A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XIII. High-z Radio Quasar Selection from HSC--VLASS over about1200 deg squared

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Next we'll be talking about the paper "A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XIII. High-z Radio Quasar Selection from HSC--VLASS over about1200 deg squared ".

Jocelyn: The paper was written by Youwen Kong, Kohei Ichikawa, Hisakazu Uchiyama, Yuxing Zhong, Xiaoyang Chen et al. from Institute of Astronomy, Graduate School of Science, The University of Tokyo and Frontier Research Institute for Interdisciplinary Sciences, Tohoku University and Astronomical Institute, Tohoku University and Department of Advanced Sciences, Faculty of Science and Engineering, Hosei University and National Astronomical Observatory of Japan and Department of Physics, School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University and Research Center for the Early Universe, Graduate School of Science, The University of Tokyo and ILANCE, CNRS – University of Tokyo International Research Laboratory and Research Center for Space and Cosmic Evolution, Ehime University and Department of Physics, Graduate School of Science, Nagoya University and Academia Sinica Institute of Astronomy and Astrophysics and National Taiwan University and Department of Physical Sciences, Ritsumeikan University and Astronomy Research Group, Institut Teknologi Bandung and Ecole Polytechnique.

Vera: Stay tuned as we take you through the paper and discuss its implications.

Title: Vera: We're starting today with a massive new paper that's been hitting the arXiv, titled "A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XIII. High-z Radio Quasar Selection from HSC--VLASS over about1200 deg two".

Jocelyn: That is quite a long title, Vera, but I see the WERGS project is moving into its thirteenth installment.

Vera: It really is, and the team behind it—led by Youwen Kong and Kohei Ichikawa—has been working on this for quite a while.

Jocelyn: I noticed they're covering about one thousand two hundred square degrees of the sky, which seems like a huge area to scan for something as rare as high-redshift quasars.

Vera: It’s an enormous footprint, especially when you consider they're combining the Subaru Hyper Suprime-Cam data with the VLASS radio survey.

Jocelyn: Does that mean they're looking for things that are visible in both optical and radio wavelengths?

Vera: Exactly, and that's how they can pinpoint these active galactic nuclei even when they're incredibly far away.

Subrahmanyan: This scale is exactly what we need because if you only look at tiny patches of the sky, you'll never find enough of these high-redshift objects to understand how the early universe actually functioned.

Jocelyn: So, Subrahmanyan, why is it so vital that we find these specific radio-detected quasars from that early epoch?

Subrahmanyan: We're trying to figure out how supermassive black holes grew so fast and how their energy output influenced the galaxies around them during the first few billion years.

Vera: And this paper seems to be setting up a much bigger search than anyone has managed before.

Jocelyn: It sounds like they're building a massive catalog to help us see that era more clearly.

Vera: They really are, and we should look at what they actually found in those one thousand two hundred degrees.

Summary: Vera: Moving into the actual findings, this team identified about four hundred high-redshift radio AGN candidates sitting out there at redshifts greater than four.

Jocelyn: That's a significant number of candidates for such a distant part of space.

Vera: It is, and most of them are quite faint in the optical bands, clustering around an i-band magnitude of twenty-four to twenty-six.

Jocelyn: If they're that faint, I'm guessing older surveys like SDSS just couldn't see them at all?

Vera: You hit the nail on the head; those objects are far too dim for shallower surveys to detect reliably.

Jocelyn: What about their radio signatures, though? Are they showing the usual steep spectra we expect from high-redshift sources?

Vera: That's actually one of the most surprising parts, because most of them show flat or only moderately steep radio spectra instead.

Subrahmanyan: This is a huge deal because it tells us that our previous understanding of these objects was biased toward a specific type.

Jocelyn: How does that bias affect the way we calculate things like their total energy output?

Subrahmanyan: Well, when they did the SED fitting, they found these AGNs cluster around bolometric luminosities of log L equal to forty-six or forty-seven.

Vera: And they also noticed a sharp decline in how many of these objects exist once you hit the i-dropout regime around redshift six.

Jocelyn: Does that drop-off suggest that these luminous radio AGNs were actually disappearing as we look further back toward the epoch of reionization?

Subrahmanyan: It's definitely a possibility, though we have to consider if it's a real physical decline or just our instruments struggling to see them.

Vera: That leads us perfectly into how they actually managed to find them without falling into that same trap.

Improvements: Jocelyn: I want to go back to what Vera said about the radio spectra not being as steep as we thought, because that implies the old way of finding these things was broken.

Vera: It wasn't necessarily broken, but it was definitely incomplete.

Jocelyn: Can you explain how they actually changed their selection process to fix this?

Vera: Traditionally, astronomers used "ultra-steep spectrum" selection, looking for sources with a spectral index less than negative one point three.

Jocelyn: So if a source had a flatter spectrum, it would just be ignored by the old searches?

Vera: Exactly, and this paper shows that most of the population they found doesn't even meet that steepness criteria.

Subrahmanyan: Instead of relying on the radio shape alone, they used this "dropout" technique with the Subaru optical data to find them based on their colors.

Jocelyn: So they're using the fact that certain light wavelengths get blocked by gas in the early universe to identify them?

Vera: Yes, they look for these g-, r-, or i-dropouts, which tells them the object is likely at a very high redshift.

Subrahmanyan: By combining that optical dropout method with the VLASS radio detections, they've created a much more complete census of the population.

Jocelyn: It sounds like this methodology allows us to catch all those "flat-spectrum" quasars that were previously invisible to us.

Vera: It really does, and it means we're finally seeing a more diverse group of early black holes.

Subrahmanyan: This shift in methodology is going to force theorists like me to rethink how we model the radio emission from these early active galaxies.

Jocelyn: We've covered a lot of ground, so let's wrap this up and see what the big picture looks like now.

Conclusion: Vera: We've reached the end of our look at "A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XIII. High-z Radio Quasar Selection from HSC--VLASS over about1200 deg two".

Jocelyn: This paper really changes the game for how we hunt for the most distant radio sources in the sky.

Vera: It provides a massive, clean catalog of hundreds of candidates that are perfect targets for future telescopes.

Jocelyn: I'm curious to see when we get the spectroscopic confirmation to prove exactly how far away these four hundred candidates really are.

Subrahmanyan: From my perspective, this is about correcting our cosmic census so we can finally understand the true growth rate of black holes in the early universe.

Vera: We'll definitely be watching for those follow-up studies to see if that decline at redshift six holds up.

Jocelyn: Thanks for joining us, everyone; we'll see you next time when we have another fascinating paper to dissect.

Subrahmanyan: It’s been a pleasure, looking forward to the next one.

Vera: Goodbye for now!