Wendy Sun

The images show examples of Little Red Dots observed by JWST, highlighting their compact structure and distinctive red colours in the early Universe. Image Credit: © NASA/CSA/ESA,  P. A. Oesch & M. Xiao (University of Geneva), J. Matthee (ISTA), G. Brammer (Niels Bohr Institute)

This week, we are hosting ISSI International Team 659, “Little Red Dots, Big Open Questions”, led by Mengyuan Xiao & Rohan Naidu @rpnaidu.bsky.social.

The team brings together observers and theorists to study one of the James Webb Space Telescope’s most intriguing discoveries: Little Red Dots. 🔴🔭

Thank u so much!! 😁

This project wouldn’t have been possible without this incredible team, featuring @rpnaidu.bsky.social @jorryt.bsky.social, and Anna de Graaff! 🙌

We also infer a BH* duty cycle of ~1% and a BH* lifetime of ~10 Myrs - they are a short-lived, and yet ubiquitous phenomenon. They may be a key phase in the origin story of possibly every massive BH!

V-shaped LRD selections are preferentially sensitive to high BH*/LRD fractions. So the population of BH*s may be more widespread than what we currently know. Hidden BH*s in e.g. broad-line AGN??

With BH*-dominated LRDs on one extreme ⬅️, the other extreme has host-dominated LRDs ➡️, which appear to be some of the most spectacular starbursts at high redshift.

BH*/LRD fractions correlate with easily accessible observables, LRD [OIII] EW and Balmer break strength. This may help accelerate the search for BH*-dominated objects.

Who dominates the LRD light? In the UV, the BH* contribution is modest relative to the host. But then there is an abrupt transition around (but not exactly at) the Balmer break, redwards of which the BH* begins to dominate the light.

LRD hosts lie above the star-forming main sequence, and display rising star-formation histories. Recent starbursts may play a role in the formation of BH*s and/or their entrapment in gas!

What about the LRD hosts? They show far stronger emission lines than a control sample, implying highly ionizing, young stellar populations.

All the features (in more detail in the paper) lead us to conclude that BH*s are indeed the central engines of LRDs!

When we look at prominent LRDs like The Cliff and MoM-BH*-1 (~“pure” BH*), they bear remarkable similarities to the (LRD - host) stack. And A2744-45924 shows several emission features (e.g., Fe II-UV) that are also apparent in the stack.

A single blackbody with Teff ~4000 K fits the rest-optical well. The inferred radius ~1300 au is 2 dex larger than the largest known stars, but comparable to local broad-line regions.

The stack has a strong Balmer break ~6.50 that far exceeds the strongest breaks seen in quiescent galaxies, as well as EW(Hα) ~850Å and Balmer decrement ~16 that are far more extreme than local AGN!

The (LRD - host) central engine stack exhibits a constellation of features that are the hallmark signatures of BH*s!

We present a novel, empirical approach to isolate the central engines of LRDs, by assuming that the [OIII] luminosity arises purely from the host galaxy. By subtracting the host, we uncover what’s remaining. LRD - Host Galaxy = ??

Paper day!! Little Red Dots (LRDs) are seen everywhere by JWST, but even after 3 years of relentless effort we are still debating what these things actually are. We decompose LRDs to show LRD - Host Galaxy = Black Hole Star (BH*)! 🧵
arxiv.org/abs/2601.20929
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I'm an undergrad student at MIT, working in astronomy

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My first bluesky post 🙌
I'm a final-year undergrad at MIT, studying Physics & AI. I'm interested in astronomy, and currently working on little red dots!