Upcoming events
Apr 26, 2024
Stellar-Stellar-Dark matter halo mass relationship
Kumail Zaidi (Tufts University)
I present the evolution of the relationship between galaxy stellar mass and their host dark matter halo mass since the Universe was only about a billion years old (z ~ 5) until today. The galaxy-halo mass relationship was constrained through the measurements of two-point clustering of galaxies and stellar mass functions in the UDS and COSMOS fields in concert with Halo Occupation Distribution (HOD) modeling. The use of two large fields covering an effective area of ~1.6 square degrees was vital in lowering the measurement uncertainties due to cosmic variance. On the other hand, I developed novel redshift scaling for the HOD model which allowed tracing a continuous evolution of stellar-halo mass relationship (SHMR) out to z~5. This study confirms the consensus in the literature that the efficiency of dark matter halos in converting baryons into stellar mass peaks at ~20%, at a halo mass of ~10^12 M⊙, out to z~5. Furthermore, from the resulting SHMR evolution, I also show how the role of Active Galactic Nuclei (AGN) feedback, in suppressing star formation, evolves with time. This is highly illuminating for the simulations.
Past events Fa’23
Apr 12, 2024
Don’t Ignore Your Systematics: A Tale In Two Epochs
Andrew Mizener (UMass Amherst)
Systematic uncertainties are unavoidable in the course of research and must be handled with great care in order to produce clear and reliable results. In this talk, I discuss my experience understanding and quantifying systematic uncertainties in two otherwise unrelated projects: one involving molecular gas and dust in a low-mass galaxy at high redshift, and one attempting to identify the systematics inherent to photometric SED fitting. In particular, I discuss how different ways of handling a priori unknown quantities such as alphaCO and Rv can alter your interpretation of the same data. We’ll make stops along the way to chat about dusty galaxies, the history of population and spectral synthesis, and some of the assumptions baked into SED fitting as a technique.
Apr 5, 2024
Learning Galaxy Intrinsic Alignment Correlations
Sneh Pandya (Northeastern University)
The intrinsic alignments (IA) of galaxies, regarded as a contaminant in weak lensing analyses, represents the correlation of galaxy shapes due to gravitational tidal interactions and galaxy formation processes. As such, understanding IA is paramount for accurate cosmological inferences from weak lensing surveys; however, one limitation to our understanding and mitigation of IA is expensive simulation-based modeling. Neural networks and deep learning have found recent wide-spread success in the physical sciences for their ability to learn complex, non-linear relationships within data and their accelerated inference capabilities with powerful GPUs.
In this talk, I will describe recent work on a deep learning approach to emulate galaxy position-position (ξ), position-orientation (ω), and orientation-orientation (η) correlation function measurements and uncertainties from halo occupation distribution-based mock galaxy catalogs.
Mar 29, 2024
Dust in galaxy formation model and SED construction pipeline
Pipit Triani from CfA | Harvard & Smithsonian
Dust is one of the main contributors to the observed spectral energy distribution (SED) of galaxies in the ultraviolet to infrared regimes. I will present the Dusty SAGE semi-analytic model which incorporates detailed dust tracking into a simulation of galaxy and halo growth. Dust SAGE follows the condensation of dust in the stellar ejecta, dust locked in star formation, removal of dust by outflows and gas reheating and dust reincorporation through gas infall. The model successfully reproduces the local dust scaling relations while simultaneously matching many observed galaxy properties. Using Dusty SAGE, I investigate the origin of dust across cosmic time. I find that grain growth is the dominant dust producer in the local Universe (z<3), while stellar dust production dominates in early galaxies. In agreement with the observation of dust abundance in the circumgalactic medium (CGM), Dusty SAGE also predicts a large dust reservoir in the halo, which is explained by the large outflow rate. Then, I use the dust and stellar properties from the model to generate the UV to IR emission self-consistently. The model reproduces the z=0 luminosity functions from observation but is less successful at higher redshift. I found that the far-UV emission is sensitive to the dust processes and AGN feedback. Altering these parameters improves the agreement of the far-UV luminosity function at z=2 and z=3. However, further study of how these parameters behave during galaxy evolution is needed to consistently reproduce the UV toIR emission across a wide redshift range.
Mar 15, 2024
Studying magnetic fields, dynamics, and fundamental physics near a black hole with current and future mm-VLBI instruments
Freek Roelofs, Center for Astrophysics | Harvard & Smithsonian
The Event Horizon Telescope (EHT) has imaged the black hole shadows of the supermassive black hole at the center of the galaxy M87 (M87*) and at the center of the Milky Way (Sgr A*). Polarimetric imaging of M87* with the EHT enabled significantly stronger inferences on the black hole and accretion parameters than total intensity data alone. Geometric modeling was a central tool for studying the structure of M87* and Sgr A* in total intensity. In the first part of the talk, I will show the results of fitting a new polarimetric “m-ring” geometric model to EHT observations of M87*. Our geometric modeling results are generally consistent with imaging methods, but they also enable studies of the black hole when imaging methods struggle, such as nights with sparse coverage or weak signals.
