by Julius Donnert

Over the last few weeks, the LOFAR collaboration dropped a bunch (20+) new papers on ArXiv - we wanted the data, now we’ve got them. This was part of the initial data release of the LOFAR Two Meter Sky survey. It is worth taking a closer look at a few of these new observations related to diffuse cluster emission. The images are spectacular and unveil the complexity of extra-galactic radio emission - a field that once was a niche in astronomy.

Before you click your way through the gallery of observations, let’s appreciate that every single image was reduced from >17 TB of complex radio data by a PhD student in Leiden, Bologna or Hamburg. Radio astronomy has a steep learning curve, this is rocket science ...

So thank you to the students for the hard work and the frustration that it takes to gnaw on the data until the noise level is micro Jansky, the bright sources clean beautifully and the flux scale is accurate to a few percent. This his hard …

This is a fantastic new glimpse into the faint radio sky at 150 MHz. Click on the image to go to ADS and read the paper.

Feedback in A1319

Amanda Wilber from Hamburg observatory looked at three galaxy clusters with LOFAR, the Chandra X-ray satellite (contours) and optical data from the SDSS survey (background). In A1319 she found this spectacular example for the three radio galaxies polluting the galaxy cluster atmosphere with magnetic field and cosmic-rays. The engine generating these AGN jets is a large black hole in the center of the galaxy. She finds that A1314 consists of two smaller clusters currently merging, which causes the long tail of IC 711 !

Diffuse Emission in A520

Duy Hoang from Leiden Observatory looked at the “Train Wreck” cluster Abell 520. Here you can see Chandra X-ray data in colors and the LOFAR data as contours. This is a massive merging system with a fantastic radio halo. Several shocks are seen in the X-rays, but the corresponding radio properties (spectral index gradient) don’t match ! Duy finds some indication for steepening, but the emission mixes quickly with the radio halo.

Thermal and non-Thermal Emission in A1914

Soumyajit Mandal from Leiden Observatory combined images from LOFAR HBA (white contours) and Chandra (image) with a weak lensing model of the matter distribution (cyan contours) in A1914. The observation very nicely shows how non-thermal radio emission and matter distribution are only weakly connected in clusters. Especially relics occur at low densities, but also radio haloes show a much flatter radial brightness profile than the density. In contrast, thermal X-ray emission and matter distribution are well correlated. This is of course, because gas density and DM density are well connected through hydrostatic forces. This argument has very deep implications for the physics of cosmic-ray electrons in the ICM.

A Cluster Chain in Abell 781

Andrea Botteon from the IRA Bologna used LOFAR, GMRT and XMM-Newton to study a chain of galaxy clusters. Here Andrea focused on the emission of the central cluster. Read his twitter thread (here) for his take on A781. LOFAR is a fantastic survey instrument with a few degree field of view and sub-arcsec resolution. Efforts are underway to try to find the cosmic radio web observationally, which arranges clusters at the knots of filaments.

Digging Deep with LOFAR HBA

Federica Savini from Hamburg observatory studied a sample of non-merging galaxy clusters using LOFAR and Chandra to searching for faint emission. And she found some ! I have taken the liberty of combining some of her images here: A central radio galaxy in A2390 (top left), no diffuse emission in A478 (top right), a new mini halo and a new ultra steep spectrum halo in RXJ1720.1 (bottom left), and a new halo in RXCJ0142.0 (bottom right). These studies can be tedious, but they are extremely important to understand cosmic-rays and magnetic field in the ICM.

The Binary Merger Abell 2146

The final image comes from Duy Hoang’s study of the Abell 2146, a famous classic merging cluster. The image shows LOFAR contours on Chandra data. You can see the cluster is well studied and has a lot going on. At least one relic, possibly another one and a halo around the southern BCG (both marked with large circles). Two shocks are well detected in the X-rays. A bridge is visible at 144 MHz that has very steep spectrum.

Of course many other papers were part of the LOFAR splash, some of them very technical, other focussing on radio galaxies (some of them always active), quasars, AGN, blazars, recombination lines, famous fields, redshifts, optical counterparts, reionization and higher radio frequency follow-ups with the Indian GMRT - pick your favorite ! The instrument pipeline especially of the LBA (70MHz) is still under heavy development, so the data quality will become even better in the next years.

As a final remark, let’s remember that the first LOTSS data release covers only ~5 percent of the northern sky, so we will see some amazing science in the next years. Stay tuned !