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Register a new userSubarcsecond LOFAR imaging of Arp299 at 150 MHz. Tracing the nuclear and diffuse extended emission of a bright LIRG
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Na\\'im Ram\\'irez-Olivencia
Publication date
2021
fields
Physics
and research's language is
English
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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.
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The nuclear starburst in the nearby galaxy M82 provides an excellent laboratory for understanding the physics of star formation. This galaxy has been extensively observed in the past, revealing tens of radio-bright compact objects embedded in a diffuse free-free absorbing medium. Our understanding of the structure and physics of this medium in M82 can be greatly improved by high-resolution images at low frequencies where the effects of free-free absorption are most prominent. The aims of this study are, firstly, to demonstrate imaging using international baselines of the Low Frequency Array (LOFAR), and secondly, to constrain low-frequency spectra of compact and diffuse emission in the central starburst region of M82 via high-resolution radio imaging at low frequencies. The international LOFAR telescope was used to observe M82 at 110-126MHz and 146-162MHz. Images were obtained using standard techniques from very long baseline interferometry. images were obtained at each frequency range: one only using international baselines, and one only using the longest Dutch (remote) baselines. The 154MHz image obtained using international baselines is a new imaging record in terms of combined image resolution (0.3$$) and sensitivity ($sigma$=0.15mJy/beam) at low frequencies ($<327$MHz). We detected 16 objects at 154MHz, six of these also at 118MHz. Four weaker but resolved features are also found: a linear (50pc) filament and three other resolved objects, of which two show a clear shell structure. We do not detect any emission from either supernova 2008iz or from the radio transient source 43.78+59.3. The images obtained using remote baselines show diffuse emission, associated with the outflow in M82, with reduced brightness in the region of the edge-on star-forming disk.
We use EDGES measurements to determine scale and zero-level corrections to the diffuse radio surveys by Guzman et al. at $45$ MHz and Landecker & Wielebinski at $150$ MHz. We find that the Guzman et al. map requires a scale correction of $1.076 pm 0.034$ ($2sigma$) and a zero-level correction of $-160 pm 78$ K ($2sigma$) to best-fit the EDGES data. For the Landecker & Wielebinski map, the scale correction is $1.112 pm 0.023$ ($2sigma$) and the zero-level correction is $0.7 pm 6.0$ K ($2sigma$). The correction uncertainties are dominated by systematic effects, of which the most significant are uncertainty in the calibration of the EDGES receivers, antenna pointing, and tropospheric and ionospheric effects. We propagate the correction uncertainties to estimate the uncertainties in the corrected maps themselves and find that the $2sigma$ uncertainty in the map brightness temperature is in the range $3.2-7.5%$ for the Guzman et al. map and $2.1-9.0%$ for the Landecker & Wielebinski map, with the largest percent uncertainties occurring at high Galactic latitudes. The corrected maps could be used to improve existing diffuse low-frequency radio sky models, which are essential tools in analyses of cosmological $21$ cm observations, as well as to investigate the existence of a radio monopole excess above the cosmic microwave background and known Galactic and extragalactic contributions.
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.
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.
We have conducted a deep survey (with a central rms of $55mutextrm{Jy}$) with the LOw Frequency ARray (LOFAR) at 120-168 MHz of the Bootes field, with an angular resolution of $3.98^{}times6.45^{}$, and obtained a sample of 10091 radio sources ($5sigma$ limit) over an area of $20:textrm{deg}^{2}$. The astrometry and flux scale accuracy of our source catalog is investigated. The resolution bias, incompleteness and other systematic effects that could affect our source counts are discussed and accounted for. The derived 150 MHz source counts present a flattening below sub-mJy flux densities, that is in agreement with previous results from high- and low- frequency surveys. This flattening has been argued to be due to an increasing contribution of star-forming galaxies and faint active galactic nuclei. Additionally, we use our observations to evaluate the contribution of cosmic variance to the scatter in source counts measurements. The latter is achieved by dividing our Bootes mosaic into 10 non-overlapping circular sectors, each one with an approximate area of $2:textrm{deg}^{2}.$ The counts in each sector are computed in the same way as done for the entire mosaic. By comparing the induced scatter with that of counts obtained from depth observations scaled to 150MHz, we find that the $1sigma$ scatter due to cosmic variance is larger than the Poissonian errors of the source counts, and it may explain the dispersion from previously reported depth source counts at flux densities $S<1,textrm{mJy}$. This work demonstrates the feasibility of achieving deep radio imaging at low-frequencies with LOFAR.
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