1. Exploring Galactic Scaling Relations with Numerical Simulations
Claire Kopenhafer with Dr. Brian O’Shea
Michigan State University, Dept. of Physics & Astronomy and
Dept. of Computational Mathematics, Science, and Engineering

2. What usually people think of when they hear “galaxy”:
What usually people think of when they hear “galaxy”: Disk of stars + gas

3. Circumgalactic Medium
(gas)
Bulge
Disk
Dark Matter Halo
Not to scale; CGM radius is ~20 larger than the disk
DM provides the gravitational potential (defines “out” and “in”)

4. Relationship of the Disk & CGM
Outflows = feedback = SNe and AGN
Hard to observe, unsure of specifics, but a good clue its important...
c/o M. Peeples, STScI

5. Example of a Scaling Relation: Stellar Mass & Metallicity
scaling relations = correlations between OBSERVABLE properties; notice how tight the relation is
metals: metallicity = metal fraction;
Metals come from stars, can trace where star stuff goes, change (cooling) properties of gas
stellar mass scales with halo mass (not directly observable)
billion yrs ago
billion
billion
(line styles - different methods for finding Z)
Zahid et al. 2013,
ApJ Letters, 771, L19

6. What is regulating feedback so that these scaling relations emerge?
Scaling relations imply regulation
Relationship between CGM & disk is a good candidate

7. Regulation via Precipitation
“Cold” gas clouds precipitate out of the CGM, initiating star formation
The largest of these stars result in supernovae, dumping energy and metals into the CGM
The CGM inflates, lowering the density and increasing the cooling time of the gas
Once the cooling time is long enough:
precipitation stops
star formation stops
metal enrichment stops
Voit et al analytic description of this process
Stellar mass levels off over time as metals enter the CGM, slowing precipitation
Explanation developed (primarily) by profs at MSU & their collaborators
Cold is ~10,000 K for the MW
Gas can also accrete on the central black hole, but AGN aren’t as important in MW sized halos (only larger)
This process limits both the stellar mass and the metallicity
(t_cool < 10 * t_ff; both cooling & t_ff affected by halo mass)

8. My Research: Testing the Analytic Predictions with Simulations
In particular, reproduce MZR from Voit et al.
(along with stellar baryon frac vs. max circular vel)

9. Kopenhafer et al., in prep
Idealized, isolated galaxies
Stars aren’t directly modeled
Instead, gas is subtracted as if stars had formed
Feedback occurs in defined regions
(~3 million ly)
1 Mpc
Simulation: cubic volume; dark matter halo
What you’re seeing: projection of gas density; disk from the edge
Full sim volume (why) & zoom in w/ feedback zones
Left: full simulation volume Above: galactic disk over time
Kopenhafer et al., in prep

10. A Different Approach Silvia et al., in prep
So you get more of an idea of what I’m going for, & what the process looks like
Temp Slices
What to watch: center; start seeing some self-regulation
That’s the goal for my sim; we’ve seen self-regulation develop in other sims from the group
Silvia et al., in prep

11. Summary
There are tight correlations between observable galaxy properties
These relationships imply self-regulation mechanism(s)
Precipitation is one possible mechanism
I’m using simulations to test precipitation’s plausibility
Importantly, the scaling relations all relate back to the halo mass
We’ve talked about precip regulating stellar mass & metallicity (but it will also work for others - PAPER)
Specifically, I’m testing in the context of MZR (+ one other but that’s not relevant rn)