Astronomy Lunch Seminar on Fridays


Seminars start at 1:30 pm and take place in Room 310 in the Collaborative Learning and Innovation Complex (CLIC).

 

SPRING 2020 semester:


Feb 14: Dr. Clara Sousa-Silva (MIT)


TITLE: Finding an Alien Biosphere with Computational Chemistry


ABSTRACT: At the edge of our present scientific frontier lies the question: “Can we identify the signs of life on an exoplanet?”. Establishing whether a planet is habitable, or inhabited, relies both on the observation of an exoplanet atmosphere and, crucially, its subsequent interpretation. This interpretation requires knowledge of the spectral behavior of every significant atmospheric molecule. However, though thousands of molecular candidates can contribute towards the spectrum of an atmosphere, data exist for only a few hundred gases. Among these, only a fraction have complete spectra (e.g. ammonia, water). This deep incompleteness in the knowledge of molecular spectra presents a pressing vulnerability in the atmospheric study of planets; there exists a strong possibility of mis-assignment, false positives, and false negatives in the detection of molecules. The work presented here combines structural organic chemistry and quantum mechanics to obtain the necessary tools for the interpretation of astrophysical spectra and, ultimately, the detection of life on an exoplanet. Whether alien life will produce familiar gases (e.g., oxygen) or exotic biosignatures (e.g., phosphine), painting a confident picture of a potential biosphere will require a holistic interpretation of an atmosphere and its molecules. In this talk Clara will describe the ongoing efforts to decipher exoplanet atmospheres through the identification of volatile molecules, in particular those that might be produced by non-Earth-like life on exoplanets.






FALL 2019 semester:


Sep 12: Dr. Jingzhe Ma (University of California - Irvine)

    TIME/DAY CHANGE - 12:30pm on Thursday Sep. 12


TITLE: The formation and evolution of high-redshift dusty galaxies


ABSTRACT: High-redshift dusty galaxies are excellent sites to investigate mass assembly and growth, total star formation rates (both obscured and unobscured star formation), chemical evolution, and physical conditions. I will present two populations of high-redshift dusty galaxies, submillimeter galaxies and quasar dusty absorbers, which are selected by their dust emission and dust absorption, respectively. First, I will talk about the physical properties (e.g. stellar masses, SFRs, etc.) of gravitationally lensed, dusty, star-forming galaxies (DSFGs) selected by the South Pole Telescope survey. Then I will present a large Spitzer follow-up program of 300 Herschel-selected ultrared (SPIRE S500>S350>S250) DSFGs at z ~3-6, and discuss their role in cosmic star formation history. Next I will talk about dusty galaxies in absorption, i.e., quasar 2175 Å dust absorbers (2DAs) and MgII absorbers. Keck, MMT, and VLT spectroscopy shows that 2DAs trace ISM gas with metallicity and dust depletion levels comparable to our own Galaxy. They are expected to be more massive than Damped Lyman-alpha (DLA) galaxies based on the mass-metallicity relation. Using HST direct imaging and grism spectroscopy, we have identified the host galaxy of a 2DA at high redshift for the first time. I have also explored the average FIR properties of MgII absorbing galaxies via Herschel image stacking. Synergies between absorption and emission selected galaxies will allow us to achieve a more complete picture of the gaseous, stellar, metal, and dust content in galaxy formation and evolution.



Sep 20: Dr. Ian Stephens (Harvard - CfA)


TITLE: Surveying the Multi-Scale Star Formation Process


ABSTRACT: The formation of stars from the interstellar medium is a complicated, multi-scale process. Survey astronomy can be used to statistically constrain the star formation process. In this talk, I will focus on some of my recent star formation surveys, showing multi-scale scientific results. I will discuss star formation on scales of entire galaxies to scales of protostellar disks. Among the results, I will discuss how dense gas of galaxies is related to the constituent dense clumps, how the initial mass function is sampled, and whether angular momentum is transferred from filaments to protostars.



Sep 27: Dr. Jeyhan Kartaltepe (Rochester Intitute of Technology) [COLLOQUIUM]



Oct 11: Dr. Britt Lundgren (University of North Carolina - Asheville) [COLLOQUIUM]



Oct 18: Nick Martis (Tufts University)


TITLE: The Prevalence and Properties of Dusty Galaxies Through Cosmic Time


ABSTRACT: From a combination of UltraVISTA and 3D-HST data we measure the evolution of the fractions of quiescent galaxies, moderately unobscured star-forming galaxies, and dusty star-forming galaxies as a function of stellar mass since z=3. Motivated by the finding that dusty star-forming galaxies become more prevalent both with increasing stellar mass and increasing redshift, we investigate the stellar and dust properties of a stellar mass complete sample of massive and dusty galaxies at 1<z<4 by modeling their UV-to-infrared spectral energy distributions (SEDs) obtained from the combination of UltraVISTA DR3 photometry and Herschel PACS-SPIRE data using MAGPHYS. We evaluate the ability of the rest-frame UVJ color-color diagram to determine the star formation and dust obscuration properties for our sample. We construct median SEDs of massive, dusty galaxies as a function of redshift and star-formation activity (quiescent vs star-forming). Simultaneous modeling of the panchromatic SED allows us to quantify the contribution to the IR emission from dust heated by star formation rather than evolved stellar populations, which we find to be a crucial element in characterizing these galaxies correctly. I summarize the stellar population and dust properties of the sample of massive and dusty galaxies at z>1, including, but not limited to, the relation between dust mass and star-formation rate, the relations between specific star-formation rate and diagnostics of the conditions of the interstellar medium, and the relation between the IRX and the UV slope. The studied sample at z>1 is finally compared to local galaxies.



