Resolving the Interstellar Medium in Ultraluminous Infrared QSO Hosts with ALMA

We present ALMA observations of the CO(1-0) line and 3-mm continuum emission in eight ultraluminous infrared (IR) quasi-stellar objects (QSOs) at z = 0.06-0.19. All eight IR QSO hosts are clearly resolved in their CO molecular gas emission with a median source size of 3.2 kpc, and seven out of eight sources are detected in 3-mm continuum, which is found to be more centrally concentrated with respect to molecular gas with sizes of 0.4-1.0 kpc. Our observations reveal a diversity of CO morphology and kinematics for the IR QSO systems which can be roughly classified into three categories, rotating gas disk with ordered velocity gradient, compact CO peak with disturbed velocity, and multiple CO distinct sources undergoing a merger between luminous QSO and a companion galaxy separated by a few kpc. The molecular gas in three of IR QSO hosts are found to be rotation-dominated with the ratio of the maximum rotation velocity to the local velocity dispersion of $V_{rm rot}/sigma=4-6$. Basic estimates of the dynamical masses within the CO-emitting regions give masses between $7.4times10^9$ and $6.9times10^{10}$ $M_odot$. We find an increasing trend between BH mass accretion rate and star formation rate (SFR) over three orders of magnitude in far-IR luminosity/SFR, in line with the correlation between QSO bolometric luminosity and SF activity, indicative of a likely direct connection between AGN and SF activity over galaxy evolution timescales.

The Impact of Star Formation and Active Nuclei on the Interstellar Medium in Ultraluminous Infrared Galaxies

The energy input into the interstellar medium in Ultraluminous Infrared Galaxies (ULIRGs) is enormous, regardless of the nature of the power source. I discuss some of the major consequences for the structure and energetics of the ISM in these galaxies. Observationally, the column densities in the nuclear regions of ULIRGs are known to be very high, which makes distinguishing starbursts from AGN quite difficult. The level of energy and momentum injection means that the pressure in the ISM must be extremely high, at least 3-4 orders of magnitude larger than in the local ISM or typical giant molecular clouds. It also means that the luminosity of GMCs in ULIRGs must be very high, as they must radiate many times their binding energy over their lifetimes. I briefly review the influence which X-ray irradiation can have on the ISM in AGN-powered ULIRGs. Finally, I show that the presence of PAH features in ULIRGs does not imply that they must be starburst-dominated, since at the column densities and pressures typical of the ISM in ULIRGs PAHs can survive even at tens of parsec distances from the AGN.

Molecular Hydrogen Excitation in Ultraluminous Infrared Galaxies

We report medium resolution VLT ISAAC K-band spectroscopy of the nuclei of seven ultraluminous infrared galaxies. After accounting for stellar absorption features, we have detected several molecular hydrogen (H_2) v=1-0, 2-1, and 3-2 vibrational emission lines, as well as the HI Brgamma and HeI 2^1P-2^1S recombination lines. The relative H_2 line intensities show little variation between the objects, suggesting that the H_2 excitation mechanisms in the nuclei are similar in all the objects. The 1-0 emissions appear thermalised at temperatures Tsim1000K. However, the 2-1 and 3-2 emissions show evidence of being radiatively excited by far-ultraviolet (FUV) photons, suggesting that the H_2 excitation in the ULIRGs may arise in dense photon dominated regions (PDRs). We show that the line ratios in the nuclei are consistent with PDRs with cloud densities between 10^4 to 10^5cm^{-3}, exposed to far ultraviolet (FUV) radiation fields at least 10^3 times more intense than the ambient FUV intensity in the local interstellar medium. We have constructed starburst models for the ULIRGs based on their H_2 properties, as well as on the intensities of the recombination lines. Our models provide a consistent picture of young 1-5Myr star clusters surrounded by relatively dense PDRs which are irradiated by intense FUV fluxes. Comparison to the inner few hundred parsecs of the Milky Way indicates that the star formation efficiency in ULIRGs is 10--100 times higher than in the Galactic Center.

Ultraluminous Infrared Galaxies

Ever since their discovery in the 1970s, UltraLuminous InfraRed Galaxies (ULIRGs; classically Lir>10^12Lsun) have fascinated astronomers with their immense luminosities, and frustrated them due to their singularly opaque nature, almost in equal measure. Over the last decade, however, comprehensive observations from the X-ray through to the radio have produced a consensus picture of local ULIRGs, showing that they are mergers between gas rich galaxies, where the interaction triggers some combination of dust-enshrouded starburst and AGN activity, with the starburst usually dominating. Very recent results have thrown ULIRGs even further to the fore. Originally they were thought of as little more than a local oddity, but the latest IR surveys have shown that ULIRGs are vastly more numerous at high redshift, and tantalizing suggestions of physical differences between high and low redshift ULIRGs hint at differences in their formation modes and local environment. In this review we look at recent progress on understanding the physics and evolution of local ULIRGs, the contribution of high redshift ULIRGs to the cosmic infrared background and the global history of star formation, and the role of ULIRGs as diagnostics of the formation of massive galaxies and large-scale structures.

Diagnosing Turbulence in the Neutral and Molecular Interstellar Medium of Galaxies

Magnetohydrodynamic (MHD) turbulence is a crucial component of the current paradigms of star formation, dynamo theory, particle transport, magnetic reconnection and evolution of structure in the interstellar medium (ISM) of galaxies. Despite the importance of turbulence to astrophysical fluids, a full theoretical framework based on solutions to the Navier-Stokes equations remains intractable. Observations provide only limited line-of-sight information on densities, temperatures, velocities and magnetic field strengths and therefore directly measuring turbulence in the ISM is challenging. A statistical approach has been of great utility in allowing comparisons of observations, simulations and analytic predictions. In this review article we address the growing importance of MHD turbulence in many fields of astrophysics and review statistical diagnostics for studying interstellar and interplanetary turbulence. In particular, we will review statistical diagnostics and machine learning algorithms that have been developed for observational data sets in order to obtain information about the turbulence cascade, fluid compressibility (sonic Mach number), and magnetization of fluid (Alfvenic Mach number). These techniques have often been tested on numerical simulations of MHD turbulence, which may include the creation of synthetic observations, and are often formulated on theoretical expectations for compressible magnetized turbulence. We stress the use of multiple techniques, as this can provide a more accurate indication of the turbulence parameters of interest. We conclude by describing several open-source tools for the astrophysical community to use when dealing with turbulence.