Charlotte Olsen to discuss galaxy histories from photometry at NASA seminar

Charlotte Olsen of CUNY City Tech is scheduled to speak at a virtual NASA Galaxies Science Interest Group seminar on 6 October 2026 at 1 p.m. Eastern Time. The talk concerns a question for astronomers studying galaxy growth: how reliably can images reveal when and where a galaxy formed its stars, given that stars can move away from their birthplaces?

NASA titles the seminar ‘A Galaxy Doesn’t Forget: Why You Should Trust Your Resolved SFHs from Photometric Observations.’ The title presents the speaker’s proposed case. NASA’s abstract describes an analysis using a galaxy simulation and reports a result, but the announcement does not establish that the result has been independently confirmed.

What the seminar will examine

A spatially resolved star-formation history divides a galaxy into regions and reconstructs how star formation changed in each one. According to NASA’s seminar abstract, these histories can show the timing and location of star formation, offering a view of how galaxies grow and how star formation slows or stops. Researchers derive spatially resolved properties through integral-field spectroscopy and, increasingly, through photometry across multiple wavelength bands.

The complication Olsen plans to address is stellar migration. If stars move between the spatial regions used for a reconstruction, their present positions may give a misleading account of where they formed. NASA says measuring the likely bias in observations is difficult because many detailed galaxy simulations do not resolve both the initial spread of stars and the rates at which they migrate.

The abstract reports that, when migration is measured in rings matched to typical spatial-pixel scales, the reconstructed radial profiles remain consistent with the actual profiles in slope and the relative shape of their scatter for lookback times up to one billion years. That is the result Olsen is due to discuss. It applies to the comparison described in the announcement; the separate studies below address other samples and methods.

What earlier studies show

An earlier study published through Oxford Academic examined roughly 970 galaxies at redshifts from 0.5 to 2.0. Its researchers compared histories obtained by fitting individual image pixels with histories inferred from a galaxy’s combined light. The spatially resolved histories generally tracked the study’s flexible combined-light reference model more closely than two simpler models, which the authors said often missed early star formation.

The same study reported median stellar-mass differences of about 0.07 dex for its spatially resolved histories relative to the flexible reference, against roughly 0.1 to 0.4 dex for the simpler combined-light fits. The flexible model was a comparison point, not a known true history. The authors also cautioned that modelling choices can change inferred histories and that simplified forms can introduce systematic bias or understate uncertainty.

A 2024 American Astronomical Society abstract by Olsen and collaborators describes another approach: fitting the light from subregions of UVCANDELS galaxies across 10 space-based imaging bands, spanning 275 nanometres to 1.6 micrometres. The team defined regions using limits on diameter, colour variation and signal-to-noise before reconstructing nonparametric histories. The abstract describes a method and research aims, rather than a test of the h277 result.

Olsen and co-authors also compared two ways of reconstructing histories in a 2021 study of 36 nearby dwarf galaxies within four megaparsecs. One method fitted galaxies’ broad-band light; the other used colour-magnitude diagrams of individually resolved stars. At the sample level, both showed similar periods of reduced and renewed star formation, although their timings differed by about one billion years. The authors described the methods’ errors as complementary and called for further study.

What remains open

The seminar is scheduled for 6 October, so NASA’s announcement describes what Olsen is expected to present, not a talk that has already taken place. The available announcement does not say whether slides, a recording or a paper will be released. Its h277 finding should therefore be read as a result reported in the seminar abstract, while the earlier studies provide context for why the method and its limits matter.