In the second part of the talk, I will focus on plans and science goals for future instruments. The Next-Generation EHT (ngEHT) will be a transformative enhancement of the EHT, with array expansions and improvements allowing for, e.g., high dynamic range imaging of AGN jets, and for real-time movie reconstructions of variable sources like Sgr A*. With the ngEHT Analysis Challenges, we explore the science capabilities of the ngEHT, and develop new analysis algorithms capable of analyzing the large and complex ngEHT datasets. Finally, I will give a brief overview of several proposals to image black holes using space-based telescopes, which achieve order-of-magnitude angular resolution improvements compared to what is attainable from the ground and allow for high-precision tests of general relativity.
Mar 13, 2024
Cracking Neutron Star Mysteries: Physics Driven Insights into Interior, surface, and Exterior by X-ray Timing results
Chun Huang (Washington University in St. Louis)
The utilization of Pulse Profile modeling techniques could simultaneously provide measurements for Neutron star Mass Radius and offer insights into their hotspot distribution. Presently, these methodologies rely on meta models of the equation of state (EOS) and employ pure geometrical cap overlapping techniques to align with observational data. This presentation aims to showcase our recent results about utilizing a physics-oriented EOS to explore the constraint of X-ray timing observations on fundamental physics parameters governing neutron stars structure.
Furthermore, I will present a Physics-motivated computation of neutron star hotspot temperature maps given by PSR J0030+0451 NICER measurement about its magnetic configuration. This computation reveals tensions between the physics-motivated temperature map and proposed hotspot configurations derived from NICER observation inference and inconsistent with its observation pulse profile itself. This motivate that we need to introduce higher multipole components and integrate them into the current computation to explain the observation data better. In the future, with better instruments like STROBE-X and eXTP, a better understanding of neutron star matter and neutron star hotspot map could be reached, by using these techniques described.
Mar 8, 2024
Feedback in Chaotic Environments: Tying Extremely Powerful AGN Feedback to Beads-on-a-String Star Formation in a Merger-Rich Galaxy Cluster
LOsase Omoruyi (CfA | Harvard & Smithsonian)
With two central galaxies engaged in a major merger and a remarkable chain of 19 young stellar superclusters wound around them in projection, the galaxy cluster SDSS J1531+3414 (z=0.335) offers an excellent laboratory to study the interplay between mergers, AGN feedback, and star formation. New Chandra X-ray imaging has revealed rapidly cooling intracluster gas, with two “wings” forming a concave density discontinuity in the cluster’s central 100 kpc. Low-frequency LOFAR observations uncover diffuse radio emission strikingly aligned with the “wings,” suggesting that they are actually the opening to a colossal X-ray supercavity. The steep radio emission is likely an ancient relic of one of the most energetic AGN outbursts observed to date, with 4pV > 10^61 erg. North of the supercavity, GMOS and ALMA detect a ~100 billion solar mass reservoir of multiphase gas surrounding the young stars but redshifted up to + 800 km/s with respect to the southern central galaxy. We propose that the multiphase gas cooled from low-entropy gas entrained by the X-ray supercavity, and suggest that tidal interactions stimulated the beads on a string” star formation morphology. Our work not only illuminates how large-scale AGN feedback and galactic dynamics regulate localized star formation processes in SDSS J1531+3414, but also provides potential insights into the feedback processes that shaped the earliest galaxy clusters, which are characterized by similarly dynamic environments.the supercavity, GMOS and ALMA detect a ~100 billion solar mass reservoir of multiphase gas surrounding the young stars but redshifted up to + 800 km/s with respect to the southern central galaxy. We propose that the multiphase gas cooled from low-entropy gas entrained by the X-ray supercavity, and suggest that tidal interactions stimulated the beads on a string” star formation morphology. Our work not only illuminates how large-scale AGN feedback and galactic dynamics regulate localized star formation processes in SDSS J1531+3414, but also provides potential insights into the feedback processes that shaped the earliest galaxy clusters, which are characterized by similarly dynamic environments.
Feb 23, 2024
A New AGN Photoionization Model and Multi-wavelength Metallicity Diagnostics
Peixin Zhu (CfA | Harvard & Smithsonian)
The photoionization model of narrow-line regions (NLRs) in active galactic nuclei (AGNs) has been investigated for decades. Many published models are restricted to simple linear scaling abundance relations, dust-free assumption, uniform AGN radiation field, and using one specific photoionization code, which restricts them from providing a satisfactory prediction on a broad range of AGN observations. Through a comprehensive investigation, here we present how the choice of abundance scaling relations, dust inclusion, AGN radiation fields, and different photoionization codes CLOUDY and MAPPINGS affect the predictions on the strength of strong UV, optical, and infrared emission lines. We find that the dust-depleted radiation pressure-dominated AGN model built with the latest non-linear abundance sets and photoionization code MAPPINGS V is consistent with AGN observations across a broad range of wavelengths. We also assess AGN metallicity diagnostics in the optical, UV, and IR wavelengths.
Feb 16, 2024
Exploring Systematic Errors in the Inferred Parameters of the Transiting High-resolution SMA and Pan-STARRS observations of IRAS 23077+6707 (“Dracula’s Chivito“) – The largest protoplanetary disk in the sky
Kristina Monsch (CfA | Harvard & Smithsonian)
I present the first resolved images of the giant edge-on protoplanetary disk IRAS 23077+6707 (“Dracula’s Chivito”), in optical scattered light with Pan-STARRS, and millimeter thermal emission with the SMA. IRAS 23077+6707 is hosted around a massive Herbig Ae/Be-type star located in an isolated region of the Cepheus star forming region, being in direct vicinity of a hot X-ray bubble that is hypothesized to have been created by a massive supernova explosion. Depending on the adopted distance estimate, IRAS 23077+6707’s dust and gas radii span enormous physical scales, ranging from 540-1100 au in scattered light, and 900-1840 au in the 1.3 mm dust emission, placing it amongst the largest protoplanetary disks ever reported physically, and the largest in angular extent yet known. As such, IRAS 23077+6707 may be the precursor of extremely massive planetary systems hosting multiple giant planets, such as HR 8799.
