Probing cold gas and dust in quiescent galaxies across cosmic time

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

Vera: We're looking at the paper: "Probing cold gas and dust in quiescent galaxies across cosmic time".

Jocelyn: The gist: A next-generation facility (ALMA2040) capable of providing transformative access to the cold interstellar medium of quiescent galaxies across redshift is essential for probing molecular gas content,

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

Title and authors: Vera: So we're looking at this paper, "Probing cold gas and dust in quiescent galaxies across cosmic time." It’s about using a new facility called ALMA2040 to study what happens to galaxies when they stop forming stars.

Jocelyn: Exactly. The authors, Zanella, Belli, Alcalde Pampliega and the others are digging into the cold gas and dust in these quiescent galaxies at different times in cosmic history. It’s a pretty big undertaking because we still don't fully understand how galaxies shut down their star formation across all epochs.

Subrahmanyan: That’s right. The core challenge is explaining this anti-hierarchical assembly of stellar mass, where the biggest halos form last, yet the most massive local galaxies look like they grew fast and then stopped. This paper addresses that by looking at how much cold gas and dust is left in these systems when they are quiescent.

Vera: Right. The paper sets up this new ALMA2040 instrument to get a detailed view of the molecular gas content, dust content, and molecular outflows from these galaxies out to high redshift. It’s essential for figuring out when and how these galaxies shut down their star formation across cosmic time.

Jocelyn: And what they are trying to probe is really about the physical conditions of that cold gas—its spatial distribution and its temperature—to give us a better idea of the quenching process, which is still quite mysterious.

Subrahmanyan: The paper focuses on understanding the mechanisms that cause star formation to cease, whether it's due to starvation from gas inflow or maybe something else entirely. It maps out several possible quenching pathways in Figure one of their work <ref:2610.21513#pg2>.

Vera: They look at a bunch of different physical processes that could be at play: cosmological starvation, gas not cooling, virial shock heating, AGN feedback preventative or ejective, gravitational heating, stellar feedback preventative or ejective, and morphological quenching.

Jocelyn: That’s a lot of possibilities for how the galaxy gets shut down. The paper then breaks it down into three categories: what is happening to the gas inflow and outflow—where gas does not accrete, or where it doesn't cool—what causes cold gas not to form stars, and what happens when the cold gas itself is rapidly consumed.

Subrahmanyan: That categorization helps organize the physics. It moves from external factors like accretion starvation to internal ones like disk instabilities or positive AGN feedback causing gas cooling. It’s a structured way to think about the quenching process.

Title and authors: Vera: The paper then gets very specific about characterizing these molecular gas reservoirs, which is key for understanding what happens right before star formation stops. They are trying to see if there’s still a lot of cold molecular gas left, because some studies suggest that up to twenty percent of the molecular gas can persist in these quenched galaxies out to redshift z equals one point four.

Jocelyn: That persistence is a big hint for us. It means that even after star formation has completely stopped, there might still be a sizable fuel supply waiting around, which tells us something about how inefficient their star formation efficiency must be.

Subrahmanyan: I agree. If there's gas left over, it complicates the models because it means we need to account for why that gas isn't turning into stars. The paper points out that current samples are limited and biased toward massive systems or recently quenched ones, so this new data set is important for expanding our conclusions.

Vera: They also discuss alternative ways to trace this gas, like the CII emission line and CI, because CO lines can be tricky at higher redshifts where they get suppressed against the cosmic microwave background.

Jocelyn: The authors note that the main issue with CII is getting a reliable conversion factor for how much it actually traces molecular gas in these quenched galaxies, and it doesn't just trace molecular gas exclusively; it can also show atomic and ionized phases.

Subrahmanyan: That’s a necessary caution. The paper then brings in the CI line as an additional probe because the dust-to-gas ratio and alpha CO are expected to change after quenching, so using CI provides a cross-check on the total molecular reservoir budget.

Vera: Then they get into constraining dust content, which has been a big headache. Dust continuum emission has often been used as an indirect tracer for the molecular gas, but they question if that relationship holds true in quiescent systems because the two components might decouple after star formation ceases.

Jocelyn: They are trying to figure out how much dust content varies with things like stellar mass and quenching timescale, because understanding this variation is essential to distinguish between different physical quenching pathways.

Subrahmanyan: That decoupling idea is important for the big picture. If the dust isn't tracking the gas anymore, then relying on dust as a proxy for molecular gas gives us a systematic error that we need to account for when building our models of galaxy evolution.

