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Star Formation and Triggered Active Nuclei in Colliding Galaxies: Using Infrared Observations and Modelling to Characterize Galaxy Formation and Evolution in the Local and Early Universe

Submitted by:
Howard Smith

Galaxy mergers are key events in galaxy evolution, generally triggering massive starbursts and stimulating active galactic nuclei (AGN). The ultraviolet produced by these processes heats the dust which re-radiates almost all the luminosity in the infrared. The dust also obscured optical light, and thus IR diagnostics are key to unraveling these processes. A major thrust of modern astrophysics is the birth and evolution of luminous galaxies in the early universe, but these objects are far away and most diagnostics of their driving evolution (star formation and AGN activity) are difficult to study however the bulk of their emission is detectable in the infrared, making the IR a key diagnostic tool.

Our extensive program of IR observing and modeling of relatively nearby merging galaxies is aimed at improving IR methods, whose tools that can then be applied to galaxies in the early universe. We have recently expanded our study to a set of over 240 galaxies including likely mergers, complemented by full photometric datasets datasets from the UV (GALEX) to the FIR (Herschel).

By modeling the spectral energy distributions (SEDs) of these objects using the Code for Investigating Galaxy Emission (CIGALE) we can quantify dozens of physical parameters in a galaxy and also apply the results to infer evolutionary stages. One prime subset of these objects consists of very late-stage mergers, a second consists of ultraluminous objects thought to contain a significant contribution from AGN.

Our results to date have uncovered a set of about twenty luminous galaxies that do not to behave like all the others, and whose physical properties have not been reliable extracted using our codes. The goal of this thesis project is a series of papers on these extreme, unusual merging galaxies, to track down and address the reasons why they are outliers and perhaps set examples for unusual galaxy behaviors in the early universe.

The masters student will review the multiwavelength data and photometry for these galaxies, learning especially the tools for astrophotometry with space missions, and then use iterated computer modelling to try to pin down key parameters like star formation rates, specific star formation rates, dust mass and temperatures, and AGN contribution. Other codes besides CIGALE can be used as well, and models of simulated mergers are also available. We will then apply the results to high-z galaxies. The results will not only clarify what processes are making these galaxies local outliers, they will help refine the modeling codes to be more complete.

Will be supported by the following grants:
NASA SOFIA research grants

Submitted on:
June 8, 2022

Anomalous Time-Domain Phenomena in X-Ray Datasets: Symbiotic X-Ray Binaries, Fast X-Ray Transients, and Beyond

Submitted by:
Rafael Martinez-Galarza

Transient X-ray phenomena (the sudden brightening of astronomical X-ray sources in short timescales) are of significant scientific importance, because they can signal exotic binary systems such as symbiotic X-ray binaries, explosive phenomena related to the mergers of neutron stars, or micro-lensing events, when a compact objects such as a black hole passes in front of an X-ray emitter. Despite the large amount of serendipitous detections by X-ray missions such as Chandra and XMM-Newton, only a few of these events have been identified. In order to constrain models of the most compact objects in the Universe, more examples are needed. In this project, the student will use archival Chandra and XMM-Newton data complemented by optical and infrared databases in order to identify additional candidates for symbiotic X-ray binaries, magnetar-related events, and lensed X-ray sources. The project will consist of three parts: First, the student will use data science tools to identify transient candidates in large archival X-ray repositories. Then, the student will cross-match the list of previously unidentified candidates with optical datasets to identify those to be more likely to be objects of interest, based on their optical properties (for example, is the optical counterpart a white dwarf? Is it a red giant? Is it an active galactic nucleus?). Finally, the student will characterize the most promising objects by employing physical models to constrain their physical properties, including lensing models, frequency analysis to search for periodicity, and X-ray spectral models. The student will work with CfA experts on X-ray datasets, data science, and physical modeling to unveil a list of newly-discovered exotic transients.