Oct 25: Dr. Catherine Espaillat (Boston University) [COLLOQUIUM]



Nov 1: Dr. Charlotte Mason (Harvard - CfA)


TITLE: What can galaxies tell us about reionization?


ABSTRACT: The reionization of intergalactic hydrogen in the universe's first billion years was likely driven by the first stars and galaxies. However, we will probably never observe these 'first light' sources directly, but their properties can be inferred through accurate measurements of the timeline and morphology of reionization. I will describe how we can use galaxies at our current observational frontiers to measure the reionization process. In particular, Lyman alpha (Lyα) emission from galaxies can be used to probe the intergalactic medium at high redshifts, but requires modelling physics from pc to Gpc scales. I will describe how we can constrain reionization from spectroscopic observations of galaxies, focusing on Lyα emission at z>6, by comparing observations to theoretical models and simulations using statistical inference methods. I will present new measurements which favour a late and relatively rapid reionization, and place these in the context of high redshift galaxy formation. I will discuss the challenges and prospects in understanding reionization and early galaxy evolution with future facilities, such as JWST and upcoming 21cm experiments.



Nov 8: Dr. Sirio Belli (Harvard - CfA)


TITLE: The Formation of Quiescent Galaxies


ABSTRACT: Why do massive galaxies in the early universe stop forming new stars and become quiescent? The nature of this quenching process is one of the most important missing pieces in the puzzle of galaxy formation. A number of physical mechanisms have been proposed, but observational evidence is still lacking. Using deep near-infrared spectroscopy, I have explored the stellar ages, stellar kinematics, and ionized gas properties for quiescent galaxies at 1 < z < 2.5. I will present recent results from these studies and discuss their implications for quenching.



Dec 6: Ms. Helena Buschermohle (Tufts)


TITLE: Digitizing the Harvard Observatory Plates Collection Dating Back to the 1880s


ABSTRACT:






SPRING 2019 semester:


Feb 1: Dr. Tracy Webb (McGill University) [COLLOQUIUM]



Apr 12: Dr. Maciek Wielgus (Harvard/SAO)


TITLE: First results from the Event Horizon Telescope


ABSTRACT: Event Horizon Telescope (EHT) is a global very long baseline interferometry array, capable of performing observations in 1 millimeter wavelength. After over a decade of tests and technical developments, in April 2017 EHT has performed first observations as a mature instrument, with sensitivity, resolution and coverage unparalleled in the history of the millimeter wavelength radioastronomy, for the first time expected to allow for the imaging of the observed sources. Observations of black hole candidates in the centers of Milky Way and M87 were conducted with a nominal resolution better than the diameter of a 'shadow of a black hole' expected from general relativity. The data set was processed and analyzed since then, leading to the announcement of the results on 10th of April 2019.



Apr 19:



Apr 26: Dr. Siyao Xu (University of Wisconsin - Madison)


TITLE: Magnetic field amplification in weakly ionized astrophysical environments


ABSTRACT: Magnetic fields exist on all length scales from the weakest fields in the intergalactic medium in the large-scale structure of the universe to the strongest fields in magnetars. Magnetic fields induce fundamental astrophysical processes, including particle acceleration, non-thermal radiation, and polarization, but their origin and evolution are not well known. It is believed that the turbulent dynamo mechanism that amplified the primordial magnetic field accounts for the observed cosmic magnetic fields. To fill the huge gap between current dynamo simulations and realistic astrophysical dynamo processes, we have analytically developed a small-scale turbulent dynamo theory that covers broad ranges of plasma parameters and can be generally applied to diverse astrophysical environments. In this talk, I will focus on the new dynamo regime in a weakly ionized medium that we have analytically discovered. Its unique features, e.g., linear-in-time growth of magnetic field strength, have been recently tested and confirmed by a computationally very expensive two-fluid dynamo simulation. I will also discuss its important astrophysical implications for problems such as the first star formation.




May 3:






FALL 2018 semester:


Sep 12: Dr. John Silverman (IPMU - Tokyo)


TITLE: Clues on forming stars at extreme rates: gas and dust in high-z starbursts with ALMA


ABSTRACT: I will present results on the molecular gas properties of starburst galaxies at z ~ 1.5. Dynamical modeling of the CO J = 5 - 4 emission, in a unique galaxy PACS-787 with rotating molecular disks, is providing an independent assessment of the gas present in a quintessential major merger. These results are then used to calibrate our larger study of the CO J = 2-1 gas properties of a larger sample at lower spatial resolution. This study is providing one of the few dynamical constraints on key parameters, alpha_CO and the gas-to-dust ratio in high-z starbursts.



Sep 14: Dr. Rachel Bezanson (University of Pittsburgh) [COLLOQUIUM]



Sep 28: Prof. Frank van den Bosch (Yale University) [COLLOQUIUM]



Oct 12: Prof. Elena D’Onghia (University of Wisconsin - Madison)


TITLE: Insights into galaxy mass quenching in the IllustrisTNG simulation using global morphological diagnostics


ABSTRACT: Among massive galaxies, the quenching of star formation is observed to correlate with morphological and structural transformations; however, the internal mechanisms by which quenching occurs are still not well constrained. The structural transformations of galaxies have so far mostly been traced using diagnostics that largely describe their innermost volumes, such as the stellar core density, or the density within 1 kpc, the growth of which reflects the dissipative history of galaxies. We use non-parametric global morphological diagnostics, such as Gini and M20, to demonstrate that structural transformations during quenching are more profound than simply the growth of the core density - indeed, the full structure of galaxies undergoes transformations during quenching. Using the IllustrisTNG simulation we test, for the first time, whether the observed relation between quenching and global morphological transformations also occurs in ΛCDM cosmological simulations. Additionally, we investigate the physical processes driving the transformations and quenching of massive galaxies.



