Mehrnoosh Tahani

Welcome to my webpage! I am a Banting Fellow, sponsored by the Canadian government. With the privilege of conducting research at any eligible university worldwide as a Banting Fellow, I chose Stanford University, where I also hold a Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) fellowship. I previously held a research associate position (Covington Fellow) at the Dominion Radio Astrophysical Observatory with the National Research Council Canada (NRC-Herzberg).

My research explores the magnetic universe through observations, theory, simulations, and instrumentation. I study how galaxies evolve to form stars by mapping interstellar magnetic fields. These field geometries act as cosmic time capsules, revealing the history of galaxies much like a dye reveals hidden patterns, offering valuable insights into our cosmic origins.

Research & Publications

I highlight some of my work below. For a full list of publications, please click on this ADS link or visit my Google Scholar profile.

An Innovative Technique for Next-Generation Radio Astronomy

Although there is an abundance of plane-of-sky magnetic field observations associated with molecular clouds, observations of their line-of-sight magnetic fields (Blos) are extremely limited. To address this issue, I pioneered a novel technique (named MC-BLOS) based on Faraday rotation to determine Blos associated with molecular clouds, outpacing Zeeman measurements. Faraday rotation is the rotation of the polarization plane of a linearly polarized electromagnetic wave as it passes through a magneto-ionic medium.

Due to the lower abundance of electrons in molecular clouds (a type of interstellar cloud known as a star-formation nursery) using the Faraday rotation technique to probe their magnetic fields was previously thought to be impossible, a challenge I sucessfully overcame during my PhD. The MC-BLOS technique utilizes the Faraday rotation of unresolved point sources (radio galaxies or pulsars), an ON-OFF approach, a chemical evolution code, and column density maps of the clouds. Please watch this video for a simplified explanation of the technique.

First Line-of-Sight Magnetic Field Maps of Some Molecular Clouds and Discovery of Magnetic Reversal

I mapped the line-of-sight magnetic fields of the Orion A, Orion B, Perseus, and California molecular clouds, using the MC-BLOS technique. Our findings are closely aligned with the available nearby molecular Zeeman measurements, thereby validating the effectiveness of our technique.

We discovered that the Blos direction in three of the clouds reverses from one side to the other across the cloud’s long axis. Previously, line-of-sight magnetic field reversals had only been detected in the Orion A cloud via Zeeman measurements, a finding consistent with our results. Subsequent theories postulated helical-shaped fields to explain these reversals. However, my subsequent research showed that an arc-shaped morphology, where the field lines bend around the filament and connect to the large-scale Galactic magnetic fields, is the most probable scenario to explain the reversal.

First Complete 3D Magnetic Field Vectors of Clouds

We successfully combined line-of-sight magnetic fields, plane-of-sky magnetic fields from the Planck mission, and Galactic magnetic field models to reconstruct the complete 3D magnetic fields of molecular clouds, encompassing full directions in all three dimensions. This approach establishes a novel framework for resolving the longstanding directional ambiguities associated with plane-of-sky magnetic fields that have long challenged astronomers. I have discussed these advancements in a review paper, highlighting potential improvements for future work.

Cloud Formation and Identification of Unknown Structure

In this work, we reconstructed the complete 3D magnetic field vectors of the Perseus cloud for the first time. This field geometry is concave from our point of view, with the field lines pointing in the direction of decreasing longitude when projected onto the plane of the sky. The determination of 3D field vectors allowed me to propose a detailed step-by-step formation and evolution scenario for the Perseus cloud, closely linked to the influence of nearby super-bubbles.

I hypothesized that the cloud's field geometry resulted from interactions with the Per-Tau bubble and another previously unidentified structure. The influence of this structure was subsequently confirmed by kinematic observations, supporting our 3D magnetic field vectors and the proposed formation scenario for the cloud. In this video, I discuss 3D fields and their formation and evolution.

The Complexity of Orion A

In reconstructing the 3D magnetic field vectors of clouds, we also consider the 3D shapes of the clouds. The Orion A cloud presents a more complex geometry compared to the Perseus cloud; its head lies parallel to the plane of the sky, while its tail is inclined at a 70-degree angle. This complexity introduces greater uncertainties in determining the 3D field vectors and complicates our understanding of the cloud's formation, history, and evolution.