Additional information:
https://cxc.cfa.harvard.edu/csc/

http://xmm-catalog.irap.omp.eu/

Will be supported by the following grants:
Collaborator Chandra grant

Submitted on:
June 1, 2022

A Search for Winking Black Holes

Submitted by:
Jack Steiner

NICER is a relatively new soft X-ray instrument on the International Space Station. It is amassing an unprecedented archive of X-ray spectral-timing data on the brightest objects in the X-ray sky - black hole (BH) X-ray binaries. Several of the BHs are persistent, but the vast majority are transient and undergo year-long outbursts which are marked by transitions through a sequence of spectral-timing states. During its outbursts, a black hole can launch relativistic jets, emit powerful winds, and produce dynamic humming patterns known as quasi-periodic oscillations (QPOs).

NICER has produced tens of thousands of observations of these BHs, often consisting of near-daily snapshots during an outburst. With an interested student, we will be building a public-facing web-interface to NICERs data archive of ~30 BH systems, and within that framework, we will be applying (python-based) machine-learning algorithms to search for anomalous (often short-lived) behavior which stands apart from its BH compatriots. Among known unusual behaviors are so-called high-frequency QPOs and a hypersoft state. We will be searching for signs of these behaviors and more. In short, we aim to automatically identify any new and unusual or hitherto unclassified X-ray phenomena with aim to better understand these remarkable objects. Or, put more cheekily, to find out whether a black hole winks.

Will be supported by the following grants:
SI have several observing proposals, including NICER, which would be suitable for use to support a AAS trip.

Submitted on:
June 1, 2022

Simulating the Stellar Galactic X-Ray Emission

Submitted by:
Cecilia Garraffo

Cool stars are, by far, the most common kind of stars in the galaxy, as they constitute ~99% of the stellar population. When cool stars rotate, they generate magnetic fields on their surface, so called magnetic activity. This activity results in X-ray emission, and it is determined primarily by the stellar mass and the rate of rotation.

X-rays, then, become a good tool to test stellar rotation models and their resulting magnetic activity. This is important, for example, to understand the space weather close-in exoplanets are exposed to, and the chances of a planet to host and retain an atmosphere. Stellar rotation evolution of cool young stars is still poorly understood. Recently, there is growing consensus on the importance of the morphology of the magnetic fields on the angular momentum loss rates. Garraffo et al 2018 proposed a physics based model that takes this into account and successfully reproduces the distribution of rotation periods in open clusters.

In this project we will generate a synthetic galactic population using the Gaia Universe Model Snapshot, and assign each star a rotation period using Garraffo et al 2018 model. From the rotation period and stellar mass one can infer the X-ray emission of each star and then compare the stellar synthetic galactic emission with X-ray survey observations.

Submitted on:
June 1, 2022

Understanding the Nature of Ultra-steep Spectrum Diffuse Radio Sources in Clusters of Galaxies

Submitted by:
Scott Randall

Recently, radio observations have identified a new class of diffuse radio sources characterized by their very steep radio spectra. These ultra-steep spectrum (USS) sources are often found in galaxy clusters, particularly in merging clusters. They often have irregular morphologies, and can be up to megaparsecs in size (on the order of the size of clusters themselves). The nature of these sources is unclear. They may arise from particle acceleration at merger shock fronts (radio relics), from adiabatic compression of fossil non-thermal electron populations (radio phoenixes), associations of old radio plasma left over from a previous AGN outburst (AGN relics), or some mix of these three.

This project will involve the analysis of XMM-Newton X-ray observations of two USS host clusters. Image analysis, as well as spectral analysis used to construct thermodynamic maps, of the diffuse intracluster medium (ICM) will be used to constrain the dynamical state of the cluster, and correlate X-ray structure with the USS radio sources. This will, in turn, provide information on the possible formation scenarios of the USS sources.

Will be supported by the following grants:
Fund 16613954

Submitted on:
June 1, 2022

Developing Accurate Molecular “Fingerprints” for High-Temperature Atmospheres: Expanding the HITEMP Database

Submitted by:
Robert Hargreaves

The HITRAN molecular spectroscopic database [1] is considered the international reference standard for molecular spectroscopic parameters. The HITEMP database [2] is analogous to HITRAN, but was specifically established for high-temperature applications. As a result, the usage of HITEMP is far-reaching: from the simulation/interpretation of atmospheric spectra, to the sensing of specific transitions in combustion processes.