Feb 8, 2024
Exploring Systematic Errors in the Inferred Parameters of the Transiting Planets
Alison Duck (Ohio State)
Transiting planet systems offer a unique opportunity to measure precise masses and radii of planets and their host stars. However, relative photometry and radial velocity measurements alone only constrain the host star density, leaving a one-parameter mass-radius degeneracy. We assess the magnitude of systematic errors in the derived system parameters relative to their statistical precision due to different methods of breaking this degeneracy. We first model extant data for the typical hot Jupiter system KELT-15 using EXOFASTv2, considering four methods of breaking the stellar mass-radius degeneracy. We find systematic differences in the inferred physical parameters of the KELT-15 system, including a 6.5% (~1.8 sigma) difference in the stellar and planetary radii based the degeneracy-breaking method. We then evaluate the systematic errors for several systems of M-dwarfs transiting FGK host stars. Finally, we introduce a homogenous analysis of ~10 hot Jupiter planetary systems with secondary eclipses observed by TESS.
Feb 2, 2024
Missing Planets around White Dwarfs?
Lou Baya Ould Rouis (BU)
Out of the 5500 confirmed exoplanets, only about a dozen are known around stars more than 3.5 times the mass of the Sun. As the descendants of stars less than 7 solar masses on the main sequence, white dwarfs provide a unique way to constrain planetary occurrence around stars that are otherwise difficult to measure with radial-velocity or transit surveys. Ultraviolet spectra collected by the Hubble Space Telescope in the past 11 years suggest that nearly half of all white dwarfs show metal pollution from remnant planetary systems. However, the fraction of white dwarfs showing metal pollution significantly decreases for massive white dwarfs (descendants of stars with masses greater than 3.5 solar masses on the main sequence). I discuss observational biases affecting specific metal diffusion timescales, as well as whether the white dwarfs of our sample are undergoing single-star evolution alone or if the evolutionary history was affected by stellar mergers. Ultimately I aim to assess the implications of missing planets around massive white dwarfs, and how these occurrence rates can connect to the formation and survival of planets around intermediate-mass stars.
Jan 26, 2024
Analyzing cosmological-hydrodynamical simulations in light of future Exploring Exoplanet Demographics with Kepler, TESS, and Beyond
Michelle Kunimoto (MIT)
Future optical surveys will provide us with detailed catalogs of billions of galaxies. We thus want robust methods with which to extract information from surveys and accurately measure the clustering of galaxies and matter. In this body of work, we use cosmological-hydrodynamical simulations to test models and study complex physical processes that affect clustering statistics. We use the IllustrisTNG simulation suite to show that machine learning tools can provide accurate models of the galaxy-halo connection. We use the CAMELS to show how feedback can affect Exoplanet surveys have been spectacularly successful in identifying thousands of planets with breathtaking diversity. These discoveries help place the Solar System in context and inform our understanding of how planets form and evolve. Finding large numbers of planets also enables statistical studies of the exoplanet population, through which we can uncover which types of planets are more common than others and find correlations between planet abundance and the properties of stars. Exoplanet demographic studies are key for constraining planet formation and evolution theories. I will highlight contributions to the field of demographics, from large and small planet populations with Kepler and its implications for the search for other Earths, to the exciting potential of TESS to significantly expand our understanding of planet populations around more diverse stellar samples than were possible before. Finally, I will identify a set of important open questions that remain to be answered and outline future goals to push the field of demographics to new frontiers.
Jan 19, 2024
Analyzing cosmological-hydrodynamical simulations in light of future optical surveys.
Ana Maria Delgado (CfA | Harvard & Smithsonian)
Future optical surveys will provide us with detailed catalogs of billions of galaxies. We thus want robust methods with which to extract information from surveys and accurately measure the clustering of galaxies and matter. In this body of work, we use cosmological-hydrodynamical simulations to test models and study complex physical processes that affect clustering statistics. We use the IllustrisTNG simulation suite to show that machine learning tools can provide accurate models of the galaxy-halo connection. We use the CAMELS to show how feedback can affect total matter clustering out to large scales and train a machine learning algorithm on baryon content and halo abundance to predict the suppression of total matter clustering. Lastly, we measure the projected correlation functions of intrinsic shape/shear of galaxies in the MillenniumTNG simulation in order to better understand the intrinsic alignment signals which contaminate weak cleansing surveys.
