Recent and Ongoing Research
Last updated: July 2012
Galaxies and the dark matter haloes in which they reside are the gravitational building blocks of the universe, and my research encompasses the study of their formation, properties, and evolution in a wide range of different environments using a combination of observational and numerical techniques. Below are brief descriptions of a few particular recent or ongoing projects:
- Kinematics of High-Redshift Star-Forming Galaxies
- Morphology of High-Redshift Star-Forming Galaxies
- (NEW!) Discovery of a Rare Grand Design Spiral Galaxy at z=2.18
- MaNGA Kinematic Survey of Low-Redshift Galaxies
- Dynamics and Stellar Populations in the Local Group
1. Kinematics of High-Redshift Star-Forming Galaxies
Over the past few years, I have led an observational program to map the kinematics of ionized gas in actively star-forming galaxies at z~2. This is a critical epoch in the formation history of galaxies as it is at such redshifts when modern day galaxies formed the bulk of their prest-day stellar mass. Although z~2 galaxies are typically less than an arcsecond in size (and hence unresolved with traditional ground-based telescopes), by pairing integral-field spectroscopy with laser-guided adaptive optics it is possible to probe their kinematics on kiloparsec scales using instruments such as the Keck/OSIRIS spectrograph. Such high angular resolution spectroscopy represents a critical advance in observational techniques by allowing full use of the diffraction limit of large-aperture telescopes, and will be a key component in next-generation telescopes such as TMT.
HAlpha flux, velocity, and velocity dispersion map of the galaxy Q2343-BX453.
Unlike in typical local galaxies, we have found that starforming galaxies in the young universe have high velocity dispersions ~ 80 km/s which are in many cases comparable to or greater than their velocity shear about a preferred kinematic axis. The v/sigma of these galaxies appears to be related to their gas fraction (or stellar mass), suggesting that galaxies dominated by gas may not have formed gravitationally relaxed stellar structures. Read more about our results in Law et al. 2007a and 2009b.
We find evidence that these high-redshift starburst galaxies may have kinematic properties extremely similar to supercompact UV luminous galaxies at lower redshifts z ~ 0.2 however, as detailed by Basu-Zych et al. 2009 and Gonçalves et al. 2010.
2. Morphology of High-Redshift Star-Forming Galaxies
A section of the GOODS-North extragalactic deep field with bright star (orange) and a nearby spiral galaxy.
I recently led a Cycle 17 program using the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST) to survey the rest-frame optical morphologies of >300 spectroscopically confirmed star-forming galaxies at redshifts z=1.5-3. In contrast to previous studies, this was the first large-scale (~ 65 arcmin^2) survey of rest-frame optical galaxy morphologies for typical-mass (Mstar ~10^10 Msun) spectroscopically-confirmed galaxies in this redshift range.
Similar to previous results from rest-UV imaging (Law et al. 2007b), z~2 star forming galaxies are predominantly irregular systems whose detailed morphology is largely uncorrelated with physical observables such as star formation rate and stellar population age. Preliminary survey results extending the mass-radius relation for star forming galaxies at z~2 down to 10^9 solar masses were published by UCLA undergraduate student Sarah Nagy (Nagy et al. 2011). Additional results have further refined the evolution of galaxy size throughout the z=1.5-3 redshift window, and demonstrated that stars and gas in these young galaxies may be distributed in a triaxial manner rather than residing in geometrically thick disks (Law et al. 2012). Such triaxial, gas-rich systems are gravitationally unstable, suggesting that z~2-3 star forming galaxies may be continually passing in and out of dynamical equilibrium. Most fundamentally, these results indicate that we have only a rudimentary understanding of the dynamical state of typical L* galaxies during the period in which they were forming the bulk of their stars, and suggest that we must revise our understanding of the relevant baryonic physics governing galaxy formation in the young universe.
A selection of z~2 galaxy morphologies.|
Followup work based on these WFC3 imaging data is ongoing and includes a variety of programs aimed at understanding the relation between morphology and the strength and characteristics of galactic-scale feedback processes, the impact of the protocluster environment on morphological evolution, and the role of AGN and faint QSOs in modifying the properties of their surrounding host galaxies.
3. Discovery of a Rare Grand Design Spiral Galaxy at z=2.18
HST/Keck false colour composite image of galaxy BX442.