Over recent years, it has become possible to observe exoplanetary atmospheres and brown dwarfs at high spectral resolution. Consequently, the radiative transfer (RT) models required to interpret these observations require accurate and reliable data. This is especially important for the underlying line lists, which contain the frequencies and intensities (among other parameters) necessary for identification of molecules – their characteristic “fingerprints”. One major attribute of the HITRAN and HITEMP databases is their accuracy, due to careful validation of the underlying datasets. For example, the HITEMP data was recently used to detect methane in transit observations of the exoplanet HD 209458b (along with five other molecules) [3]. Furthermore, HITEMP data is an integral component for RT models used in the interpretation of cool brown dwarf spectra [4].

The proposed work aims to develop accurate line lists for HITEMP. This will require the development of physical models for validation and interpolation (or extrapolation) of disparate data, as well as efficient representation of weakest spectroscopic features [5], thereby allowing for fast calculations of reference opacities. Comparison with temperature-appropriate observations (both laboratory and astronomical) is an essential and valuable step, which will form a key part of the work. The resultant line lists from this work will be used to advance understanding of stellar, brown dwarf, and exoplanetary atmospheres.

[1] I. E. Gordon et al. (2022), JQSRT 277, 107949 [doi: 10.1016/j.jqsrt.2021.107949]

[2] L. S. Rothman et al. (2010), JQSRT 111, 2139-2150 [doi: 10.1016/j.jqsrt.2010.05.001]

[3] P. Giacobbe et al. (2021), Nature 592, 205-208 [doi: 10.1038/s41586-021-03381-x]

[4] M. Tannock et al. (2022), MNRAS in press [doi: 10.1093/mnras/stac1412]

[5] R. J. Hargreaves et al. (2020), ApJS 247, 55 [doi: 10.3847/1538-4365/ab7a1a]

Additional information:
whttps://hitran.org

https://www.cfa.harvard.edu/research/hitran-and-hitemp-database

Will be supported by the following grants:
NASA PDART 80NSSC20K1059

Submitted on:
May 31, 2022

X-ray Luminosity Function of Normal Galaxies

Submitted by:
Dong Woo Kim

The X-ray luminosity function (LF) of normal galaxies is a key observational measurement, which can be used to test theoretical models and numerical simulations. However, they can only be built from large, unbiased samples. Most X-ray LFs published so far are based on a small number of (preferentially X-ray bright) galaxies. The previous LFs (Kim et al. 2006, ApJ, 644, 829) are unconstrained both at the low and high luminosity end and could not investigate dependencies on mass or redshift convincingly.

We have built the most extensive and well-characterized Chandra X-ray Galaxy Catalog, utilizing the Chandra Source Catalog (v2), cross-correlating the X-ray sources with major all-sky survey data, and applying the multi-wavelength (from radio to X-ray) characteristics of galaxies.

Using the large galaxy sample, we can apply the modified 1/VMAX (non-parametric) method and the (parametric) maximum likelihood techniques, with special care for the uncertainty in photo-z, e.g., by using deconvolution- or convolution-based estimators and the Eddington bias. We will build the X-ray luminosity function in multiple bins of different masses, redshifts, and morphological types. Then, we can compare the observed X-ray luminosity function with cosmological simulations to critically constrain the model parameters, e.g., feedback recipes.

Will be supported by the following grants:
Chandra GO grant

Submitted on:
May 31, 2022

Planets Orbiting X-Ray Binaries in External Galaxies and in the Milky Way

Submitted by:
Rosanne Di Stefano

During the past year, our group has discovered a candidate planet orbiting an X-ray binary in an external galaxy, the Whirlpool Galaxy, M51. We are engaged in several types of projects to take this result further. One crucial direction is the search for additional planets orbiting X-ray binaries in a wide range of galaxies, including our own. These searches rely primarily on archived X-ray data. Another type of project consists of analyzing transit events from a theoretical perspective. For example, we now know that some transits repeat over time intervals shorter than the planetary orbit. The repeats are due to the motion of the components of the X-ray binary. We want to better understand the role of three-body dynamics and to determine what can we learn from such phenomena. And of course the discovery of moons orbiting planets in X-ray binaries is another frontier that we are beginning to explore with existing data.