Nov 17, 2023
Searching for Black Holes From the Nearby to the Faraway Universe
Fabio Pacucci (CfA | Harvard & Smithsonian)
I will provide an overview of our quest to explore, with theory and observations, undetected populations of black holes, proceeding from the nearby to the faraway Universe. First, I will unveil a new multiwavelength observational campaign likely leading to the discovery of the second closest supermassive black hole in the Milky Way satellite Leo I. I will then describe the properties of intermediate-mass black holes potentially wandering in local galaxies, including the Milky Way. Moving higher in the redshift ladder, I will detail the detection of the farthest lensed quasar to date and investigate why we may be missing a significant fraction of this population. Furthermore, based on JWST data of galaxies hosting black holes at z=4-7, I will present a new, detailed statistical analysis that reveals a high-z M_bh-M_star relation that deviates at >3sigma confidence level from the local relation. Black holes are overmassive by 10-100 times compared to their low-z counterparts; this fact is not due to a selection effect in surveys. I will then conclude with the observational properties of the sources that have started the reionization at z ~ 20-30: the first population of stars and black holes, and how we may soon detect them directly.
Oct 31, 2023
The effects of the ICM on gas and star formation histories of galaxies at low redshift
Benedetta Vulcani (Osservatorio di Padova)
GAs Stripping Phenomena in galaxies (GASP) is a program aimed at studying gas removal processes in nearby and z~0.5 galaxies in different environments, using observations at different wavelengths (X, UV, optical, sub-mm, radio). Integral-field spectroscopic observations with MUSE at the VLT are the core of the program, as they allow us to study both the integrated and spatially resolved properties of galaxies. I will present some breakthrough results based on the GASP survey on the effects of the IntraCluster Medium (ICM) on gas propertied and star formation histories. I will focus both on specific galaxies undergoing strong gas stripping and on some general trends that help us to understand galaxy quenching and evolution in general. I will also focus on galaxies in low density environments, showing the multitude of mechanisms that can affect the gas distribution in non cluster galaxies.
Oct 27, 2023
Tidal Synchronization and Stellar Activity on Low Mass Eclipsing Binaries with TESS
Ritika Sethi (MIT)
Stellar binaries are ubiquitous in the galaxy and an excellent laboratory for understanding tidal evolution and synchronization. In this talk, I will discuss my recent work involving the study of photometric modulations in the TESS lightcurves of 162 unequal-mass eclipsing binaries from the EBLM (Eclipsing Binary Low Mass) survey, comprising F/G/K primaries and M-dwarf secondaries. We detected modulations on 81 eclipsing binaries and estimated their rotation rates. We also discovered ellipsoidal variations (tidally-driven distortions) in 17 of our binary systems. Using our rotation period measurements, we investigated the dependence of tides on orbital period, eccentricity, primary star parameters, and mass ratios. This allowed us to probe the transition regime of when tides are and are not significant in tight unequal mass binaries. Additionally, in a large portion (at least ~51%) of our sample, we detected photometric modulations consistent with two over-densities of spots on the primary star that are roughly 180◦ apart. These so-called active longitudes are preferentially at the sub- and anti-stellar points on the primary star. Physically, this means that the spots on the primary star preferentially face directly towards and away from the secondary star.
Oct 20, 2023
Multiple Patchy Cloud Layers in the Atmosphere of SIMP0136
McCarthy Allison (Boston University)
Multi-wavelength photometric monitoring of brown dwarfs and planetary-mass objects provides insight into their atmospheres and cloud layers. Since different bands can probe different layers of the atmosphere, simultaneous multi-wavelength photometry enables us to measure multiple cloud layers in the atmospheres of these objects. Here, we present simultaneous infrared J- and Ks-band observations of the highly variable, T2.5, planetary-mass object, SIMP J013656.5+093347. For the first time, we detect a phase shift between J and Ks-band light curves. Using the Sonora Bobcat models, we expand on this idea to show that at least two different patchy cloud layers must be present to explain this phase shift. In an attempt to explain the J-Ks color change, as well as a previously measured J-H lackof color change, we use a combination of the Sonora Bobcat cloudless atmospheric models and the Sonora Diamondback cloudy atmospheric models at different temperatures to create an atmosphere that may resemble that of SIMP0136. Here we show that the combinations of Sonora models we tested are not able to explain both the J-Ks color change and previously measured J-H color change, suggesting that our understanding of the cloud layers between 1 and 20 bar is incomplete.
Sep 29, 2023
Early Results from the CANUCS JWST Observations
Dr. Nicholas Martis (Saint Mary’s University)
I will present an overview of the CAnadian NIRISS Unbiased Cluster Survey, a Guaranteed Time Observation program carried out in the first year of operation of the James Webb Space Telescope (JWST). The first year of JWST observations has already enabled new discoveries and opened unexpected questions regarding the evolution of galaxies at all epochs in the history of the universe. I will highlight early results from the CANUCS team utilizing both our own observations and the Early Release Observations of the SMACS0723 lensing galaxy cluster. Topics range from resolved studies of the stellar populations of galaxies at cosmic noon to extreme galaxies in the early universe. I will conclude by presenting a more detailed look at my own research into the spatial distribution of globular clusters and intracluster light in the SMACS0723 cluster.
Sep 28, 2023
ML x Cosmology with 50 Million Galaxies
Dr. ChangHoon Hahn (Princeton University)
The 3D spatial distribution of galaxies encodes key cosmological information that can be used to probe the nature of dark energy and measure the sum of neutrino masses. The next generation of galaxy surveys, such as the Dark Energy Spectroscopic Instrument (DESI) and the Prime Focus Spectrograph (PFS), will observe 50 million galaxies over unprecedented cosmic volumes and produce the most precise measurements of galaxy clustering across 10 billion years of cosmic history. In my talk, I will present how we can leverage machine learning (ML) to go beyond current analyses and extract the full cosmological information of these galaxy surveys. In particular, I will present SimBIG, a framework for analyzing galaxy clustering using ML-based simulation-based inference. I will show the latest results from applying SimBIG to SDSS-III: BOSS observations and demonstrate that we can more than double the precision of current analyses. Lastly, I will present the status of the DESI and PFS surveys and how I will apply SimBIG to them to produce the leading constraints on dark energy and the sum of neutrino masses.
