The Evolution of Star-Forming Galaxies Using the Largest Narrowband Surveys
- Author(s): Khostovan, Ali Ahmad
- Advisor(s): Mobasher, Bahram
- et al.
Observations of the star formation history of the Universe shows that galaxies have evolved considerably over the past 13 Gyrs, but what is causing such an evolution to occur? To answer this question is to delve deeper and ask more fundamental questions: How are the statistical properties of galaxies changing with time? What mechanisms/drivers are involved? How does the host halo influence the residing galaxy’s evolution? What are the progenitors of the present-day galaxies?
To address these questions, I use unique, narrowband selected samples of 3475 Hβ+[Oiii]- and 3298 [Oii]-selected emission line galaxies from the High-z Emission Line Survey (HiZELS) and ∼ 4000 Lyα-selected galaxies from the Slicing COSMOS 4K (SC4K) survey. The HiZELS survey covers samples divided in four discrete redshift slices (∆z ∼ 0.01 − 0.03) between z ∼ 0.8 − 3.3 and z ∼ 1.5 − 5 for Hβ+[Oiii] and [Oii] emitters, respectively, and the SC4K covers Lyα samples divided in 15 discrete redshift slices (∆z ∼ 0.02 − 0.15) between z ∼ 2.5 − 6.
Measurements of the [Oiii] and [Oii] stellar mass and luminosity functions are presented for the first time up to z ∼ 3 and 5, respectively. Both [Oiii] and [Oii] emitters show stellar mass and luminosity functions that strongly evolve with redshift with [Oii] emitters observed to become rarer with increasing redshift. I present measurements of the star formation history of the Universe using only [Oii] from the local Universe to z ∼ 5. I confirm that star formation rates strongly increase from z ∼ 5 to a peak around z ∼ 3 and gradually decrease to the present-day. Stellar mass densities of [Oiii] and [Oii] emitters are found to trace the stellar mass buildup of star-forming galaxies, but diverge at z < 1 from the global population of galaxies, suggesting a decreasing fraction of star forming galaxies due to quenching mechanisms.
I investigate the evolution of the equivalent widths of Hα, [Oiii], and [Oii] emitters and find different evolutions for both lines. Hα and [Oiii] emitters are found to continuously increase with redshift, while [Oii] equivalent widths increase up to z ∼ 4 and decrease quickly at higher redshifts. Comparing the evolution of the [Oiii] and [Oii] equivalent widths suggests an increasingly energetic ionization state of the gas in the interstellar medium of star-forming galaxies. This explains the rapid drop in number densities of [Oii] emitters in respect to [Oiii]-selected emitters where due to higher [Oiii]/[Oii] line ratios, [Oii] emitters become increasingly rare due to changes in the physical conditions of the interstellar medium.
I conclude my work by investigating the clustering and halo properties of [Oiii], [Oii], and Lyα emitters. Strong evolutions in the clustering lengths for all redshift samples is observed with galaxies being more clustered at higher redshifts. Applying models to convert clustering lengths to halo masses shows that galaxy properties and halo masses are tightly correlated with one another. Strong, redshift-independent trends between halo mass and line luminosity, stellar mass, rest-frame 1500 ̊A luminosity, and UV star formation rate are observed. This signifies the important role that halos play in the overall evolution of star-forming galaxies. Lastly, I find that Lyα emitters at high-z are progenitors of a wide range of present-day galaxies, ranging from dwarf-like, to Milky Way, to large cluster galaxies making them ideal tools to study the formation of galaxies in the local Universe.