Dusty Galaxies

 If there is one galaxy story from JWST that has caught the news, it’s all the big galaxies at high redshift, forcing a rethink on how galaxies form in the very early Universe. If there a second story from JWST, it is how many galaxies have lots and lots of dust in them apparently in the epochs right after. This was not completely a surprise, the Madau plot already had the option that the lead up to cosmic noon (z=1–2), a lot of the star-formation is hidden by dust. But with JWST, we are really getting a handle on how much of it is hidden. 

For example, dust is apparent in early galaxies. This is the point on D. Burgarella’s recent paper [astroph]. Or this recent paper by Tarasse [astroph]. And this week a paper came out on the hidden star-formation in galaxies at 2.5<z<3.5. Cheng et al [astroph]:

Unveiling the Dark Side of UV/Optical Bright Galaxies: Optically Thick Dust Absorption

This uses the CEERS observations of galaxies at this redshift to explore how many are dust-dominated and hiding star-formation. 

Quick primer: what does a certain amount of reddening look like in the JWST filters of CEERS at z=3? Note that there is an optically thick regime.

One can select objects that are candidate/not-a-candidate based on their SEDs and compare against the color-color diagram (the original way to select different galaxy populations) and the stellar-mass vs star-formation plot (with the *shudder* “main sequence”)

The candidate and not-a-candidate optically dark galaxies with rest-fram optical colors and the stellar mass and star-formation plot with the MS, the “star-forming galaxy main sequence”, a poor choice or term but we’re stuck with it now.

So how do the authors select candidates, by identifying galaxies with an excess in the longerst wavelength filter (F444W) compared to the SED. Below is an example SED with the MIRI contribution highlighted. 

The interesting point here is the F444W just short of that. If there is more flux than expected, that is an indication there is radiation being reprocessed by dust inside this galaxy.

An example SED of a z=2.5 galaxy in CEERS

One of the sanity checks is to see if we are not looking at a disk edge-on. Easy to select for opaque disks in that case.

Now the results show that there is a sizeable fraction of star-formation that is obscured, not just dimmed, by star-formation at z>2. This is something we see in the local Universe, some 40% of disks is not transparent. 

You can see that as well in the slope of the blue colors, beta. 

And just to remind myself which way beta goes again, here is the slopes for aging (therefore redder) populations from Qin+ 2022 [astroph]: negative means bluer. positive means redder. The obscured structures redden on average. This is the age/reddening degeneracy that plagues SED fitting. 

Two examples from Qin+ 2022. Either a burst of star-formation ages ago or constant star-formation over the last 500 million years. As populations age, their blue/ultraviolet slope \beta becomes redder (positive). 

Long story short: we see more and more obscured star-formation in galaxies; either in dust-obscured dwarfs galaxies, or part of galaxies (this paper), or even completely optically thick “dark galaxies” not detected with Hubble but now observed with JWST. The early Universe is a lot more dusty place than previously thought. Should be interesting to look at in the coming years.