Past events Fa’22- Sp’23
Apr 3, 2023
When and where does star formation stop? Stellar Mass Assembly and Quenching in High-Redshift Galaxies
Gourav Khullar (Samuel P Langley PITT PACC Postdoctoral Fellow,University of Pittsburgh)
A major challenge in the field of extragalactic astrophysics is understanding when the most massive galaxies assemble the bulk of their stellar mass and quench star formation, across a wide range of redshifts and environments. The combination of a new discovery space in redshift and spatial resolution offered by new galaxy surveys, and space telescopes like Hubble and JWST, is set to fundamentally change our understanding of quenching mechanisms in galaxies. In this talk, I describe my work to characterize stellar populations in massive galaxies in distant galaxies, both in a) representative populations of quiescent galaxies, as well as b) individual bright gravitationally lensed systems with spatially resolved observations.
I use spectra of galaxies in massive South Pole Telescope (SPT) galaxy clusters to address the question: on what timescales do galaxies that end up in clusters form their stars, and does the cluster sample matter when studying these properties? This mass-limited cluster sample across redshifts 0.3 < z < 1.5 allows us to constrain star formation histories and formation redshifts of 900 quiescent galaxies in clusters, as a function of cluster environment and mass. In conjunction, I also discuss the discovery and characterization of ~100 robust quiescent galaxy candidates at redshifts > 2 in the ultradeep (29-30 AB mag) JWST UNCOVER Treasury Survey (PIs: Bezanson, Labbe; Co-Is: Marchesini, Pan). This dataset contains 10s of low-mass (logM < 10) systems at z > 2, making it the largest such candidate sample to-date, and allowing us to explore new pathways for quenching in an uncharted parameter space.
I also describe my work with gravitationally lensed systems, including COOL J1241+2219 (CJ1241), a lensed galaxy at z = 5.04 that is the brightest galaxy known at z > 5 (at AB magnitude z~20.5). Using ground-based spectrophotometric data and SED fitting analyses, we find CJ1241 to be an intrinsically luminous and massive star-forming galaxy near the epoch of reionization. With an approved JWST Cycle 1 Program (GO 2566, PI: Khullar), I describe the anticipated improvements in constraints on old stellar populations, dust and metallicity in CJ1241, and finally explore these results in the context of the underlying mass assembly in other JWST-observed bright lensed galaxies.
Mar 27, 2023
Light Dark Sectors Attempting to Restore Cosmological Concordance
Itamar Allali (Tufts University)
There exist tensions between observations of the early and late Universe in the determination of the cosmological parameters S8 and H0. In this talk, I will review the status of these tensions, and I will present various solutions. After reviewing the well-known Early Dark Energy (EDE) model, I will discuss two new frameworks involving dark sectors with light particle species. First is a phenomenological model featuring: (i) a decaying dark energy fluid, to raise today’s value of the Hubble parameter, and (ii) an ultra-light axion, to suppress the matter power spectrum and address the S8 tension. Markov Chain Monte Carlo analyses show that such a Dark Sector model fits a combination of early time datasets slightly better than the ΛCDM model, while reducing both the H0 and S8 tensions to <∼3σ level. Combined with measurements from cosmic shear surveys and local supernovae measurements, we find that the H0 and S8 tensions are reduced to the 1.5σ and 1.1σ level respectively, with a significant improvement Δχ2≃−17 compared to the ΛCDM model. This work is summarized in 2104.12798. Next, I will discuss the Stepped Dark Radiation models put forth in 2111.00014 and 2207.03500, which include a dark radiation (DR) fluid with a step in its abundance and optional interactions with dark matter. This model improves upon other models, in particular EDE, by having a straight-forward realization in particle physics: a pair of light particle species, one of which becomes massive and decays. I will present the results of a new MCMC analysis of these models with two goals: (i) evaluating all new parameters without bias, and (ii) fitting to Large Scale Structure data. This analysis overall indicates a decrease in the evidence from its goodness-of-fit to data (increase in Δχ2) compared to the literature.
Mar 13, 2023
Shocking tales of structure formation: Evolving galaxies and black holes in evolving environments
Andra Stroe (CfA | Harvard & Smithsonian)
Understanding the interplay between galaxy evolution, star formation, and black hole activity from the perspective of structure formation remains one of the most fascinating challenges in modern astrophysics. On the largest scales, pairs of galaxy clusters colliding drive the growth of structure. Cluster mergers are the most energetic events since the Big Bang, which release 10^64 ergs over 1-2 billion years and produce dramatic, long-lasting effects. By bringing together panchromatic observations, I will discuss how the merger of galaxy clusters can trigger star formation and black hole activity in cluster galaxies, shape the evolution of cluster galaxies, and reverse typical environmental trends observed in relaxed clusters at low redshift. With approximately half the galaxy clusters in the local Universe undergoing mergers, this recent work has revealed gaps in our understanding of the growth of structure in the Universe and showed the potential for discovery in this understudied field. I will draw parallels between the fundamental drivers of galaxy and black hole evolution in low-redshift clusters and the processes relevant in the context of proto-clusters and high-redshift clusters, where mergers and associated non-thermal phenomena were far more common than in the nearby Universe. I will conclude by discussing how the treasure trove of cluster samples at increasingly large redshifts which will be delivered by new generation of instruments will help guide discoveries in the field of gas, galaxy and black hole evolution at the epoch when structures first formed.