In our study, we reconstructed the 3D magnetic fields of the Orion A cloud, allowing us to investigate the cloud's formation and evolution. I found that both the Orion-Eridanus bubble and a nearby dust ring have influenced Orion A's formation and evolution. Interestingly, the famous Barnard's Loop has only interacted with the head of the cloud, which explains the higher rate of star formation observed in the head.

Ionized Hydrogen (HII) Regions

Dust polarization offers a powerful method for observing interstellar magnetic fields. This technique is based on the alignment of the long axis of elongated interstellar dust grains perpendicular to magnetic fields and reveals only the plane-of-sky component of magnetic fields (though not their direction). To apply this method, we conducted observations of the NGC 6334 molecular cloud, a massive star-forming region within the Galactic disk, using the James Clerk Maxwell Telescope (JCMT) within the BISTRO collaboration. These observations not only allowed us to map the magnetic field structure of this cloud but also enabled an analysis of the hot ionized atomic hydrogen (HII) regions surrounding newly formed massive stars, providing a comprehensive view of the cloud's magnetic environment.

In this study, I found that HII regions influence the magnetic fields of their parent molecular clouds, aligning these fields tangentially to their boundaries. I also developed an algorithm to characterize this alignment and quantify the impact of HII regions on magnetic fields. The orientation of magnetic field lines relative to the HII bubbles provides crucial insights into the dynamic processes that drive cloud evolution and trigger secondary star formation, underscoring the complex interplay between magnetic fields, feedback processes, and interstellar medium dynamics.

Magnetic Fields from Clouds to Cores

Magnetism plays important roles in the evolution of galaxies and the formation of clouds and stars, yet observing magnetic fields presents significant challenges. In this Protostars and Planets VII book chapter, we provide a comprehensive review of the past decade's advancements in studying interstellar magnetic fields, from Galactic clouds to prestellar cores. For an overview of this decade-long progress in interstellar magnetic field research, you can watch my Protostars and Planets VII conference talk here.

Magnetic Field Observations with the Fred Young Submilimeter Telescope

Within the Galactic Polarization Working Group of the CCAT-Prime collaboration, we are planning comprehensive surveys of plane-of-sky magnetic fields, including both shallow and deep observations. Our goals include determining the 3D magnetic fields of various molecular clouds and exploring magnetohydrodynamic (MHD) turbulence, dust properties, and the interconnected evolution of clouds and their magnetic fields. This research extends to both Galactic and extraGalactic environments. For a more detailed overview of this work, please refer to this publication. I also briefly discuss these topics in this talk.

Dragonfly Polarimetry

In an exciting collaboration with th e Dragonfly team and Prof. Leo Hollberg at Stanford, I am working on adding full-Stokes polarimetry capabilities to a sub-array of the Dragonfly Telephoto Array. This enhancement could significantly expand the array's observational capacities, enabling novel astrophysical studies. The image on the left showcases one of the array's cameras that I have been closely examining as part of this project.

Discovery of a 3D Filament

This exciting work is currently in progress. Updates and publication details will be shared here upon completion.

Faraday Tomography

In several ongoing projects, my students and I are investigating Faraday tomography techniques, both theoretically and observationally. We are particularly focused on utilizing GMIMS (Global Magneto-Ionic Medium Survey) observations to study the magnetic fields of interstellar bubbles and molecular clouds. This research is currently in progress, aiming to enhance our understanding of magnetic field structures in various interstellar environments.

Magnetohydrodynamic Simulations for 3D Studies

To complement observational studies, I employed FLASH Magnetohydrodynamic simulations to investigate the 3D magnetic field structures of isolated filaments. These simulations modeled isothermal filaments with various initial conditions, including different rotation frequencies, magnetic field strengths, and orientations. The results consistently supported my earlier observational findings of arc-shaped morphologies in molecular clouds, reinforcing the robustness of this conclusion across a range of parameters.

Machine Vision for Bubble Identification

In a few ongoing projects, my students and I are applying and further developing machine vision algorithms to identify and characterize interstellar bubbles within the Milky Way and in extragalactic environments. These approaches automate the detection of these structures and their impact on surrounding environments.

