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Register a new userSub-arcsecond imaging of Arp,299-A at 150 MHz with LOFAR: Evidence for a starburst-driven outflow
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Na\\'im Ram\\'irez-Olivencia
Publication date
2018
fields
Physics
and research's language is
English
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We report on the first sub-arcsecond (0.44 $times$ 0.41 arcsec$rm ^2$) angular resolution image at 150 MHz of the A-nucleus in the Luminous Infrared Galaxy Arp$,$299, from International Low Frequency Array (LOFAR) Telescope observations. The most remarkable finding is that of an intriguing two-sided, filamentary structure emanating from A-nucleus, which we interpret as an outflow that extends up to at least 14 arcseconds from the A-nucleus in the N-S direction ($approx$ 5 kpc deprojected size) and accounts for almost 40% of the extended emission of the entire galaxy system. We also discuss HST/NICMOS [FeII] 1.64 $rm mu m$ and H$rm_2$ 2.12 $rm mu m$ images of Arp$,$299-A, which show similar features to those unveiled by our 150 MHz LOFAR observations, thus giving string morphological support for the outflow scenario. Finally, we discuss unpublished NaI D spectra that confirm the outflow nature of this structure. From energetic arguments, we rule out the low-luminosity active galactic nucleus in Arp$,$299-A as a driver for the outflow. On the contrary, the powerful, compact starburst in the central regions of Arp$,$299-A provides plenty of mechanical energy to sustain an outflow, and we conclude that the intense supernova (SN) activity in the nuclear region of Arp299-A is driving the observed outflow. We estimate that the starburst wind can support a mass-outflow rate in the range (11-63) $rm M_{odot} yr^{-1}$ at speeds of up to (370 - 890) $rm km , s^{-1}$, and is relatively young, with an estimated kinematic age of (3 - 7) Myr. Those results open an avenue to the use of low-frequency (150 MHz), sub-arcsecond imaging with LOFAR to detect outflows in the central regions of local luminous infrared galaxies.
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We analyse new observations with the International Low Frequency Array (LOFAR) telescope, and archival data from the Multi-Element Radio Linked Interferometer Network (MERLIN) and the Karl G. Jansky Very Large Array (VLA). We model the spatially resolved radio spectrum of Arp 220 from 150 MHz to 33 GHz. We present an image of Arp 220 at 150 MHz with resolution $0.65times0.35$, sensitivity 0.15 mJy beam$^{-1}$, and integrated flux density $394pm59$ mJy. More than 80% of the detected flux comes from extended ($6approx$2.2 kpc) steep spectrum ($alpha=-0.7$) emission, likely from star formation in the molecular disk surrounding the two nuclei. We find elongated features extending $0.3$ (110 pc) and $0.9$ (330 pc) from the eastern and western nucleus respectively, which we interpret as evidence for outflows. The extent of radio emission requires acceleration of cosmic rays far outside the nuclei. We find that a simple three component model can explain most of the observed radio spectrum of the galaxy. When accounting for absorption at 1.4 GHz, Arp 220 follows the FIR/radio correlation with $q=2.36$, and we estimate a star formation rate of 220 M$_odottext{yr}^{-1}$. We derive thermal fractions at 1 GHz of less than 1% for the nuclei, which indicates that a major part of the UV-photons are absorbed by dust. International LOFAR observations shows great promise to detect steep spectrum outflows and probe regions of thermal absorption. However, in LIRGs the emission detected at 150 MHz does not necessarily come from the main regions of star formation. This implies that high spatial resolution is crucial for accurate estimates of star formation rates for such galaxies at 150 MHz.