Mar 6, 2023
What’s inside astrophysical black holes?
Kevin Croker (University of Hawaii)
Over the past ten years, observational evidence has continued to mount that astrophysical black holes grow too quickly or are more massive than easily explained by physical processes such as merger and accretion. With the advent of gravitational wave astronomy, this evidence now spans all observational channels, over ten orders of magnitude in mass, and over cosmological timescales. The simplest black hole model, Kerr, cannot be applied over cosmological timescales because it has non-cosmological boundary conditions. Aspects of the many known black hole models in General Relativity suggest dynamical features on cosmological timescales that can help to ease these observational tensions. We review recent formal advances in General Relativity that permit investigation of candidate black hole models through their possible cosmological interactions, and present the first observational evidence for cosmologically coupled mass growth in the supermassive black holes of quiescent elliptical galaxies. The measured growth is consistent with a vacuum energy equation of state for astrophysical black hole contributions in aggregate, leading to the striking prediction that astrophysical black holes may well be the material source that drives accelerated late-time expansion.
Feb 27, 2023
Presentations on Telescope by Tufts Students
Series of student presentations on telescopes at different wavelengths. The presentations will focus on the instrument specifications and science capabilities.
Feb 13, 2023
The atomic interstellar medium’s role in the star formation lifecycle: sharpening our view of nearby galaxies with LGLBS and PHANGS-JWST
Eric Koch (CfA | Harvard & Smithsonian)
Encoded in the atomic interstellar medium (ISM) are the physical processes that govern how neutral gas is accreted or removed from galaxies, how molecular clouds are born and evolve through star formation, and how various feedback processes drive interstellar turbulence. Understanding the atomic ISM is a crucial component in building a complete picture of the star formation lifecycle, but this remains an observational challenge as these processes act on a wide range of spatial scales. Because of this challenge, fundamental questions remain unanswered: How are molecular clouds formed? What fraction of the atomic ISM is cold versus warm? Which processes drive interstellar turbulence? Nearby galaxies are an ideal laboratory to answer these questions as we gain a top-down view that enables us to link the fine-scale structure to galaxy scales and beyond. New facilities such as ALMA and VLT/MUSE have revolutionized our understanding of the molecular gas and star formation across the nearby galaxy population, and long-standing unknowns on the atomic ISM are now a limiting factor in building a complete view of the star formation lifecycle.
I will present the first results from two ongoing surveys that enable a new view of the atomic ISM and the processes that govern it. First, I will introduce the Local Group L-band Survey (LGLBS), a VLA “extra”-large program to study the northern Local Group star-forming galaxies in the 21-cm HI line and the 1-2 GHz radio continuum with exquisite detail. The data reveal intricate spectral structure in the 21-cm HI spectra, approaching the complexity we see in Galactic HI. We find that how we model these spectra impacts measurements of basic properties like the cold and warm atomic gas fractions. Our detailed modeling also reveals that HI and CO kinematics are strongly correlated, which may provide a roadmap to observationally constraining molecular cloud formation mechanisms. Second, I will present the first results on the diffuse dust emission from the PHANGS-JWST Treasury, a Cycle 1 project using NIRCam and MIRI to map the inner portions of 19 nearby star-forming galaxies. We find that mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) is likely an accurate tracer of the total gas column density and, with JWST’s incredible sensitivity, enables a new direct tracer of the atomic ISM at resolutions far exceeding current radio interferometers. JWST opens the door to resolving the atomic ISM structure across a representative sample of the local Universe star-forming galaxy population, while LGLBS will unveil detailed atomic ISM kinematics across the Local Group. Together, these surveys lay the groundwork for observations of the atomic ISM in the SKA and ngVLA eras.
Feb 6, 2023
Probing Extreme Accretion Physics with Changing-Look AGN and Tidal Disruption Events
Megan Masterson (MIT)
Recent discoveries of extreme accretion events around supermassive black holes are defying what we thought we knew about active galactic nuclei (AGN) and the accretion process. In this talk, I will highlight recent work on two extreme changing-look AGN and new infrared-selected tidal disruption event (TDE) candidates that present new avenues for studying extreme accretion phenomena. In addition to broad emission lines appearing in their optical spectra, the two changing-look AGN I will discuss show unique X-ray variability, both captured by extensive NICER monitoring. In one of these systems, 1ES 1927+654, we witnessed the disappearance and reformation of the putative X-ray corona for the first time in an AGN. I will present the X-ray spectral evolution of this source throughout its entire outburst, as well as a new soft reflection model for a broad 1 keV line in the early X-ray spectra. The other system, AT2019cuk/Tick-Tock, was first proposed as an imminent binary supermassive black hole merger. After following this event extensively in the X-rays, we do not detect the periodicity that was initially used to support this claim. However, the X-rays do reveal unique hard X-ray flaring, with repetitive, day-long flares occurring during the entirety of an eight month high-cadence monitoring campaign with NICER. I will present potential models for these flares, which have never been seen before in AGN. Lastly, I will show preliminary results on a new class of dust-obscured TDEs discovered with the infrared WISE all-sky monitor, which may help elucidate some of the open questions surrounding host galaxy properties of optically-selected TDEs.
