Browsing by Autor "A. Saintonge"

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Identifying Anomalous DESI Galaxy Spectra with a Variational Autoencoder
(Oxford University Press, 2026) C. Nicolaou; Rowina S Nathan; O Lahav; A. Palmese; A. Saintonge; J. Aguilar; S. P. Ahlen; C. Allende Prieto; S. Bailey; S. BenZvi
ABSTRACT The tens of millions of spectra being captured by the Dark Energy Spectroscopic Instrument (DESI) provide tremendous discovery potential. In this work we show how Machine Learning, in particular Variational Autoencoders (VAE), can detect anomalies in a sample of approximately 200 000 DESI spectra comprising galaxies, quasars and stars. We demonstrate that the VAE can compress the dimensionality of a spectrum by a factor of 100, while still retaining enough information to accurately reconstruct spectral features. We detect anomalous spectra as those with high reconstruction error and those which are isolated in the VAE latent representation. The anomalies identified fall into two categories: spectra with artefacts and spectra with unique physical features. Awareness of the former could improve the DESI spectroscopic pipeline; whilst the latter could help us discover new and unusual objects. To further curate the list of outliers identified, we use the Astronomaly package which employs Active Learning to provide personalized outlier recommendations for visual inspection. In this work we also explore the VAE latent space, finding that different object classes and subclasses are separated despite being unlabelled. We inject controlled synthetic anomalies and analyse their locations in the latent space to illustrate how the VAE responds to atypical spectral features; and we demonstrate the interpretability of this latent space by identifying tracks within it that correspond to various spectral characteristics. In upcoming work we hope to apply the methods presented here to search for both systematics and astrophysically interesting objects in much larger datasets of DESI spectra.
The atomic gas sequence and mass–metallicity relation from dwarfs to massive galaxies
(Oxford University Press, 2024) D. Scholte; A. Saintonge; John Moustakas; Barbara Catinella; Hu Zou; Biprateep Dey; J. Aguilar; S. P. Ahlen; Abhijeet Anand; Robert Blum
ABSTRACT Galaxy scaling relations provide insights into the processes that drive galaxy evolution. The extension of these scaling relations into the dwarf galaxy regime is of particular interest. This is because dwarf galaxies represent a crucial stage in galaxy evolution, and understanding them could also shed light on their role in reionizing the early Universe. There is currently no consensus on the processes that dominate the evolution of dwarfs. In this work, we constrain the atomic gas sequence (stellar mass versus atomic gas fraction) and mass–metallicity relation (stellar mass versus gas-phase metallicity) from dwarf ($10^{6.5} \, {\rm M}_{\odot }$) to massive ($10^{11.5} \, {\rm M}_{\odot }$) galaxies in the local Universe. The combined optical and 21-cm spectroscopic observations of the Dark Energy Spectroscopic Instrument and Arecibo Legacy Fast ALFA surveys allow us to constrain both scaling relations simultaneously. We find a slope change of the atomic gas sequence at a stellar mass of ${\sim} 10^{9} \, \textrm{M}_{\odot }$. We also find that the shape and scatter of the atomic gas sequence and mass–metallicity relation are strongly linked for both dwarfs and more massive galaxies. Consequently, the low-mass slope change of the atomic gas sequence is imprinted onto the mass–metallicity relation of dwarf galaxies. The mass scale of the measured slope change is consistent with a predicted escape velocity threshold below which low-mass galaxies experience significant supernova-driven gas loss, as well as with a reduction in cold gas accretion onto more massive galaxies.