Richard Anthony D'Souza (Vatican Observatory)

Constraining the assembly time of the stellar haloes of nearby Milky Way-mass galaxies

Studying the past significant mergers of Milky Way-mass galaxies out to 10-20 Mpc through resolved stellar populations of their stellar halos allow us to gain several valuable insights into the effects of mergers onto the properties of those galaxies.   Recent data has confirmed that the median mass of the most significant progenitor that a Milky Way-mass galaxies accretes in its lifetime is around the mass of the LMC.  While such mergers are known to cause the thickening or destruction of the stellar disk, bursts of star formation and bring in groups of smaller satellite galaxies, without constraints on the time of information of the significant merger for galaxies further than 3 Mpc, it is difficult to correlate these effects with the significant merger. Here, we show that ratio of brighter AGB to RGB stars in the stellar halo can be used to time the shutdown of star formation in the significant progenitor and constrain the time of merger.  These time constraints demonstrate for the first time that stellar haloes of Milky Way mass galaxies build up hierarchically, and that the significant progenitors bring in with them a number of dwarf galaxies.

Callum Donnan (NOIRLab)

Observing the Formation of the First Galaxies with JWST

The launch of JWST has enabled the robust detection of galaxies at z>10 for the first time. This allows the measurement of the evolution of the UV luminosity function (LF) as cosmic star-formation rate density (SFRD) to within ~300 Myr of the Big Bang. I will present my latest measurement of the UV LF at z=8-15 from ~380 sq. arcmin of JWST/NIRCam imaging reaching depths of ~30 mag in the deepest regions. This shows a slow, shallow evolution in the UV LF and SFRD out to z~13. I will then present a simple theoretical model which demonstrates a lack of change in the star-formation efficiency needed from both the stellar mass function and the UV LF. Instead, ever younger stellar populations can explain this slow evolution, counteracting the rapid evolution of the halo-mass function at early times. It then follows that a key prediction of this model is that there is a more rapid evolution in the UV LF at z>14 where this rapid evolution of the halo-mass function takes over. Finally, I will present the CAPERS survey which will obtain NIRSpec spectra for ~100 galaxies at z>9.5 where I will present some initial results.