DAO Colloquium Schedule
We have a mixture of fully remote and in person talks. In person talks will take place in the LCR and will also be streamed live online via Zoom.
Tuesdays at 11am unless otherwise indicated with (***)
Tuesday August 29: Neal Evans (U. Texas at Austin), in person
Tuesday September 5: Xuan Du (DRAO), in person Recording
Tuesday September 12: Anna Ordog (UBC-O), in person Recording
Tuesday September 19: Yifan Zhou (University of Virginia), remote Recording
Tuesday September 26: Jason Rowe (Bishop’s Unversity), remote Recording
Tuesday October 3: Mike Walmsley (U. Toronto), in person Recording
Tuesday October 10: Break
Tuesday October 17: Dan Zucker (Macquarie U.), in person Recording
Tuesday October 24:Mathew Lehnert (Centre de Recherche Astrophysique de Lyon), remote Recording
Tuesday October 31: Sebastiaan Haffert (U. Arizona), remote Recording
Tuesday November 7:Jacqueline Antwi-Danso (U. Toronto), remote Recording
Tuesday November 14: Christina Williams (U. Arizona), remote
Tuesday November 21: Matt McQuinn (U. Wash.), in person
Tuesday November 28: Caitlin Casey (U. Texas at Austin), remote
Tuesday December 5: Rowan Smith (Manchester), remote
Tuesday December 12: Stella Offner (U. Texas at Austin), remote
Tuesday December 19: Kristen Dage (McGill), remote
Tues January 17, 11am, Norm Murray (CITA), in person Recording
Why the day is 24 hours long
The length of the day and the month are seen to be increasing, due to gravitational tidal torques. Geologic data suggests, however, that between about 2,000 million years ago (Ma) and 1,000 Ma, the length of day (LOD) was fixed at about 19.5 hours, while the length of the month was increasing. Following a suggestion by Zahnle and Walker in the 1980’s, I and my co-workers explore the hypothesis that the fixed LOD results from the Solar thermal atmospheric tide, which was stronger in the past, due in part to a resonance in Earth’s atmosphere. Absent this resonance, the LOD today would be around 60 hours. We use two global circulation models (or GCMs), PlaSim and LMD, to estimate the frequencies of normal modes (or free oscilliations) in Earth’s atmosphere, finding excellent agreement with recent measurements. Using the GCMs, we show that an atmospheric resonant period of 19.5 hours corresponds to a mean global surface temperature T in the range 40-50 C; the GCMs show that T could have been that high despite the lower Solar flux 2,000 Ma, if the partial pressure of CO2 was of order a tenth of a bar, compared to the present day value of 0.0004 bar. This is at the upper range of estimates 1,500 Ma based on geochemical and paleosoil evidence. Thermal tides are likely to have affected the length of day of many exoplanets.
Tues January 24, 11am, Kevin Casteels (HAA), in person Recording
Complex Spacetime and Luminosity Driven Expansion
The two pillars of modern physics, Quantum Mechanics and General Relativity, interpret reality in very different ways, the former as purely statistical, and latter purely geometrical. This apparent incompatibility can be reconciled through the use of complex math to describe spacetime. We will explore previous work done on Complex Spacetime including recent studies which show imaginary numbers aren’t merely a mathematical convenience, but essential to describe observation. A new model will be described which treats real spacetime as composed of two imaginary parts, or layers, which mix to create real space. One of the predictions of this model is that when mass is converted to energy, the real space metric expands by an amount proportional to the photon’s half wavelength. We will explore the consequences of this prediction, and calculate expansion rates for various objects. It is found that the Local Group of Galaxies produces a luminosity driven expansion rate of ~70 km/s, which is comparable to measurements of the Hubble flow. When considering galaxy clusters, the luminosity driven real space expansion would act to increase the observed velocity dispersions, giving virial mass estimates several times greater than the observed baryonic mass, potentially removing the need for large dark matter components in these systems.
