Has JWST Seen Everything? Hunting for "NIRCam-dark" Dusty Star-Forming Galaxies at z 6 - 9 with ALMA2040

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

Vera: We're looking at the paper: "Has JWST Seen Everything? Hunting for "NIRCam-dark" Dusty Star-Forming Galaxies at z 6 - 9 with ALMA2040".

Jocelyn: The gist: A blind 1.3-mm continuum survey with ALMA2040 can provide the first high-significance census of this elusive NIRCam-dark DSFG population across z ≃ 6 − 9.

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

Paper summary: Vera: So, this paper, "Has JWST Seen Everything? Hunting for "NIRCam-dark" Dusty Star-Forming Galaxies at z six - nine with ALMA2040," is looking at something specific that optical and near-infrared surveys often miss <ref:2610.20727#pg1,Hunting for "NIRCam-dark" Dusty Star-Forming Galaxies at z 6 - 9>.

Jocelyn: They’re talking about these "NIRCam-dark" dusty star-forming galaxies, or DSFGs, which are super faint even when you look with the James Webb Space Telescope's Near-Infrared Camera.

Subrahmanyan: The core idea is that ALMA, using a blind one point three-mm continuum survey with ALMA2040, might find these objects across redshifts from z equal to six to nine which are currently invisible to JWST/NIRCam alone.

Vera: The paper claims this "NIRCam-dark" population could be tracing a missing rapid growth phase that connects the very early, UV-luminous galaxies at z greater than ten with the more massive quiescent galaxies we see at redshifts greater than four.

Jocelyn: They suggest these aren't just random faint objects, but potentially the earliest stages in how massive galaxies evolve into what they look like later on.

Subrahmanyan: That connects the very high redshift starbursts to the systems that become massive and relatively quiescent by z greater than four, which is a key puzzle in structure formation theory.

Vera: The paper points out that so far, the small sample we have of NIRCam-dark galaxies isn't enough to figure out how many there are or what their role really is in this evolution.

Jocelyn: They show that if you use ALMA2040, they can get a high-significance census of these elusive DSFGs across that z six to nine range.

Subrahmanyan: To make that happen, they calculated the required sensitivity and survey speed, specifically needing a detection with a dust-obscured star formation rate of six times one solar mass per year at redshift eight.

Vera: And they show that at a like-for-like seven point five gigahertz normalization, ALMA2040 could reach that required one point three millimeter RMS depth of twenty one microjanskys per beam square about ninety times faster than the current ALMA instruments.

Jocelyn: That speed calculation is impressive, showing how much quicker this new survey would be compared to what we have now for finding these faint sources.

Subrahmanyan: With one hundred hours on source, ALMA2040 could cover an area of about zero point five square degrees and detect about sixteen of these NIRCam-dark DSFGs over the redshift range seven point five to nine.

Vera: They assume a fiducial number density of three times ten to the negative six cubic megaparsecs per cubic centimeter for that calculation.

Jocelyn: And they also mention using deep JWST coverage in independent fields would help reduce cosmic variance and allow for immediate characterization with NIRCam, MIRI, and spectroscopy.

Subrahmanyan: Plus, the survey's expected instantaneous bandwidth of greater than five hundred twelve gigahertz will detect the C II line across a wide redshift range which helps confirm those continuum discoveries with spectroscopic data.

Paper summary: Vera: So what this means for us is that we can finally start to statistically characterize this population instead of just looking at individual bright examples like AC-two thousand one hundred sixty-eight at z equals six point six three <ref:2610.20727#pg2>.

Jocelyn: They’ve highlighted some specific examples, like AC-two thousand one hundred sixty-eight and ND1, which are found through joint ALMA and JWST surveys <ref:2610.20727#pg2>.

Subrahmanyan: These sources show star formation rates of approximately eighty to two hundred fifty times one solar mass per year, giving us a good sense of the activity level in this phase.

Vera: The paper suggests that these NIRCam-dark starbursts might be representing that missing rapid growth phase between the earliest UV-luminous galaxies at z greater than ten and the massive quiescent systems established by redshift greater than four.

Jocelyn: That points toward a timeline where star formation ramps up incredibly fast before settling down into a more stable, massive galaxy structure.

Subrahmanyan: The authors suggest that these NIRCam-dark and potentially "JWST-dark" galaxies might be among the first systems where over ninety percent of ongoing star formation is hidden at rest frame ultraviolet wavelengths.

Vera: They propose that patchy dust-stellar geometries could leave a blue but intrinsically faint rest ultraviolet component, which is what makes them hard to see in NIRCam.

Jocelyn: The open questions they raise are about the true number density of these starbursts and how fast they evolve their stellar mass over time.

Subrahmanyan: They also ask if these systems are common progenitors of those early quiescent galaxies at redshift greater than four, which we can test by looking at things like abundance or size consistency.

Vera: The authors admit that getting a proper number density measurement is still very challenging given the small and heterogeneous sample collected so far.

Jocelyn: They conclude that deep, blank-field surveys with ALMA2040 are the necessary next step to get a statistical characterization of this emerging population.

Vera: So, to wrap up this paper on "Has JWST Seen Everything? Hunting for "NIRCam-dark" Dusty Star-Forming Galaxies at z six - nine with ALMA2040," the main point is that we need a statistical census of these objects <ref:2610.20727#pg1,Hunting for "NIRCam-dark" Dusty Star-Forming Galaxies at z 6 - 9>.

Jocelyn: They argue that the current methods are too small and scattered to know how common this population really is or what it does in cosmic history.

Subrahmanyan: The implication for structure formation theory is that these NIRCam-dark DSFGs might be the missing link showing how we transition from the very first star-forming galaxies into the massive, older systems observed later on.

Vera: They are connecting the earliest building blocks to the massive end of galaxy evolution through this hidden phase.

Jocelyn: The authors are pushing for that blind ALMA2040 survey as a way to get that statistical characterization they need and test those evolutionary links with spectroscopic follow-up.

Conclusion: Vera: So we're wrapping up this look at the paper "Has JWST Seen Everything? Hunting for 'NIRCam-dark' Dusty Star-Forming Galaxies at z six - nine with ALMA2040."

Jocelyn: Yeah, the authors are trying to find these super faint star-forming galaxies that JWST just can’t see well.

Subrahmanyan: They are focusing on a gap in our census between the very early universe and the more massive galaxies we see later.

Vera: Exactly, they’re suggesting these "NIRCam-dark" sources might be filling in a missing growth phase.

Jocelyn: It’s about connecting those first UV-luminous galaxies at z greater than ten to those huge quiescent galaxies by z greater than four.

Subrahmanyan: That implies there was a really rapid period of star formation hidden from us, which we haven't fully modeled yet.

Vera: The paper shows that with ALMA2040, they can get the first high-significance count for these across redshifts six to nine.

Jocelyn: It’s about using that one point three millimeter continuum survey to actually find them when optical and infrared look like they are missing them entirely.

Subrahmanyan: The methodology hinges on requiring a dust-obscured star formation rate of sixty times one solar mass per year at redshift eight for a good detection.

Vera: And the numbers show that ALMA2040 could survey about zero point five square degrees in one hundred hours and find about sixteen of these sources over that redshift range.

Jocelyn: That’s a lot of new candidates, but they also mention using deep JWST fields to make sure we aren't just seeing some local artifacts.

Subrahmanyan: The implication for structure formation is that these could be the common progenitors of those massive systems we observe later on if the statistics hold up.

Vera: They are asking about the true number density, which is still really hard to pin down with what we have now.

Jocelyn: If they can get that census right, it will tell us a lot about how common this hidden phase of galaxy growth actually is in cosmic history.