Dec 9, 2022
Into the Dusty Universe: Obscured Star-formation from Today to Cosmic Noon and Beyond
Jed McKinney (University of Texas)
The formation of stars in galaxies has led to what we see in the Universe today, and most star-formation is obscured by dust. I will begin this talk discussing the characteristics of dust-obscured star-formation up to cosmic noon (z~2) when galaxies were the most active. Between z=0 and z~2, massive dusty galaxies form their stars under similar gas and dust conditions. In addition to increases in total gas mass, the higher incidence of spatially compact dust-obscured star-formation along the main-sequence may help explain the higher star-formation rates at z~2. Strong gravitational lenses can help characterize dusty star-formation in lower stellar mass galaxies and at earlier times, but coordination between upcoming sub-millimeter/millimeter surveys and large extragalactic JWST programs will be crucial for building statistical samples at z>3. I will introduce the COSMOS-Web survey and highlight challenges to selecting dusty galaxies from NIRCam and MIRI maps, and subsequent implications for ultra-high redshift catalog fidelity at z>10. Finally, I will touch on technology and mission design requirements that a future space-based far-infrared telescope must satisfy to fully unveil the dust-obscured Universe.
Dec 2, 2022
Rapid quenching of galaxies at Cosmic Noon: How galaxies grow and die rapidly in the early Universe
Minjung Park (CfA | Harvard & Smithsonian)
In the early phase of cosmic history, the global star formation and galaxy growth peak at z~2, the epoch so-called “Cosmic Noon”. While most of the galaxies at this epoch are actively star-forming, massive quiescent galaxies are still found. One of the important questions in galaxy evolution is to understand how galaxies stop forming stars and become quiescent, especially at such high redshifts. Given that the age of the Universe is only a few billion years at z>2, these massive quiescent galaxies must have been quenched very rapidly. To understand the rapid quenching process in detail, we study galaxies that are thought to be rapidly quenched. In this talk, I will talk about how we select the rapid quenching galaxy candidates, how we investigate their star formation histories, and finally the possible physical mechanisms behind the rapid quenching.
Nov 18, 2022
Disk, corona, jet connection in black hole X-ray binaries
Jingyi Wang (MIT)
Astrophysical black holes tell us about accretion and ejection physics in the strongest gravity regime in the Universe. Reverberation mapping is a tool for probing those compact regions, in which X-ray light echoes off the accretion disk near the innermost stable circular orbit. The NICER Observatory is revolutionizing reverberation mapping of the inner accretion disks of black hole X-ray binaries in outburst. In this talk, I will present the discovery of reverberation lags in 7 new black hole transients, increasing our total sample to 10 sources. In particular, I will present our key finding that the reverberation lags get longer during the hard-to-soft state transition when a ballistic radio jet is launched. This may suggest an increase in the size of the X-ray emitting region, possibly due to an expanding corona. I will also present the data of the ongoing intriguing outburst of IGR J17091-3624, the little sister of GRS 1915+105, that shows “heartbeat” exotic variability along with disk winds, and put the results into the context of understanding the disk-corona-jet connection in black hole transients.
Nov 4, 2022
Tufts Alumni: Where Are They Now and Tips for Grad School Application
Adina Feinstein (University of Chicago)
Lilianna Houston (University of Denver)
Justin Hudson (Colorado State University)
We have invited Tufts alumni to talk about their experiences of navigating the grad school application process and transitioning from undergrad to grad school. The speaker(s) will also share their work experiences both in Astrophysical researches and other fields. For those who are applying (or considering to apply) to grad school, it will be a great opportunity to learn from them and ask questions.
Zoom link: https://tufts.zoom.us/j/98639969232?pwd=Rk82NEpBSHlHQTBxR3haNmZtV0oxQT09
Meeting ID: 986 3996 9232
Passcode: 078910
Oct 28, 2022
The First Glimpse of the First Galaxies with JWST
Rohan Naidu (NASA Hubble Fellow at MIT Kavli Institute)
One of the last great unknowns in our history of the Universe is when and how the first galaxies emerged after the Big Bang. These galaxies transformed the cosmos — they illuminated the invisible scaffolding of dark matter that underpins the Universe, they ionized the intergalactic reservoirs of hydrogen, and they synthesized the elements that would one day seed life on Earth. Thanks to JWST, these enigmatic galaxies are finally coming into view. In this talk I will present early JWST results on these systems, and preview upcoming Cycle 1 programs I am leading. I will discuss: (i) emerging constraints on the nature of the first galaxies both via direct searches for the most distant galaxies as well as via archaeological methods; (ii) outstanding questions around how these early galaxies reionized the Universe (e.g., was reionization driven by numerous faint galaxies, or by rare, bright “oligarch” galaxies?) and the pathway to settle them with JWST.
