Scott C. Chapman

The remnants of galaxy formation from a panoramic survey of the region around M31.

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than Mv = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

Spatially-Resolved View of High-Redshift Starbursts: the case of Sub-mm Galaxies

Ultra-luminous infrared galaxies (LI R > 10 L ) are locally rare, but appear to dominate the co-moving energy density at higher redshifts (z > 2). Many of these are opticallyfaint, dust-obscured galaxies that have been identified by the detection of their thermal dust emission at sub-mm wavelengths. Multi-wavelength spectroscopic follow-up observations of these sub-mm galaxies (SMGs) have shown that they are massive (Mste l la r ∼ 10 M ) objects undergoing intense star-formation (SFRs ∼ 10 − 10 M yr−1 ) with a mean redshift of z ∼ 2, coinciding with the epoch of peak quasar activity. The large fraction of AGNs in SMGs and the derived SMBH masses (M• < 10 M ) in these galaxies suggest that the submm phase may play an important role in the rapid growth of SMBHs. When both AGN and star-formation activity are present, long-slit spectroscopic techniques face difficulties in disentangling their contributions and may result in SFR and mass overestimates. We present an integral field view of the Hα emission in a sample of 3 SMGs at z ∼ 1.4 − 2.4 with the IFU instrument OSIRIS on Keck. Designed to be used with Laser Guide Star Adaptive Optics, OSIRIS allows a spatial resolution of up to 10× higher than what has been possible in previous seeing-limited studies of the ionized gas in these galaxies. Our main results are the following: (1) We detect multiple galactic-scale sub-components: the compact, broad Hα emission (FWHM > 1000 km s−1 ) likely associated with an AGN, the more extended narrow-line Hα emission (FWHM 500 km s−1 ) of star-forming regions; the latter are dominated by multiple 1−2 kpc sized Hα-bright clumps, each contributing 1-25% of the total clump-integrated Hα emission. (2) We derive clump dynamical masses ∼ 1 − 10 × 10M , 1 − 2 orders of magnitude larger than the kpc-scaled stellar clumps uncovered in optically-selected z ∼ 2 star-forming galaxies. (3) We determine high star-formation rate surface densities (ΣSFR ∼ 1 − 50 M yr−1 kpc−2 , after extinction correction), similar to local starbursts and luminous infrared galaxies. In contrast to these local environments, SMGs undergo such intense activity on significantly larger spatial scales as revealed by extended Hα emission over 4− 16 kpc. (4) We find no evidence of ordered global motion as it would be found in a disk, but rather large velocity offsets ( ∼ few ×100 km s−1 ) between the distinct stellar clumps. The merger interpretation is likely the most accurate scenario for the SMGs in our sample. However, the final test of whether an underlying disk structure is present will come from studies of the cold gas at the high spatial resolutions possible with ALMA. We refer the reader to Menéndez-Delmestre et al. (2012) for more details.