Galaxies Over Cosmic Time
Contents
12. Galaxies Over Cosmic Time¶
12.1. Identifying Galaxies¶
Some notes:
For the same intrinsic size, galaxies are smaller with increasing \(z\), and then they become larger again (angular diameter distance)
12.1.1. Lyman Break Technique¶
Relatively low \(u\) flux and a relatively blue continuum color of an observed galaxy can be explained by the Lyman break being redshifted to \(z\sim 2\) ot \(3\)
The galaxy will emerge from observations as you go to higher redshift
From here:
A photon emitted with wavelength shorter than 912 Angstroms (Lyman continuum) will be completely absorbed by hydrogen gas both in a galaxy and along the line of sight to us. Essentially, there will be no light making it to us from a galaxy with wavelengths shorter than that. We see a “break” (the Lyman break) in the spectrum of the galaxy. From 912-1216 A (Lyman alpha), photons can also be absorbed by intervening gas.
For high redshift galaxies, this Lyman break redshifts into the optical. By looking at the colors of galaxies, we should see high redshift star forming galaxies as objects that “disappear” in the bluest filters, or more correctly are red in blue colors (ie U-B) and blue in red colors (B-V or V-I). We can identify high redshift candidates this way and do follow-up spectroscopy using big telescopes to confirm their redshifts.
This cannot be used for dusty star forming galaxies or quiescent galaxies at high \(z\)
Instead use balmer break at NIR wavelengths
12.1.2. Balmer Break Technique¶
QUiescent and dust star-forming galaxies cannot be identified using the Lyman break tech- nique. Quiescent because there are no stars producing UV emission. Dusty because the light is being absorbed in-situ. How do we take this into account?
The light is thus mostly in NIR wavelengths! The stellar light of quiescent galaxies is shifted to NIR wavelengths beyond z ∼ 2, Balmer break between J and H band filter. Dusty star forming beyond z ∼ 2 also require NIR surveys.
Quiescent galaxies at \(z\sim 2\) have the Balmer break between the J and H filters, allowing for a similar technique to Lyman break galaxies
A primary feature is the strong break at 4000A (Angstroms), caused by the blanket absorption of high energy radiation from metals in stellar atmospheres and by a deficiency of hot, blue stars. A smoothly varying function can easily be fit to spectra, with a clear drop off in intensity at 4000A.
12.1.3. Sub-mm Galaxies¶
ALMA has allowed for dust temperature measurements to really high redshfit.
12.1.4. Lyman Alpha Emitters¶
12.2. Evolution of Global Properties¶
12.2.3. Stellar Mass Density¶
12.2.4. Mean Metallicity of the Universe¶
12.2.5. Star Forming Main Sequence¶
Star formation rate at fixed mass was higher at earlier times
Starforming main sequence was already in place at earlier times
12.2.6. Specific Star Formation Rate¶
Decreases over cosmic time, and we also see we cannot get it even close with our best fit model.
12.2.7. SFH and Massive BH Accretion History¶
12.3. Summary¶
There are many criteria to identify distant galaxies
The ‘zoo’ of galaxies provides a biased, incomplete census of the distant galaxy population
Complete photometric and spectroscopic surveys provide unbiased views of distant galaxies
Photometric surveys are large, but suffer from systematic uncertainties (no accurate photo-\(z\))
Spectroscopic surveys are accurate but small
Medium band photometry or low res spectroscopy compromise between techniques
Star formation rate density increas to \(z\sim 2\) and then falls again
The stellar mass density increases gradually, with 50% of the mass formed by \(z\sim 1.2\)
The main sequence of star formation was already in place at \(z\sim 6\) (or higher), and the specific star formation rate decreases with time
The accretion history of SMBHs follows the star formation history of the Universe