List of Past Astronomy Colloquia : 01-Jan-2018 to 01-Jun-2018

Date:   Wednesday 31-Jan-2018
Speaker:   Dr. Niel Brandt (Penn State)
Title:  A Good Hard Look at Cosmic Supermassive Black Hole Growth

The Chandra exposure on the Chandra Deep Field-South (CDF-S) has recently been increased to 7 Ms, making it the most sensitive extragalactic X-ray survey by a wide margin. About 1050 X-ray sources have been detected, primarily distant active galactic nuclei (AGNs) and starburst/normal galaxies. The unmatched deep multiwavelength coverage for these sources allows superb follow-up investigations, revealing the details of supermassive black hole growth over most of cosmic time. I will briefly describe the sources in the 7 Ms CDF-S and some exciting first science results. The latter will include (1) relations between black-hole growth and host stellar mass; (2) constraints on supermassive black hole growth in the first galaxies as revealed by direct detection and stacking; and (3) the discovery of a representative of a new population of faint, fast X-ray transient sources. Finally, I will discuss some future prospects for X-ray surveys of AGNs in the distant universe.

Date:   Wednesday 07-Feb-2018
Speaker:   Dr. Don Brownlee (University of Washington)
Title:  What comet samples tell us about the origin of the solar system

The analysis of collected comet samples has provided direct information on the detailed nature of rocky grains that existed in cold regions of the early solar system. The typical rocky solids in the 2-50 µm size range are anhydrous materials that formed in the solar system at incandescent temperatures. It is clear that ices, organics and presolar interstellar grains could not have survived these high temperature environments. Many of the comet solids are complex assemblages that are directly comparable, in detail, with high temperature materials found in primitive meteorites. It appears that most comet grains >2µm formed in the inner solar system and were transported to distant cold regions where ices could form. The rocky components of comets are a broader mix of materials than are found in specific meteorite classes; evidence that comets contain materials from a broad range of nebular environments. It seems likely that most outer solar system bodies received similar mixes of rocky inner solar system grains. The sample-based view of comet solids is quite different from many expectations.

Date:   Wednesday 14-Feb-2018
Speaker:   Dr. Christine Wilson (McMaster University)
Title:  Nearby Galaxies with the Atacama Large Millimeter Array

ALMA is a revolutionary new telescope that is dramatically expanding our understanding of the high-frequency radio sky. Astronomers are using ALMA to make exciting new discoveries in topics ranging from star and planet formation to galaxies in the distant universe. I will begin by highlighting some of the recent exciting results that illustrate the breadth of ALMA's science. I will then describe my recent work with my students and collaborators probing the relationship between molecular gas properties and star formation in merging galaxies.

Date:   Wednesday 21-Feb-2018
Speaker:   Dr. Nora Troja (UMD/GSFC)
Title:  Neutron Star Mergers in the New Era of Multi-Messenger Astrophysics

The second run of advanced LIGO led to the breakthrough discovery of the first gravitational wave signal from a neutron star merger, GW170817, coincident with a short duration gamma-ray burst (GRB), at a distance of 40 Mpc. The discovery of GW170817 and its electromagnetic counterparts marked the beginning of a new era of multi-messenger astrophysics, in which photons and gravitational waves provide complementary views of the same source. We now have direct evidence that neutron star mergers are the progenitors of short duration gamma-ray bursts, and the dominant sites for the production of r-process elements. On-going observations of the GRB afterglow are completing the picture by probing the structure and geometry of the relativistic outflow launched after the merger. I will describe the discovery of this unique source, its evolution over the last six months, and present the first results of our multi-messenger analysis. Finally, I will discuss how this event changed our view of neutron star mergers, and why similar cases might have been missed in the past.

Date:   Wednesday 28-Feb-2018
Speaker:   Dr. Lori Glaze (GSFC)
Title:  Venus: the forgotten, mysterious planet

Compared to Earth and Mars, we know very little about the early history and evolution of Venus. Despite the likelihood that Venus was very similar to Earth after formation, the two planets followed vastly different evolutionary pathways. Venus is a planet of extremes -- it now hosts a runaway greenhouse atmosphere composed almost entirely of carbon dioxide, sulfuric acid clouds, and surface temperatures that could melt lead. When, how, and why Venus' evolution diverged from Earth is unknown, yet better understanding of why Venus is the way it is today is critical to interpreting new observations of exoplanets that have been found around other stars in our galaxy. This talk will take attendees on a tour of what we know about Venus, what mysteries we need to solve, and what future spacecraft and instrument technologies could help us answer our questions.