A student interested in this project could work on data mining large sets of light curves analyzing multi-wavelength, multi-mission data on those transit candidates that have been identified confirmation or elimination of the transit model for individual events theoretical studies to explore one of several interesting directions, including the probabilities of event detection, modeling of planetary orbits within X-ray binaries, and the reaction of planetary atmospheres to significant X-ray and UV irradiation.

During the summer of 2022 a group of eight students and seven senior researchers are collaborating on a suite of related projects, establishing a deep and broad foundation for further work in this new and exciting field.

Will be supported by the following grants:
ITC

Submitted on:
May 28, 2022

Formation of Most Massive Stars in the Galaxy

Submitted by:
Qizhou Zhang

Massive protostars must accrete more than 20 Msun of material to become early O-type stars, but they begin core hydrogen burning well before reaching their final masses. Once a protostar reaches a late O-type, the stellar surface temperature is high enough to create an HII region of ionized hydrogen gas. In the classical theory of HII regions, the outward pressure differential between the hot ionized gas and the cold molecular gas of 100 K terminates the accretion and ends the star formation. This raises a fundamental question: how do stars with masses greater than 20 Msun form?

The classical theory of HII regions ignores gravity, which assists accretion by pulling the ionized gas within the gravitational radius despite the thermal pressure. The goal of this project is to identify and analyze the ionized accretion flow in massive protostars. The project makes use of high-angular resolution observations obtained with Atacama Large Millimeter/submillimeter Array (ALMA). The data revealed, for the first time, mass accretion in the ionized gas. This project involves processing the ALMA data using CASA and run radiative transfer models using the Markov chain Monte Carlo (MCMC) technique to constrain the basic parameters (stellar mass, gas densities, etc.) of the system. The student will have the opportunity to learn the basics of interferometry, to process data obtained from ALMA, the most advanced millimeter/submillimeter interferometer in the world, and the theory of star formation. The project also involves radiative transfer modeling (the code is fully developed and is ready for use), and some limited coding in python. Since cases of ionized accretion around massive O-type stars are extremely rare, the results from this project have the potential to be published in a high-impact journal.

Additional information:
https://lweb.cfa.harvard.edu/~qzhang/

Will be supported by the following grants:
SMA

Submitted on:
May 27, 2022

A Data Mining-powered Atlas of X-ray Annotated Spectral Energy Distributions

Submitted by:
Raffaele D'Abrusco
617-229-9732

The relationship between the X-ray properties of astronomical sources and the nature of their multi-wavelength emission is encoded in their Spectral Energy Distributions (SEDs). While used extensively in astronomy, SEDs have never been investigated on a purely data-driven basis, using a statistically large, diverse population of sources. Thanks to the Chandra Source Catalog 2.0 (CSC2), which has detected and characterized in detail the X-ray behavior of ~317,000 sources from archival Chandra observations, the systematic study of this relationship is finally possible.

In this project, we will collect photometric data from radio to the highest energy available for a large subset of CSC2 sources, using the Virtual Observatory (~80% of CSC2 sources already have a multi-wavelength counterpart in the NED database). A set of curation steps will be applied to the photometric data associated with each source to homogenize them, reconcile differences due to different aperture sizes and intrinsic variability, and produce science-ready SEDs. These SEDs will be then grouped together according to their shape using unsupervised Machine Learning (ML) techniques for dimensionality reduction, clustering and visualization, resulting in a low-dimensionality, easy-to-explore distribution of Chandra sources that reflects their topological structure in the high-dimensionality SEDs space.

Labeling these sources with their photometric, spectral and time-variability X-ray properties, we will study the correlations between the SED-based common features in groups of nearby sources in the SED space and their corresponding X-ray properties. Moreover, we will use the X-ray/SED correlations determined for known CSC sources, to shed light on the nature of the still unidentified CSC sources. This novel approach will allow us to produce a multi-dimensional, multi-wavelength characterization of the X-ray Universe never accomplished before on such scale and scope.

