Anna Y. Q. Ho

Research Projects


Science Policy

Code on GitHub


I am a third-year graduate student in Astronomy at Caltech and the 2017 Garmire Scholar in the Division of Physics, Math, and Astronomy. My advisor is Shri Kulkarni. For my thesis, I am using the Zwicky Transient Facility (ZTF) to conduct the first large-scale optical survey dedicated to finding engine-driven explosions. For the last half-century, relativistic outflows accompanying the deaths of massive stars have predominantly been discovered via high-energy emission (i.e. gamma-ray bursts, or GRBs), but this is changing with the advent of wide-field high-cadence optical facilities. We are pursuing these rare explosions via two avenues, afterglow emission and broad-lined Ic supernovae (Ic-BL SNe).

Before coming to Caltech, I obtained a B.S. in Physics at MIT and spent three summers at the National Radio Astronomy Observatory (NRAO). At the NRAO, I worked with Scott Ransom on using millisecond pulsars in the globular cluster Terzan 5 to measure small-scale structure in the magnetic field of the Milky Way. I then spent a year on a Fulbright at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany. At the MPIA, I worked with Hans-Walter Rix, Melissa Ness, and David Hogg on a data-driven approach to spectral modeling called The Cannon. By using The Cannon to reanalyze the LAMOST spectra of 450,000 giants, my work resulted in the largest catalog to-date of stellar masses, ages, and individual abundances (alpha enhancement, carbon, and nitrogen).


Afterglows to Relativistic Explosions with ZTF

In the death of a massive star, the core collapses into a black hole and a spherical shock unbinds the outer layers in a core-collapse supernova. In rare cases (0.1%), the rapidly spinning black hole acts as an engine that drives a relativistic bipolar jet, which tunnels through the star and is visible to us as a gamma-ray burst (GRB). Recent evidence suggests that GRBs are just the tip of the iceberg of a much broader landscape of jet and central engine phenomena. This landscape could include jets with a higher baryon loading fraction ("dirty fireballs"), choked jets, and jets that are viewed off-axis.

Undertaking the first census of these phenomena is one of the primary goals of the Zwicky Transient Facility (ZTF). The survey begins in February 2018 and will run for three years. We will address key physical questions such as: How many SNe actually produce central engines? Why do ``classical" GRB jets accelerate only a tiny fraction of their mass? Do low-luminosity GRBs result from jets getting choked within the star? And, finally, how accurate is our understanding of beaming in GRBs?

False positives such as flaring stars and AGN present an impediment to discovering relativistic transients. To address this issue, I conducted a systematic search of the iPTF archive to develop a strategy for mitigation. I have also been using the Very Large Array (VLA) to follow up a mysterious population of short soft X-ray flashes discovered by the Monitor of All-sky X-ray Image (MAXI). The short duration, flux, hardness ratio, and lack of known counterpart to these events makes them promising candidate dirty fireballs.

Data-Driven Spectral Modeling using The Cannon

A new generation of large-scale spectroscopic stellar surveys (e.g. SEGUE, APOGEE, Gaia-ESO) are measuring spectra for millions of stars across the Milky Way. Ideally, each spectrum would be used to "label" the corresponding star with its physical characteristics (e.g. temperature) and physical labels would be independent of the pipeline used to measure them. However, in practice, survey pipelines measure inconsistent labels for the same stars, because their methodologies are sensitive to properties that differ between surveys (e.g. wavelength range and resolution).

The Cannon is a data-driven approach to spectral modeling that can be used to bring two discrepant surveys onto the same scale. The Cannon requires no physical models, is very fast, and achieves comparable accuracy to existing survey pipelines but with significantly lower SNR spectra. It relies only on the existence of a subset of objects within a survey for which the corresponding labels are known and sufficiently cover the label space of interest.

I used The Cannon to reanalyze the spectra of 450,000 giant stars from the LAMOST survey. I improved the precision on existing measurements and made the first measurements of several other parameters. Taken together, this work represents the largest catalog to date of stellar [alpha/M], [C/M], [N/M], mass, and age. This work resulted in two first-author publications, several co-author publications, and several invited and contributed conference talks. I also wrote a version of The Cannon for use by the community, along with detailed documentation and two tutorials.

Rotations Measures of Terzan 5 Pulsars as a Unique Probe of the Small-Scale Galactic Magnetoionized Medium

Our position within the Milky Way gives us a unique vantage point: we can measure the galactic magnetic field shape and structure over several orders of magnitude, ranging from parsec to several parsec scales. The dense millisecond pulsar (MSP) populations in globular clusters (GCs) provide a unique opportunity to measure and map the galactic magnetic field on small (pc) scales, using the Faraday rotation technique.

The globular cluster Terzan 5 has the highest-known population of MSPs: radio flux measurements predict a much higher total than the current population of 35. Ransom (2007) used timing solutions for each MSP to calculate precise positions and precise DMs to demonstrate that differences in DM are dominated by electron density fluctuations in the ISM, rather than within the cluster.