Date:   Wednesday 07-Mar-2018
Speaker:   Dr. Andrew Fox (STScI)
Title:  The Galactic Atmosphere: the Gaseous Halo of the Milky Way

The Milky Way provides an ideal opportunity to study the circumgalactic medium (CGM) of a star-forming spiral galaxy. High velocity clouds (HVCs) seen in H I 21 cm emission and UV absorption probe the multi-phase gas flows that circulate material from the disk to the CGM and back. In this talk I will review recent observations of HVCs and the Milky Way halo, including absorption-line spectra from the Cosmic Origins Spectrograph (COS) on Hubble and H I studies from the Green Bank Telescope. We will take a tour of different parts of the Galactic ecosystem, including the biconical outflow from the Galactic Center that drives gas into the giant Fermi Bubbles, and the Magellanic Stream, a massive tidal tail stripped from the LMC and SMC that is bearing enough gas to potentially elevate the Galactic star formation rate.

Date:   Wednesday 14-Mar-2018
Speaker:   Dr. Brad Peterson (STScI)
Title:  Exploring the Inner Structure of Active Galactic Nuclei by Reverberation

The innermost structure of active galactic nuclei (AGNs) consists of an accretion disk surrounding a supermassive black hole and, on somewhat larger scales, rapidly moving diffuse gas. The ultraviolet through near IR spectrum of AGNs is dominated by thermal continuum emission from the accretion disk and broad emission lines and absorption features from the diffuse gas. The continuum flux from the accretion disk varies with time, and the emission lines also change in brightness, or “reverberate,” in response to these variations, with a delay due to the light-travel time across the line-emitting region. Measurement of the emission-line time delay yields the size of the line-emitting region and by combining this with the emission-line Doppler width, the central black hole mass can be inferred. I will discuss results from recent “reverberation mapping” experiments, including a 179-orbit Hubble Space Telescope program, that have been designed to explore the dynamics of the emission-line gas and are yielding a wealth of new and quite surprising information about AGN structure.

Date:   Wednesday 21-Mar-2018

Date:   Wednesday 28-Mar-2018
Speaker:   Dr. Edo Berger (Harvard University)
Title:  Rattle and Shine: Joint Detection of Gravitational Waves and Light from the Binary Neutron Star Merger GW170817

The much-anticipated joint detection of gravitational waves and electromagnetic radiation was achieved for the first time on August 17, 2017, for the binary neutron star merger GW170817. This event was detected by Advanced LIGO/Virgo, gamma-ray satellites, and dozens of telescopes on the ground and in space spanning from radio to X-rays. In this talk I will describe the exciting discovery of the optical counterpart, which in turn led to several detailed studies across the electromagnetic spectrum. The results of the observations carried out by our team include the first detailed study of a "kilonova", an optical/infrared counterpart powered by the radioactive decay of r-process nuclei synthesized in the merger, as well as the detection of an off-axis jet powering radio and X-ray emission. These results provide the first direct evidence that neutron star mergers are the dominant site for the r-process and are the progenitors of short GRBs. I will also describe how studies of the host galaxy shed light on the merger timescale, and describe initial constraints on the Hubble Constant from the combined GW and EM detection.

Date:   Wednesday 04-Apr-2018
Speaker:   Dr. Rachael Beaton (Princeton University)
Title:  Engineering the Measurement of the Hubble Constant

The local expansion rate of the Universe, the Hubble constant, is one of the fundamental parameters in our current concordance cosmology and one that anchors the expansion history of the Universe. The resolution of the historical factor-of-two controversy in the Hubble constant nearly two decades ago (e.g., the Hubble Space Telescope Key Project; Freedman et al. 2001) has evolved into a 3.4-sigma tension between the traditional Cepheid-distance ladder measurements (Riess et al. 2016, Freedman et al. 2012, Freedman et al. 2001) and that determined from modelling anisotropies in the cosmic microwave background (CMB; Planck Collaboration et al. 2016). At the heart of the tension, is not only a difference in method, but also a fundamental difference in the state of the observed Universe: the distance ladder measures the local rate in the nearby universe (e.g., z~0), whereas the CMB anisotropy measurements uses the very young Universe (z ~1100).

Resolution of the tension requires (i) a full scale evaluation of the systematic effects in either technique or (ii) “new physics” added to the standard cosmological model. The trigonometric parallaxes provided by Gaia in the near term permit an unprecedented opportunity to use alternative standard candles and construct a full end-to-end distance ladder without Cepheids. The Carnegie-Chicago Hubble Program is doing just that; we are in the middle of building a new distance ladder that relies on the tip of the red giant branch (TRGB; Beaton et al. 2016). As I will demonstrate, this not only provides a direct cross-check on the Cepheid path, but there are numerous advantages to using a distance indicator that, as a standard candle from old stellar populations, is nearly ubiquitously present low-crowding and low-extinction components of galaxies. More specifically, by being able to calibrate every ‘local’ SNe Ia and easily probing ever-larger volumes with JWST and WFIRST, the TRGB-based distance ladder paves a clear path to a 1% measurement within the foreseeable future.

