EVN observations of the farthest and brightest ULIRGs in the local Universe: the case of IRAS 23365+3604

We present high-resolution, high-sensitivity radio images of the ultra-luminous infrared galaxy (ULIRG) IRAS 23365+3604. We performed contemporaneous observations at 1.7 and 5.0 GHz, in three epochs separated by one year from each other, with the European very long baseline interferometry Network (EVN). We also present complementary Multi-Element Radio Linked Interferometry Network (MERLIN) at 1.6 and 5.0 GHz, and archival Very Large Array (VLA) data, taken at 1.4 and 4.9 GHz. We find that the emission at ~5.0 GHz remains quite compact as seen at different resolutions, whereas at ~1.7 GHz, high resolution imaging reveals some extended structure. The nuclear region has an approximate linear size of 200 pc and shows the presence of two main emission components: i) one with a composite spectrum due to ongoing non-thermal activity (probably due to recently exploded supernovae and AGN activity), ii) another one with a steep spectrum, likely dominated by an old population of radio emitters, such as supernova remnants (SNRs). Radiative losses are important, so re-acceleration or replenishment of new electrons is necessary. We estimate a magnetic field strength of 18 \mu G at galactic, and 175 \mu G at nuclear scales, which are typical for galaxies in advanced mergers.Comment: 12 pages, 4 figures and 6 tables. Accepted for publication in MNRA

Star formation and AGN activity in a sample of local Luminous Infrared Galaxies through multi-wavelength characterization

Nuclear starbursts and AGN activity are the main heating processes in luminous infrared galaxies (LIRGs) and their relationship is fundamental to understand galaxy evolution. In this paper, we study the star-formation and AGN activity of a sample of 11 local LIRGs imaged with subarcsecond angular resolution at radio (8.4GHz) and near-infrared ($2.2\mu$m) wavelengths. This allows us to characterize the central kpc of these galaxies with a spatial resolution of $\simeq100$pc. In general, we find a good spatial correlation between the radio and the near-IR emission, although radio emission tends to be more concentrated in the nuclear regions. Additionally, we use an MCMC code to model their multi-wavelength spectral energy distribution (SED) using template libraries of starburst, AGN and spheroidal/cirrus models, determining the luminosity contribution of each component, and finding that all sources in our sample are starburst-dominated, except for NGC6926 with an AGN contribution of $\simeq64$\%. Our sources show high star formation rates ($40$ to $167M_\odot\mathrm{yr}^{-1}$), supernova rates (0.4 to $2.0\mathrm{SN}\mathrm{yr}^{-1}$), and similar starburst ages (13 to $29\mathrm{Myr}$), except for the young starburst (9Myr) in NGC6926. A comparison of our derived star-forming parameters with estimates obtained from different IR and radio tracers shows an overall consistency among the different star formation tracers. AGN tracers based on mid-IR, high-ionization line ratios also show an overall agreement with our SED model fit estimates for the AGN. Finally, we use our wide-band VLA observations to determine pixel-by-pixel radio spectral indices for all galaxies in our sample, finding a typical median value ($\alpha\simeq-0.8$) for synchrotron-powered LIRGs.Comment: Accepted for publication in MNRAS. 20 pages, 12 figure

The persistent shadow of the supermassive black hole of M87:I. Observations, calibration, imaging, and analysis

In April 2019, the Event Horizon Telescope (EHT) Collaboration reported the first-ever event-horizon-scale images of a black hole, resolving the central compact radio source in the giant elliptical galaxy M 87. These images reveal a ring with a southerly brightness distribution and a diameter of ∼42 μas, consistent with the predicted size and shape of a shadow produced by the gravitationally lensed emission around a supermassive black hole. These results were obtained as part of the April 2017 EHT observation campaign, using a global very long baseline interferometric radio array operating at a wavelength of 1.3 mm. Here, we present results based on the second EHT observing campaign, taking place in April 2018 with an improved array, wider frequency coverage, and increased bandwidth. In particular, the additional baselines provided by the Greenland telescope improved the coverage of the array. Multiyear EHT observations provide independent snapshots of the horizon-scale emission, allowing us to confirm the persistence, size, and shape of the black hole shadow, and constrain the intrinsic structural variability of the accretion flow. We have confirmed the presence of an asymmetric ring structure, brighter in the southwest, with a median diameter of 43.3−3.1+1.5 μas. The diameter of the 2018 ring is remarkably consistent with the diameter obtained from the previous 2017 observations. On the other hand, the position angle of the brightness asymmetry in 2018 is shifted by about 30° relative to 2017. The perennial persistence of the ring and its diameter robustly support the interpretation that the ring is formed by lensed emission surrounding a Kerr black hole with a mass ∼6.5 × 109 M⊙. The significant change in the ring brightness asymmetry implies a spin axis that is more consistent with the position angle of the large-scale jet

Unveiling the dominant gas heating mechanism in local LIRGs and ULIRGs

We show preliminary results from a sample of Luminous and Ultra-Luminous Infrared Galaxies (LIRGs and ULIRGs, respectively) in the local universe, obtained from observations using the Very Large Array (VLA), the Multi-Element Radio Link Interferometer Network (MERLIN), and the European VLBI Network (EVN). The main goal of our high-resolution, high-sensitivity radio observations is to unveil the dominant gas heating mechanism in the central regions of local (U)LIRGs. The main tracer of recent star-formation in (U)LIRGs is the explosion of core-collapse supernovae (CCSNe), which are the endproducts of the explosion of massive stars and yield bright radio events. Therefore, our observations will not only allow us to answer the question of the dominant heating mechanism in (U)LIRGs, but will yield also the CCSN rate and the star-formation rate (SFR) for the galaxies of the sample.Comment: 6 pages, 2 figures, uses PoS.cls. To appear in Proceedings of Science, Proc. of the 9th European VLBI Network Symposium on the Role of VLBI in the Golden Age for Radio Astronomy and EVN Users Meeting, Bologna, September 200

TXS 2116−-077: A Gamma-ray Emitting Relativistic Jet Hosted in a Galaxy Merger

What triggers collimated relativistic outflows or jets, from the centers of galaxies remains a fundamental question in astrophysics. The merging of two galaxies has been proposed to realize the conditions to successfully launch and drive such jets into the intergalactic medium. However, evidences for the operation of this mechanism are scarce. Here we report the first unambiguous detection of an ongoing merger of a narrow-line Seyfert 1 galaxy, TXS 2116$-$077, hosting a closely aligned, $\gamma$-ray emitting relativistic jet with a Seyfert 2 galaxy at a separation of $\sim$12 kpc, using the observations taken with 8.2 m Subaru telescope. Our subsequent followup observations with 10.4 m Gran Telescopio Canarias, 4.2 m William Herschel Telescope, and Chandra X-ray observatory have provided what is likely to be the first glimpse of the merging environment hosting a closely aligned relativistic jet. Our finding that the jet is considerably younger than the merger demonstrates that jet activity can be triggered by galaxy mergers and that $\gamma$-ray detected narrow-line Seyfert 1 galaxies represent the beginning phase of that activity. These results also highlight the crucial role of mergers in shaping the fate of galaxies in their cosmological evolution and are consistent with recent studies focused on the host galaxy imaging of this enigmatic class of active galactic nuclei.Comment: 13 pages, 7 figures, 1 table, published in the Astrophysical Journa