Systematically Measuring Ultra-Diffuse Galaxies. VII. The HI Survey Overview

TL;DR

This study uses the Green Bank Telescope to survey 378 ultra-diffuse galaxy candidates, detecting HI in 110, revealing insights into their gas content, star formation, and environment, and providing a large dataset for future detailed analysis.

Contribution

It presents the largest HI follow-up survey of UDG candidates, offering new data on their gas properties, environments, and scaling relations, and highlighting the importance of size-based definitions.

Findings

HI detections are bluer and more irregular than non-detections.

HI detections are mostly in isolated environments.

Gas richness correlates with physical size of UDGs and LSB dwarfs.

Abstract

We present the results from the neutral hydrogen (HI) follow-up survey of 378 optically-detected UDG candidates from the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) survey using the Robert C. Byrd Green Bank Telescope (GBT). We detect HI in 110 targets and determine 37 to be UDGs and 73 to be low surface brightness (LSB) dwarfs based on their effective radii and central surface brightnesses. In line with previous studies, we find that: i) our HI detections have on average bluer $g-r$ colors and more irregular morphologies than our HI non-detections, ii) our HI detections populate the tail end of the star-forming main sequence from the ALFALFA catalog with marginally lower specific star formation rates, and iii) HI detections are mostly in relatively isolated (i.e. field) environments, while most non-detections have at least one nearby neighbor in projection. We find that the HI mass to stellar mass ratios (i.e. gas richnesses) scale with the physical sizes for UDGs and LSB dwarfs alike, suggesting that mechanisms other than bursty star formation feedback may be at play for UDGs. However, we find a stronger trend between gas richnesses and physical sizes if we define UDGs using their effective surface brightness instead of their central surface brightness. We are in the process of using this unprecedented sample of UDG candidates to carry out detailed follow-up studies (i.e. star-formation and environmental analysis, comparisons to simulations) and are obtaining resolved HI observations for several of them.