Research

Gas Content Over Cosmic Time

Motivation

""How does the molecular gas content of galaxies evolve over cosmic time?". Since molecular hydrogen gas is the direct fuel for star formation, a change in molecular hydrogen (H2) content (traced by the CO molecule) in galaxies can be a potential driver of the observed cosmic star formation history. Recent "blind" surveys of CO-emitting galaxies on the Very Large Array and the Atacama Large Millimeter Array have made significant improvements in terms of survey size and the number of sources detected; however, the time required to observe large fields is expensive for current facilities. For this reason, I explore the feasibility of using existing observations of targeted galaxies to perform a search for serendipitous CO sources. Re-using existing data for this purpose provides an alternative way to build up a large co-moving volume of space for exploration, as long as there is existing ancillary data that can be used to find counterparts to CO sources and constrain their redshifts. The second Plateau de Bure High-z Blue Sequence Survey (PHIBSS2) data set is very well suited to this task.

Figure 1: The evolution of the molecular gas mass density with redshift, where the black boxes represent the constraints from the PHIBSS2 data, compared to previous observational constraints, and to model predictions. The constraints derived from serendipitous detections of CO in the PHIBSS2 fields are consistent with those of previous blind surveys. Adopted from Lenkić et al. 2020.

Results

The work involves searching for CO-emitting galaxies that are either chance alignments with the targeted galaxies, or physically associated with it. The goal of the line search is to systematically select candidate sources from the noisy data, and assess their significance in terms of their corresponding signal-to-noise ratio (SNR) and the completeness of the search algorithm. Identifying IR/optical counterparts for serendipitous CO detections in each PHIBSS2 data cube constrains the rotational transition and redshift of each. By then dividing all sources into their respective transitions and redshift ranges, and weighting each by the statistical measures corresponding to reliability and completeness, I was able to construct a range of CO luminosity functions spanning the redshift range of z ~ 0.4 - 5.2, and hence establish constraints for the molecular gas mass density evolution. These findings are summarized in Figure 1, where the constraints derived from the PHIBSS2 data (black boxes) are compared to all previous observational constraints and to theoretical predictions. The PHIBSS2 constraints are consistent with all previous observations, validate the novel re-purposing of existing data, and show that this method is a possible way of better constraining this problem.

The Nature of Star Forming Clumps

Motivation

"How long can massive star forming clumps, as are found in galaxies at z ~ 1-3, survive before they are dissipated by stellar feedback, and is this timescale long enough for the clumps to have an important role in the evolution of the galaxy?". Simulations of galaxies at these redshifts show that massive star forming clumps can migrate to the centers of galaxies and contribute to bulge growth. They also show that the light-weighted ages of clumps plateau at around 200-250 Myr, due to the loss of low-mass stars and ongoing star formation within the clumps. However, studies show that clumps in nearby galaxies, blurred to the resolution of high-redshift observations, merge into the large structures that are seen in high-redshift galaxies, which renders it difficult to investigate this problem. I address this problem with a sample of six z ~ 0.1 galaxies selected for their resemblance in terms of gas fractions, star formation rates, and velocity dispersions, to galaxies at the peak of cosmic star forming activity, than present day ones. These galaxies were observed with the Hubble Space Telescope in three filters: F225W, F336W, and F467M. This bypasses the difficulty of performing resolved observations of clumps at high-redshift.

Figure 2: Clump ages estimated from the clump integrated colors (left), the outer 75% of the clump by area (middle), and the inner 25% of the clump by area (right), as a function of galactocentric radius, compared to the galaxy disk estimated ages (colored lines). Error bars represent the spread in measurements when shifting the inner and outer clump boundaries by 10%. Adopted from Lenkić et al. in prep.

Results

I measure the 225-336 and 336-467 colors of each clump, which are sensitive to changes in extinction and age respectively. I use these measurements to constrain the stellar population ages and extinction of individual clumps. Unique to this work, the resolution of the HST data (0.09'') allows me to also probe how these properties change across each clump. I find that extinction varies very little across any individual clump, while the ages vary significantly. The inner regions of clumps appear to be, generally, less than or about 50 Myr old, while the outer regions appear to be as old as 250 Myr (Figure 2). This finding is consistent with high-redshift simulations and suggests that clumps continue to form stars in their interiors, even after it has slowed down in the outer regions. If clumps are long-lived and continue to accrete gas as they migrate through the disk, then this internal star formation could be maintained.