Spatially Resolved Stellar Spectroscopy of the Ultra-diffuse Galaxy Dragonfly 44. III. Evidence for an Unexpected Star Formation History under Conventional Galaxy Evolution Processes

Alexa Villaume,Aaron J Romanowsky,Christopher Martin,Shany Danieli,Charlie Conroy,Pieter Van Dokkum,Matt Matuszewski,Jean Brodie,Duncan A Forbes

doi:10.3847/1538-4357/ac341e

Abstract

Abstract We use the Keck Cosmic Web Imager integral field unit spectrograph to (1) measure the global stellar population parameters for the ultra-diffuse galaxy (UDG) Dragonfly 44 (DF44) to much higher precision than previously possible for any UDG and (2) for the first time measure spatially resolved stellar population parameters of a UDG. We find that DF44 falls below the mass–metallicity relation established by canonical dwarf galaxies both in and beyond the Local Group. We measure a flat radial age gradient ( m logage = + 0.01 − 0.08 + 0.08 log Gyr kpc−1) and a flat to positive metallicity gradient ( m [ Fe / H ] = + 0.09 − 0.12 + 0.11 dex kpc−1), which are inconsistent with the gradients measured in similarly pressure-supported dwarf galaxies. We also measure a negative [Mg/Fe] gradient ( m [ Mg / Fe ] = − 0.20 − 0.18 + 0.18 ) dex kpc−1 such that the central 1.5 kpc of DF44 has stellar population parameters comparable to metal-poor globular clusters. Overall, DF44 does not have internal properties similar to other dwarf galaxies and is inconsistent with it having been puffed up through a prolonged, bursty star formation history, as suggested by some simulations. Rather, the evidence indicates that DF44 experienced an intense epoch of “inside-out” star formation and then quenched early and catastrophically, such that star formation was cut off more quickly than in canonical dwarf galaxies.

Highlights

While low-surface brightness (LSB) galaxies have been recognized since Sandage & Binggeli (1984), the inherent difficulties in observing them have meant that only a handful of LSB galaxies were studied over decades (Impey et al 1988; Bothun et al 1991; Dalcanton et al 1997)
We make use of the spectroscopic dataset described in van Dokkum et al (2019b), to which interested readers should refer for detailed descriptions of the observations and data reductions
We obtained integral field unit (IFU) spectroscopy of DF44 with Keck Cosmic Web Imager (KCWI) and extracted spectra in nine elliptical apertures following the isophotes of the galaxy

Summary

Introduction

While low-surface brightness (LSB) galaxies have been recognized since Sandage & Binggeli (1984), the inherent difficulties in observing them have meant that only a handful of LSB galaxies were studied over decades (Impey et al 1988; Bothun et al 1991; Dalcanton et al 1997). Subsequent follow-up work focused first on attempting to measure the total masses of UDGs. Subsequent follow-up work focused first on attempting to measure the total masses of UDGs This included both dynamical mass measurements, and proxy measurements through the number of globular clusters (GCs, e.g, van Dokkum et al 2017; Lim et al 2018). Many UDGs were found to have more populous GC systems than galaxies with similar luminosities, and given empirical scaling relations between GCs and dark matter halos, it was inferred that these UDGs are overmassive for their luminosities (e.g., Beasley et al 2016; Forbes et al 2020a), albeit not as massive as L∗ galaxies. With implied halo masses of ∼ 1011M , these UDGs could be considered as failed M33-mass galaxies (e.g., Gannon et al 2020)

Results

Discussion

Conclusion