Tues January 31, 11am, Melissa Graham (UWash), in person Recording
Supernova Science with Large Sky Surveys
As the endpoints of stellar evolution, sources of dust, and cosmological standard candles, supernovae are a useful and important astrophysical phenomenon to understand. In this talk I will describe how I use large sky surveys and targeted follow-up to constrain the physical nature of Type Ia supernovae (SN Ia), the thermonuclear explosions of carbon-oxygen white dwarf stars. I will also provide a look towards the future with the Rubin Observatory, which will detect millions of supernovae over its 10-year Legacy Survey of Space and Time (LSST). I am currently a research staff scientist at the University of Washington in Seattle, and I hold the roles of Data Management Science Analyst and Lead Community Scientist for the Rubin Observatory. This talk will also cover how individuals without Rubin data rights can participate in LSST science, and provide an inside look at how Rubin staff are preparing themselves — and the science community — for the data revolution of the LSST.
Tues February 7, 11am, Joan Najita (NOIRLab), on zoom Recording
Clues to Our History from Debris Disks and the Dynamics of Andromeda’s Halo
Abstract: The patterns we notice in astronomical data can provide simple but valuable clues to how systems evolve and to our own origins. I’ll share two examples. (1) The similar sizes of the spectacular rings observed in protoplanetary disks and debris disks suggest that they share a common origin. New calculations of the evolution of rings of pebbles and planetesimals lead us to a simple picture in which large protoplanetary disks evolve into the known bright debris disks, with our Solar System following a distinct evolutionary path that originates in compact disks. (2) Data recently obtained by DESI, the highly multiplexed multi-object spectrograph on the 4m Mayall Telescope on Kitt Peak, reveal delicate structures—streams, wedges, and chevrons—in the positions and velocities of individual stars: evidence of a recent galactic immigration event in exquisite detail. The observations may open a window onto our past, offering a view of what our own galaxy may have looked like billions of years ago.
Tues February 14, 11am, Jessie Christiansen (Caltech), in person Recording
Towards an Exoplanets Demographics Ladder
The NASA Kepler mission has provided its final planet candidate catalogue, the K2 mission has contributed another four years’ worth of data, and the NASA TESS mission has been churning out new planet discoveries at a rapid pace. The demographics of the exoplanet systems probed by these transiting exoplanet missions are complemented by the demographics probed by other techniques, including radial velocity, microlensing, and direct imaging. I will walk through the progress of the Kepler occurrence rate calculations, including some of the outstanding issues that are being tackled. I will present our new results from K2 and TESS, and outline how K2 and TESS will able to push the stellar parameter space in which we can explore occurrence rates beyond that examined by Kepler. Finally, I will highlight some of the pieces of the larger demographics puzzle – occurrence rate results from the other techniques that probe different stellar and exoplanet regimes – and progress to be made working to join those pieces together.
Thursday March 9, 11am, Mark Voit (MSU), in person Recording
Whether or not the mass of a supermassive black hole (MBH) is causally linked to the mass of the halo around it (Mhalo) has been hotly debated for at least two decades. I will present evidence supporting a direct proportionality between MBH and the binding energy of the halo’s baryons that extends from Milky Way scales up to at least Mhalo ~ 10^14 MSun. That relationship is consistent with models of black-hole feedback that rely on cumulative kinetic energy injection to quench star formation by lifting the halo’s baryons. I will also show why the IllustrisTNG simulations do not reproduce the observed MBH-Mhalo relationship, even though baryon lifting through cumulative kinetic energy injection appears to be what quenches star formation in those simulations.
Tues March 14, 11am, Alex Cameron (Oxford), in person Recording
Chemical evolution of galaxies from the early Universe to the present day
Rest-frame optical emission lines offer one of our most powerful and well-studied probes of chemical abundances in the ISM of galaxies. With the advent of JWST/NIRSpec, we can now study rest-optical emission spectra at z>9, within the first 0.5 Gyr after the Big Bang, allowing us to measure metallicities in the interstellar medium (ISM) of galaxies over almost the entirety of cosmic history.
Meanwhile, integral-field unit (IFU) spectrographs on current and future ground-based telescopes are enabling new constraints to be placed on chemical abundances in different phases of galaxies, such as from emission lines in extended diffuse emission. The combination of these cutting-edge facilities presents an opportunity to track chemical enrichment throughout the Baryon cycle in unprecedented detail.
