Image of an LSB galaxy in the near-infrared (Spitzer 3.6μ)

Research

Low Surface Brightness Galaxies
The Properties of LSB Galaxies
The Stellar Populations of LSB Galaxies
The Number Density of LSB Galaxies

Disk Galaxy Dynamics
The Mass Discrepancy in Disk Galaxies
The Baryonic Tully Fisher Relation



Low Surface Brightness Galaxies

Much of my research centers on Low Surface Brightness (LSB) galaxies. LSB galaxies are dim objects with less light per unit area than typical of bright spiral galaxies. Their stars are spread thin, so they are hard to see in contrast against the glow of the terrestrial sky - even before light pollution.

For many years, LSB galaxies were thought not to exist, but this proved to be a selection effect. Many examples have now been discovered. Indeed, LSB galaxies have become central to many areas of research as they provide new insight into theories designed to explain brighter galaxies. The data for LSB galaxies bends and sometimes breaks some of our cherished ideas about the universe.


The Physical Properties of LSB Galaxies

LSB galaxies outside the Local Group are typically late type, rotating disk gaalxies with have approximately exponential light distribtions.

The LSB galaxy UGC 1230 in various wavelengths: from left to right, U, B, V (top); I, Hα, continuum subtracted Hα (bottom). Loose spiral arms are apparent in the bluer bands; a color gradient is obvious to the eye. The overall colors are quite blue, typical of late type LSB galaxies. There is some sporadic star formation, indicated by the isolated HII regions (McGaugh & Bothun 1994).

Position of slit for spectroscopic observations of HII regions in UGC 1230 (McGaugh 1994; Kuzio de Naray et al. 2004).

The surface brightness profile derived from the B-band image of UGC 1230. Note that the central surface brightness does not exceed 23 mag. arcsec-2, well below what was once thought to be the "universal" (Freeman) value of 21.65 mag. arcsec-2. The scale length of this galaxy is 3 kpc, comparable to the Milky Way. It is not a tiny dwarf.

The spectrum of one of the HII regions in UGC 1230 showing the bright emission lines of oxygen and hydrogen. These allow an abundance estimate to be made, showing that LSB galaxies are metal poor.


The Stellar Populations of LSB Galaxies

LSB disk galaxies exhibit a variety of morphologies and have a range of stellar populations. Typically they are rather blue. Both a young age and low metallicity appear to be necessary to explain the observed blue colors.
High and low contrast images (top row) of the LSB galaxies D572-5 (left) and D646-9 (right). This dim, fuzzy, irregular morphology is typical of the smallest, lowest mass, latest type disk galaxies. Also shown are color maps (lower left corners) and Hα maps (lower right corners). The latter show the location of current sites of star formation, which can range from active & multiple to single HII regions to none at all.

See more at Jim Schombert's website.


The Number Density of LSB Galaxies

The size and surface brightness of disk galaxies as quantified by exponential disk fits, showing the central surface brightness in the B-band against the disk scale length. The data show the progression towards discovery of lower surface brightness galaxies as surveys press deeper. Solid lines show the theoretical limits of a hypothetical very deep survey. The diagonal dotted lines shows the luminosity of an L* galaxy whicle the horizontal dotted line shows the apparent threshold for disk stability.

The number density (top) and luminosity density (bottom) of disk galaxies as a function of central surface brightness. The disk stability line at left corresponds approximately to the classic "Freeman Law." Rather than all disks sharing this surface brightness, Freeman's limit is an upper limit to disk surface brightness above which the numbers of galaxies declines rapidly. Below the Freeman limit, there are approximately constant numbers of galaxies at each surface brightness (top). These are, on average, less luminous than higher surface brightness galaxies, so their contribution to the luminosity density declines steadily (bottom).



Disk Galaxy Dynamics

The physics of gravitationally bound, rotating disks of stars is very rich. Understanding spiral structure, bar formation, and the mass discrepancy problem in galactic disks is an endlessly fascinating - and challenging - topic.

The Mass Discrepancy in Disk Galaxies

The missing mass problem in galaxies remains one of the most serious and compelling problems in all of physics. For some time, there had been indications that lower luminosity galaxies tended to need more dark matter than bright ones. We discovered that the most important variable is the surface brightness: the lower the surface brightness of a disk, the more dark matter dominated it was inferred to be. The apparent correlation with luminosity is just a result of lower surface brightness galaxies tending to be lower luminosity.

The mass-to-light ratio enclosed by galactic disks as a function of absolute magnitude. Fainter galaxies tend to require more dark matter, but there is considerable real scatter in the mass-to-light ratio at a given absolute magnitude.

The enclosed mass-to-light ratio as a function of central surface brightness. Lower surface brightness disks are progressively more dark matter dominated. This correlation appears to be the sole driver of the weaker correlation with absolute magnitude.


The Baryonic Tully Fisher Relation

The Tully-Fisher relation has long been an important tool for estimating extragalactic distances. I have been interested in the physics that underlies the relation, which remains poorly understood. Why does this relation exist?

The stellar mass (left) and baryonic (right) Tully-Fisher relations. The baryonic mass is the sum of stellar and gas mass. Originally a relation between line-width and luminosity, the underlying physical variables appear to be rotation velocity and baryonic mass. Physics does not care whether the mass is in the form of stars or gas. If an important part of the total is omitted, the relation appears to break down. Low mass galaxies are frequently gas rich (colored points have more than half their baryonis in the form of gas), and deviate from the stars-only relation (left panel). Adding the gas into the total baryonic mass restores a relation that is continuous over five decades in mass.