Active galactic nuclei (AGNs) show episodic activity, evident in galaxies that exhibit restarted radio jets. These restarted jets can interact with their environment, leaving signatures on the radio spectral energy distribution. Tracing these signatures requires resolved spectral index measurements over a broad frequency range including low frequencies. We present such a study for the radio galaxy 3C 293. Using the International LOFAR telescope (ILT) we probed spatial scales as fine as ~0.2 at 144 MHz, and to constrain the spectrum we combined these data with Multi-Element Radio Linked Interferometer Network (MERLIN) and Very Large Array (VLA) archival data. In the inner lobes (~2 kpc), we detect the presence of a spectral turnover that peaks at ~225 MHz and is most likely caused by free-free absorption from the rich surrounding medium. We confirm that these inner lobes are part of a jet-dominated young radio source (spectral age $lesssim$0.17 Myr), which is strongly interacting with the rich interstellar medium (ISM) of the host galaxy. The outer lobes (~100 kpc) have a spectral index of $alpha$~0.6-0.8 from 144-4850 MHz with a remarkably uniform spatial distribution and only mild spectral curvature ($Deltaalphalesssim$ 0.2). We propose that intermittent fuelling and jet flow disruptions are powering the mechanisms that keep the spectral index in the outer lobes from steepening and maintain the spatial uniformity of the spectral index. Overall, it appears that 3C 293 has gone through multiple (two to three) epochs of activity. This study adds 3C 293 to the new sub-group of restarted galaxies with short interruption time periods. This is the first time a spatially resolved study simultaneously studies a young source as well as the older outer lobes at such low frequencies. This illustrates the potential of the ILT to expand such studies to a larger sample of radio galaxies.
We study for the first time the low-frequency ($sim$150 MHz) radio brightness distribution of Arp~299 at subarcsecond resolution, tracing in both compact and extended emission regions the local spectral energy distribution (SED) in order to characterize the dominant emission and absorption processes. We analysed the spatially resolved emission of Arp 299 revealed by 150 MHz international baseline Low-Frequency Array (LOFAR) and 1.4, 5.0, and 8.4 GHz Very Large Array (VLA) observations. We present the first subarcsecond (0.4$sim$100~pc) image of the whole Arp~299 system at 150~MHz. The high surface brightness sensitivity of our LOFAR observations ($sim$100 $mu$Jy/beam) allowed us to detect all of the nuclear components detected at higher frequencies, as well as the extended steep-spectrum emission surrounding the nuclei. We obtained spatially resolved, two-point spectral index maps for the whole galaxy: the compact nuclei show relatively flat spectra, while the extended, diffuse component shows a steep spectrum. We fitted the radio SED of the nuclear regions using two different models: a continuous free-free medium model and a clumpy model. The continuous model can explain the SED of the nuclei assuming a population of relativistic electrons subjected to synchrotron, bremsstrahlung, and ionization losses. The clumpy model fits assuming relativistic electrons with negligible energy losses, and thermal fractions that are more typical of star-forming galaxies than those required for the continuous model. Our results confirm the usefulness of combining spatially resolved radio imaging at both MHz and GHz frequencies to characterize in detail the radio emission properties of LIRGs from the central 100 pc out to the kiloparsec galaxy-wide scales.
We report on simultaneous observations of the local starburst system Arp 299 with NuSTAR and Chandra, which provides the first resolved images of this galaxy up to energies of ~ 45 keV. Fitting the 3-40 keV spectrum reveals a column density of $N_{rm H}$ ~ 4 x10^{24} cm^{-2}, characteristic of a Compton-thick AGN, and a 10-30 keV luminosity of 1.2x 10^{43} ergs s^{-1}. The hard X-rays detected by NuSTAR above 10 keV are centered on the western nucleus, Arp 299-B, which previous X-ray observations have shown to be the primary source of neutral Fe-K emission. Other X-ray sources, including Arp 299-A, the eastern nucleus which is also thought to harbor an AGN, as well as X-ray binaries, contribute $lesssim 10%$ to the 10-20 keV emission from the Arp 299 system. The lack of significant emission above 10 keV other than that attributed to Arp 299-B suggests that: a) any AGN in Arp 299-A must be heavily obscured ($N_{rm H}$ > 10^{24} cm^{-2}) or have a much lower luminosity than Arp 299-B and b) the extranuclear X-ray binaries have spectra that cut-off above ~10 keV. Such soft spectra are characteristic of ultraluminous X-ray (ULX) sources observed to date by NuSTAR.
The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30006 observations of 24713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of $geq$ 200 km baselines is identified by the detection of compact flux density, for declinations north of 30 degrees and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2 arcsec, for sources observed in the survey. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2 arcsec and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the Murchison Widefield Array (MWA) shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.
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