In this talk I will present early results from the JWST Advanced Deep Extragalactic Survey (JADES), a combined NIRSpec + NIRCam large GTO program studying galaxy evolution from z~2 out to the highest redshifts observable with JWST. In particular, I will focus on what these (and other) early JWST observations are already telling us about the ISM conditions in high redshift galaxies, including some very intriguing findings from a recently published spectrum of GN-z11 (an ultra-luminous galaxy at z=10.60).
I will also present results from the DUVET survey, an IFU survey targeting low-redshift starburst galaxies, in which we have present direct measurements of outflow metallicity, affording new constraints on how these galaxy-scale gas flows are shaping the chemical evolution of galaxies.
However, deriving chemical abundances is notoriously susceptible to systematic uncertainties.
I will discuss our recent efforts to model the emission lines measurements from outflows, which are helping to unpick the systematic uncertainties involved in making these novel outflow metallicity measurements.
Tues March 28, 11am, Tarraneh Eftekhari (Northwestern), in person
Uncovering the Elusive Origin of Fast Radio Bursts
Upgrades in multiple fast radio burst (FRB) experiments have led to a growing sample of precisely localized events, enabling host galaxy associations and detailed observations of the immediate environments surrounding FRBs. Such observations play a key role in elucidating the stellar populations that give rise to FRB progenitors. Indeed, host galaxy demographics, as well as the spatial offsets of FRBs from their host galaxy centers, can be used to inform progenitor channels. The localizations of two repeating FRBs to dwarf galaxies, their coincidence with persistent radio sources, and their large observed excess dispersion measures (DMs) stand in stark contrast to other localized events, which generally reside in more massive galaxies and exhibit modest excess DMs. Understanding this dichotomy among FRB hosts will provide critical insight into the stellar populations prevalent in FRB host galaxies and hence their likely progenitors. In this talk, I will review our current knowledge of FRB progenitors based on the properties of a small, but growing sample of host galaxies. I will also outline key follow-up observations that will lend to detailed characterizations of the galactic and local environments of FRBs, thereby shedding light on their progenitor channels.
Tues April 4, 11am, Kartheik Iyer (Columbia), on zoom Recording
From cosmic dawn to cosmic noon: studying galaxy evolution using star formation histories in the era of JWST
A diverse range of physical processes are responsible for regulating star formation across galaxies. Understanding their relative contributions to galaxy growth and quenching at different epochs is one of the key questions in galaxy evolution today. Since these processes are thought to have characteristic timescales, studying their imprints on galaxy star formation histories (SFHs) helps quantify their contributions for a population of galaxies. In this talk, I will describe a framework for modeling galaxies as Gaussian processes, that can be applied to both theory and observations to (i) establish a formalism to study the variability of star formation across different timescales and analyze cosmological galaxy evolution simulations, and (ii) reconstruct the star formation histories of galaxies observed using HST and JWST (from the CANUCS and CEERS surveys). Taken together, simulations and observations leverage predictive power against observational constraints, and allow us to develop a fuller picture of how galaxies evolve over time.
Tues April 11, 11am, Melissa Amenouche (HAA), in person Recording
Tues April 18, 11am, Leonardo Ferreira (UVic), in person Recording
The Optical Morphologies of Galaxies beyond z~3 with JWST
The launch of JWST with its powerful infrared capabilities has opened up a new window for studying the resolved optical morphologies of galaxies beyond z~3. Prior to this, morphology studies in the early Universe were limited to the ultraviolet portion of the electromagnetic spectrum. This narrowed our ability to track the bulk of stellar masses in high redshift galaxies, as the UV mainly captures clumpy star formation and other energetic phenomena. We present our findings from a visual classification effort of 4265 galaxies observed with JWST at $1.5 < z < 8$ in the JWST CEERS observations that overlap with the CANDELS EGS field. By employing a simple classification scheme to produce disk/spheroid/peculiar classifications, we find a strong dominance of morphologically selected disk galaxies up to z~8, a far higher redshift than previously thought possible. Our results suggest that most stars in the Universe were likely formed in a disk galaxy, and that these morphological populations are consistent with predictions from cosmological simulations.