As a summer student at the NRAO (a program funded by the NSF) I used nearly five full days of 1.5 GHz and 2 GHz Green Bank Telescope data to calculate precise rotation measures (RMs) for 24 of the 35 millisecond pulsars in Terzan 5. Dividing these values by the corresponding DMs enabled me to measure the mean line-of-sight galactic magnetic field strength to each pulsar. I found a 15-20% gradient in line-of-sight magnetic field strength across the cluster with fluctuations on the order of 0.1 microGauss and measured a structure function power law index of alpha = 1.13 +/- 0.15.


3 lead-author, 11 co-author

Ho, A. Y. Q., Kulkarni, S.R., Nugent, P. E. et al. 2017, iPTF Archival Search for Fast Optical Transients, arXiv:1712.00949

Ho, A. Y. Q., Rix, H.-W., Ness, M. K., Hogg, D. W., et al. 2017, Masses and Ages for 230,000 LAMOST Giants, via Their Carbon and Nitrogen Abundances, ApJ, 841, 40 arXiv:1609.03195

Ho, A. Y. Q., Ness, M. K., Hogg, D. W., et al. 2017, Label Transfer from APOGEE to LAMOST: Precise Stellar Parameters for 450,000 LAMOST Giants, ApJ, 836, 5 arXiv:1602.00303

Adams, S. M., et al. (including Ho, A. Y. Q.) 2017, iPTF Survey for Cool Transients, submitted to PASP, arXiv:1711.10501

Guglielmo, M., Lane, R. R., Conn, Blair C., Ho, A. Y. Q. et al. 2017, On the Origin of the Monoceros Ring - I: Kinematics, proper motions, and the nature of the progenitor, accepted by MNRAS, arXiv:1711.06682

Abbott, B. P., et al. (including Ho, A. Y. Q.) 2017, Multi-messenger Observations of a Binary Neutron Star Merger, ApJ,848,12 arXiv:1710.05833

Kasliwal, M. M., et al. (including Ho, A. Y. Q.) 2017, Illuminating Gravitational Waves: A Concordant Picture of Photons from a Neutron Star Merger, accepted by Science, arXiv:1710.05436

Hallinan, G., et al. (including Ho, A. Y. Q.) 2017, A Radio Counterpart to a Neutron Star Merger, accepted by Science, arXiv:1710.05435

Ting, Y.-S., Rix, H.-W., Conroy, C., Ho, A. Y. Q., & Lin, J. 2017, Measuring 14 elemental abundances with R=1,800 LAMOST spectra, ApJ, 849, 9 arXiv:1708.01758

Blagorodnova, N., et al. (including Ho, A. Y. Q.) 2017, iPTF16fnl: a faint and fast tidal disruption event in an E+A galaxy, ApJ, 844, 46 arXiv:1703.00965

Casey, A. R., Hogg, D. W., Ness, M., Rix, H.-W., Ho, A. Y. Q., Gilmore, G. 2016 The Cannon 2: A data-driven model of stellar spectra for detailed chemical abundance analyses , submitted to ApJ, arXiv:1603.03040

Ness, M., Hogg, D. W., Rix, H.-W., Martig, M., Pinsonneault, M. H., Ho, A. Y. Q. 2016, Spectroscopic Determination of Masses (and Implied Ages) for Red Giants, ApJ, 823, 114 arXiv:1511.08204

Hogg, D. W., Casey, A. R., Ness, M., Rix, H.-W., Foreman-Mackey, D., Hasselquist, S., Ho, Anna Y. Q. et al. 2016, Chemical tagging can work: Identification of stellar phase-space structures purely by chemical-abundance similarity, ApJ, 833, 262 arXiv:1601.05413

Ness, M., Hogg, D.W., Rix, H-W., Ho, A. Y. Q., Zasowski, G. 2015, The Cannon: A data-driven approach to stellar label determination, ApJ, 808, 16 arXiv:1501.07604

Talks and Workshops

I enjoy public speaking, and have given numerous talks at academic conferences and at our department's journal club, as well as for the general public. On invitation, I taught a 2-day workshop on using The Cannon at Gemini Observatory in La Serena, Chile: I gave a talk and led a series of hands-on sessions, guiding the participants through Jupyter notebooks and datasets that I prepared.

This section lists my talks for the astronomy community. You can click here to see my talks for a general audience.

Invited and Contributed Talks

Dirty Fireballs and Orphan Afterglows: A Broader Landscape of Relativistic Explosions
Presented Oct 2017 at the GROWTH Annual Meeting in Milwaukee, WI

The Cannon: Data-Driven Spectral Modeling in the Era of Large Stellar Surveys
Presented at 5 international conferences (1 invited, 4 contributed)

Rotation Measures of Globular Cluster Pulsars as a Unique Probe of the Galactic Magnetic Field
Presented at the MPIA Galaxy Coffee (Heidelberg) and the NRAO lunch seminar (Socorro)

Caltech Astro Journal Club

A tale of 115 photons: "A New, Faint Population of X-ray Transients" (Bauer et al. 2017)
Presented on 14 April 2017

Ultraluminous X-Ray Bursts: A Challenge to High-Energy Astrophysics Lore (Irwin et al. 2016)
Presented on 4 November 2016


Public Outreach

I am a AAS Astronomy Ambassador and the co-coordinator for the astronomy department outreach program at Caltech.

Talks and Volunteering

Classroom Teaching


Science Policy

I am the Vice President of Science and Engineering Policy at Caltech (SEPAC).