Deep Observations with the Very Large Array

As principal investigator, I led a deep observational survey of the Perseus molecular cloud using the Very Large Array. These observations, spanning over 100 hours, aim to reconstruct the most detailed 3D magnetic field map of the cloud to date (Hajizadeh et al., in preparation). This effort promises to significantly advance our understanding of magnetic field structures in molecular clouds and their role in star formation processes.

Software

MC-BLOS For Next-Generation Radio Observations

We have developed the MC-BLOS software to determine line-of-sight magnetic fields in molecular clouds, enhancing our understanding of interstellar magnetism. The software is available on GitHub.

BLOSMapping

The BLOSMapping software complements MC-BLOS by offering more flexibility in selecting OFF points. It serves as an earlier version of our current tools. BLOSMapping is accessible through the Astrophysics Source Code Library and GitHub.

Students

I have had the privilege of mentoring and supervising a number of students and researchers.

Current:

Fyza Parviz Jazra, research assistant, Stanford, USA
Haleh Hajizadeh, PhD student, University of Calgary, Canada
Khwaish Billore, Undergraduate student, Stanford, USA
Minjie Lei, PhD Candidate, Stanford, USA
Mohammad Reza Nasirzadeh, research assistant, IPM, Iran

Alumni:

Anthony Nunez, Undergraduate student, Stanford, USA
Carlos Rodriguez, undergraduate student, Stanford, USA
Charles Feng Yang, PhD rotation student, Stanford, USA
Gabriel Munoz Zarazua, Undergraduate student, California State University, USA; Now graduate student at Cal State University
Gabriel Munoz Zarazua, Undergraduate student, California-Bridge program, Stanford, USA
George Halal, PhD candidate, Stanford, USA
Gisselle Jimenez, Undergraduate student, California-Bridge program, USA
Hsin-Yu Chen, Undergraduate student, National Taiwan University, Taiwan
Jennifer Glover, co-op student, National Research Council Canada, DRAO, Canada; Now MSc student at McGill University
John Ming Ngo, co-op student, National Research Council Canada, DRAO, Canada; Now MSc student at the University of Alberta
Lucas Bayouri, MSc intern student, Nagoya University, Japan
Matthew Bouchard, Undergraduate student, University of Calgary, Canada
Patrick Tupoumalohi, Undergraduate student, Stanford, USA
Ryan Anthony Clairmont, Undergraduate student, Stanford, USA
Violet Zhou, undergraduate student, Stanford, USA
Wednesday Lupypciw, co-op student, National Research Council Canada, DRAO, Canada

Teaching & Outreach

Teaching

I have taught twice as a Sessional Instructor at the University of Calgary, where I was responsible for all aspects of course delivery, including lecture preparation, course design, content development, and assessment creation. I also have served as a teaching assistant and guest lecturer in various courses across multiple universities, teaching in two languages (English and Farsi/Persian).

I have received teaching awards, most notably from the Taylor Institute for Teaching and Learning at the University of Calgary. My teaching portfolio includes courses such as: Introduction to Astronomy, Introduction to Astrophysics, Electromagnetism & Thermal Physics, Optics and Electromagnetism, Introduction to Optics and Waves, Electromagnetic Theory, Computational Physics I & II, First/Second Year Physics, and Physical Optics.

Outreach

I am enthusiastic about sharing the wonders and excitement of exploring physics and astronomy. Over the years, I've embraced more than 35 service and outreach roles, ranging from founding initiatives to organizing conferences, and from leading to supporting various events. One of these efforts includes co-founding the Open Cultural Astronomy Forum (OCAF) with Ray Norris, an initiative dedicated to exploring the interplay between astronomy and cultures.

We welcome anyone interested to join the OCAF seminar series and mailing list. You can participate by sending an empty email to ocaf-seminar+subscribe[at]googlegroups.com or by contacting us directly through our website.

Contact

Email: mtahani[at]stanford.edu

I am passionate about discussing my research and have delivered around 85 invited and contributed conference talks, colloquia, and seminars. If you would like to learn more about my work, please don't hesitate to get in touch.