Global Properties and Galaxy Correlations
Contents
2. Global Properties and Galaxy Correlations¶
2.1. Spectra¶
Early type galaxies have redder spectra and deeper absorption features
Later-type galaxies have spectra that are dominated by emission from hot stars
2.2. General Trends¶
Late Type Galaxies:
High star formation rate
High gas fraction
High rotational support
Higher gas-phase metallicity
Early type galaxies
Mass is higher
Environment is more complicated
Mass surface density higher
Metallicity higher
2.3. Synethic Spectra¶
Note that spectra vary much more with age than metallicity, and the two quantities are degenerate.
2.3.1. with Age¶
For the same stellar mass population, younger populations are bluer and more luminous per unit mass.
2.3.2. with Metallicity¶
For a fixed mass and galaxy type, lower metallicites are bluer and brighter. This is due to opacity. Higher metallicities give higher opacity, and thus we have less light at bluer wavelenghts.
2.4. K Corrections¶
SEDs change with reedshift. Ultimately, we want to measure the rest frame color \(u-r\) value, but we need to correct for both expansion and distance.
Due to redshift effects, both the wavelength and bandpasses change!
Looking at higher z galaxies in the same filter, we receive emission from shorter wavelengths at a slower rate.
You need to assume that you know the intrinsic spectrum shape, though.
2.5. Stellar Mass-Color Relation¶
Redder galaxies have larger mass-to-light ratios. The redder the band you observe, the less sensitive the inferred mass to light is with color
There is no one-to-one mapping between mass and luminosity. You need a color measurement as well. The redder bands are better since the spectrum is less dominated by single, hot stars.
If you want age measurements though, you should go to bluer wavelengths.
2.6. Color Sequences¶
The color of a galaxy is really the slope of the SED. Measuring a galaxy color allows us to measure the mass-to-light ratio.
We see two groups of galaxies when we plot color versus mass:
Early type galaxies span 2 dex in mass but hardly change color
The color slope for early type galaxies is driven primarily by metallicity
The color slope for late-type galaxies is driven by star formation properties (dominant over dust effects)
See the Mass-Metallicity relations below for more info on why the slopes for the two groups are the way they are. Fundamentally these are metallicity effects.
2.7. 4000 Angstrom Decrement¶
Another interesting way to classify galaxies is by looking just before and just after 4000 angstroms.
Lots of age sensitive features in the spectrum here, allowing us to make this plot;
2.8. Stellar Mass-Metallicity Relation¶
2.8.1. Early Type Galaxies¶
As you go to higher mass galaxies, star formation had generated more metals making stars more metal rich. Massive galaxies have more star formation, to zeroth order.
You measure mostly Mg, Fe stellar absorption features via Lick indices for example, to measure this in ETGs.
2.8.2. Late Type Galaxies¶
For late type galaxies, you can measure ionized HII regions which emit oxygen lines. This doesn’t happen in early type galaxies!
2.9. SFR-Stellar Mass Relation¶
2.10. sSFR-Stellar Mass Relation¶
As you go to higher mass galaxies, the star formation rate per unit mass decreases. That is to say – star formation is less efficient at higher masses.
This effect drives the slope of the color-mass plots from above. Most efficient star formation is in low mass galaxies and thus those are bluer.
More interpretable:
Low mass galaxies have younger populations and formed more recently. Thus they have not yet converted mass to stars because potential is shallower.
Larger galaxies, though, have had time to spread mass out over the halo where gas density is too low to form stars.
2.11. Stellar-Mass Halo-Mass Relation¶
One of the key ideas in galaxy formation is that the dark matter halo drives galaxy formation moreso than anything else. The halo mass should thus determine the galaxy evolution as a whole.
Should set the potential, the kinetic gas temperature, accretion rate, etc.
Measuring halo mass is really difficult.
Per unit mass, really massive haloes and really low mass haloes are inefficient at star formation.
2.12. Stellar-Mass Size Relation¶
2.12.1. Sersic Profile¶
Higher Sersic \(n\) means a more “peaky” inner profile.
Low Sersic \(n\) means flatter profile.
ETGs are typically high \(n\) and have peaky inner profiles. Late type galaxies are flatter near the center and exponential in the outskirts.
2.13. Color-Environment Relation¶
Redder galaxies are found in denser environments.
Color doesn’t depend too much on environment, though!