The project will produce an extensive atlas of X-ray annotated SEDs of CSC sources. This dataset will represent a valuable contribution to the astronomical community and will enable further scientific inquiry into one of the richest high-energy datasets ever produced.

Will be supported by the following grants:
Collaborators grant

Submitted on:
May 20, 2022

Probing High-Mass Star Formation in HII Regions with SOFIA/FORCAST

Submitted by:
Joseph Hora
617-719-5764

High mass star formation is much more poorly understood than low mass star formation. This is in part due to the greater distance, extinction and confusion toward regions of high mass star formation and due in part to the rapid evolution of high mass protostars and difficulty of finding such objects in the earliest stages of their evolution. A remaining mystery is how the stars assemble large masses in their turbulent and chaotic environments.

The massive star-forming cluster NGC 3603 is the largest and closest example of a giant H II region in our Galaxy. The HII region G104.52+1.24 is a lower-mass region in the outer Galaxy. We used the FORCAST instrument on SOFIA to image these regions in the 20 40 m range to locate embedded objects and measure their emission. The new data complements existing Spitzer and Herschel data by enabling us to construct spectral energy distributions (SEDs) of objects which span a wide range of mass and evolutionary stages, allowing us to probe with some of the best statistics available the earliest stages of massive protostar formation. With SOFIA we can find massive protostars and study the Galactic equivalent of objects that JWST will observe in more distant Galactic regions and in other galaxies. The SOFIA results will enable us characterize the high end of the mass distribution in NGC 3603 and G104.52+1.24. We will also be able to examine the effect that these stars have on their surroundings through outflows and radiation.

Additional information:
https://lweb.cfa.harvard.edu/~jhora/
https://lweb.cfa.harvard.edu/~jhora/SFOG/

Will be supported by the following grants:
SOFIA grant for program 09_0031

Submitted on:
May 17, 2022

How Magnetic Fields Guide Star-forming Flows

Submitted by:
Phil Myers

Star formation is the primary baryonic process driving the evolution of the Universe. Despite its central importance to astrophysics, cosmology, and life in the Universe, star formation is still poorly understood. Magnetic fields are thought to play a crucial role in the puzzle. Strong fields can prevent gravitational collapse and star formation, while moderate fields can affect the masses and numbers of stars a cloud will form. We have recently obtained unprecedented high quality polarization maps of star-forming regions and now have a good chance to solve the star formation puzzle. The maps were made at far infrared wavelengths with NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) - a remarkable 2.7m infrared telescope flown on a specially modified Boeing 747 airplane.

The student will analyze these new SOFIA polarization maps of molecular clouds, from immense filaments forming massive star clusters, down to small dense cores forming just a few stars like our Sun. The magnetic maps will be combined with observations of molecular spectral lines and will tell us how magnetic fields prevent collapse in different gas clouds, whether inflowing gas is dominated by chaotic, turbulent flows or is regulated and inhibited by magnetic fields, and in what conditions both inflowing gas and gas ejected by newly forming protostars is channeled along field lines. The research should provide exciting new results on star formation that the student will be encouraged to lead publication of in a professional astrophysics journal.

Recent SOFIA Observing Programs with P. Myers as co-I:

FIELDMAPS - cycle 8 magnetic studies of large cluster-forming filaments in the Milky Way (PI Stephens)

SIMPLIFI - cycle 9 magnetic studies of local star-forming filaments with finer resolution (PI Pillai)

Recent Papers:

Myers et al. 2018, ApJ, 868, 51 Myers et al. 2020, ApJ, 896, 163 Myers Basu 2021, ApJ 917,35 Pillai et al. 2020, NatAs 4, 1195, Stephens et al. 2022, ApJ, 926L, 6.

Project Advisors: Philip Myers (CfA) Ian Stephens (Worcester State University), Thushara Pillai (Boston University)

Additional information:
https://www.sofia.usra.edu/data/legacy-programs

Will be supported by the following grants:
Student AAS travel will be supported by SOFIA grants to proposal ID 08_0186 (FIELDMAPS) and 09_0215 (SIMPLIFI)

Submitted on:
May 14, 2022