Ji Hoon Kim

Say hi to @lellifede.bsky.social. 😉

A meeting nearby Firenze?

Merger fraction as a function of distance from the cluster center, separated by AGN presence. Red circles show galaxies hosting AGNs, and blue squares show non-AGN galaxies. While overall merger fractions are similar, AGN-host galaxies exhibit a mild increase in merger fraction toward the cluster center. Alt text: The x axes show the R/R200 from 0.0 to 1.0. The y axes show the merger fraction from 0.22 to 0.6.

We also present tentative evidence that mergers contribute to AGN triggering in cluster cores.

Anri put a great effort into this work!

This paper is accepted for publication in PASJ.

Merger fraction as a function of redshift, shown for cluster member galaxies (blue circle), galaxy pairs (orange square), and field galaxies (green triangle). All subsamples show a positive correlation with redshift.

We find that the merger fraction increases with redshift for all environments, showing a strong radial gradient in clusters. This suggests that galaxy merger activity is enhanced both at earlier cosmic times and in dense environments, highlighting the role of groups.

Distribution of galaxies in the Gini–M20 plane. The solid line (G = −0.14 M20 + 0.33) indicates the merger/non-merger boundary. The color scale represents shape_asymmetry (Ashape). Galaxies above the line and with high asymmetry tend to be classified as mergers under our criterion. Alt text: A scatter graph, the x axes shows the M20 from -3.0 to 0.0, the y axes shows the Gini coefficient from 0.3 to 0.9.

Using a new non-parametric classification scheme that combines Gini-M20 and shape asymmetry, we identify 12,666 mergers from a sample of 33,320 galaxies, validating our method against GALAXY CRUISE visual classifications.

It's paper day! 🧪🔭
Our latest paper investigates galaxy merger rates and their environmental dependence at z<0.2 using high-resolution Hyper Suprime-Cam (HSC) data.
ui.adsabs.harvard.edu/abs/2025arXi...

Non-equilibrium dynamics in galaxies that appear to lack dark matter: tidal dwarf galaxies There are a number of galaxies that have been reported to lack dark matter. This is weird in a universe made of dark matter. It is also weird in MOND, which (if true) is what causes the inference o…

Tidal dwarfs & non-equilibrium dynamics 🔭🧪
tritonstation.com/2025/08/20/n...

These bright AGNs fill key gaps in the SDSS and Quaia catalogs and will serve as prime targets for future multi-wavelength studies.

Congratulations, Yunyi!

Distribution of redshift and bolometric luminosity for AllBRICQS quasars in this work (Northern Hemisphere; red circles) and from Onken et al. (2023) (Southern Hemisphere; blue triangles), in comparison to the SDSS DR16 Quasar Catalog (SDSS DR16Q; Lyke et al. 2020), shown as grey contours. The AllBRICQS quasars occupy the bright end of the quasar luminosity distribution, exhibiting significantly higher luminosities than the bulk of the SDSS DR16Q sample. The grey contours correspond to a two-dimensional histogram constructed using 10,000 bins (100 bins per axis), effectively illustrating the density distribution of the SDSS quasars.

It's paper day! 🧪🔭

Our latest ApJS paper presents the second AllBRICQS catalog — 62 newly confirmed luminous quasars in the Northern Hemisphere (0.09 < z < 2.48), selected via WISE + Gaia DR3. Highlights include the most luminous FeLoBAL quasar ever found.

arxiv.org/pdf/2508.06028

What's more?

Post a song from the year you turned 12.

youtu.be/djV11Xbc914?...

I often disagree with him, but Stacy’s physics posts are always thoughtful and informative, and the observational truth of Milgrom’s law — which Stacy has been trying to get people to take seriously for decades — really does seem to say something fundamental about the Universe. 🧪

Here's a little side-by-side of a patch of our Virgo imaging (from Mihos+17) compared to today's Rubin release. I knew that Rubin field looked familiar! Nice to see they recover a lot of that low surface brightness structure. 🔭

All this time & Mike Turner still making promises he can’t keep. A quarter century ago it was “we will detect dark matter in five years.”

‘This is bull——”: National Science Foundation employees protest HUD's takeover Dozens of National Science Foundation employees crowded the agency’s headquarters in Alexandria, Virginia, Wednesday to protest the news that the Department of Housing and Urban Development, or HUD, w...

Another day testing the limits of resilience.

The Alexandria NSF building was designed for the NSF merit review process (gold standard), complete with a floor devoted to panels.

Now we aren’t allowed to have in person panels.

The loss of US science leadership will haunt this country for decades.

I got to wonder if this is the end of an era for SExtractor. So, be ready for Tractor?

Ever-changing Universe Revealed in First Imagery From NSF–DOE Vera C. Rubin Observatory From distant stars and galaxies to asteroids whizzing through the Solar System, this next-generation facility unveils its first imagery and brings the night sky to life like never before.

Holy smokes.🔭🧪
noirlab.edu/public/news/...

Taeho Ryu is providing a clue on the formation of blue straggler stars via magnetic field amplification. 🔭🧪

Eun-jin Shin is giving a talk on IMBH formation in dwarf galaxies during IAUS398/MODEST-25. 🔭🧪

Michela Mapelli is starting the very first scientific talk for IAU Symposium 398/MODEST-25 “Compact Stars and Binaries in Dense Stellar Systems”. 🔭🧪

Exit polls show that the outcome of this election will be what has been obvious. Or, rather what should have been obvious. What’s not obvious is how quickly the country will overcome the harms done by the previous president.

After we impeached the president second time in 9 years, we are voting for the 21st president. While the entire globe is slowly turning conservative, we may have someone central. I mean not far right.

Based on my previous skeet five months ago, pot?

Crisis at NASA It's very bad news all round for NASA science, but the worst hit is Astrophysics (which includes cosmology) where the proposed cut is about two-thirds, which would be devastating.

Crisis at NASA

It's very bad news all round for NASA science, but the worst hit is Astrophysics (which includes cosmology) where the proposed cut is about two-thirds, which would be devastating.

Woong-seop Jeong is giving a talk on SPHEREx at Seoul National University. 🔭🧪

This is what we (South-) Koreans should be worried about.

Advertise your sky account in 3 words:

So it goes

Old galaxies in the early universe Continuing our discussion of galaxy formation and evolution in the age of JWST, we saw previously that there appears to be a population of galaxies that grew rapidly in the early universe, attainin…

New blog post - Old galaxies in the early universe
tritonstation.com/2025/01/14/o...

Sort of an audiobook for a journal article. 🔭🧪