Dr Janet Preston

We present the first comprehensive spectroscopic study of the Andromeda galaxy’s Eastern Extent. This ∼4° long filamentary structure, located 70–90 kpc from the centre of M31, lies perpendicular to Andromeda’s minor axis and the Giant Stellar Stream and overlaps Stream C. In this work, we explore the properties of the Eastern Extent to look for possible connections between it, the Giant Stellar Stream and Stream C. We present the kinematics and photometry for ∼50 red giant branch stars in seven fields along the Eastern Extent. We measure the systemic velocities for these fields and find them to be −368  km s−1 ≲ vv ≲ −331  km s−1, with a slight velocity gradient of −0.51 ± 0.21  km s−1 kpc−1 towards the Giant Stellar Stream. We derive the photometric metallicities for stars in the Eastern Extent, finding them to be metal-poor with values of −1.0 ≲ [Fe/H]phot ≲ −0.7 with an 〈[Fe/H]phot〉 ∼ −0.9. We find consistent properties for the Eastern Extent, Stream B and one of the substructures in Stream C, Stream Cr, plausibly linking these features. Stream Cp and its associated globular cluster, EC4, have distinctly different properties indicative of a separate structure. When we compare the properties of the Eastern Extent to those of the Giant Stellar Stream, we find them to be consistent, albeit slightly more metal-poor, such that the Eastern Extent could plausibly comprise stars stripped from the progenitor of the Giant Stellar Stream.

We present a kinematic and spectroscopic analysis of 38 red giant branch stars, in 7 fields, spanning the dwarf spheroidal galaxy Andromeda XXVII and the upper segment of the North West Stream. Both features are located in the outer halo of the Andromeda galaxy at a projected radius of 50-80 kpc, with the stream extending for ∼3° on the sky. Our data is obtained as part of the PAndAS survey and enables us to confirm that Andromeda XXVII’s heliocentric distance is 827 ± 47 kpc and spectroscopic metallicity is -2.1+0.4−0.5⁠. We also re-derive Andromeda XXVII’s kinematic properties, measuring a systemic velocity = -526.1+10.0−11.0 kms−1 and a velocity dispersion that we find to be non-Gaussian but for which we derive a formal value of 27.0+2.2−3.9 kms−1. In the upper segment of the North West Stream we measure mean values for the metallicity = -1.8±0.4, systemic velocity = -519.4 ±4.0 kms−1 and velocity dispersion = 10.0±4.0 kms−1. We also detect a velocity gradient of 1.7±0.3 kms−1 kpc−1 on an infall trajectory towards M31. With a similar gradient, acting in the same direction, in the lower segment we suggest that the North West Stream is not a single structure. As the properties of the upper segment of the North West Stream and Andromeda XXVII are consistent within 90% confidence limits, it is likely that the two are related and plausible that Andromeda XXVII is the progenitor of this stream.

With a central surface brightness of μ0 = 29.3 mag. per sq. arcsec, and half-light radius of rhalf=3.1+0.9−1.1kpc, Andromeda XIX (And XIX) is an extremely diffuse satellite of Andromeda. We present spectra for ∼100 red giant branch stars in this galaxy, plus 16 stars in a nearby stellar stream. With this exquisite dataset, we re-derive the properties of And XIX, measuring a systemic velocity of ˂vr˃ = − 109.0 ± 1.6 kms−1 and a velocity dispersion of σvr=7.8+1.7−1.5σvr=7.8+1.7−1.5 kms−1(higher than derived in our previous work). We marginally detect a velocity gradient along the major axis of dvdχ=−2.1±1.8 kms−1dvdχ=−2.1±1.8 kms−1kpc−1. We find its mass-to-light ratio is higher than galaxies of comparable stellar mass (⁠[M/L]half=278+146−198M⊙/L⊙), but its dynamics place it in a halo with a similar total mass to these galaxies. This could suggest that And XIX is a “puffed up” dwarf galaxy, whose properties have been altered by tidal processes, similar to its Milky Way counterpart, Antlia II. For the nearby stream, we measure vr = −279.2 ± 3.7 kms−1, and σv=13.8+3.5−2.6 kms−1. We measure its metallicity, and find it to be more metal rich than And XIX, implying that the two features are unrelated. Finally, And XIX’s dynamical and structural properties imply it is a local analogue to ultra diffuse galaxies (UDGs). Its complex dynamics suggest that the masses of distant UDGs measured from velocity dispersions alone should be carefully interpreted.