CoRGI in MaNGA - II. NGC 5717: cold-gas accretion and disk growth in a BGG

summary

Video file (mp4)

The gist

The gist NGC 5717 hosts a prominent dynamically cold counterrotating stellar disk embedded in a dynamically hot elliptical component, and its properties support an in situ origin from externally

In short

The study analyzed NGC 5717, a massive galaxy in a group, which features a cold counter-rotating stellar disk inside a hot elliptical component. Using multi-wavelength data, researchers found the disk is young and metal-poor. The results support the idea that this disk grew from gas accreted externally from the cosmic web filament over several billion years.

Key concepts

Counter-rotation
This describes a stellar disk where stars rotate in opposite directions relative to each other. In NGC 5717, one component rotates in one direction while the other rotates in the opposite direction, suggesting they formed from different gas sources or accretion events.
Dynamically Cold Disk
This refers to a stellar disk component that has a low velocity dispersion (how much stars are moving randomly) and is kinematically cold, meaning its stars move in organized, coherent rotational patterns rather than being randomly dispersed.
In Situ Origin
This means the secondary stellar component (the counter-rotating disk) formed from gas that was brought into the galaxy externally (accreted), rather than forming inside the galaxy itself. The chemical evolution model supports this by tracing the metal-poor gas back to cosmic filaments.
Brightest Group Galaxy (BGG)
NGC 5717 is identified as one of the most massive galaxies known to reside in a group environment. This context is important because it tests whether external cold gas accretion can occur even in environments where hot gas might otherwise strip or heat incoming material.

Terminology used across episodes

This episode discusses

The paper

CoRGI in MaNGA - II. NGC 5717: cold-gas accretion and disk growth in a BGG · Read on arXiv

Damir Gasymov, Ivan Yu Katkov, Eva K Grebel, Anna Pasquali, Evgenii V Rubtsov, Katarina Kraljic, Igor V Chilingarian, Kirill A Grishin, Anna S Saburova

Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg · Sternberg Astronomical Institute, Lomonosov Moscow State University · New York University Abu Dhabi Abu Dhabi · Center for Astrophysics and Space Science New York University Abu Dhabi Center for Astrophysics and Space Science Harvard and Smithsonian Cambridge Massachusetts · Observatoire Astronomique de Strasbourg UMR 7550 CNRS Université de Strasbourg France · Center for Astrophysics Harvard and Smithsonian Cambridge Massachusetts Institute of Technology Paris Centre National de la Recherche Scientifique France

Large-scale stellar counter-rotation, in which two coplanar stellar components rotate in opposite directions, is usually interpreted as a signature of external gas accretion. Such systems are most often found in relatively sparse environments, where cold gas can be acquired and remain dynamically undisturbed long enough to settle into a disk. It is less clear whether this channel can operate in massive central galaxies, whose group or cluster environments can heat, strip, or perturb cold gas. Here we present NGC 5717, one of the most massive galaxies known to host a counter-rotating stellar disk and the first identified case in a brightest group galaxy. Using integral-field spectroscopy from SDSS-IV Mapping Nearby Galaxies at Apache Point Observatory (MaNGA), together with CFHT imaging, ancillary HI data, and SDSS/DESI spectroscopy of the surrounding group, we show that the galaxy contains a dynamically cold counter-rotating disk embedded in an old dynamically hot elliptical component. The disk is young (about 1.3 Gyr), metal-poor ([Fe/H] about-0.7 dex), extended (h about 2.4 kpc), and massive (M/M about 9.9); the associated ionized gas co-rotates with it, while the available HI data indicate no significant remaining reservoir of neutral atomic gas (M HI/M 9). These properties support an in situ origin of the secondary component from externally accreted gas. The chemical evolution model favors gas accretion from a cosmic filament, which is consistent with the surrounding cosmic-web geometry, and indicates highly efficient burst-like star formation with substantial metal-enriched outflows. NGC 5717 may therefore represent a rare example of disk growth in a massive central galaxy, enabled by external cold-gas accretion during an earlier phase of the group assembly history.

Transcript

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

Vera: We're looking at the paper: "CoRGI in MaNGA - II. NGC 5717".

Jocelyn: The gist NGC 5717 hosts a prominent dynamically cold counterrotating stellar disk embedded in a dynamically hot elliptical component,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So to wrap up this paper "CoRGI in MaNGA - II. NGC five thousand seven hundred seventeen: cold-gas accretion and disk growth in a BGG," the main conclusion is that NGC five thousand seven hundred seventeen hosts a prominent dynamically cold counter-rotating stellar disk embedded in an old, dynamically hot elliptical component > <ref:2610.20704#pg1,NGC 5717: cold-gas accretion and disk growth in a BGG>

Jocelyn: And the authors conclude that this system’s properties strongly support an in situ origin for that secondary component from externally accreted gas >

Subrahmanyan: Their preferred interpretation, based on the chemical evolution model and how cosmic web geometry works, is that the accreted material was supplied through prolonged accretion of metal-poor gas, possibly linked to a cosmic filament starting about two point three Gyr ago > <ref:2610.20704#pg1>

Vera: It’s important to remember that this requires several conditions to align: cold gas needs to be accreted onto a future central galaxy, it needs to survive and settle into a disk during a relatively short window in the group's assembly history, and it has to evolve without major heating or removal by things like AGN activity or the intra-group medium >

Jocelyn: The challenge is that the cold gas must not only be accreted but also survive long enough against heating or removal by those things throughout its history >

Subrahmanyan: So what we learn is that these conditions—external accretion, survival, and undisturbed evolution—have to line up for something like NGC five thousand seven hundred seventeen to exist > <ref:2610.20704#pg1>

Conclusion: ---: Conclusion ---

Vera: So we’ve been looking at NGC five thousand seven hundred seventeen this galaxy that hosts that counter-rotating disk and embedded in a hot elliptical component. The paper's title is "CoRGI in MaNGA - II," which points us right back to how these counter-rotating systems fit into the larger picture of galaxy evolution.

Jocelyn: And the authors, they’re using data from MaNGA, plus CFHT imaging and HI data from the surrounding group. They really put together a lot of pieces here to see what's going on in that environment.

Subrahmanyan: The big implication is that this isn't just about a weird feature in one galaxy; it’s about how massive central galaxies can grow when they get cold gas from outside, especially in crowded environments like groups.

Vera: It suggests that the history of a galaxy isn't always just internal mergers; sometimes, what we see as disk growth is actually driven by external accretion happening during a specific phase of group assembly.

Jocelyn: That means we need to look at how cold gas gets funneled into these massive systems and if they have enough time to keep it settled against the background noise of the group environment.

Subrahmanyan: The chemical evolution model they run is pretty telling, suggesting that this secondary component formed in a burst of star formation fueled by metal-poor gas coming in from a cosmic filament about two point three Gyr ago.

Vera: So, we’re looking at this as a rare window where external cold-gas accretion and internal disk settling happen simultaneously in a massive galaxy.

Jocelyn: It really makes you think about the conditions necessary for that survival, those delicate requirements for the gas to stay cool and settled against all the heating from things like an AGN or the intra-group medium.

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