MMT Observatory, Arizona

Undergraduate and Graduate Mentoring

For a list of current students performing research with Prof. Whitaker, please refer to our Research Group. Former students mentored include:

Graduate Students:
  • Claire Dickey (Yale University, 2015-2016)
Undergraduate Students:
  • Rochelle Horanzy (University of Connecticut, 2016-2018)
  • Mohammad Ashas (University of Connecticut, 2016-2018)
  • Warren Sharpp (UMass Amherst/FCAD Internship, June-August 2016)
  • Daniel Lange-Vagle (Tufts University, 2014 - 2016)
  • Michael Alburger (NASA/GSFC Summer Internship, June-August 2014)

The Star Formation Sequence: Redshift Evolution and the Connection to Galactic Structure

Whitaker et al. (2012, 2014b, 2015, 2017)

A wealth of data from deep extragalactic surveys have revealed a picture where star-forming galaxies follow a tight relation between star formation rate (SFR) and stellar mass (M*), known as the “star formation sequence” (see figure to right from Whitaker et al. 2014b).  In Whitaker et al. (2012b), we showed that this relation has a roughly constant scatter with cosmic time, and that galaxies show strong trends of increasing dust attenuation with stellar mass.  By leveraging the factor of ten lower mass-completeness limits in the 3D-HST photometric catalogs, we further constrained the low-mass slope of this relation in Whitaker et al. (2014b).  For the first time, we showed that while the slope of the star formation sequence is unity for low-mass galaxies out to z=2.5, it becomes increasingly shallower towards later times for massive galaxies, indicating either less efficient star formation or decreasing gas accretion. These self-consistent empirical measurements of the evolving relation between star formation rate and stellar mass enable powerful constraints on theoretical models across different mass regimes.  Most recently, we have explored the correlations between the star formation sequence and various metrics of galactic structure, including the Sersic index (Whitaker et al. 2015)  and size and central density (Whitaker et al. 2017). 

Figure 2; Whitaker et al. (2014b)
Figure 4; Whitaker et al. (2012a)

The Quenching of Star Formation in Massive Galaxies

Whitaker et al. (2010, 2011, 2012a, 2013)

My research is focused on mapping the stellar population properties and spatial structures of galaxies, in order to reconstruct their life cycles and identify their driving mechanism. How do massive galaxies grow? And what regulates their star formation? Unifying both of these questions, one of the most outstanding problems in galaxy formation theory is understanding how actively star-forming galaxies quench their star formation. To make sense of the formation and evolution of the most massive galaxies in our universe, we must directly observe these galaxies at the pivotal epoch of 1<z<3 when they are most rapidly shutting off star formation. At these early times, roughly half of massive galaxies had already stopped forming new stars within the last one to two gigayears (see figure on left from Whitaker et al. 2012a).  One approach to illuminating the dominant quenching mechanism requires first discriminating the distant galaxies that most recently shut off. This is now possible with the technique of Whitaker et al. (2010).  Using this color-color selection, we are working to quantify the rest-frame optical, observed in the near-infrared (NIR), structural and stellar population properties of these recently quenched galaxies relative to star-forming and older quiescent galaxies (Whitaker et al. 2012a, 2013).  The next step, and ongoing research, involves spatially resolving the stellar populations to disentangle the unique observational signatures that different quenching models predict. 

Resolved Star Formation and Stellar Populations of Distant Galaxies

Whitaker et al. (2014a)

RCS0327 is one of the best-studied star-forming galaxies at the peak of cosmic star formation. Due to the strong gravitational lensing magnification and enhanced star formation from an ongoing interaction (see figure to right from Whitaker et al. 2014a), RCS0327 provides a unique opportunity to study a distant galaxy in detail on the ∼100 pc spatial scales characteristic of star-forming regions. In Whitaker et al. (2014a), we use HST/WFC3 grism observations to spatially-resolved the Hγ/Hβ, [OIII]λ5007/Hβ and HeIλ5887/Hβ emission line ratios, mapping the variations in the extinction and ionization conditions for 7 individual star-forming regions at z=1.7.  Working with the Sloan Giant Arcs Survey collaboration (PI: M. Gladders), we are now performing a detailed census of the stellar populations of a pilot sample of distant gravitationally lensed galaxies.  Synergizing the science possible with the large representative 3D-HST galaxy sample and the high spatial resolution case-studies of lensed galaxies will have durable value and guide the focus of the next generation of telescopes.

Figure 1; Whitaker et al. (2014a)