arXiv subjects

Marisa Brienza

Publications and source records attributed to Marisa Brienza.

26 records · Page 2Linked to original sources

arXiv2020

New constraints on the 1.4 GHz source number counts and luminosity functions in the Lockman Hole field

We present a study of the 1173 sources brighter than $S_{1.4\,\rm GHz}= 120\,μ$Jy detected over an area of $\simeq 1.4\,\hbox{deg}^{2}$ in the Lockman Hole field. Exploiting the multi-band information available in this field for $\sim$79% of the sample, sources have been classified into radio loud (RL) active galactic nuclei (AGNs), star forming galaxies (SFGs) and radio quiet (RQ) AGNs, using a variety of diagnostics available in the literature. Exploiting the observed tight anti-correlations between IRAC band 1 or band 2 and the source redshift we could assign a redshift to 177 sources missing a spectroscopic measurement or a reliable photometric estimate. A Monte Carlo approach was used to take into account the spread around the mean relation. The derived differential number counts and luminosity functions at several redshifts of each population show a good consistency with models and with earlier estimates made using data from different surveys and applying different approaches. Our results confirm that below $\sim300\,μ$Jy SFGs$+$RQ AGNs overtake RL AGNs that dominate at brighter flux densities. We also confirm earlier indications of a similar evolution of RQ AGNs and SFGs. Finally, we discuss the angular correlation function of our sources and highlight its sensitivity to the criteria used for the classification.

astro-ph.GA↗

arXiv2020

Unveiling the rarest morphologies of the LOFAR Two-metre Sky Survey radio source population with self-organised maps

The Low Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS) is a low-frequency radio continuum survey of the Northern sky at an unparalleled resolution and sensitivity. In order to fully exploit this huge dataset and those produced by the Square Kilometre Array in the next decade, automated methods in machine learning and data-mining will be increasingly essential both for morphological classifications and for identifying optical counterparts to the radio sources. Using self-organising maps (SOMs), a form of unsupervised machine learning, we created a dimensionality reduction of the radio morphologies for the $\sim$25k extended radio continuum sources in the LoTSS first data release, which is only $\sim$2 percent of the final LoTSS survey. We made use of \textsc{PINK}, a code which extends the SOM algorithm with rotation and flipping invariance, increasing its suitability and effectiveness for training on astronomical sources. After training, the SOMs can be used for a wide range of science exploitation and we present an illustration of their potential by finding an arbitrary number of morphologically rare sources in our training data (424 square degrees) and subsequently in an area of the sky ($\sim$5300 square degrees) outside the training data. Objects found in this way span a wide range of morphological and physical categories: extended jets of radio active galactic nuclei, diffuse cluster haloes and relics, and nearby spiral galaxies. Finally, to enable accessible, interactive, and intuitive data exploration, we showcase the LOFAR-PyBDSF Visualisation Tool, which allows users to explore the LoTSS dataset through the trained SOMs.

astro-ph.IM↗

arXiv2020

Spectro-polarimetric observations of the CIZA J2242.8+5301 northern radio relic: no evidence of high-frequency steepening

Observations of radio relics at very high frequency (>10 GHz) can help to understand how particles age and are (re-)accelerated in galaxy cluster outskirts and how magnetic fields are amplified in these environments. In this work, we present new single-dish 18.6 GHz Sardinia Radio Telescope and 14.25 GHz Effelsberg observations of the well known northern radio relic of CIZA J2242.8+5301. We detected the relic which shows a length of $\sim$1.8 Mpc and a flux density equal to $\rm S_{14.25\,GHz}=(9.5\pm3.9)\,mJy$ and $\rm S_{18.6\,GHz}=(7.67\pm0.90)\,mJy$ at 14.25 GHz and 18.6 GHz respectively. The resulting best-fit model of the relic spectrum from 145 MHz to 18.6 GHz is a power-law spectrum with spectral index $α=1.12\pm0.03$: no evidence of steepening has been found in the new data presented in this work. For the first time, polarisation properties have been derived at 18.6 GHz, revealing an averaged polarisation fraction of $\sim40\%$ and a magnetic field aligned with the 'filaments' or 'sheets' of the relic.

astro-ph.GA↗

arXiv2020

The duty cycle of radio galaxies revealed by LOFAR: remnant and restarted radio source populations in the Lockman Hole

Feedback from radio jets associated with Active Galactic Nuclei (AGN) plays a profound role in the evolution of galaxies. Kinetic power of these radio jets appears to show temporal variation, but the mechanism(s) responsible for this process are not yet clear. Recently, the LOw Frequency ARray (LOFAR) has uncovered large populations of active, remnant and restarted radio jet populations. By focusing on LOFAR data in the Lockman Hole, in this work we use the Radio AGN in Semi-Analytic Environments (RAiSE) dynamical model to present the first self-consistent modelling analysis of active, remnant and restarted radio source populations. Consistent with other recent work, our models predict that remnant radio lobes fade quickly. Any high (>10 percent) observed fraction of remnant and restarted sources therefore requires a dominant population of short-lived jets. We speculate that this could plausibly be provided by feedback-regulated accretion.