Vera: To tackle this, ALMA2040 offers independent measurements of both dust continuum emission and molecular gas across cosmic time, down to dust masses Mdust equal to ten to the power seven point five solar masses and dust fractions fdust equal to ten to the power minus four in less than twenty hours per galaxy.

Title and authors: Jocelyn: That level of sensitivity means they can get a complete picture of how dust evolves after star formation shuts down, which is crucial for understanding the interstellar medium in these systems.

Subrahmanyan: And this resolution, around zero point five to zero point eight arcseconds achievable with an intermediate or compact configuration, should be enough for spatially integrated detections on galaxy scales. It’s about getting a complete view of the interstellar medium in galaxies closer to the primary episodes of quenching.

Vera: So they are looking at gas and dust detection, outflow characterization, and constraining how dust content varies with physical properties. That’s the main scientific goal driving this work on "Probing cold gas and dust in quiescent galaxies across cosmic time."

Jocelyn: It sounds like a big piece of the puzzle for us trying to connect the dots between galaxy growth and feedback mechanisms over time.

Subrahmanyan: Exactly. The paper sets up the observational foundation needed to test whether star formation shuts down because gas is expelled or because its physical properties prevent new stars from forming.

Vera: Before we wrap up, I just want to say that the increased sensitivity of ALMA2040 will allow for CO SLED measurements on substantially larger samples out to higher redshifts, which lets us directly assess the mechanisms keeping galaxies quenched.

Jocelyn: And it connects nicely with other work from JWST and the ELT because those facilities are mapping different parts of this gas cycle, like atomic outflows and warm dust emission.

Subrahmanyan: The synergy between ALMA2040 and those other facilities will be key to delivering that complete, multi-phase view of the gas cycle in massive quiescent galaxies.

Vera: This paper really lays out exactly what’s needed to build those models for galaxy formation and evolution. It’s a solid foundation for understanding how these systems transition from active star formation to quiescence.

Jocelyn: So we've seen the setup, the measurements they are aiming for, and the limitations of using some of their tracers. Now we have a much clearer idea of what needs to be observed next in this field.

Subrahmanyan: It’s about establishing a physical basis for galaxy formation models by characterizing the cold gas reservoirs and outflows that drive these quenching processes across cosmic time.

Vera: That’s the gist of "Probing cold gas and dust in quiescent galaxies across cosmic time." We'll keep an eye on how this new data comes in.

The paper's summary: Vera: So, this paper is basically saying that we need to see how much cold gas and dust is left in galaxies when they stop forming stars across all cosmic time to understand why they shut down so fast.

Jocelyn: Right, and it’s using this new ALMA2040 instrument to get a really detailed look at the molecular gas reservoirs, the dust content, and even those molecular outflows from these quiescent galaxies way out there at high redshift.

Subrahmanyan: It really tackles that "anti-hierarchical" stuff we talked about—how massive dark matter halos form last but their biggest galaxies look like they grew super fast and then stopped.

Vera: They are looking at the gas fractions, which they found can be up to twenty percent in some of these quenched galaxies out to redshift z equals one point four. That persistence is a big data point, showing that even after star formation stops, there's still a lot of fuel around.

Jocelyn: And it’s not just about the total gas; they are trying to figure out the physical conditions of that cold gas—like its temperature and density—to see if it’s what prevents new stars from forming or if something else is pushing that material out.

Subrahmanyan: The paper breaks down the quenching process into different pathways, looking at things like whether the gas gets starved from inflow, or if internal processes like AGN feedback prevent cloud formation.

Vera: They also tackle the dust issue head-on. They question whether using dust as a proxy for molecular gas still works in these quiescent systems because they suggest those two things might actually decouple after star formation ends.

Jocelyn: So, the implication is that we can’t just look at total gas and assume it means star formation will continue; we need to measure the dust and gas together to get a complete picture of what’s happening in the interstellar medium.

Subrahmanyan: And they are using alternative tracers like CII and CI because CO lines can be suppressed at high redshift, and they want to check their budget by cross-referencing those different probes.

Vera: The real power of this work is that it provides independent measurements of both the dust continuum emission and the molecular gas in statistically representative samples across cosmic time.

Jocelyn: That level of sensitivity means they can map out how the interstellar medium evolves after star formation shuts down, which is crucial for building better galaxy evolution models.

Subrahmanyan: This data will help us constrain those models by telling them exactly what the cold gas reservoirs look like right before and during the quenching phase.

Vera: So, this study gives us a much more detailed view of the physical drivers behind how galaxies transition into a quiescent state across cosmic history.