Oct 19: Prof. Eric Gawiser (Rutgers) [COLLOQUIUM]



Nov 6 (SPECIAL TIME 9:30am): Dr. David Goz (OA-Trieste / INAF)


TITLE: Cosmological Simulations in Exascale Era


ABSTRACT: The ExaNeSt H2020 EU-funded project aims at the design and development of an exascale (10^18 flops) ready supercomputer with a low energy consumption profile but able to support the most demanding scientific and technical applications. The project will produce a prototype based on hybrid hardware (CPUs+accelerators) implementing a co-design approach where scientific applications requirements are driving the hardware design. Astrophysical codes are playing a fundamental role to validate the exascale platform.

I will present strategies adopted by our team at the Department of Astronomy in Trieste (Italy) in order to port on the prototype a state-of-the-art N-body code, derived from HiGPUs (Capuzzo-Dolcetta+13), and based on high order Hermite's integration scheme with a direct evaluation of pair-wise particle forces. Furthermore , I will talk about our activities, in order to exploit an exascalable platform, on other codes suited to perform large-scale, high-resolution numerical simulations of cosmic structures formation and evolution (GADGET, Springel+05), and to generate catalogues of collapsed dark-matter halos in cosmological volumes (PINOCCHIO, Monaco+02).



Nov 9: Emmet Golden-Marx (Boston University)


TITLE: Seeing Red: The Search for High-z Galaxy Clusters in the Clusters Occupied by Bent Radio AGN (COBRA) Survey


ABSTRACT: Galaxy clusters are the largest gravitationally-bound systems in the universe.  Although many spectroscopically confirmed low-redshift clusters are known, few high-redshift clusters have been found.  To probe the earliest eras of cluster formation, we need high-redshift clusters with a variety of morphological states and masses.  One known tracer of high-z clusters is radio loud active galactic nuclei (RLAGN).  Particularly, bent, double-lobed radio sources have been shown to be an excellent tracer of galaxy clusters.  These bent radio AGN have a distinct “c” shape indicative of ram pressure caused by a gaseous medium; specifically the relative motion of the host galaxy with respect to the intracluster medium bends the lobes.  I’ll present results from the Clusters Occupied by Bent Radio AGN (COBRA) Survey, which consists of 646 bent, double-lobed radio sources selected from the VLA FIRST Survey and has observations in the infrared from Spitzer and optical from the 4.3m Discovery Channel Telescope.  The COBRA survey spans the redshift range 0.5 < z < 3.0 and includes candidates with a wide range of masses and dynamical states.  Using our IR and optical data, we measure galaxy excesses, locate red sequence galaxies, and determine photometric redshifts.  As bent radio AGN are not necessarily found in brightest cluster galaxies, we use local galaxy surface density measurements to analyze the spatial offset between our bent radio AGN and newly determined cluster centers, which are estimated using the overdensity of red galaxies.  We measure the surface density of all galaxies and red galaxies to better trace large-scale cluster morphologies and dynamical states.  Additionally, we compare the projected surface density distributions of our clusters to a projected NFW profile to further estimate the morphological state of our clusters.  We find that at least 30% of our high-z bent radio sources are in cluster environments and that these clusters appear to be in a variety of morphological states. 



Nov 30:


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Dec 7:


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SPRING 2018 semester:


May 11: Dr. Mojegan Azadi (CfA - Harvard)


TITLE: Active supermassive black holes identification and their host galaxies' physical properties


ABSTRACT: Active galactic nuclei (AGN) are powered by the accretion activity of supermassive black holes residing at the centers of galaxies. While observations propose that growth of AGN and galaxies are globally tied, I investigate whether this connection exists in individual galaxies. I also investigate various AGN selection techniques and star formation rate (SFR) estimates using multi-wavelength data from Chandra, Spitzer and rest-frame optical spectra from the Keck telescope. I find that combining multi-wavelength identification techniques provides a complete AGN sample, as each selection method suffers from selection biases. In particular, all selection techniques are biased against identifying AGN in low mass galaxies. Once stellar mass selection biases are taken into account, I find that AGN reside in galaxies with similar physical properties (i.e., SFR) as inactive galaxies. I find that while AGN are prevalent in both star-forming and quiescent galaxies, they are more likely to reside in star-forming galaxies. The probability of fueling an AGN does not strongly depend on SFR for a star-forming galaxy, though it decreases when star formation is shut down in quiescent galaxies. I find no evidence for a strong correlation between SFR of the host galaxy and AGN luminosity. These results indicate that while both AGN and galaxy growth are reliant on the same fuel, enhanced star formation activity does not necessarily go hand-in-hand with increased AGN activity. While the star formation activity of galaxies can be traced with various indicators, my investigations indicate that at z~2 a combination of mid-infrared and far-infrared data provide a more reliable SFR estimation than the mid-infrared data alone. I also find that the robustness of UV-based SFRs depends on the extinction correction method used.  I find that about 30% of z ∼ 2 galaxies have SFRs from infrared observations that are elevated relative to the dust corrected UV-based SFR. My investigations show that this infrared excess is not due to any contribution from AGN, and is primarily due to the enhanced polycyclic aromatic hydrocarbon (PAH) emission.



