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Except that LRDs are not short-lived transients and Much more luminous than those stars. The sheer luminosity still make it most likely that accretion onto a black hole is the engine powering the emission, but the mass scale is likely lower than line-widths suggest - new mass tracers are needed!

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A key implication of these results is that line-profiles (and line-widths specifically) are not set by the dynamics of the gas, but rather by radiative transfer effects. This is not something uncommon to happen and well understood in stellar phenomena as Type IIn supernova or luminous blue variables

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As illustrated in these sketches, our trends could suggest that differences in viewing angle (and a non-spherically symmetric geometry) and/or different evolutionary phases (linked to accretion and outflow cycles) drive the variety among observed LRDs, warranting such geometries in new models.

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To me the newest result is that the central velocity of the Balmer absorption correlates with the UV-optical color (Balmer break, HI column density). The reddest sources show redshifted absorption from (primarily) inflowing gas, whereas bluer sources have outflowing gas with lower column densities

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The exponential wings are increasingly more dominant in redder sources. This means that the photons that experience higher column of HI gas also experience more free electrons: gas is partially ionised, likely clumpy. Scattering in a clumpy medium explains stronger Hbeta absorption.

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The Balmer break strength depends on the column density of HI atoms. The HI column density also sets the resonant scattering opacity. From blue to red colors we indeed see line cores (subtracting the wing for visualisation purposes) showing increasingly strong scattering effects.

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From blue to red (i.e. with increasing Balmer break strength), sources have broader and more dominant wings, steeper Balmer decrements and stronger Balmer absorption. Seeing absorption is not "random". Important: blue sources still lack various standard AGN indicators (X-ray, IR, MgII, [NeV])

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This figure summarizes the results: the cores of the Balmer lines depend on UV to optical color, wings are exponential. Blue sources have core-dominated lines. Redder sources show P Cygni features with blue-shifted absorption and redshift emitted. The reddest sources have dominant central absorption

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Jorryt Matthee @jorryt.bsky.social · 06/02/2026

Paper Day! You may have heard about these "Little Red Dots" in JWST data. Here we use new high quality spectra to show that the their spectral appearance is primarily determined by the properties of surrounding gas. LRDs are hot engines covered by dense gas arxiv.org/abs/2603.17667

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One of the main conclusions so far is that a majority in the room thinks the powering engine in little red dots should be called quasars (and that the class of objects historically known as quasars could perhaps be called DABs; disk accreting black holes).

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Reposted by Jorryt Matthee

Wendy Sun @wendysun.bsky.social · 01/02/2026

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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Jorryt Matthee @jorryt.bsky.social · 28/01/2026

This was also announced by the ESO DG at an ESO workshop in Garching this week!

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Reposted by Jorryt Matthee

Emily Hunt @emily.space · 05/01/2026

Vienna has a lively astronomy department - consider applying to the below if you'd be interested in a four year (!!) position here: 🔭☄️

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Jorryt Matthee @jorryt.bsky.social · 04/12/2025

You know this is just called "Money", right? :D

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Jorryt Matthee @jorryt.bsky.social · 04/12/2025

Happy to share my introductory review article "JWST provides a new view of cosmic dawn: latest developments in studies of early galaxies" has now been published in Contemporary Physics! www.tandfonline.com/doi/full/10.... arxiv.org/abs/2511.04843

tandfonline.com

JWST provides a new view of cosmic dawn: latest developments in studies of early galaxies

Studies of the distant Universe are providing key insights into our understanding of the formation of galaxies. The advent of the James Webb Space Telescope (JWST) has significantly enhanced our ob...

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The FMR appears to be weaker at high-redshift, possibly because the gas flow response to variations in star formation rate are different than in the local Universe. However, this needs to be tested with larger samples of individual measurements rather than stacks -- ideally to lower masses as well!

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With toy models, we investigate how selection effects impact the observed MZR, and whether an intrinsically steep MZR may be masqueraded. The direction of this effect depends on whether the 3D correlation between stellar mass, star formation rate and metallicity known at z=0 evolves or not.

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However, our measurements (and thus the literature implied) do not match recent galaxy simulations, especially once we account for our known selection effects. This implies that the MZR is flatter than expected, suggestive of more rapid chemical enrichment in low-mass galaxies.

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The mass-metallicity relation of our sample confirms that early galaxies have a lower oxygen abundance than galaxies in the local Universe, with values and a mass-dependence in broad agreement with recent measurements in the literature based on other surveys and instruments.

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We use these stacks to measure the direct Te-based gas-phase oxygen abundance in various stacks of galaxies, and we demonstrate that previously calibrated strong line calibrations between the [OIII]/Hb ratio and metallicity are also applicable to our sample.