astro-ph.GA↗

arXiv2019

The low-frequency properties of FR0 radio galaxies

Using the Alternative Data Release of the TIFR GMRT Sky Survey (TGSS), we studied the low-frequency properties of FR0 radio galaxies, the large population of compact radio sources associated with red massive early-type galaxies revealed by surveys at 1.4 GHz. We considered TGSS observations from FR0CAT, a sample formed by 104 FR0s at z<0.05: all but one of them are covered by the TGSS, and 43 of them are detected above a 5 sigma limit of 17.5 mJy. No extended emission has been detected around the FR0s, corresponding to a luminosity limit of < 4 10^23 W/Hz over an area of 100 kpc x 100 kpc. All but eight FR0s have a flat or inverted spectral shape (alpha < 0.5) between 150 MHz and 1.4 GHz: this spectral behavior confirms the general paucity of optically thin extended emission within the TGSS beam, as is expected for their compact 1.4 GHz morphology. Data at 5 GHz were used to build their radio spectra, which are also generally flat at higher frequencies. By focusing on a sub-sample of FR0s with flux density > 50 mJy at 1.4 GHz, we found that ~75% of them have a convex spectrum, but with a smaller curvature than the more powerful gigahertz peaked-spectrum sources (GPS). The typical FR0s radio spectrum is better described by a gradual steepening toward high frequencies, rather than to a transition from an optically-thick to an optically-thin regime, possibly observed in only ~15% of the sample.

astro-ph.GA↗

arXiv2017

Studying the late evolution of a radio-loud AGN in a galaxy group with LOFAR

Feedback by radio-loud active galactic nuclei (AGN) in galaxy groups is not fully understood. Open questions include the duty cycle of the AGN, the spatial extent of the radio lobes, the effect they have on the intragroup medium, and the fate of the cosmic rays. We present the discovery of a 650 kpc-radio galaxy embedded in steep diffuse emission at $z = 0.18793 \pm 5 \times 10^{-5}$ located at the center of the galaxy group MaxBCG J199.31832+51.72503 using an observation from the LOFAR Two-meter Sky Survey (LoTSS) at the central frequency of 144 MHz. Subsequently, we performed a GMRT observation at the central frequency of 607 MHz to study the spectral properties of the source. The observations reveal a radio galaxy with a total radio power $P_{\rm tot, 1.4} \sim 2.1 \times 10^{24}$ W Hz$^{-1}$, exhibiting two asymmetrical jets and lobes. The derived spectral index map shows a steepening toward the inner regions and a steep-spectrum core region. We model the integrated radio spectrum, providing two possible interpretations: the radio source is evolved but still active or it is just at the end of its active phase. Finally, in the same field of view we have discovered Mpc-sized emission surrounding a close pair of AGN located at a redshift $z = 0.0587 \pm 2 \times 10^{-4}$ (SDSS J131544.56+521213.2 and SDSS J131543.99+521055.7) which could be a radio remnant source.

astro-ph.GA↗

arXiv2016

FR II radio galaxies at low frequencies I: morphology, magnetic field strength and energetics

Due to their steep spectra, low-frequency observations of FR II radio galaxies potentially provide key insights in to the morphology, energetics and spectrum of these powerful radio sources. However, limitations imposed by the previous generation of radio interferometers at metre wavelengths has meant that this region of parameter space remains largely unexplored. In this paper, the first in a series examining FR IIs at low frequencies, we use LOFAR observations between 50 and 160 MHz, along with complementary archival radio and X-ray data, to explore the properties of two FR II sources, 3C452 and 3C223. We find that the morphology of 3C452 is that of a standard FR II rather than of a double-double radio galaxy as had previously been suggested, with no remnant emission being observed beyond the active lobes. We find that the low-frequency integrated spectra of both sources are much steeper than expected based on traditional assumptions and, using synchrotron/inverse-Compton model fitting, show that the total energy content of the lobes is greater than previous estimates by a factor of around 5 for 3C452 and 2 for 3C223. We go on to discuss possible causes of these steeper than expected spectra and provide revised estimates of the internal pressures and magnetic field strengths for the intrinsically steep case. We find that the ratio between the equipartition magnetic field strengths and those derived through synchrotron/inverse-Compton model fitting remains consistent with previous findings and show that the observed departure from equipartition may in some cases provide a solution to the spectral versus dynamical age disparity.

astro-ph.GA↗

arXiv2016

Dying radio galaxies in the LOFAR Lockman Hole

After the jets have switched off, radio galaxies undergo a fading phase which is often named the dying phase. The luminosity evolution of the remnant plasma during this period is still poorly constrained because of the paucity of objects detected. Using the new 150-MHz deep LOFAR observations of the well-known extragalactic field the Lockman Hole, we performed a systematic search of dying radio sources aiming to provide good statistics on their detection and properties. To avoid selection biases towards any specific class of dying sources we used both morphological and spectral selection criteria. To do this we combined the LOFAR data with publicly available surveys at other frequencies as well as dedicated deep observations. Our preliminary results, show that the fraction of candidate dying radio sources is < 6-8% of the entire radio source population and is dominated by steep spectrum sources. By comparing these observational results with statistical modelling of the radio sky population we will be able to constrain the main mechanisms contributing to the dying radio galaxy luminosity evolution.

astro-ph.GA↗