Dr. Jordan Mirocha
Senior NASA Postdoctoral Fellow
Jet Propulsion Laboratory
Hello!
I am an astronomer working in the area galaxy formation theory -- I build models that are used to interpret of a variety of observations, including galaxy surveys (e.g., those conducted with the Hubble and James Webb space telescopes), probes of the intergalactic medium in the early Universe (e.g., the Hydrogen Epoch of Reionization Array), and maps of the extragalactic background light (via SPHEREx). My primary goal is to use this diversity of observations to constrain the properties of the first stars and black holes to form after the Big Bang, as well as the feedback processes that regulate galaxy evolution throughout cosmic time.
Background
2022-present
Senior NASA Postdoctoral Fellow
2018-2022
Research Associate
CITA National Fellow
Jet Propulsion Laboratory
Section 3266: Structure of the Universe
McGill University
Department of Physics and McGill Space Institute
2015-2018
Postdoctoral Scholar
Postdoctoral Scholar
2009-2015
Graduate Student
NASA Earth & Space Sciences Fellow
UCLA
Department of Physics and Astronomy
University of Colorado Boulder
Department of Astrophysical & Planetary Science
Research Interests
First stars
My main interest for a long time has been the formation of the first stars and black holes. The lack of heavy elements in the early Universe is expected to affect how gas clouds cool and collapse, which could result in unusually massive stars by today's standards. This has a slew of implications, e.g., how the Universe came to be "reionized," how and when more typical stars began to form, and how/when the first black holes formed. The massive first-stars hypothesis is exceedingly difficult to test directly, as even the brightest stars in the early Universe are too faint to be detected by our best telescopes.
I see this challenge as the motivation for much of my work: I spend my time trying to figure out how to indirectly constrain the very early phases of galaxy formation, primarily via the extragalactic background light and 21-cm background. This means I also spend a lot of time trying to understand the galaxies we can see, so that we can better isolate the effects of sources beyond the detection thresholds of traditional galaxy surveys.
Black holes
Observations indicate that super-massive black holes (BHs), i.e., those a billion times more massive than the Sun, are already in place less than a billion years after the Big Bang. Their existence is difficult to reconcile with how we think most BHs form: via the collapse of massive stars in supernovae explosions, which yield BHs that are ~10s of solar masses, or perhaps ~100s if the first stars are very massive.
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Like the first stars, the first BHs will be nearly impossible to see directly. But, if BHs grow by accreting gas, they will produce X-rays that can heat and ionize the intergalactic medium (IGM). I work on understanding the signatures of this X-ray production in the 21-cm background radiation. Experiments like HERA now disfavor scenarios in which the high-z IGM is heated by sources similar to high-mass X-ray binary systems in nearby galaxies, which are powered by ~10 solar mass BHs. Improved limits (and eventually detections!) will help us constrain the nature of the Universe's first X-ray emitters, and whether they were the remnants of the first stars.
Stellar feedback
If two galaxies -- one more massive than the other -- are each given the same amount of gas, they will not in general form the same number of stars. The more massive galaxy will form stars more efficiently than the less massive galaxy, at least up to a point. Feedback from supernovae explosions is thought to drive this behavior, but it's not clear in detail how it works, or whether it should be effective in all galaxies at all cosmic times. A breakdown of feedback in small galaxies at high redshift, for example, is not unexpected, and could dramatically change how we interpret upcoming datasets.
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Stars in the early Universe don't just feedback on their own host galaxies -- they can actually affect the formation other galaxies! This is through the radiation they produce, which can travel a much greater distance than the blastwaves from their supernovae explosions. I'm interested in finding ways to see this feedback in action, and determine if events unfolding > 13 billion years ago could effect the properties of galaxies even today.
Research Projects
Most of my work fits under the umbrella of a mission or experiment, though I also build models that speak to projects with which I have no formal involvement (see, e.g., recent papers' predictions for Roman and constraints from JWST).
SPHEREx
I work as part of the SPHEREx Science Team, developing models of the Extragalactic Background Light (EBL), which is the cumulative emission from galaxies across all of cosmic history. I am focused in particular on separating out the different contributions to the EBL, and figuring out what we can learn through cross correlations with galaxy samples.
ARES
I started building ARES 10+ years ago, and it remains the code I most actively use and develop. It is designed to generate rapid 'simulations' of galaxy formation without generating 3-D realizations. This enables a massive speed-up, and so permits rapid -- albeit approximate -- explorations of parameter space and parameter inference. ARES can be used to make predictions for the 21-cm background (see also, e.g., this), as well as galaxy surveys and the EBL.
HERA
I also work as part of the HERA Theory team, developing models for the 21-cm power spectrum. This signal encodes fluctuations in the neutral hydrogen content and temperature of the early Universe, and so can be used to indirectly constrain the properties of high redshift black holes and galaxies.
micro21cm
I also spend time developing phenomenological models, which provide a way to test the assumptions baked into our physically-motivated models (like ARES). This code is an attempt to do that, and provide ~model-independent constraints on reionization with observations of the 21-cm background. It has been used to interpret upper limits from HERA (and more recently, here), but is modular and could be extended to other problems without much trouble.
DARE/DAPPER
I am involved in efforts to measure the global 21-cm signal from the Moon (e.g., Dark Ages Radio Explorer and DAPPER mission concepts). This signal encodes the average ionization and thermal history of the Universe, so can be used to constrain the formation of the first stars and in principle cosmic structures at even earlier times.
In the past few years I've begun developing a hybrid model that borrows techniques from N-body simulations and "semi-numerical" models of reionization. My collaborators and I have used this technique to assess the impact of detailed galaxy histories in models of reionization and make predictions for signatures of 'reionization feedback' in the EBL. Contact me if you're interested in collaborating on the code!