The Keck Baryonic Structure Survey (KBSS)

Quasar field

The Keck Baryonic Structure Survey (KBSS) is a large, targeted spectroscopic survey designed to jointly probe galaxies and their gaseous environments at the peak of galaxy assembly (z~2-3). The survey comprises 15 independent fields centered on a bright background quasar; the total survey area is 0.24 square degrees, comparable to many of the legacy fields. The KBSS galaxy sample is selected from deep optical and near-infrared imaging and subsequently followed up with spectroscopic observations in the rest-UV (with Keck/LRIS) and rest-optical (with Keck/MOSFIRE) bandpasses. My role since MOSFIRE's commissioning in 2012 has been to lead the near-infrared component of the KBSS survey, which now encompasses observations of more than 1100 individual galaxies.

Above image courtesy Gwen Rudie (Carnegie)

Pic 01

Near-infrared spectroscopy of KBSS galaxies was conducted in the early 2000s with Keck/NIRSPEC, but follow-up with MOSFIRE over the last several years has been significantly more efficient, thanks to its more sensitive detector and multiplexing capability. The figure above compares the spectra of two different galaxies taken with NIRSPEC (on the left) and MOSFIRE (on the right). With MOSFIRE, we now have multiple strong-line measurements, including H-alpha and [N II] as shown, for hundreds of individual galaxies. These measurements are critical for constraining galaxies' gas-phase metallicities, electron densities, and ionization parameters.

Near-infrared spectoscropy of individual high-z galaxies


The KBSS contains more than 700 galaxies at z~2-3, with quality near-infrared spectroscopic observations of ~380 individual systems. The above figure comes from a paper submitted in August 2016, in which I detail the nebular properties of the z~2-3 KBSS galaxies and conclude that the primary difference with respect to local galaxies is an increase in the overall degree of excitation. At the same time, high-z KBSS galaxies appear to be more chemically evolved (with higher N/O and O/H) than local galaxies with similar excitation conditions, meaning that galaxies in the early Universe must have harder ionizing radiation fields than z~0 galaxies at fixed oxygen abundance. The most likely explanation for this trend is a systematic difference in the star-formation histories of galaxies at z~2-3 and z~0, even at fixed stellar mass.

Measuring O/H in high-redshift galaxies

The nebular spectra of galaxies originates in the ionized gas surrounding young, massive stars and thus reflects the combined properties of both the gas and stars. Both local galaxies and high-redshift galaxies occupy relatively tight loci in multi-dimensional line-ratio space, which implies strong correlations between the physical properties driving their spectra. However, it remains unclear if such correlations (for example, between ionization parameter and metallicity) are redshift-invariant, limiting the usefulness of empirical abundance calibrations based on z~0 samples.

There is also evidence to suggest that many of the emission line ratios observed for high-excitation nebulae respond more sensitively to changes in the shape and normalization of the ionizing radiation field than to changes in the gas-phase oxygen abundance. This effect is more pronounced at high-redshift because nearly all z~2-3 galaxies exhibit high levels of nebular excitation. Thus, efforts to measure their O/H must also consider their special ionization and excitation conditions.

Part of my ongoing work with the KBSS sample involves using a combination of BPASSv2 stellar population models and photoionization modeling with Cloudy to obtain self-consistent estimates of O/H, N/O, and ionization parameter U for individual galaxies. In a paper published earlier this year, we proved the utility of this approach using deep composite spectra constructed from the rest-UV and rest-optical spectra of a representative subset of KBSS galaxies.

MOSPEC, an IDL-based analysis tool for MOSFIRE spectra


MOSPEC is an interactive analysis tool developed in IDL specifically for MOSFIRE spectroscopy and is designed to reproduce many of the central functionalities of the splot task in IRAF. MOSPEC allows the user to extract 1D spectra from the 2D spectrograms produced by the MOSFIRE data reduction pipeline, using either the default aperture based on the CSU mask design file or an aperture defined in real-time by the user. In the case of emission-line galaxies (such as those in the KBSS), MOSPEC can also be used to model and measure line fluxes for a specified list of emission lines.

If you are interested in using MOSPEC for your observations, please contact me, as it is currently being adapted for more general use.

About me

Allison L. Strom


My research focuses on analyzing the rest-optical spectra of high-redshift star-forming galaxies and what we can learn about their massive stellar populations and the conditions in their HII regions. I am also interested in the overlap between extragalactic observational science and theoretical predictions, not only of galaxy formation and evolution, but also concerning stellar evolution. Galaxies in the early Universe are powerful laboratories for studying stellar populations with unique properties, particularly with respect to their chemical composition and energetic feedback on their surroundings.


In addition to research, I also seek to promote equity and inclusion in the scientific community. As Vice Chair of Caltech's Graduate Student Council, I organized a campus-wide Student-Faculty Colloquium designed to address issues related to campus climate; secured administrative and financial support for affinity groups; and served as a liaison between the Title IX office and student groups. As a new alumna, I have advocated for enhanced support for graduate alumni, especially through mentorship opportunities for underrepresented STEM professionals (including women of all ethnicities, racial minorities, and LGBTQ persons).


Carnegie Observatories
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Telephone: (626) 304-0226
Email: astrom [at]