Multiphase turbulent interstellar medium: some recent results from radio astronomy

The radio frequency 1.4 GHz transition of the atomic hydrogen is one of the important tracers of the diffuse neutral interstellar medium. Radio astronomical observations of this transition, using either a single dish telescope or an array interferometer, reveal different properties of the interstellar medium. Such observations are particularly useful to study the multiphase nature and turbulence in the interstellar gas. Observations with multiple radio telescopes have recently been used to study these two closely related aspects in greater detail. Using various observational techniques, the density and the velocity fluctuations in the Galactic interstellar medium was found to have a Kolmogorov-like power law power spectra. The observed power law scaling of the turbulent velocity dispersion with the length scale can be used to derive the true temperature distribution of the medium. Observations from a large ongoing atomic hydrogen absorption line survey have also been used to study the distribution of gas at different temperature. The thermal steady state model predicts that the multiphase neutral gas will exist in cold and warm phase with temperature below 200 K and above 5000 K respectively. However, these observations clearly show the presence of a large fraction of gas in the intermediate unstable phase. These results raise serious doubt about the validity of the standard model, and highlight the necessity of alternative theoretical models. Interestingly, numerical simulations suggest that some of the observational results can be explained consistently by including the effects of turbulence in the models of the multiphase medium. This review article presents a brief outline of some of the basic ideas of radio astronomical observations and data analysis, summarizes the results from recent observations, and discusses possible implications of the results.Comment: 20 pages, 10 figures. Invited review accepted for publication in the Proceedings of the Indian National Science Academy. The definitive version will be available at http://insaindia.org/journals/proceedings.ph

Next Generation Very Large Array Memo No. 6, Science Working Group 1: The Cradle of Life

This paper discusses compelling science cases for a future long-baseline interferometer operating at millimeter and centimeter wavelengths, like the proposed Next Generation Vary Large Array (ngVLA). We report on the activities of the Cradle of Life science working group, which focused on the formation of low- and high-mass stars, the formation of planets and evolution of protoplanetary disks, the physical and compositional study of Solar System bodies, and the possible detection of radio signals from extraterrestrial civilizations. We propose 19 scientific projects based on the current specification of the ngVLA. Five of them are highlighted as possible Key Science Projects: (1) Resolving the density structure and dynamics of the youngest HII regions and high-mass protostellar jets, (2) Unveiling binary/multiple protostars at higher resolution, (3) Mapping planet formation regions in nearby disks on scales down to 1 AU, (4) Studying the formation of complex molecules, and (5) Deep atmospheric mapping of giant planets in the Solar System. For each of these projects, we discuss the scientific importance and feasibility. The results presented here should be considered as the beginning of a more in-depth analysis of the science enabled by such a facility, and are by no means complete or exhaustive.Comment: 51 pages, 12 figures, 1 table. For more information visit https://science.nrao.edu/futures/ngvl

Sensitivity Projections for Dark Matter Searches with the Fermi Large Area Telescope

The nature of dark matter is a longstanding enigma of physics; it may consist of particles beyond the Standard Model that are still elusive to experiments. Among indirect search techniques, which look for stable products from the annihilation or decay of dark matter particles, or from axions coupling to high-energy photons, observations of the $\gamma$-ray sky have come to prominence over the last few years, because of the excellent sensitivity of the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope mission. The LAT energy range from 20 MeV to above 300 GeV is particularly well suited for searching for products of the interactions of dark matter particles. In this report we describe methods used to search for evidence of dark matter with the LAT, and review the status of searches performed with up to six years of LAT data. We also discuss the factors that determine the sensitivities of these searches, including the magnitudes of the signals and the relevant backgrounds, considering both statistical and systematic uncertainties. We project the expected sensitivities of each search method for 10 and 15 years of LAT data taking. In particular, we find that the sensitivity of searches targeting dwarf galaxies, which provide the best limits currently, will improve faster than the square root of observing time. Current LAT limits for dwarf galaxies using six years of data reach the thermal relic level for masses up to 120 GeV for the $b\bar{b}$ annihilation channel for reasonable dark matter density profiles. With projected discoveries of additional dwarfs, these limits could extend to about 250 GeV. With as much as 15 years of LAT data these searches would be sensitive to dark matter annihilations at the thermal relic cross section for masses to greater than 400 GeV (200 GeV) in the $b\bar{b}$ ($\tau^+ \tau^-$) annihilation channels.Comment: Updated with a few additional and corrected references; otherwise, text is identical to previous version. Submitted on behalf of the Fermi-LAT collaboration. Accepted for publication in Physics Reports, 59 pages, 34 figures; corresponding author: Eric Charles ([email protected]