Jan 26: Dr. Sandro Tacchella (CfA - Harvard)


TITLE: The Formation and Evolution of Bulges and Disks in High-Redshift Galaxies


ABSTRACT: I will start by presenting a simple model that links dark matter halos to galaxies for gaining a better understanding of the evolution of the cosmic star-formation rate density, describing why redshift z~2 marks a key epoch for the formation of galaxies. I will then use state-of-the-art cosmological hydrodynamical zoom-in simulations to explore the physical processes that are responsible for the formation and evolution of bulges in galaxies at these early cosmic times. I will present evidence from the simulations that gas-driven compaction is a key phase in the life of galaxies while they evolve along the so-called star-forming Main Sequence. The main part of my talk will then present observations of z~2 star-forming galaxies combining rest-frame Far-UV-to-optical HST imaging and VLT SINFONI Adaptive-Optics spectra of the H-alpha line emission. In addition to comparing these observations with the simulations, I demonstrate not only that these massive galaxies at the peak of cosmic star-formation activity have already achieved the stellar density distributions that we see in massive spheroids at z=0, but also that they have outward-increasing specific SFR radial profiles, i.e., suppressed star-formation activity in their central bulge regions.







FALL 2017 semester:


This semester was reserved for current Tufts graduate students presenting the research projects they were engaged with during the summer and the progress.






SPRING 2017 semester:


Jan 27: Dr. Or Graur (CfA - Harvard)


TITLE: Using Supernova Rates to Reveal their Progenitors


ABSTRACT: We still do not know what types of stellar systems end up exploding as most types of supernovae (SNe). In my talk, I will show how we can use observed correlations between the SN explosion rates and various host-galaxy properties to constrain the progenitor scenarios of different types of SNe. I will focus on Type Ia supernovae, as well as stripped-envelope supernovae (i.e., SNe Ib, Ic, and IIb). The rates, which come from a variety of surveys (of both nearby and high-redshift galaxies, as well as both imaging and spectroscopic surveys), consistently point in the same direction: most Type Ia supernovae probably come from the explosion of two white dwarfs. Stripped-envelope supernovae probably emerge from binary systems as well, and not single, massive stars.



Feb 3: Dr. James Guillochon (CfA - Harvard)


TITLE: Tidal disruptions of stars by supermassive black holes: dynamics, light, and relics


ABSTRACT: Most supermassive black holes in the local universe lie dormant, with only one in a hundred accreting at their Eddington limits. Aside from this active minority, and the black holes in nearby galaxies that we can observe to influence the dynamics of stars and gas, most remain difficult to study. Tidal disruptions of stars by supermassive black holes give these dormant black holes a chance to be seen once every ~10,000 years, and each tidal disruption brings along with it a host of observable signatures that can be studied from gigaparsecs away, from the moment of the disruption to millennia after a disruption has occurred. In my talk I will present work I have done on tidal disruptions of stars, and describe their dynamics, observational signatures from real-time monitoring, and relics of disruption that may exist in plain sight.



Feb 10: CANCELLED DUE TO SNOW STORM...



Feb 17: Dr. Anna Pancoast (CfA - Harvard)


TITLE: Measuring Black Hole Masses in AGN by Revealing the Hidden Broad Line Region


ABSTRACT: Echoes from the broad line region in active galactic nuclei (AGN) allow for the measurement of supermassive black hole masses outside the local Universe. However, the detailed structure of the broad line region is difficult to constrain due to the very small scales involved. With a new generation of high-quality reverberation mapping datasets, we can substitute time resolution for spatial resolution and begin to model echoes from the broad line region directly. I will discuss the development of a direct modeling approach for reverberation mapping data capable of measuring the absolute black hole mass and the geometry and dynamics of the broad line region.  Applying this approach to recent datasets we find that the broad line region emission comes from a thick disk from gas that is generally inflowing towards the black hole.  Finally, I will discuss how this work can improve AGN black hole mass measurements at all redshifts.



Feb 24: Dr. Joel Leja (CfA - Harvard)


TITLE: The Next Generation of Galaxy SED Fitting: Accurate Physical Properties from Broadband Photometry with Prospector


ABSTRACT: Broadband photometry of galaxies measures an unresolved mix of complex stellar populations, gas, and dust. Interpreting these data is a challenge for models: I will show several studies demonstrating that properties derived from modeling galaxy photometry are uncertain by a factor of two or more. Yet, answering key questions in the field now requires higher accuracy than this. Here, we present a new model framework specifically designed for these complexities. Our model, Prospector-α, includes dust attenuation and re-radiation, a flexible attenuation curve, nebular emission, stellar metallicity, and a 6-component nonparametric star formation history. The flexibility and range of the parameter space, coupled with MCMC sampling within the Prospector inference framework, is designed to provide unbiased parameters and realistic error bars. We assess the accuracy of the model with aperture-matched optical spectroscopy, which was excluded from the fits. We compare spectral features predicted solely from fits to the broadband photometry to the observed spectral features. Our model predicts Hα luminosities with a scatter of ∼0.16 dex and an offset of 0.08 dex across a wide range of morphological types and stellar masses. This agreement is remarkable, as the Hα luminosity is dependent on accurate star formation rates, dust attenuation, and stellar metallicities. The model also accurately predicts dust-sensitive Balmer decrements, spectroscopic stellar metallicities, PAH mass fractions, and the age- and metallicity-sensitive features Dn4000 and Hδ.



