Research

It is well-established that the environment of a galaxy plays a crucial role in its evolution. In the local universe, galaxies in galaxy clusters are mainly red and no longer forming stars. They have early-type morphologies (a way of saying they are approximately ellipsoidal), which is very different from the disky late-type morphology that star-forming galaxies, like our Milky way, have. This implies a morphological change must have taken place at some point in their lifetime. Understanding when and what drives this morphological transformation is an important question in galaxy evolution.

In a study I led in 2021 as part of the Gemini Observation of Galaxies in Rich Early Environments (GOGREEN) survey, I measured the projected axis ratio (the ratio of the minor to the major axis) of cluster galaxies in a sample of galaxy clusters at 1.0<z<1.4 and compared them with a sample of galaxies in the field. Comparing the axis ratio distributions between clusters and the field in four mass bins, the distributions for star-forming galaxies in clusters are consistent with those in the field. But interestingly, the distributions for quiescent galaxies in the two environments are distinct, most remarkably in 10.1<log(M_sun)<10.5 where clusters show a flatter distribution, with an excess at low axis ratios.

Modelling the distribution with oblate and triaxial components, we find that the cluster and field sample difference is consistent with an excess of flattened oblate quiescent galaxies in clusters. The oblate population contribution drops at high masses, resulting in a narrower q distribution in the massive population than at lower masses. Our results suggest that environmental quenching mechanism(s) likely produce a population with a different morphological mix than those resulting from the dominant quenching mechanism in the field.

In the local universe, the galaxy population in the high-density environment (galaxy clusters and groups) comprises mainly red, passive galaxies that are no longer actively forming stars. The red galaxies in the highest-density environment, i.e., galaxy clusters, reside in a distinct region of the color–magnitude space known as the red sequence. Understanding how these red-sequence galaxies form and evolve and the physical processes involved remains one of the primary goals in extragalactic astronomy.

The extent of the evolution of the faint red-sequence population is under debate. Various studies have revealed that clusters at intermediate and high redshifts show a continual decrease in the fraction of the faint red-sequence population with redshift, which indicates a gradual buildup of the faint red-sequence population over time. Contrary to the studies mentioned above, several studies have reported that there is little or no evolution of the faint end of the red-sequence cluster luminosity function up to redshift z~1.5, which in turn suggests an early formation.

In a study I led in 2019, I investigated the abundance of faint red-sequence galaxies in a sample of galaxy clusters taken from the Gemini Observation of Galaxies in Rich Early Environments (GOGREEN) survey at 1.0<z<1.3. By looking into the rest-frame H-band luminosity function of the clusters, we showed that there is a deficit of faint red-sequence galaxies in clusters at 1.0<z<1.3. By comparing our sample with a sample of clusters at z~0.6, we find an evolution of the faint end of the red sequence over the ∼2.6 Gyr between the two samples, with the mean faint-end red-sequence luminosity growing by more than a factor of 2. The ratio of faint to luminous red sequence galaxies (faint-to-luminous ratio) of our sample is consistent with the trend of decreasing ratio with increasing redshift proposed in previous studies. We also found that the faint-to-luminous ratios in clusters are consistent with those in the field at z~1.15 and exhibit a stronger redshift dependence. Our results support the picture that the buildup of faint red-sequence galaxies occurs gradually over time and suggest that faint cluster galaxies, similar to bright cluster galaxies, already experience the quenching effect induced by the environment at z~1.15.

Massive passive galaxies at high redshift appear to be extremely compact in size. Those at z~2 have sizes three to four times smaller than their counterpart with a similar mass in the local Universe, suggesting a size evolution over cosmic time. The exact degree of the evolution and the role of the environment to galaxy size are not yet clear. Studies at high-z often show conflicting results.

Size measurements are mostly derived from 2D parametric fitting (e.g. Sérsic profiles) of the galaxy surface brightness profiles from imaging data, i.e. luminosity-weighted. At high-z, the star formation history and stellar population gradients of the galaxies play a huge role in their internal color gradients and luminosity-weighted sizes, making luminosity-weighted sizes not always a reliable proxy of the mass distribution of galaxies.

I therefore developed a novel technique to derive 2D spatially resolved stellar mass maps and mass-weighted sizes for galaxies. Stellar masses can be derived either through spectral energy distribution (SED) fitting techniques or through relations between the stellar mass-to-light ratio (M/L) and color. Following the latter, I exploit the high-resolution imaging and an M/L-color relation, calibrated using public survey catalogues. This allowed me to study the mass distribution and derive mass-weighted structural parameters of the galaxies via parametric fitting. This method, described in detail in my 2016 work, has the advantage of obtaining mass-weighted sizes for a large sample of galaxies with the sole requirement of a suitable combination of imaging and M/L-color relation.

In my 2018 work, we applied this method to a sample of passive galaxies residing in high redshift galaxy clusters at redshift z~1.5 from the KMOS Cluster Survey (KCS) and showed that their mass-weighted sizes are smaller than their luminosity-weighted sizes measured in rest-frame R-band. The average mass-weighted size to luminosity-weighted size ratio (size ratio) that varies between ~0.45 and 0.8 among the clusters. Comparing the size ratio with those in local galaxies and those in field galaxies at a similar redshift, we found that this ratio depends on the environment where the galaxies reside. The average ratio is larger in galaxies residing in more evolved clusters in our sample, showing that these galaxies experience environmental effects that can alter their internal M/L gradients.