Galaxy Collisions - Dawn of a New Era

The study of colliding galaxies has progressed rapidly in the last few years, driven by observations with powerful new ground and space-based instruments. These instruments have used for detailed studies of specific nearby systems, statistical studies of large samples of relatively nearby systems, and increasingly large samples of high redshift systems. Following a brief summary of the historical context, this review attempts to integrate these studies to address the following key issues. What role do collisions play in galaxy evolution, and how can recently discovered processes like downsizing resolve some apparently contradictory results of high redshift studies? What is the role of environment in galaxy collisions? How is star formation and nuclear activity orchestrated by the large scale dynamics, before and during merger? Are novel modes of star formation involved? What are we to make of the association of ultraluminous X-ray sources with colliding galaxies? To what do degree do mergers and feedback trigger long-term secular effects? How far can we push the archaeology of individual systems to determine the nature of precursor systems and the precise effect of the interaction? Tentative answers to many of these questions have been suggested, and the prospects for answering most of them in the next few decades are good.Comment: 44 pages, 9 figures, review article in press for Astrophysics Update Vol.

Working Papers: Astronomy and Astrophysics Panel Reports

The papers of the panels appointed by the Astronomy and Astrophysics survey Committee are compiled. These papers were advisory to the survey committee and represent the opinions of the members of each panel in the context of their individual charges. The following subject areas are covered: radio astronomy, infrared astronomy, optical/IR from ground, UV-optical from space, interferometry, high energy from space, particle astrophysics, theory and laboratory astrophysics, solar astronomy, planetary astronomy, computing and data processing, policy opportunities, benefits to the nation from astronomy and astrophysics, status of the profession, and science opportunities

A census of Herbig Ae/Be stars: new candidates and analysis from a Gaia perspective

Herbig Ae/Be stars are pre-main sequence sources, canonically defined as having masses 2 Msun < M < 10 Msun, which are key to understanding the differences in formation mechanisms between low- and high-mass stars. The study of the general properties of these objects is hampered by the lack of a well-defined, homogeneous sample, and because few and mostly serendipitously discovered sources are known. As a consequence, many open problems involving high-mass star formation suffer from biases and lack of completeness. I study the general properties of the 252 known and proposed Herbig Ae/Be stars with parallaxes from Gaia DR2 at the start of this doctoral thesis. High-mass stars were found to have a much smaller infrared excess and optical variability compared to lower-mass stars, with the break at around 7 Msun. Different or differently acting (dust-) disc-dispersal mechanisms are proposed for this break. The variability indicator developed in this dissertation shows that approximately 25% of all Herbig Ae/Be stars are strongly variable. Evidence is provided to support the hypothesis that this variability is in most cases due to asymmetric dusty disc structures seen edge-on. Using that well-characterised sample of Herbig Ae/Be stars as a training sample for a bespoke machine learning algorithm, a homogeneous and well-defined catalogue of 8470 new pre-main sequence candidates was obtained. In parallel, a catalogue of 693 new classical Be candidates was produced. At least 1361 sources are potentially new Herbig Ae/Be stars according to their position in the Hertzsprung-Russell diagram. This increases the number of known objects of the class by an order of magnitude. In addition, I discuss the results of independent spectroscopic observations conducted for a selection of 145 new Herbig Ae/Be and 14 new classical Be stars. These independent observations further confirm the quality and the accuracy of the classification. I conclude with an analysis of the general properties of the new catalogues that validates the results and conclusions obtained for the set of previously known Herbig Ae/Be stars

Modeling the Dust Properties of z ~ 6 Quasars with ART^2 -- All-wavelength Radiative Transfer with Adaptive Refinement Tree

The detection of large quantities of dust in z ~ 6 quasars by infrared and radio surveys presents puzzles for the formation and evolution of dust in these early systems. Previously (Li et al. 2007), we showed that luminous quasars at z > 6 can form through hierarchical mergers of gas-rich galaxies. Here, we calculate the dust properties of simulated quasars and their progenitors using a three-dimensional Monte Carlo radiative transfer code, ART^2 -- All-wavelength Radiative Transfer with Adaptive Refinement Tree. ART^2 incorporates a radiative equilibrium algorithm for dust emission, an adaptive grid for inhomogeneous density, a multiphase model for the ISM, and a supernova-origin dust model. We reproduce the SED and dust properties of SDSS J1148+5251, and find that the infrared emission are closely associated with the formation and evolution of the quasar host. The system evolves from a cold to a warm ULIRG owing to heating and feedback from stars and AGN. Furthermore, the AGN has significant implications for the interpretation of observation of the hosts. Our results suggest that vigorous star formation in merging progenitors is necessary to reproduce the observed dust properties of z~6 quasars, supporting a merger-driven origin for luminous quasars at high redshifts and the starburst-to-quasar evolutionary hypothesis. (Abridged)Comment: 26 pages, 22 figures, accepted by ApJ. Version with full resolution images is available at http://www.cfa.harvard.edu/~yxli/ARTDUST/astroph0706.3706.pd