Oct 21, 2022
Our supermassive black hole Sagittarius A* across the radio band
Sara Issaoun (NHFP Einstein Fellow at the Center for Astrophysics | Harvard & Smithsonian)
Last Spring, the Event Horizon Telescope collaboration revealed the first images of the shadow of our Milky Way supermassive black hole, Sagittarius A* (Sgr A*). Since its detection in the mid-70s, this bright radio source in the Galactic Center was shrouded in a veil of mystery. The Nobel-awarded stellar orbits research in the Galactic Center pinned down its mass and distance, showing evidence of an extremely compact 4 million-solar-mass object at the heart of our Galaxy. The EHT then provided the first direct evidence that this object is indeed a black hole and resolved its shadow for the first time. In this talk, I will explain the challenges of imaging Sgr A* and how these were overcome by the EHT, and I will walk through important milestones of discovery across the radio band that laid the foundation for the first image of our black hole.
Oct 18, 2022
Being Captain of Your PhD
Prof. Kim-Vy Tran (University of New South Wales – Australia)
Obtaining your PhD is likely the most difficult academic challenge you will ever face, yet few people are truly prepared for how to best navigate their journey. This interactive discussion helps identify the most critical elements needed for a successful PhD regardless of your particular field of study. Topics include mapping a timeline, strategies for success, how to make the most of your supervisory team, how to tackle writing, and identifying warning signs. This discussion is appropriate for students at all levels, in particular students at the beginning of their PhD.
Oct 18, 2022
Galaxy Evolution with ASTRO 3D
Prof. Kim-Vy Tran (University of New South Wales – Australia)
ASTRO 3D is an Australian Centre of Excellence with 300+ members united in understanding the evolution of the matter, light, and elements from the Big Bang to the present day. Here I highlight results from two of our flagship surveys. The MOSEL survey confirms a population of Extreme Emission Line Galaxies in the distant universe and suggests that most galaxies go through a strong starburst phase. By comparing galaxy kinematics to cosmological simulations, we show that massive galaxies in the distant universe build their stellar mass mostly by accreting other galaxies, and that puffy galaxies quench slower than compact galaxies.
Gravitational lensing is a powerful cosmic tool for exploring a wide range of astrophysical phenomena including understanding how ordinary and dark matter couple and finding the earliest galaxies and stars. Using Convolutional Neural Networks to sift through legacy imaging data-sets, the AGEL survey confirms about 100 strong gravitational lenses. I provide an overview of the AGEL survey and describe AGEL projects based on observations from the Hubble Space Telescope, Keck Observatory, and the Very Large Telescope.
Zoom link: https://tufts.zoom.us/j/98303897153?pwd=SndmL1NCdEFEYjdyUXlSWVRad3JBZz09
Meeting ID: 983 0389 7153
Passcode: 551048
Oct 14, 2022
Testing Feedback Models: Resolved Star Formation in Simulated TNG100 Galaxies
Bryanne McDonough (Boston University)
Galaxies in the observed universe exhibit a bimodal distribution of star formation properties, with most either actively star forming or passive. This implies that mechanisms for quenching star formation occur over relatively short timescales. Simulations have only been able to reproduce this distribution by implementing feedback models, including supernova winds and AGN heating. In upgrading the feedback models in the Illustris-1 simulation, its successor, IllustrisTNG, is now better able to reproduce the observed bimodal distribution of galaxy colors. Seeking a further test of these feedback models, this work examines resolved properties of star formation by analyzing radial profiles of star formation and stellar age of galaxies in the TNG100 simulation box. By comparing these profiles to those observed by the IFU survey, MaNGA, we find that the simulation reproduces general trends for most populations of galaxies, with the exception of those undergoing star bursts.
Oct 7, 2022
X-ray detection of a nova in the fireball phase (König et al., Nature 605, 248, 2022
Ole König (Remeis Observatory)
Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which leads to a rapid expansion of the envelope and the ejection of most of its mass. Theory has predicted the existence of a ‘fireball’ phase following directly on from the runaway fusion. This should be observable as a short, bright and soft X-ray flash before the nova becomes visible in the optical. I report of the discovery of this X-ray flash in the classical Galactic nova YZ Reticuli. The fireball phase happened 11 hours before the source’s 9mag optical brightening. No X-ray source was detected 4h before and after the event, constraining the duration of the flash to shorter than 8h. In agreement with theoretical predictions, the source’s spectral shape is consistent with a black-body of 327,000 K (28.2 eV), or a white dwarf atmosphere, radiating at the Eddington luminosity, with a photosphere that is only slightly larger than a typical white dwarf. I also discuss a novel simulation approach to mitigate pile-up that was used to derive these results.
Zoom link: https://tufts.zoom.us/j/98952989200?pwd=UHZoLzZjS2RoM1RyWUpKME80TmJSdz09
Meeting ID: 989 5298 9200
Passcode: 630152
Sep 30, 2022
Fake Redshift-Space Distortions: How intrinsic alignment of galaxies will bias clustering statistics for the DESI survey
Claire Lamman (Center for Astrophysics | Harvard & Smithsonian)
The Dark Energy Spectroscopic Instrument (DESI) is surveying 40 million galaxies and will put new, stricter constraints on cosmological parameters. This means it’s more important than ever to consider every possible source of systematic error. I’ll describe how an orientation-dependent selection bias in DESI’s Luminous Red Galaxy (LRG) sample combines with the intrinsic alignment of LRGs to mimic a RSD signal. I estimate the impact this will have on DESI’s measurement of the quadrupole of the 2pt correlation function, demonstrating how a subtle effect can impact measurements on the largest scales.