The First Homogeneous Measurement of the Evolution of the Luminosity Function of Galaxies in the Last 12.6 Billion Years

by Victor Soto Castro

mentor: Danilo Marchesini, Physics and Astronomy; funding source: NSF TRIPODS


Today, 13.8 billion years after the big bang, we notice that not all of the galaxies in our local universe look the same. Some are actively forming new stars while others have been dead for ages. From studying the fossil light that originated from the beginning of our universe, (the cosmic background radiation) when it was roughly 380,000 years old, astronomers have inferred that matter in the infant universe was not homogeneously distributed. Instead, there existed regions where matter was slightly over dense and under dense. One of the major issues in astrophysics is understanding how these tiny seeds in matter grew to form the stars, galaxies, and clusters we see today. The current working hypothesis is that galaxies form under the influence of gravity. First, the gravity of dark matter (DM) causes tiny seeds in the distribution of matter across regions of spacetime to grow larger with age. As these seeds grow, the gravitational attraction between matter becomes stronger making it easier for these structures to attract additional matter. As time proceeds, the DM distribution becomes clumpier, and begins to pull in gas, mainly Hydrogen and Helium. These two gases are the primary ingredients required for the formation of stars that eventually form the stellar content of galaxies.

The evolution of the LF as a function of time provides insights into the physical process that govern the assembly and evolution of galaxies. The mean space density of galaxies per unit luminosity (i.e., number of galaxies per unit of volume per unit of luminosity), or luminosity function (LF), is one of the most fundamental of all cosmological observables, and it is one of the most basic descriptors of a galaxy population. The shape of the LF retains the imprint of all galaxy formation and evolution processes. In our first figure, we have a comparison between the dark matter mass function (red) and the evolution of the Luminosity Function (LF) of galaxies
(blue). The bright end of the LF (in blue) is believed to be shaped by the AGN feedback (accreting supermassive black holes), whereas the faint end of the LF is shaped by stellar feedback (intense radiation from massive stars and supernova explosions heating or ejecting gas that prevent the formation of further stars).

By combining several extragalactic surveys (namely, UVISTA DR1/DR3, CANDELS-3DHST, and HFF-DeepSpace) in a “wedding cake” layer fashion we are able to measure the evolution of the LF of galaxies over the last 12.6 billion years (i.e., ∼93% of cosmic history). The combination of the aforementioned surveys allowed us to probe simultaneously both the bright and faint end of the LF with unprecedented statistics. In particular, HFF-DeepSpace, the deepest imaging program to have ever been taken by the Hubble Space telescope, allowed us to probe the LF of distant galaxies down to incredibly faint luminosities, hence allowing us to constrain the stellar
feedback in shaping galaxies even better.

The method we used to calculate the LF is called the Vmax method. After finding the largest distance of an observed galaxy with absolute magnitude and calculating M i the volume of the sample corresponding to that distance, we get V max . This volume for the galaxy is used to calculate the LF of all galaxies with absolute magnitude in the range (M + MdM). After calculating the LF for each individual survey and for their respective fields, I combined all of the surveys into one large LF plot containing all the points in our “wedding cake” layer combination. After completing this, I plotted it on top of the individual surveys in order to see the differences in points we gained by combining all the LFs. Subesquentially, we realized that we
were not obtaining the extra points from the Hubble Frontier Fields. This was an issue in the code that had to be fixed. Unfortunately, this occurred towards the end of the summer program and I was unable to continue the research due to hardships caused by Covid-19. In the future I plan to complete these figures by including HFF and then prepare them for publication.

Leave a Reply