Dust in Galaxies over Cosmic Times

This week’s blog post had to be about Trevor Butrum’s paper coming out on astro-ph!

A comparison of dust content and properties in GAMA/G10-COSMOS/3D-HST and SIMBA cosmological simulations

Trevor Butrum (University of Louisville), Benne Holwerda (University of Louisville), Romeel Dave (University of Edinburgh), Kyle Cook (University of Louisville), Clayton Robertson (University of Louisville), Jochen Liske(University of Hamburg)

https://arxiv.org/abs/2505.02359

This all started with an off-the wall idea I had following the work by Driver+ 2018. There the emphasis was on the volume densities of some of the main components of galaxies we can infer from their light; their stellar mass, their current star-formation and their dust content.

The neat trick for that paper was to sum the luminosity functions and get a volume density. This was a neat approach as it used a heterodyne mix of surveys, GAMA, G10/COSMOS and the 3D-HST survey for ever increasing distances. This creates some gaps in the coverage as you can see very nicely in Trevor’s first plot:

The complete data sets of GAMA, G10-COSMOS, 3D-HST with stellar mass

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But the mass range 7 to 11 is reasonably covered. The idea I had was to compare the dust masses inferred for galaxies by the SED fits from Driver+ 2018 with those predicted in The Simba simulations. These are good volume and reasonable physics SAMs that seem to be doing a good job on the dust household (including ejection).

The dust mass ranges selected for this study. We plot GAMA/G10-COSMOS/3D-HST data as black dots. We plot the selected data with the ranges applied in green dots with a box surrounding them. The red dotted line represents the dust mass volume limits of the surveys. Note that we exclude GAMA data at z=0.5 due to volume-related issues in the selections. Selection effects were a real challenge. 

So we picked four slices in the Driver+ data and compared them to The Simba catalog at the same redshift/epoch. Simple no?

Normalized counts of stellar mass from Simba and GAMA/G10-COSMOS/3D-HST. The observational dataset is separated into individual surveys to highlight the distinctions between them. Trevor figured out how to make these mass functions completely on his own! 

And that is what Trevor did! And it worked. Reasonably well. It is of course quite instructive as well to see where itdidn’t work as well. And that occupied quite a bit of Trevor’s time.

And there is some work for the next generations of galaxy evolutionary models. Simba misses dust-rich galaxies for all epochs above z=0 and Simba does not accurately model low-dust mass galaxies at earlier epochs.

The planetarium as a teaching space

 I love the Rauch Planetarium at the University of Louisvlle as the place to teach Astronomy in. That probably doesn’t sound controversial to you when an astronomer says so but apparently it is?

A space dedicated to showing the night sky and our Universe gets students already in an astronomy frame of mind. But it goes beyond that. So much beyond that.

Not only can we show how the night sky looks without the city lights or the ever multiplying artificial satellites thanks to Starlink and others mega-constellations. We can show how it looked to ice age hunter-gatherers or people in the Southern hemisphere. That links us all across the globe and along the age of the human race. It also allows me to link what you see in the night sky directly to physics. We have a program that shows the (now familiar) stars in the night sky which then move to their place on the color-luminosity diagram and the class learns the link between temperature and luminosity of stars around us. That kind of linking between sky and physics is what my class is all about. More planetarium resources like this are made available by federal projects like the Rubin Observatory and NASA.

Alternatively I can link stars and navigation like the student have seen in the Disney movie Moana. Or we can work on the different constellations across global cultures. Shared exploration like we do in the planetarium is what we aim for in higher education and the Planetarium supercharges that for my subject astronomy.

One of the NASA shows showing the ISS with its cosmic ray detectors. 

This shared experience is teaching them collectively, a superpower for learning. When we learn together, we do it better and faster. If we do all our learning on individual screens, it remains an isolating experience.

If you want to enjoy the planetarium, you cannot have your screen out. It makes a difference.

Unfortunately, my class is the first and often only time students at UofL see inside this amazing space. The Louisville schools have not had the opportunity to organize school trips to the Planetarium because it closed and stayed closed since March 2020. And that is a shame but also a missed opportunity for UofL. These school trips brought those middle- and high school students to campus who might consider UofL for their immediate future.

This is a traditional slide, similar to the ones I use in class. And then we switch to the dome projection of the same data. It makes a huge difference!

The obstacle is two things, staff and equipment, but really that’s both a money issue. Staff got reassigned within UofL but we are at risk of not having people who know how to run a planetarium anymore. And the equipment is now well over 20 years old. It’s starting to show its age (“why is half the sky brighter than the other?”) The shows aged a bit too. We have a show where Liam Neeson confidently states there is no direct picture of a black hole. My science hasn’t sat still since that show came out: it made a picture of a black hole, found the first galaxies and more than 5000 planets around nearby stars. Just to give you an idea. More up-to-date material would be great.

The planetarium does two things that are increasingly rare in the modern world: a good look at the night’s sky and space and a shared learning experience for students.

The author with the Sloan Digital Sky Survey exhibit in the hall of the Planetarium. I got the SDSS plate but the staff turned it into the amazing exhibit.