A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XIII. High-Redshift Radio Quasar candidates beyond Ultra-Steep Spectrum Selection: Dropout selection from HSC--VLASS over $\sim$1200 deg$^2$
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| Natura: | Preprint |
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2026
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| author | Kong, Youwen Ichikawa, Kohei Uchiyama, Hisakazu Zhong, Yuxing Chen, Xiaoyang Kohno, Kotaro Nagao, Tohru Lee, Kianhong Vijarnwannaluk, Bovornpratch Matsuoka, Yoshiki Toba, Yoshiki Fitriana, Itsna Khoirul Obuchi, Sakiko Ishikawa, Yuta Kadri, Victor |
| author_facet | Kong, Youwen Ichikawa, Kohei Uchiyama, Hisakazu Zhong, Yuxing Chen, Xiaoyang Kohno, Kotaro Nagao, Tohru Lee, Kianhong Vijarnwannaluk, Bovornpratch Matsuoka, Yoshiki Toba, Yoshiki Fitriana, Itsna Khoirul Obuchi, Sakiko Ishikawa, Yuta Kadri, Victor |
| contents | We report the results of $g-$, $r-$, and $i-$dropout selections based on optical identifications of Very Large Array Sky Survey (VLASS) radio sources using the Hyper Suprime-Cam Subaru Strategic Program survey (HSC--SSP). By positional crossmatching within $1''.5$ between the VLASS Epoch~2 catalog and the HSC--SSP Wide-layer catalog ($i \lesssim 26$), we obtain $\sim$400 high-redshift radio AGN candidates at $z \gtrsim 4$ over a $\approx1200~\mathrm{deg}^2$ survey footprint, extending optimistically to $z \sim 7$. Optical magnitudes cluster at $i_\mathrm{AB} \simeq 24$--26, indicating that these sources are largely inaccessible to shallower surveys such as SDSS. By further cross-matching the HSC--VLASS dropout catalog with VLA Faint Images of the Radio Sky at Twenty-centimeters (FIRST) at 1.4~GHz, the LOFAR Two-metre Sky Survey (LoTSS) at 144~MHz, and the TIFR GMRT Sky Survey (TGSS) at 150~MHz, the majority of the high-$z$ candidates show flat to moderately steep radio spectra ($-1 \lesssim α\lesssim 0$, with $f_ν\propto ν^α$), and some also exhibit turnover radio spectra, demonstrating that conventional ultra-steep-spectrum (USS; $α<-1.3$) selection would miss most of the population selected in this study. Building on this, we perform SED fitting and obtain AGN luminosities, which show a clustering at typical bolometric luminosities of $\log(L_{\rm bol}/{\rm erg~s^{-1}})\sim46$--47. We also examine the comoving number density distribution of our samples and find a sharp decline around the $i$-dropout regime ($z \sim 6$), suggesting the possible disappearance of luminous radio AGNs toward the epoch of reionization. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_28044 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XIII. High-Redshift Radio Quasar candidates beyond Ultra-Steep Spectrum Selection: Dropout selection from HSC--VLASS over $\sim$1200 deg$^2$ Kong, Youwen Ichikawa, Kohei Uchiyama, Hisakazu Zhong, Yuxing Chen, Xiaoyang Kohno, Kotaro Nagao, Tohru Lee, Kianhong Vijarnwannaluk, Bovornpratch Matsuoka, Yoshiki Toba, Yoshiki Fitriana, Itsna Khoirul Obuchi, Sakiko Ishikawa, Yuta Kadri, Victor Astrophysics of Galaxies We report the results of $g-$, $r-$, and $i-$dropout selections based on optical identifications of Very Large Array Sky Survey (VLASS) radio sources using the Hyper Suprime-Cam Subaru Strategic Program survey (HSC--SSP). By positional crossmatching within $1''.5$ between the VLASS Epoch~2 catalog and the HSC--SSP Wide-layer catalog ($i \lesssim 26$), we obtain $\sim$400 high-redshift radio AGN candidates at $z \gtrsim 4$ over a $\approx1200~\mathrm{deg}^2$ survey footprint, extending optimistically to $z \sim 7$. Optical magnitudes cluster at $i_\mathrm{AB} \simeq 24$--26, indicating that these sources are largely inaccessible to shallower surveys such as SDSS. By further cross-matching the HSC--VLASS dropout catalog with VLA Faint Images of the Radio Sky at Twenty-centimeters (FIRST) at 1.4~GHz, the LOFAR Two-metre Sky Survey (LoTSS) at 144~MHz, and the TIFR GMRT Sky Survey (TGSS) at 150~MHz, the majority of the high-$z$ candidates show flat to moderately steep radio spectra ($-1 \lesssim α\lesssim 0$, with $f_ν\propto ν^α$), and some also exhibit turnover radio spectra, demonstrating that conventional ultra-steep-spectrum (USS; $α<-1.3$) selection would miss most of the population selected in this study. Building on this, we perform SED fitting and obtain AGN luminosities, which show a clustering at typical bolometric luminosities of $\log(L_{\rm bol}/{\rm erg~s^{-1}})\sim46$--47. We also examine the comoving number density distribution of our samples and find a sharp decline around the $i$-dropout regime ($z \sim 6$), suggesting the possible disappearance of luminous radio AGNs toward the epoch of reionization. |
| title | A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XIII. High-Redshift Radio Quasar candidates beyond Ultra-Steep Spectrum Selection: Dropout selection from HSC--VLASS over $\sim$1200 deg$^2$ |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2603.28044 |