Mar 3: Mackenzie Jones (Dartmouth)


TITLE: Exploring the consequences of selection effects in black hole-galaxy evolution studies 


ABSTRACT: In probing the connection between active galactic nuclei (AGN) and their host galaxies, an understanding of the full underlying AGN population is presently limited by complex observational biases that can be challenging to model theoretically. Determining the intrinsic Eddington ratio distribution in the optical, in particular, can be strongly influenced by selection effects and dilution from the host galaxy star formation. Using spectroscopic galaxies from SDSS, we show that an optically selected AGN population is consistent with being drawn from a universal AGN Eddington ratio distribution in the form of the broad Schechter function that is observed in the X-ray surveys. To more completely explore selection effects in the context of galaxy formation, we use a semi-numerical galaxy evolution simulation along with this universal Eddington ratio distribution to model the AGN population. We explicitly model selection effects to produce the “observed” AGN population for comparison with both theoretical and observational X-ray data. We investigate the impact on the “observed” population of selecting AGN in the X-rays based on a thresholds in luminosity (as they are selected in most surveys) as well as Eddington ratio. We find that we can broadly reproduce the host galaxies and halos of the X-ray AGN population, and that selecting AGN using these luminosity or Eddington ratio limits yield samples with very different host galaxy properties.



Mar 31: Dr. Francesca Civano (CfA - Harvard)


TITLE: X-rays from the “COSMOS” to understand black hole and galaxy co-evolution


ABSTRACT: Observations indicate that supermassive black holes (SMBHs) ordinarily dwell at the centers of local galaxies. Scaling relations have been identified between SMBHs and many large scale properties of the host galaxies that point to a joint galaxy and black hole cosmic evolution. Is this evolution regulated by the galaxy or by the black hole? Is star-formation triggering black hole activity or vice versa? To address these questions, in this talk I will present my work testing this co-evolution scenario, focusing on SMBH growth mechanisms, accretion and mergers. I will use the extraordinarily rich multiwavelength dataset of the Cosmic Evolutionary Survey (COSMOS). I will concentrate on the highest energy data available, the X-ray ones, from the surveys I have led using both the Chandra and NuSTAR NASA satellites. These data provide us with a unique and powerful tool to find and study accreting SMBHs in the distant Universe.



Apr 14: Dr. Duncan Farrah (Virginia Tech)


TITLE: Insights into galaxy assembly from luminous AGN in the high redshift Universe


ABSTRACT: There is a deep connection between star formation and AGN activity which profoundly impacts the mass assembly history of galaxies. The nature of the connection however remains controversial, due to, for example, evolution in the AGN and starburst duty cycles, and the obscuring effect of dust. In this talk I will briefly review our current knowledge of galaxy assembly across cosmic history, and then discuss two recent results. First is an observed scaling relation between star formation rates and AGN luminosities in luminous type 1 quasars at high redshift, which offers insights into how star formation may be triggered and quenched in these systems. Second is the morphological properties of the most luminous obscured AGN in the Universe, whose nature offers extremely stringent tests for galaxy evolution models and which may be signposting brief but critical phases in massive galaxy assembly. 






FALL 2016 semester:


Sep 30: Dr. Matteo Bonato (Tufts University)


TITLE: Exploring the evolution of star formation, black hole accretion and dwarf galaxy properties with JWST/MIRI serendipitous spectroscopic surveys


ABSTRACT: I will show predictions for spectroscopic observations with the MIRI Medium Resolution Spectrometer on board JWST. Specifically, pointed observations of Herschel sources will require only a few minutes for detections of several star-forming and AGN lines (out to z=3 and beyond), allowing us to efficiently investigate early phases of the star formation rate history and of the galaxy/AGN (co-)evolution. But the same data will also include tens of serendipitous 0<z<4.5 galaxies per field with IR luminosities down to about 10^6 L_sun. Therefore, for the first time and for free, we will be able to detect very low-luminosity galaxies (L_IR<10^9 L_sun) at high redshifts, and with good statistics. This will allow us to study the properties of these extreme galaxies and to test galaxy evolution models. Such surveys will achieve an improvement over Herschel of about three orders of magnitude in the study of the evolution of the cosmic star formation rate and about one order in black hole accretion rate. These results hold for a wide range in the modeled low-L end of the IR luminosity function, and accounting for the PAH deficit in low-L, low-metallicity galaxies.



Oct 14: Mr. Jeremy Bradford (Yale University)


TITLE: Isolated Low-Mass Galaxies: A Control Sample for Galaxy Formation and Evolution


ABSTRACT: Low-mass galaxies with stellar masses between 10^7 and 10^9 M_sun represent an interesting regime of galaxy formation. These galaxies are more susceptible to environmental processes, less efficient at forming stars, and often inconsistent with simulations. When selected with a strict isolation criterion, all low-mass galaxies are found to be star-forming. This implies that feedback processes are insufficient for shutting down star formation at these masses. Therefore, isolated galaxies are an excellent control sample for comparisons to predictions of galaxy formation. I have selected a sample of isolated, low-mass galaxies from the Sloan Digital Sky Survey and obtained neutral hydrogen observations for these galaxies. To compliment this study with high-mass galaxies, I have also re-measured data from the ALFALFA survey. I will present this sample and explore the neutral gas content and the scaling relations of isolated, gas-rich, late-type galaxies. Because it is a commonly used test of galaxy formation theory, I will focus on the baryonic Tully-Fisher scaling relation and the effect of systematic uncertainties on the relation when comparing observations to predictions.



