The Pristine Dwarf-Galaxy survey -- IV. Probing the outskirts of the dwarf galaxy Boötes I

Abstract

We present a new spectroscopic study of the dwarf galaxy Bootes I (Boo I) with data from the Anglo-Australian Telescope and its AAOmega spectrograph together with the Two Degree Field multi-object system. We observed 36 high-probability Boo I stars selected using Gaia Early Data Release 3 proper motions and photometric metallicities from the Pristine survey. Out of those, 29 are found to be Boo I's stars, resulting in an excellent success rate of 80% at finding new members. Our analysis uses a new pipeline developed to estimate radial velocities and equivalent widths of the calcium triplet lines from Gaussian and Voigt line profile fits. The metallicities of 18 members are derived, including 3 extremely metal-poor stars ([Fe/H] < −3.0), which translates into an exceptional success rate of 25% at finding them with the combination of Pristine and Gaia. Using the large spatial extent of our new members that spans up to 4.1 half-light radii and spectroscopy from the literature, we are able to detect a systemic velocity gradient of 0.15 ± 0.10 km s −1 arcmin −1 and a small but resolved metallicity gradient of −0.007 ± 0.003 dex arcmin −1 . Finally, we show that Boo I is more elongated than previously thought with an ellipticity of = 0.68 ± 0.15. Its velocity and metallicity gradients as well as its elongation suggest that Boo I may have been affected by tides, a result supported by direct dynamical modelling.

The Far-Away Blues: Exploring the Furthest Extents of the Boötes I Ultra Faint Dwarf Galaxy

“…Their candidate, and confirmed, member stars are RGB and HB stars redder than (g−i) 0,SDSS ∼ 0.2. As such, there is no overlap between the sample of Longeard et al (2021) and the candidate member stars identified in the present paper. Longeard et al (2021) found spectroscopically confirmed member stars out to ∼ 4 half-light radii from the center of Boo I and thirteen of the 29 total member stars identified in their study reside at projected distances further than two half-light radii from the center of Boo I.…”

“…After the analysis presented in this work was completed, the complementary investigation of Longeard et al (2021) was posted to the arXiv. These authors identified candidate Boo I member stars far from that galaxy's center using the metallicity-sensitive Pristine photometric filters (see e.g.…”

“…Starkenburg et al 2017) in combination with a fiducial isochrone and Gaia proper motions. Longeard et al (2021) obtained spectroscopic follow-up of a subset of these stars, all within the 2-degree fieldof-view of the 2dF/AAOmega instrument on the Anglo-Australian Telescope (AAT). Their candidate, and confirmed, member stars are RGB and HB stars redder than (g−i) 0,SDSS ∼ 0.2.…”

“…After we completed the analysis presented below, the complementary analysis of Longeard et al (2021) was posted to the arXiv. These authors similarly identify candidate Boo I member stars far from that galaxy's center using a combination of photometry and Gaia astrometry.…”

“…Starkenburg et al 2017) and focus on RGB and HB stars redder than (g − i) 0, SDSS ∼ 0.2. There is no overlap between the sample of Longeard et al (2021) and the candidate member stars identified using our technique below. We discuss the results from Longeard et al (2021) further, within the context of our findings, in Section 4 below.…”