Tues April 25, 11am, Hao He (McMaster), on zoom Recording
Evolution of Giant Molecular Clouds in Nearby Starburst Mergers
Starburst mergers represent a common population of galaxies in early universe and hence are great laboratories to study the star formation (SF) in these most extreme environments. Due to the rarity of mergers in the local universe, we do not have large observed samples that are close enough to study individual giant molecular cloud (GMC) properties. In this work, we employ idealized galaxy merger simulation based on the Feedback In Realistic Environments (FIRE-2) physics model to study how the properties of GMCs evolve at different merging stages. We conduct a pixel-by-pixel analysis of molecular gas properties in both the simulated control galaxies and galaxy major mergers. The simulated GMCs in the control galaxies follow a similar trend in a diagram of velocity dispersion (σv) versus gas surface density (Σmol) to the one observed in local spiral galaxies in the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. For GMCs in simulated mergers, we see a significant increase of factor of 5 – 10 in both Σmol and σv during the second passage along with the fact that more than 50% of molecular gas is concentrated in the central 1 kpc region. GMCs during the second passage also exhibit much higher virial parameters (αvir) reaching 10 – 100, which suggests they are much less gravitationally bound compared to GMCs in normal spiral galaxies. Furthermore, we find that the increase in αvir happens at the same time as the increase in global star formation rate (SFR), which suggests stellar feedback is responsible for dispersing the gas. We also find that the gas depletion time (tdep) is significantly lower for high αvir GMCs during a starburst event. This is in contrast to the simple physical picture that low αvir GMCs are easier to collapse and form stars on shorter depletion times. This might suggest that some other physical mechanisms besides self-gravity are helping the GMCs in starbursting mergers collapse and form stars. Instead, we find significant anti-correlation between average Σmol for GMCs and tdep, which might be due to higher fraction of dense gas that leads to shorter tdep. In the future, we will gather a larger sample of observed starburst mergers (~40) with GMC-resolution CO 2-1 ALMA archival data to perform a comprehensive comparison between observation and simulation.
Friday April 28, 11am, Katherine de Kleer (CalTech), in person Recording
Tues May 9, 11am, Pallavi Patil (NRAO), in person Recording
Tues May 16, 11am, Alberto Saldana-Lopez (University of Geneva), in person Recording
A low-redshift look to reionization with star-forming galaxies
One of the current hot topics in galaxy evolution is the so-called Cosmic Reionization, the last major cosmic phase by which the Universe transitioned from a neutral to a (mostly) ionized state, happening around z = 6-9. The main responsible sources for reionization are still under debate, but star-forming galaxies seem to be the likeliest due to their high number density at early epochs respect to Active Galactic Nuclei.
However, directly probing ionizing (also called Lyman Continuum, LyC) radiation from galaxies at the Epoch of Reionization is difficult, since the escaping photons are likely absorbed by the remaining neutral gas in the intergalactic medium (IGM). Therefore, in order to study the role of star-forming galaxies during Reionization, we need to rely on indirect indicators of such emergent LyC radiation. The low-redshift Universe offers a unique window to study the properties of LyC-emitting galaxies, since the IGM attenuation is negligible and, at the same time, a full suite of multi-wavelength observations is available both from space and the ground.
Recent HST campaigns such as the Low-Redshift Lyman Continuum Survey (LzLCS) has nearly tripled the number of LyC detections at low-redshift. The LzLCS has revealed that compact star formation, high ionization parameters, strong and narrow LyA emission, low dust-attenuation and the presence weak absorption lines seem to characterize the spectra of the strongest LyC emitters. In this talk, I will present some of the main results of the LzLCS and explain how this data set has changed our understanding of the mechanisms for LyC escape. We will also discuss the implications of these results for the high redshift community in the JWST era.
Tues June 6, 11am, Vincent Henault-Brunet (St Mary’s), remote Recording
Tues June 20, 11am, Matías Bravo (McMaster), In person Recording