Oct 28: Ms. Kathryn Grasha (UMass - Amherst)


TITLE: The Hierarchical Distribution of Young Stellar Clusters in Nearby Galaxies


ABSTRACT: A turbulent interstellar medium will drive the hierarchical nature of star formation, resulting in a smoothly varying distribution of substructure, where bound star clusters occupy the smallest, densest regions.  We use young stellar clusters to trace the unbound hierarchical star-forming structures for several nearby galaxies drawn from the Legacy ExtraGalactic UV Survey (LEGUS).  We implement the angular two point correlation function to quantify the clustering among stellar clusters as a function of spatial scale and age to establish whether the clustering strength and the survival timescale of the clustered substructure depends on the properties of the stellar populations.  We separate the clusters into different classes, compact (bound) clusters and associations and compare the clustering among the different classes.  We find that younger star clusters are more strongly clustered over small spatial scales and that the clustering disappears rapidly for ages as young as a few tens of Myr. We also find gravitationally bound clusters exhibit less clustering compared to stellar associations, and are no longer spatially correlated after ~40-50 Myr, and beyond 100-300 pc.  Thus, clusters are more evolved and have travel significantly from their birth site within a few tens of Myr, and associations show evidence of disruption occurring very quickly after formation, dispersing over the same timescale.



Nov 18: Dr. Rafael Martinez-Galarza (CfA - Harvard)


TITLE: Bayesian SED-fitting in the Era of Multi-wavelength Astronomy: Applications to Star-Forming Systems.


ABSTRACT: In the study of galactic and extragalactic star-forming systems, spectral energy distribution (SED) fitting is a powerful tool, boosted in recent years by a plethora of multi-wavelength observations covering the full (UV to sub-millimeter) thermal spectrum and by the development of new computational techniques that allow scientist to formulate and solve the problem in a Bayesian framework. Yet, the fundamentals of such Bayesian interpretation are not uniformly understood and applied across the many publications dealing with SED analysis in star formation and this is in part due to a lack of  sufficient statistics background among astronomers. In this talk I intend to provide an overview of a particular approach to the problem of Bayesian SED fitting. I discuss the basic elements of  Bayesian analysis in this context,  describe why Monte Carlo Markov Chains (MCMC) methods appear naturally in the process, and show two applications of Bayesian parameter estimation in star formation: the evolution of star formation in merging galaxies as a function of their interaction stage, and the physical properties of clustered Young Stellar Objects (YSOs) that appear blended together within the beam of infrared telescopes. The latter (SED-fitting of confused sources) is a common problem at all wavelengths, and I describe an algorithmic path to solve it by combining spatial and spectral information.






SPRING 2016 semester:


Feb 12: Mr. Vicente Rodriguez-Gomez (CfA - Harvard)


TITLE: The stellar mass assembly of galaxies in the Illustris simulation


ABSTRACT: In the Lambda-CDM cosmological model, structure grows hierarchically, potentially making mergers one of the primary drivers of galaxy growth. We use the Illustris cosmological simulation to study the relative contributions of in situ star formation and stellar accretion to the build-up of galaxies over an unprecedentedly wide range of masses (Mstar = 10^9−10^12 Msun), galaxy types, environments, and assembly histories. We find that the ‘two-phase’ picture of galaxy formation predicted by some models is a good approximation only for the most massive galaxies in our simulation – namely, the stellar mass growth of galaxies below a few times 10^11 Msun is dominated by in situ star formation at all redshifts. The fraction of the total stellar mass of galaxies at z = 0 contributed by accreted stars shows a strong dependence on galaxy stellar mass, ranging from about 10 per cent for Milky Way-sized galaxies to over 80 per cent for Mstar ~ 10^12 Msun objects, yet with a large galaxy-to-galaxy variation. At a fixed stellar mass, elliptical galaxies and those formed at the centers of younger halos exhibit larger fractions of ex situ stars than disk-like galaxies and those formed in older halos. On average, ~50 per cent of the ex situ stellar mass comes from major mergers (stellar mass ratio \mu > 1/4), ~20 per cent from minor mergers (1/10 < \mu < 1/4), ~20 per cent from very minor mergers (\mu < 1/10), and ~10 per cent from stars that were stripped from surviving galaxies (e.g. flybys or ongoing mergers). These components are spatially segregated, with in situ stars dominating the innermost regions of galaxies, and ex situ stars being deposited at larger galactocentric distances in order of decreasing merger mass ratio.



Feb 19: Dr. Dan Milisavljevic (CfA - Harvard)


TITLE: Reverse Engineering Supernovae


ABSTRACT: Core-collapse supernovae mark the catastrophic deaths of massive stars. They are among the most powerful explosions in the universe that affect many aspects of their host galaxies, and produce a variety of exotic objects including neutron stars, black holes, and some gamma-ray bursts.  Somewhat unsettling, however, is the fact that fundamental properties of supernovae with regard to their progenitor systems and explosion mechanisms remain poorly understood. Indeed, questions such as "Which stars explode?" and "How do stars explode?" do not have clear answers. I will review recent radio-through-X-ray observations that are revolutionizing our understanding of supernova explosions, with some emphasis on my own research that utilizes investigations of the entire supernova life cycle - from progenitor star, to explosion, to remnant - in order to "reverse engineer" key aspects of their dramatic and turbulent nature.



