[Physics-talks] Special Seminar Wednesday 6/24 11am: Dr. Ming-Feng Ho of UMichigan

[Celina Yamakawa]

Dear all,

Please join us for a special seminar next Wednesday 6/24 at 11am in the
Reading Room. We will have a very special visitor: Ming-Feng Ho! Ming-Feng
graduated from my group in September 2024 and is now a prize fellow at the
Leinweber Institute for Theoretical Physics (LITP), University of Michigan.
Please come and hear what he has been up to since then. His title and
abstract are below.

Simeon

==Mountains in the forest: DLAs and their role in Lyman-α forest cosmology
at small and large scales==

Abstract:
The Lyman-α forest, the dense pattern of neutral hydrogen absorption lines
seen in distant quasar spectra, is one of the most powerful tools for
measuring the growth of structure at z = 2–5 on ~Mpc scales. Embedded in
this forest are rarer and much bigger "mountains": damped Lyman-α systems
(DLAs). These systems trace the densest neutral hydrogen around galaxies.
The galaxy-formation community studies them as objects in their own right.
Cosmologists, by contrast, see them as unwanted contamination to be masked
out before measuring the forest. In this talk, I will argue that it would
be a missed opportunity not to pay these mountains a little more attention.

I will first describe my somewhat awkward journey helping to build the DLA
catalog for DESI's DR2 Lyα BAO measurement, combining three independent
finders with human validation. Even at this level of effort, the weakest
DLAs and the noisiest spectra remain difficult to identify cleanly, and I
will discuss how this incompleteness affects (or does not affect) the BAO
and the large-scale modes of the 1D power spectrum. Turning to small
scales, I will present my recent analysis of the high-resolution
KODIAQ-SQUAD dataset with the PRIYA emulator, which finds that weaker DLAs
(sub-DLAs and LLSs) are strongly degenerate with the small-scale
cosmological signal and cannot be cleanly marginalized as simple
contaminants. A subsequent DESI DR1 P1D analysis reaches a similar
conclusion.

If these absorbers are this entangled with the forest cosmological signal,
perhaps they are better thought of as part of the signal than as
contamination. This suggests two things for the community: (1) modeling
their physics directly in simulations, and (2) measuring them alongside the
forest rather than masking them one by one. On the latter, I will present
my new DESI Y3 measurements of the DLA, sub-DLA, and LLS abundance and
redshift evolution, using a probabilistic detection pipeline calibrated
with mock spectra. I will then discuss how the community can use this
measurement to improve the forest signal, and how these absorbers
themselves could serve as a complementary cosmological probe in their own
right.
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