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ABSTRACT In order to facilitate the future study of ultra-diffuse galaxies (UDGs), we compile a catalogue of their spectroscopic properties. Using it, we investigate some of the biases inherent in the current UDG sample that have been targeted for spectroscopy. In comparison to a larger sample of UDGs studied via their spectral energy distributions (SED), current spectroscopic targets are intrinsically brighter, have higher stellar mass, are larger, more globular cluster-rich, older, and have a wider spread in their metallicities. In particular, many spectroscopically studied UDGs have a significant fraction of their stellar mass contained within their globular cluster (GC) system. We also search for correlations between parameters in the catalogue. Of note is a correlation between alpha element abundance and metallicity, as may be expected for a ‘failed galaxy’ scenario. However, the expected correlations of metallicity with age are not found, and it is unclear if this is evidence against a ‘failed galaxy’ scenario or simply due to the low number of statistics and the presence of outliers. Finally, we attempt to segment our catalogue into different classes using a machine learning K-means method. We find that the clustering is very weak and that it is currently not warranted to split the catalogue into multiple, distinct subpopulations. Our catalogue is available online, and we aim to maintain it beyond the publication of this work.
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The star formation histories of quiescent ultra-diffuse galaxies and their dependence on environment and globular cluster richness
ABSTRACT We derive the stellar population parameters of 11 quiescent ultra-diffuse galaxies (UDGs) from Keck/KCWI data. We supplement these with 14 literature UDGs, creating the largest spectroscopic sample of UDGs to date (25). We find a strong relationship between their α-enhancement and their star formation histories: UDGs that formed on very short time-scales have elevated [Mg/Fe] abundance ratios, whereas those forming over extended periods present lower values. Those forming earlier and faster are overall found in high-density environments, being mostly early infalls into the cluster. No other strong trends are found with infall times. We analyse the stellar mass–metallicity, age–metallicity, and [Mg/Fe]–metallicity relations of the UDGs, comparing them to other types of low mass galaxies. Overall, UDGs scatter around the established stellar mass–metallicity relations of classical dwarfs. We find that GC-rich UDGs have intermediate-to-old ages, but previously reported trends of galaxy metallicity and GC richness are not reproduced with this spectroscopic sample due to the existence of GC-rich UDGs with elevated metallicities. In addition, we also find that a small fraction of UDGs could be ‘failed-galaxies’, supported by their GC richness, high alpha-abundance, fast formation time-scales and that they follow the mass–metallicity relation of z ∼2 galaxies. Finally, we also compare our observations to simulated UDGs. We caution that there is not a single simulation that can produce the diverse UDG properties simultaneously, in particular the low metallicity failed galaxy like UDGs.
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- Award ID(s):
- 2308390
- PAR ID:
- 10469839
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 526
- Issue:
- 3
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 4735-4754
- Size(s):
- p. 4735-4754
- Sponsoring Org:
- National Science Foundation
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