Jocelyn: And knowing that, we can start to better understand whether star formation is shut down because the gas is expelled or because its physical properties prevent new stars from forming.

Subrahmanyan: It sets up a strong need for follow-up observations to confirm these mechanisms, and it points us toward using these next-gen facilities to truly map out the entire cycle.

The paper's improvements: Tom: So, we're talking about how this paper suggests we can take what they found and push it further for future work on these quiescent galaxies.

Vera: They suggest that to really nail down the quenching mechanisms, we need to move beyond just looking at the total gas mass in a single snapshot.

Jocelyn: That makes sense. They are pointing toward combining this ALMA data with other observations from facilities like JWST and the ELT for a multi-phase view.

Subrahmanyan: It means we don't just get one piece of evidence on how these galaxies shut down; we get the whole picture of the gas cycle from different angles.

Vera: They suggest using those high-resolution stellar kinematic maps from JWST to see if the gas is actually being expelled or just heated up internally.

Jocelyn: That links nicely with what they're doing now with ALMA, which probes those molecular outflows down to masses around two to five times ten to the eighth solar masses.

Subrahmanyan: That outflow mass measurement is critical because it’s a direct way to see if the supermassive black hole feedback is actually supplying enough energy and momentum.

Vera: They are also suggesting that we need better dust modeling, specifically looking at how dust content changes depending on the stellar mass or the environment of these galaxies.

Jocelyn: That helps us figure out which quenching pathway—whether it’s a violent event or something more subtle—is dominant for a given galaxy.

Subrahmanyan: And they emphasize that we need to use those alternative tracers like CII and CI, not just CO, because those lines give us different information about the molecular gas reservoir after quenching.

Vera: The paper flags that the current samples are limited in size and stellar mass, so future work needs to be focused on building much larger, more statistically representative samples across a wider range of redshifts.

Jocelyn: That’s the practical part—we need more data points from different cosmic epochs to make these conclusions robust.

Subrahmanyan: From a cosmological standpoint, if we can constrain the gas physics this well, it gives us a physical basis for those galaxy formation models we've been struggling with.

Vera: So, the paper sets up a clear path: use ALMA2040 for the detailed gas and dust measurements, then combine that with JWST and ELT data to build a comprehensive view of how these galaxies transition out of star formation.

Jocelyn: It’s about using different tools to check each other's work and really see the full story of how galaxies end up being quiescent.

Subrahmanyan: This kind of integrated approach is what we need if we want to truly establish when, how, and why galaxies shut down their star formation across cosmic time.

Conclusion: Vera: So we're wrapping up on "Probing cold gas and dust in quiescent galaxies across cosmic time." Basically, this paper shows us how crucial it is to get a full picture of the cold gas and dust in these galaxies when they stop forming stars.

Jocelyn: It’s really about setting up the observational groundwork for understanding the entire process, from accretion to quenching.

Subrahmanyan: I think what this study means for structure formation models is that we can finally start to constrain the physical conditions of the interstellar medium right at cosmic noon and beyond.

Vera: The main implication is that we need these next-generation capabilities—like ALMA2040—to get independent measurements of dust and gas across cosmic time.

Jocelyn: And that sensitivity allows us to probe molecular outflows, which is essential for understanding the baryon cycle in these massive galaxies.

Subrahmanyan: If we can map those outflows, it gives us a direct measure of how much momentum is being driven by the supermassive black hole feedback.

Vera: The authors flag that their current samples are biased toward massive systems or those recently quenched, so future work needs to focus on building much larger, more complete datasets.

Jocelyn: That’s right; we need more data points from different redshifts to make the conclusions robust about how quenching works across all cosmic time.

Subrahmanyan: This paper provides a strong physical basis for those models, and it points us toward the kind of observations that will confirm or kill our current ideas about galaxy growth.

Vera: It’s a solid foundation for seeing how these systems transition from active star formation to a state of quiescence.

Jocelyn: And knowing that, we can start to better understand whether star formation is shut down because the gas is expelled or because its physical properties prevent new stars from forming.

Subrahmanyan: That distinction is exactly what’s needed for our theoretical work, and it pushes us toward needing observations that can resolve those physical differences.

Vera: So, in summary, "Probing cold gas and dust in quiescent galaxies across cosmic time" gives us the necessary measurements of gas reservoirs and outflows to map out the quenching process.

Jocelyn: It’s a big step forward for connecting our observations with the underlying physics of galaxy evolution.

Subrahmanyan: We can't wait to see what these next observations reveal about how dark matter halos assemble and how they regulate star formation in the early universe.