High-resolution imaging paired with IFU spectroscopy is a particularly powerful combination that allows us to
explore the structure of z~2-3 galaxies in exceptional detail. In our most recent such study detailed in a study published in Nature (Law et al. 2012b), we obtained OSIRIS IFU spectroscopy of Q2343-BX442, a z=2.18 star forming galaxy
that we identified from our WFC3 imaging survey to have ultra-rare grand design spiral structure similar to the M 51
Whirlpool Galaxy. Our 13-hour OSIRIS data (one of the deepest IFU observations of a single z~2 galaxy taken to date)
indicate that the spiral arms of the galaxy are embedded within a massive rotating disk. Unlike modern spiral galaxies however,
the vertical velocity dispersion of the disk is extremely high (~ 70 km/s), indicating (contrary to expectations) both that dynamically hot z~2 disk
galaxies can form spiral structure and that such structure can be characterized with current instrumentation. Using our
deep IFU spectroscopy we have studied the chemical enrichment patterns within BX442, isolated emission from the warm ionized medium (WIM),
and identified a nearby dwarf companion with a few percent the mass of the primary whose perturbative
influence is likely the primary mechanism exciting the observed spiral structure.
Read more about this discovery online at the BBC, Time magazine, Scientific American, our local press release, or in the original Nature paper.
Kinematic velocity and velocity dispersion maps of BX442.
Artist's rendering of galaxy BX442 and companion. Credit: Dunlap Institute for Astronomy & Astrophysics; Joe Bergeron.|
4. MaNGA Kinematic Survey of Low-Redshift Galaxies
One of the major challenges to understanding kinematic maps of galaxies in the young universe is the comparative lack of spatially resolved 2D velocity data for galaxies in the nearby universe. Although surveys such as the SDSS (Sloan Digital Sky Survey) have obtained spectra of huge catalogs of nearby galaxies, these spectra represent just a single small sampling point in the centers of the galaxies, and are completely devoid of spatial information regarding velocity and composition gradients across the galaxies. Such a local comparison sample will be particularly critical for maximizing the scientific return from next-generation facilities such as JWST and extremely large ground based telescopes.
The Mapping Nearby Galaxies at APO (MaNGA) program (see the SDSS project description) aims to remedy this situation by gathering together invidual fibers from the fiber-fed BOSS spectrograph into 15 or more bundles to obtain integral-field spectroscopy of 10,000 relatively nearby (z ~ 0.05) galaxies representing a wide range in stellar masses, morphological types, and star formation histories. With a wavelength coverage of 3700-10,000 Angstroms and a spectral resolution R ~ 2000, MaNGA will obtain both kinematic and chemical information from a host of emission and absorption line features.
As one of three programs in After-Sloan III (AS3), MaNGA will be on-sky from 2014-2020 and yield millions of individual spectra. With non-contiguous fibers, significant atmospheric differential refraction over the wavelength range of the spectra, and all of the calibration complexities of both an imager and a fiber spectrograph, MaNGA represents a significant technological and scientific challenge. I am presently leading the development of the MaNGA data reduction and analysis pipeline, along with efforts to optimize bundle design and observing strategy.
5. Dynamics and Stellar Populations in the Local Group
One of the best laboratories for testing cosmological structure formation models is the Local Group (LG), consisting primarily of the Milky Way, Andromeda, and associated dwarf galaxies. I am actively involved with multiple projects to study the dynamics, stellar populations, and mass distribution within LG galaxies.
With the advent of deep photometric surveys, it has become apparent that galactic haloes (including that of the Milky Way) are threaded with the phase-mixed detritus of multiple generations of dwarf satellites that have been destroyed by the inexorable tides of their hosts' gravitational potential. By constraining models of the Sagittarius (Sgr) dwarf spheroidal galaxy with observational measurements of its stellar tidal streams from 2MASS and SDSS, we have been able to construct mass maps of the Milky Way and its cold dark matter halo (see Johnston et al. 2005, Law et al. 2005, Law & Majewski 2010a). This recent work suggests (Law et al. 2009a) that the Milky Way may have a significant non-axisymmetric component to its gravitational potential. Click here to see a movie of how we think the Milky Way --- Sgr system may have evolved, here to read our latest press release, or see our online Sgr web resource.
Diagram of the Sgr --- Milky Way system.|
Read more about the discovery of a triaxial dark matter halo for the Milky Way online at the BBC, Sky & Telescope, or in the original ApJ paper, or listen to it on NPR's Morning Edition.
This work has broad applications for understanding the interplay between galaxy morphology, kinematics, and component stellar population in the well-resolved environment of the Local Group. By dynamically associating specific clusters with the Sgr system for instance, we have determined that the strength of the second-parameter effect on the horizontal branch morphology of clusters may be uniquely tied to their subhalo of origin (see discussion in Law & Majewski 2010b). Similarly, it is possible to use proper motion measurements of stellar streams to constrain the local circular speed of the Milky Way, obtaining a values of > 220 km/s via a method completely independent of standard techniques (Carlin et al. 2012). Additional collaborative efforts with which I have been involved are described by Geha et al. 2009, Lokas et al. 2010, and Siegel et al. 2011.
Diagram of age-metallicity relation and horizontal branch type (HBR) for Sgr clusters (from Law & Majewski 2010b).