosity AGN, and they agree well with the black hole fundamental plane relation. There are three AGN-dominated X-ray sources, whose X-ray hardness-ratio derived column densities show that two are unobscured and one is obscured. The low obscured fraction suggests these dAGN are no more obscured than single AGN, in contrast to the predictions from simulations. These three sources show an apparent X-ray deficit compared to their mid-infrared continuum and optical [OIII] line luminosities, suggesting higher levels of obscuration, in tension with the hardness-ratio derived column densities. Enhanced mid-infrared and [OIII] luminosities from star formation may explain this deficit. There is ambiguity in the level of obscuration for the remaining five components since their hardness ratios may be affected by non-nuclear X-ray emissions, or are undetected altogether. They require further observations to be fully characterized.
We investigate the astrophysics of radio-emitting star-forming galaxies and ac- tive galactic nuclei (AGNs), and elucidate their statistical properties in the radio band including luminosity functions, redshift distributions, and number counts at sub-mJy flux levels, that will be crucially probed by next-generation radio continuum surveys. Specifically, we exploit the model-independent approach by Mancuso et al. (2016a,b) to compute the star formation rate functions, the AGN duty cycles and the conditional probability of a star-forming galaxy to host an AGN with given bolometric luminosity. Coupling these ingredients with the radio emission properties associated to star formation and nuclear activity, we compute relevant statistics at different radio frequencies, and disentangle the relative con- tribution of star-forming galaxies and AGNs in different radio luminosity, radio flux, and redshift ranges. Finally, we highlight that radio-emitting star-forming galaxies and AGNs are expected to host supermassive black holes accreting with different Eddington ratio distributions, and to occupy different loci in the galaxy main sequence diagrams. These specific predictions are consistent with current datasets, but need to be tested with larger statistics via future radio data with multi-band coverage on wide areas, as it will become routinely achievable with the advent of the SKA and its precursors.
Context. It will soon become possible to directly link the most accurate radio reference frame with the Gaia optical reference frame using many common extragalactic objects. It is important to know the level of coincidence between the radio and optical positions of compact active galactic nuclei (AGN). Aims. Using the best catalogues available at present, we investigate how many AGN with significantly large optical-radio positional offsets exist as well as the possible causes of these offsets. Methods. We performed a case study by finding optical counterparts to the International Celestial Reference Frame (ICRF2) radio sources in the Sloan Digital Sky Survey (SDSS) Data Release 9 (DR9). The ICRF2 catalogue was used as a reference because the radio positions determined by Very Long Baseline Interferometry (VLBI) observations are about two orders of magnitude more accurate than the optical positions. Results. We find 1297 objects in common for ICRF2 and SDSS DR9. Statistical analysis of the optical-radio differences verifies that the SDSS DR9 positions are accurate to ~55 mas in both coordinates, with no systematic offset with respect to ICRF2. We find 51 sources (~4% of the sample) for which the positional offset exceeds 170 mas (~3{sigma}). Astrophysical explanations must exist for most of these outliers. There are 3 known strong gravitational lenses among them. Dual AGN or recoiling supermassive black holes may also be possible. Conclusions. The most accurate Gaia-VLBI reference frame link will require a careful selection of a common set of objects by eliminating the outliers. On the other hand, the significant optical-radio positional non-coincidences may offer a new tool for finding e.g. gravitational lenses or dual AGN candidates. Detailed follow-up radio interferometric and optical spectroscopic observations are encouraged to investigate the outlier sources found in this study.
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