A detailed study of Andromeda XIX, an extreme local analogue of ultradiffuse galaxies

Abstract

With a central surface brightness of µ 0 = 29.3 mag. per sq. arcsec, and half-light radius of r half = 3.1 +0.9 −1.1 kpc, 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 < v r >= −109.0 ± 1.6 kms −1 and a velocity dispersion of σ vr = 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 dv dχ = −2.1 ± 1.8 kms −1 kpc −1 . We find its mass-to-light ratio is higher than galaxies of comparable stellar mass ([M/L] half = 278 +146 −198 M /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 v r = −279.2±3.7 kms −1 , and σ v = 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.

A photometric and kinematic analysis of UDG1137+16 (dw1137+16): Probing ultradiffuse galaxy formation in a group environment

“…However, tidal stripping operates outside-in, affecting the outskirts of the galaxy and leaving the central velocity dispersion approximately constant (Bender et al 1992;Chilingarian et al 2009;Blom et al 2014;Penny et al 2015). For example, an extreme Local Group UDG analogue with suspected tidal origins, Andromeda XIX, exhibits a largely flat velocity dispersion profile (Collins et al 2020). Additionally, our stellar velocity dispersion measurement is made within the half-light radius of the central stellar component, which would be subject to lessened tidal forces in comparison to the outskirts of the galaxy.…”

“…In Figure 6 we plot our stellar velocity dispersion for UDG1137+16, along with measurements for other UDGs (van Dokkum et al 2017(van Dokkum et al , 2019bMartín-Navarro et al 2019;Danieli et al 2019;Gannon et al 2020;Forbes et al 2020a) and objects called UDGs where some debate remains as to their precise classification/properties (Chilingarian et al 2019;Collins et al 2020). Additional to this, we include UDGs from Toloba et al (2018) 3 andvan Dokkum et al (2019a) with GC kinematics under the assumption their measured GC kinematics are representative of the stellar velocity dispersion of their associated UDG (see further Forbes et al 2020a).…”

“…We plot a sample of non-UDGs: early type galaxies (Cappellari et al 2013), elliptical/S0/irregular/spiral galaxies (Harris et al 2013), dwarf ellipticals (Chilingarian et al 2009) and Local Group dwarfs (McConnachie 2012) (all black points). We also plot our data (red star) along with other UDGs with central stellar velocity dispersions (blue points -van Dokkum et al , 2018Danieli et al 2019;van Dokkum et al 2019b;Martín-Navarro et al 2019;Gannon et al 2020;Forbes et al 2020a) and 'UDGs' from the literature where some debate remains as to their precise classification/properties (green triangles - Chilingarian et al 2019;Collins et al 2020). We also include measured GC velocity dispersions of UDGs assuming they are comparable to the stellar velocity dispersion (yellow triangles - Toloba et al 2018;van Dokkum et al 2019a).…”

Andromeda XXI – a dwarf galaxy in a low-density dark matter halo

“…All three of these "feeble giants" also appear to reside in surprisingly low mass halos. Through measured velocity dispersions, their mass within 𝑟 half are far lower than expected for systems of their size or brightness (Caldwell et al 2017;Fu et al 2019;Torrealba et al 2019;Collins et al 2020), raising questions about whether they can be understood in the context of the Λ Cold Dark Matter (ΛCDM) framework. For these systems, it is unlikely that star formation feedback alone can explain their low densities.…”

“…Indeed, a plunging orbit for Antlia 2 could also explain observed disturbances in the outer HI disc of the Milky Way (Chakrabarti et al 2019). A precise orbit for Andromeda XIX has not been determined, but from its structural and dynamical properties, it is possible that this galaxy is also on a similarly plunging orbit (McConnachie et al 2008;Martin et al 2016;Collins et al 2013Collins et al , 2020. To determine whether all low mass extended galaxies can be explained in this manner in ΛCDM, however, we need to study more systems.…”