Mar 4: Dr. Brendan Griffen (MIT)


TITLE: There's no place like home: the assembly history of Milky Way-sized halos, from z=100 to z=0


ABSTRACT: In my talk I will present the largest number of Milky Way sized dark matter halos simulated at extremely high mass and temporal resolution done to date, more than quadrupling what is currently found in the literature (www.caterpillarproject.org). Our current set of 30+ halos of the Caterpillar Project whose project goal of 70 halos will eventually be made public. Our detailed contamination study of over thousands of low resolution halos has resulted in obtaining very large and unprecedented, high-resolution regions around our host halos (sphere of radius 2 Mpc) for our flagship resolution allowing for accurate studies of the assembly of the stellar halo, low mass dwarf galaxies at large galactocentric radii and the very first stellar systems at high redshift. In my talk I will discuss the prospects of carrying out "stellar archeology" by using semi-analytic models to identify Pop. III/II star formation sites and tracing their descendants down to the present day to probe the high-z Universe without the need for expensive high-z observations. [http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1509.01255]. 



Apr 8: Alex Ji (MIT)


TITLE: A rare and prolific r-process event in the ultra-faint dwarf galaxy Reticulum II


ABSTRACT: Ultra-faint dwarf galaxies contain a simple fossil record of early chemical enrichment that can probe the astrophysical site of r-process nucleosynthesis. Previous measurements found very low levels of neutron-capture elements in ultra-faint dwarfs, possibly supporting supernovae as the primary r-process site. I present high-resolution chemical abundances of nine stars in the recently discovered ultra-faint dwarf Reticulum II, seven of which display extremely enhanced r-process abundances 2-3 orders of magnitude higher than the other ultra-faint dwarfs. Stars with such extreme r-process enhancements are only rarely found in the Milky Way halo. The r-process abundances imply that the neutron-capture material in Reticulum II was synthesized in a single prolific event that is incompatible with r-process yields from ordinary core-collapse supernovae. Reticulum II provides an opportunity to discriminate between different candidates for rare and prolific r-process production, such as neutron star mergers or magnetorotationally driven supernovae. The single event is also a uniquely stringent constraint on the metal mixing and star formation history of this ultra-faint dwarf galaxy.



May 13: Dr. Heath Shipley (Tufts University)


TITLE: The Luminous Polycyclic Aromatic Hydrocarbon Emission Features: Applications to High Redshift Galaxies and Active Galactic Nuclei


ABSTRACT: For decades, significant work has been applied to calibrating emission from the ultra-violet, nebular emission lines, far-infrared, X-ray and radio as tracers of the star-formation rate (SFR) in distant galaxies.  Understanding the exact rate of star-formation and how it evolves with time and galaxy mass has deep implications for how galaxies form.  The co-evolution of star-formation and supermassive black hole (SMBH) accretion is one of the key problems in galaxy formation theory.  But, many of these SFR indicators are influenced by SMBH accretion in galaxies and result in unreliable SFRs.  Utilizing the luminous polycyclic aromatic hydrocarbon (PAH) emission features, I provide a new robust SFR calibration using the luminosity emitted from the PAHs at 6.2μm, 7.7μm and 11.3μm to solve this.  The PAH features emit strongly in the mid-infrared (mid-IR; 5-25μm) mitigating dust extinction, containing on average 5-10% of the total IR luminosity in galaxies.  The PAH luminosity correlates linearly with the SFR as measured by the extinction-corrected Hα luminosity, with a tight scatter of <0.15 dex.  The scatter is comparable to that between SFRs derived from extinction-corrected Paα and Hα emission lines. This new SFR indicator will be useful for probing the peak of the SFR density in the universe (1 < z < 3) and for studying the co-evolution of star-formation and SMBH accretion contemporaneously in a galaxy. 






FALL 2015 semester:



Sep 18: Dr. Annalisa Pillepich (CfA-Harvard)


TITLE: Building Galaxies and their Stellar Haloes by In-Situ and Ex-Situ Star Formation: insights from Eris and the Illustris Simulations


ABSTRACT: In the standard LambdaCDM scenario, hierarchical clustering leads to complex galaxy assembly histories, and the paths leading to the build up of each major stellar component may be different. In particular, galactic stellar haloes are thought to be the direct evidence of the hierarchical growth of structure in the CDM paradigm and numerical simulations have been able to reproduce their broad features from the debris of accreted and disrupted satellite galaxies. In the talk, I will adopt two complementary techniques with the specific purpose of understanding how baryonic physics and the hierarchical clustering leave imprints on the stellar assembly and the stellar distributions within haloes. On the one side, I will show the results of the detailed analysis of the Eris simulation, the first N-body+SPH simulated galaxy to result in a close analog of the Milky Way: I will give an account of the relative contributions of “in-situ” (within the main host) and “ex-situ” (within satellite galaxies) star formation to each major Galactic component of such close Milky Way analog. On the other side, I will support such findings by tackling a statistically-meaningful analysis of a large sample of galaxies from the Illustris Simulation. Illustris is a state-of-the-art simulation which combines the statistical power of a ∼106 Mpc-side cosmological volume with gasdynamics, prescriptions for star formation, feedback, and kpc resolution. For example, in the first work of a series (Pillepich et al. 2014), we show that by solely measuring the power-law slope of the stellar density profile of the stellar haloes, quantitative information can be obtained regarding the total mass, density profile, formation time, and merger history of the underlying DM haloes.