Date:   Wednesday 11-Apr-2018
Speaker:   Dr. Alex Hayes (Cornell University)
Title:  A Guide to Lakefront Vacationing on Titan: Glassy Seas, Fish Finders, and Magic Islands

While the spacecraft may be gone, the Cassini mission continues to deliver new and exciting discoveries throughout the Saturn system. Titan, in particular, has been unveiled as one of the most exciting and complex worlds in the solar system. In this seminar, we will review the latest discoveries surrounding Titan’s methane-based hydrologic cycle and describe the culmination of a campaign conducted through the end of the Cassini mission to map the bathymetry and composition of its hydrocarbon lakes and seas. These observations include transient phenomena, affectionately dubbed Titan’s Magic Islands, and fantastically transparent lakes that confound expectation. We will end with a review of unanswered questions and justify why returning to Titan is compelling from both geologic and astrobiologic perspectives.

Date:   Wednesday 18-Apr-2018
Speaker:   Dr. Meredith MacGregor (Carnegie/DTM)
Title:  Debris Disks as Probes of Planetary System Evolution

At least 20% of nearby main sequence stars are surrounded by disks of dusty material resulting from the collisional erosion of planetesimals, larger bodies similar to asteroids and comets in our own Solar System. The resulting dust can be observed via scattered light at visible to near-infrared wavelengths or thermal emission at mid-infrared to millimeter wavelengths. Since the dust-producing planetesimals are expected to persist in stable regions like belts and resonances, the locations, morphologies, and physical properties of dust in these ‘debris disks’ provide probes of planet formation and subsequent dynamical evolution. Observations at millimeter wavelengths are especially critical to our understanding of these systems, since the large grains that dominate emission at these long wavelengths do not travel far from their origin and therefore reliably trace the underlying planetesimal distribution. I will present ongoing work that uses observations of the angularly resolved brightness distribution and the spectral dependence of the flux density to constrain both the structure and grain size distribution of nearby debris disks. In particular, I will show new ALMA observations that place constraints on the position, width, surface density gradient, and any asymmetric structure of several well-known debris disks (including Fomalhaut, HD 32297, and HD 61005). Together these results provide an exciting foundation to investigate the dynamical evolution of planetary systems through multi-wavelength observations of debris disks.

Date:   Wednesday 25-Apr-2018
Speaker:   Dr. Brett McGuire (NOAO)
Title:  From One to Two Dimensional Interstellar Carbon: A Synthesis of Laboratory, Observations, and Theory

In the last 50 years of astrochemical research, the realm of one-dimensional carbon chemistry (i.e. carbon chain molecules) has been well explored. Life, however, relies on two and three-dimensional carbon - branches, rings, bridges, and so forth. Here, we present the first rotational detection of a six-membered ring molecule in the interstellar medium (ISM), benzonitrile (c-C6H5CN), using deep Green Bank Telescope observations of TMC-1 combined with high-precision laboratory spectroscopy. We then explore the formation chemistry of this two-dimensional carbon molecule using a combined laboratory, quantum chemical, and modeling approach. We demonstrate the synthesis of cyclic species (benzene [c-C6H6] and benzonitrile) from simple, acyclic precursors, providing definitive evidence for facile bottom-up generation of two-dimensional carbon chemistry in the ISM. The results show that benzonitrile can already be used as a reliable proxy for the presence of benzene in the ISM, and that there may exist a much larger array of 'hidden' aromatic species just beyond the current sensitivity of spectral surveys.

Date:   Wednesday 02-May-2018
Speaker:   Dr. Dana Hurley (JHUAPL) & Marcia Segura (GSFC/UMD)
Title:  Unconscious Bias and Privilege – and their impacts in our community.

Space science is a discipline that relies on collaboration of teams to make large advances in the state of knowledge. Research shows that diverse teams are more innovative and produce better results than homogenous teams. Often in space science, team members are often selected without ormal competitions, where factors such as unconscious bias, homophily, and privilege can affect the personnel selected to be on a team. We have shown that team membership by gender is statistically anomalous in planetary science spacecraft missions, with women underrepresented by 4 sigma compared to their population in the AAS Division of Planetary Science. We present best practices to be aware of these effects so that they can be consciously countered. Space sciences will achieve more innovation and discovery by working as a community to create a more inclusive environment.

Date:   Wednesday 09-May-2018
Speaker:   Dr. Aaron Evans (University of Virginia)
Title:  The Great Observatories All-sky LIRG Survey

Luminous Infrared Galaxies (LIRGs) are observed primarily to be interacting and merging galaxies. They are the sites of rampant star formation and active galactic nuclei (AGN) which are fed by abundant supplies of molecular gas. However, the very property that led to their initial discovery by IRAS as a significant galaxy population - their high infrared luminosity - also makes them difficult to study; the majority of the UV and optical light from young, massive stars and AGN is absorbed by obscuring dust and re-emitted in the infrared. The Great Observatories All-sky LIRGs Survey thus makes use of the diversity in wavelength coverage of the present space-based telescopes to probe the activity in a large (~ 100 - 200), flux-limited sample of LIRGs from the Revised Bright Galaxy Sample (RBGS). The first part of the talk will be devoted to discussing the survey as a whole. The latter part of the talk will be focussed on how our JVLA and Herschel Space Telescope observations are helping to shape the picture of star formation in extreme environments.

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