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By combining deep NIRCam grism data from the EIGER, ALT and COLA1 surveys, we assembled a clean oxygen emission-line selected sample of nearly 1000 galaxies in the early Universe at z~6. In stacked spectra, we detect feature as faint as [OIII]4364.

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Rapid, out of equilibrium metal enrichment indicated by a flat mass-metallicity relation at z~6 from NIRCam grism spectroscopy

Time to spotlight the first paper of my PhD student Gauri Kotiwale that appeared on arXiv last week: Rapid, out of equilibrium metal enrichment indicated by a flat mass-metallicity relation at z ∼ 6 from NIRCam grism spectroscopy arxiv.org/abs/2510.19959

arxiv.org

We aim to characterise the mass-metallicity relation (MZR) and the 3D correlation between stellar mass, metallicity and star-formation rate (SFR) known as the fundamental metallicity relation (FMR) fo...

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Each of these effects should be quantified with future observations, to improve our ability to use Halpha-based SFRs to map the variability of galaxy star formation histories as a more precise test of galaxy formation models.

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2) a significant population of quenched galaxies at low masses and/or 3) an under-estimate in the dust attenuation from SED modeling at the massive end.

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With boundary constraints from simulations ("the slope must be 1"), our data does prefer an increasing scatter towards low masses, which confirms the picture of bursty star formation in low mass galaxies.

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We show that the flat slope is partly due to the flux-limited nature of our survey. Yet, even correcting for this with a Bayesian model, our data prefers a flatter slope, with a scatter that we cannot constrain on our data alone.

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At face value, our data yields a relatively flat slope of the main sequence, which other work recently also reported, but which is at odds with virtually every galaxy simulation and with the relative constant low mass slope of the galaxy stellar mass function.

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Thanks to JWST grism spectroscopy behind a lensing cluster, we can now obtain large samples of uniformly selected galaxies at redshifts z~5, whose star formation rates we accurately estimate with the Halpha line, and characterize the main sequence across three orders of magnitude in mass.

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Early JWST results indicate that the first galaxies had "bursty" star formation histories, this impacts the observed UV luminosity function, leads to detection of "napping"/mini-quenched galaxies but also causes a larger scatter in the main sequence.

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Delayed paper day! In work lead by my postdoc Claudia Di Cesare as part of the ALT survey, we provide a detailed census of the so-called star formation main sequence in the early Universe. arxiv.org/pdf/2510.19044

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Puzzled by all the latest developments and observed features in LRDs? We tried to summarize the key features, our interpretation, alternatives and challenges in this table:

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What does this mean? This very luminous LRD likely resides in a very low mass galaxy. The BH mass is unlikely as high as the virial indicators suggest, because the broadening is not dynamical. What the BH mass is.. a wide range is still possible, and we should find new ways of figuring that out!

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Despite most of the light being dominated by the "LRD" (which we think is an accreting SMBH highly covered by dense gas), there are some indications of host galaxy light, in particular the extremely narrow [OIII] emission and some (similarly) narrow Hgamma emission that pops up

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Using Cloudy modeling, we can connect the various observed features: [FeII], scattering & collisionally dominated Balmer emission and the now well-known Balmer break of LRDs: a warm layer of dense gas (T~7000 K, ne~10^9 cm3) is responsible.

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The most surprising observed feature is a forest of [FeII] emission lines -- these lines are commonly observed in quasars but they tend to be much broader. This is further evidence that the dense gas is not moving as rapidly as the line-widths of the Balmer lines suggest.

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Together, this is a radically different explanation than is common, where line-profiles are explained as a composite of narrow emission from low density gas and broad emission from fast moving regions of gas, and the absorber would be a decoupled cloud of gas.

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The wings are exponential with an indistinguishable width, suggesting they emerge from electron scattering -- likely in the same layer of dense gas. Another exciting feature are the line-ratios: Ha/Hb~10. This is likely because of collisional effects in this dense gas.

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The P Cygni profile emerges because there is a layer of partially excited, very dense gas with a significant population of H atoms in the n=2 state. Resonant scattering in this layer (and complicated decay effects of Hb photons) cause the differences in the cores.

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With our new deep JWST NIRspec IFU Prism+G395H data, we can characterize the Balmer lines in unprecedented detail; We confirm the absorption seen in grism data, show it's also present in other transitions, and we argue the line-profiles are best explained by exponential wings with P Cygni cores.

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Object of interest: FRESCO-GN-9771, the most luminous LRD from my discovery paper (arxiv.org/abs/2306.05448). This object shows such luminous Halpha emission that I initially thought it must have been a very nearby object when I spotted it in the FRESCO data back in 2023.. Yet it's at z~5.5.

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