Oct 23: Dr. Matteo Bonato (Tufts University)


TITLE: Predictions for imaging and spectroscopic surveys of galaxies and Active Galactic Nuclei in the mid-/far-Infrared


ABSTRACT: While continuum imaging data at far-infrared to sub-millimeter wavelengths have provided tight constraints on the population properties of dusty star forming galaxies up to high redshifts, future space missions like the Space Infra-Red Telescope for Cosmology and Astrophysics (SPICA) and ground based facilities like the Cerro Chajnantor Atacama Telescope (CCAT) will allow detailed investigations of their physical properties via their mid-/far-infrared line emission. In this talk I present my predictions for SPICA imaging and spectroscopic surveys, obtained using both a phenomenological approach and physically grounded models. These predictions are useful to optimize the planning of the surveys and exploit a recent upgrade of evolutionary models, that include the effect of strong gravitational lensing, in the light of the most recent Herschel and South Pole Telescope data. The relations between line and continuum infrared luminosity were worked out considering differences among source populations, with the support of extensive simulations that take into account dust obscuration. My reference model for the redshift dependent IR luminosity functions was the one worked out by Cai et al. (2013), based on a comprehensive hybrid approach combining a physical model for the progenitors of early-type galaxies with a phenomenological one for late-type galaxies. I present new estimates of redshift-dependent luminosity functions of IR lines detectable by SPICA and excited both by star formation and by AGN activity. To this end I worked out a major upgrade of the Cai et al. (2013) model, by dealing in a self consistent way with emission of galaxies as a whole, including both the starburst and the AGN component. These spectroscopic observations will allow us to probe all phases of the interstellar medium (ionized, atomic and molecular). Measurements of these lines will provide redshifts and key insight on physical conditions of dust obscured regions and on the energy sources controlling their temperature and pressure. This information is critically important for investigating the complex physics ruling the dust-enshrouded active star forming phase of galaxy evolution and the relationship with nuclear activity. Observations of strongly gravitationally lensed galaxies, described in detail by my reference model, will be of special interest, because strong lensing allows us to measure their total mass distribution up to very large distances and to gain information on sources too faint to be detected with current instrument sensitivities, thus testing models for galaxy formation and dark matter.



Oct 30: Dr. Aleksander Sadowski (MIT-Kavli)


TITLE: Simulations of radiative black hole accretion in general relativity


ABSTRACT: In this talk I will summarize the recent progress in simulating optically thick black hole accretion. I will discuss the methods used to evolve gas together with radiation field, their power and limitations. In detail, I will describe simulations of super-critical black hole accretion disks applicable to ultra luminous X-ray sources and tidal disruption events. Special attention will be paid to their luminosities and mildly relativistic, radiative jets formed in such systems. In the end I will discuss challenges related to simulating thin disks and show preliminary simulations of sub-critical accretion disks.



Nov 19: Dr. Remco van der Burg (Paris - CEA Saclay)


TITLE: The distribution of stellar mass in galaxy clusters over cosmic time


ABSTRACT: Galaxy clusters are the most massive gravitationally collapsed structures in the universe, and they have important cosmological and astrophysical applications. Given their extreme overdensities, they allow us to study how the evolution of galaxies depends on their environment. I will present results on the properties of galaxies in clusters around redshift 1, at a time where a significant fraction of the cluster galaxies is still forming stars. These measurements provide constraints on the times when, and the locations in the clusters where, their star formation is being quenched. I will also present measurements on the radial distribution of galaxies in two cluster samples, which span about 8 Gyrs of lookback time. By matching local galaxy clusters to their progenitors at high redshift, we study how clusters grow their stellar mass content. Interestingly, this suggests that the central part of the stellar mass distribution of local galaxy clusters is already in place at redshift, and any further growth seems to happen in an inside-out fashion. I will put these findings into context by comparing them to the results from dark matter simulations. I will also discuss future directions, both on the observational side as on the interpretation of these results.



Dec 4: Andrew Battisti (Umass - Amherst)


TITLE: Characterizing Dust Attenuation in Local Star Forming Galaxies


ABSTRACT: The presence of dust in galaxies causes their spectral energy distribution to experience reddening, a consequence of the highest attenuation occurring in the ultraviolet and decreasing towards longer wavelengths out to the infrared. If not properly accounted for, this effect can lead to incorrect values of derived physical quantities such as the star formation rate, stellar mass, and photometric redshift. Virtually all studies of star forming galaxies, both local and distant, make use the attenuation curve derived for local starburst galaxies (Calzetti et al. 2000) to correct for the effects of dust. However, it is not clear how reasonable the general application of this curve is for more typical star forming galaxies. In this talk, I will present results from a study utilizing ~10000 local (z ≤ 0.1) star forming galaxies to identify the extent to which attenuation varies as a function of galactic physical properties.



Dec 11: Allison Kirkpatrick (Umass - Amherst)


TITLE: What Lies Beneath: Dust Obscured Star Formation and Black Hole Growth


ABSTRACT: The evolution of massive dusty galaxies is driven internally by AGN growth and star formation, but due to the copious amounts of dust, it is often impossible to observe these processes directly. Instead, we must disentangle information about the inner workings of galaxies from the observed dust emission. Using a large sample of z=0.5-2.8 Ultra Luminous Infrared Galaxies, I quantify the contributions to the dust heating from AGN activity and star formation using deep Spitzer mid-IR spectroscopy. I also explore composite galaxies--those galaxies which show both strong mid-IR star formation and AGN signatures. These galaxies exhibit a clear increase of dust temperature in the far-IR as the AGN grows stronger, indicating that a growing AGN is significantly contributing to LIR, although the galaxy is still strongly star-forming. Furthermore, these composite systems are prevalent even at faint 24 micron flux thresholds, and could be an important source of AGN contamination in any high redshift sample of star forming galaxies, if care is not taken to identify them. I will discuss ways to identify composite galaxies and how to correct infrared luminosities and star formation rates for dust emission heated by an obscured AGN.