Chris J. Lintott

Massive Elliptical Galaxies: From Cores to Halos

In the context of recent observational results that show massive ellipticals were in place at high redshifts, we reassessthestatusofmonolithiccollapseinaCDMuniverse.Usingasampleofover2000galaxiesfromtheSloan Digital SkySurvey,bycomparing thedynamical mass andstellar mass (estimated fromcolors)wefindthat ellipticals have ‘‘cores’’ that are baryon-dominated within their half-light radius. These galaxies correspond to 3 � peaks in the spherical collapse model if the total mass in the halo is assumed to be 20 times the dynamical mass within the halflight radius. This value yields stellar mass‐to‐total massratios of 8%, compared to a cosmological baryon fraction of 18% derived from the first 3 years ofWMAPobservations alone. We further develop a method for reconstructing the concentration halo parameterc of the progenitors of these galaxies by utilizing adiabatic contraction. Although the analysis is done within the framework of monolithic collapse, the resulting distribution ofc is lognormal with a peakvalueofc � 3 10anda distributionwidth similartothe results ofN-bodysimulations. We alsoderive scaling relationsbetweenstellaranddynamicalmassandthevelocitydispersion,andfindthatthesearesufficienttorecover the tilt of the fundamental plane. Subject headingg dark matter — galaxies: elliptical and lenticular, cD — galaxies: evolution — galaxies: formation — galaxies: fundamental parameters

Rapid star formation in the presence of active galactic nuclei

Recent observations reveal galaxies in the early universe (2 < z < 6.4) with large reservoirs of molecular gas and extreme star formation rates. For a very large range of sources, a tight relationship exists between star formation rate and the luminosity of the HCN J = 1-0 spectral line, but sources at redshifts of z ~ 2 and beyond do not follow this trend. The deficit in HCN is conventionally explained by an excess of infrared radiation due to active galactic nuclei (AGNs). We show in this Letter not only that the presence of AGNs cannot account for the excess of IR over molecular luminosity, but also that the observed abundance of HCN is in fact consistent with a population of stars forming from near-primordial gas.

Molecular abundance ratios as a tracer of accelerated collapse in regions of high-mass star formation

Recent observations suggest that the behaviour of tracer species such as N_2H+ and CS is significantly different in regions of high and low mass star formation. In the latter, N_2H+ is a good tracer of mass, while CS is not. Observations show the reverse to be true in high-mass star formation regions. We use a computational chemical model to show that the abundances of these and other species may be significantly altered by a period of accelerated collapse in high mass star forming regions. We suggest these results provide a potential explanation of the observations, and make predictions for the behaviour of other species.

Tracing High-Density Gas in M82 and NGC 4038

We present the first detection of CS in the Antennae galaxies toward the NGC 4038 nucleus, as well as the first detections of two high-J (5-4 and 7-6) CS lines in the center of M82. The CS(7-6) line in M82 shows a profile that is surprisingly different from those of other low-J CS transitions we observed. This implies the presence of a separate, denser and warmer molecular gas component. The derived physical properties and the likely location of the CS(7-6) emission suggest an association with the supershell in the center of M82.

Hot cores: probes of high-redshift galaxies?

The very high rates of second generation star formation detected and inferred in high redshift objects should be accompanied by intense millimetre-wave emission from hot core molecules. We calculate the molecular abundances likely to arise in hot cores associated with massive star formation at high redshift, using several independent models of metallicity in the early Universe. If the number of hot cores exceeds that in the Milky Way Galaxy by a factor of at least one thousand, then a wide range of molecules in high redshift hot cores should have detectable emission. It should be possible to distinguish between independent models for the production of metals and hence hot core molecules should be useful probes of star formation at high redshift.