Astrophysics Colloquium

Virtual Colloquium Meetings are held via Zoom.  Meeting information will be sent in email.  


5/20/20  10am - Günther Hasinger (European Space Agency) 
Title:  Is the Dark Matter made of Primordial Black Holes?
Abstract:  The recent interpretation of cold dark matter as the sum of contributions of different mass Primordial Black Hole (PBH) families could explain a number of so far unsolved mysteries, like e.g. the massive seed black holes required to create the supermassive black holes in the earliest QSOs, the ubiquitous massive LIGO/VIRGO massive binary black holes, or even the putative "Planet X" PBH in our Solar System. The most abundant family of PBH should be around the Chandrasekhar mass (1.4 Msun). This prediction may already have been vindicated by the recent OGLE/GAIA discovery of a sizeable population of putative black holes in the mass range 1-10 Msun.
PBH can also have an important contribution to the extragalactic background light in several wavebands. To check this hypothesis I assume a realistic 1E-8 -1E10 Msun PBH mass distribution providing the bulk of the dark matter, consistent with all observational constraints. I estimate the contribution of baryon accretion onto this PBH population to various cosmic background radiations, concentrating first on the cross-correlation signal between the Cosmic X-ray and the Cosmic infrared background fluctuations discovered in deep Chandra and Spitzer surveys. I assume Bondi capture and advection dominated disk accretion with reasonable parameters like baryon density and effective relative velocity between baryons and PBH, as well as appropriate accretion and radiation efficiencies, and integrate these over the PBH mass spectrum and cosmic time. The prediction of the PBH contribution in X-rays is indeed consistent with the residual X-ray background signal and the X-ray/infrared fluctuation signal. The predicted flux peaks at z~17-30, consistent with other constraints requiring the signal to come from such high redshifts. The PBH contribution to the 2-5 micron cosmic infrared background fluctuations is only about 1%, so that these likely come from star formation processes in regions associated with the PBH.
I discuss a number of other phenomena, which could be significantly affected by the PBH accretion. Magnetic fields are an essential ingredient in the Bondi capture process, and I argue that the PBH can play an important role in amplifying magnetic seed fields in the early universe and maintaining them until the galactic dynamo processes set in. Next I study the contribution of the assumed PBH population to the re-ionization history of the universe and find that they do not conflict with the stringent ionization limits set by the most recent Planck measurements. X-ray heating from the PBH population can provide a contribution to the entropy floor observed in groups of galaxies. The tantalizing redshifted 21-cm absorption line feature observed by EDGES could well be connected to the radio emission contributed by PBH to the cosmic background radiation. Finally, the number of intermediate-mass black holes and the diffuse X-ray emission in the Galactic Center region are not violated by the assumed PBH dark matter, on the contrary, some of the discrete sources resolved in the deepest Chandra observations of the Galactic Ridge could indeed be accreting PBH.

6/3/20 12pm - Mark Wyatt (Institute of Astronomy, Cambridge)
Title:  Exocomets and their effect on planet atmospheres
Abstract:  It is known that ~20% of nearby stars host planetesimal belts orbiting 10s of au from the star. For a growing number CO gas has been detected coincident with the planetesimal belts showing that their planetesimals have a similar composition to Solar System comets. It is expected that some of these planetesimals may be perturbed into the inner regions of the system where they may collide with any planets residing there. The hot dust seen in several systems may be evidence of such comet-like dynamics, and in one system this picture is reinforced by the detection of CO close to the CO2 sublimation radius. This talk will present the evidence for the aforementioned scenario and consider the effect of collisions with such an exocomet population on the atmospheres of inner planets. These atmospheres can be stripped in collisions, but can also be enhanced by the delivery of volatiles, in a way that can be quantified from simulations of impacts. It will be shown that whether an atmosphere grows or depletes can be inferred from the planet’s mass and semimajor axis (for given assumptions about the cometary impactors); the atmospheres of close-in exoplanets like those of TRAPPIST-1 will deplete while those at larger separation will grow in impacts. The Earth sits at the boundary, where more detailed consideration of the impactor populations finds that its bombardment history would lead to an atmosphere similar to the present one regardless of its initial mass.

6/11/20  12pm - Ewine van Dishoeck (Leiden Observatory)
Title:  Zooming in on planet-forming disks with ALMA
Abstract:  Thanks to ALMA, the study of protoplanetary disks is undergoing a revolution, with a wide variety of results being reported on statistics of disks and fascinating substructures in dust and gas. This talk will present a few recent results from our group and collaborators on ALMA observations and associated models of disks. First, the latest statistics on disk masses in nearby star-forming regions will be presented, including low vs high-mass
regions and young vs more evolved disks. An important conclusion from comparison of disk dust masses with the solid mass in exoplanets is that planet formation must start early.
Second, the differences in sizes of gas vs dust disks will be discussed. Do the much smaller dust disks indeed provide evidence for radial drift of mm-sized dust grains as often claimed?  How do the large disks imaged by the DSHARP team fit into the overall disk population? Third, possible explanations for the surprisingly weak CO
emission will be discussed, including models in which CO is chemically transferred into other species. Consequences for the C/O ratios in gas and ice will be presented. Special attention will be given to transitional disks, which are a subset of disks with evidence for sharp-rimmed cavities (gaps or holes).  They are the best candidate sources for harboring just-formed giant planets.

6/17/20  12pm - Renske Smit (Institute of Astronomy, Cambridge)
Title:  Galaxies growing up in the Epoch of Reionisation
Abstract:  During the first billion years of cosmic time the first galaxies in the Universe form. As these systems grow, they are thought to reionise the pervading neutral gas in the intergalactic medium. In the last decade, large samples of these galaxies have been identified with the Hubble Space Telescope. Despite this remarkable progress, the physical properties of these galaxies are still largely unknown. I will describe the step-by-step progress that has been made over the last fews years in uncovering the stellar populations, radiation fields and even the first dynamical measurements of some of these earliest-known sources of light in the Universe. In particular, I will show how the Atacama Large Millimetre Array is currently transforming this field by identifying and characterising some of the most massive ISM reservoirs in the first billion years.

6/24/20  12pm - Rodrigo Ibata (University of Strasbourg)
Title:  Following the Streams of the Gaping Abyss
Abstract: The Galactic halo is criss-crossed by long stellar streams that are probably the remnants of defunct globular clusters and dwarf galaxies. I will present the recent discoveries of these structures including several streams that can be traced back to well known halo features. While streams clearly inform us in a direct way about past accretions onto our Galaxy, their most promising property is that they allow us to measure the Galactic acceleration field and they may possibly allow us to reveal the presence of small-scale of dark matter overdensities in the halo. I will present our initial results on the acceleration field, and discuss the future prospects of this work.