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We present different chemical evolution models for the ultrafaint dwarf galaxy Boötes I. We either assume that the galaxy accretes its mass through smooth infall of gas of primordial chemical composition (classical models) or adopt mass accretion histories derived from the combination of merger trees with semi-analytical modelling (cosmologically-motivated models). Furthermore, we consider models with and without taking into account inhomogeneous mixing in the interstellar medium within the galaxy, i.e. homogeneous versus inhomogeneous models. The theoretical predictions are then compared to each other and to the body of the available data. From this analysis, we confirm previous findings that Boötes I has formed stars with very low efficiency but, at variance with previous studies, we do not find a clear-cut indication that supernova explosions have sustained long-lasting galactic-scale outflows in this galaxy. Therefore, we suggest that external mechanisms such as ram pressure stripping and tidal stripping are needed to explain the absence of neutral gas in Boötes I today. [ABSTRACT FROM AUTHOR]

This paper concludes a systematic search for evidence of the Monoceros Ring and Canis Major dwarf galaxy around the Galactic plane. Presented here are the results for the Galactic longitude range of l= (280–025)°. Testing the claim that the Monoceros Ring encircles the entire Galaxy, this survey attempts to document the position of the Monoceros Ring with increasing Galactic longitude. Additionally, with the discovery of the purported Canis Major dwarf galaxy, searching for more evidence of its interaction is imperative for tracing its path through the Galaxy and understanding its role in the evolution of the Milky Way. Two new detections of the Monoceros Ring have been found at and (300, +10)°. Interestingly, in general, there seem to be more detections above the plane than below it; in this survey, around 2/3 of the firm Monoceros Ring detections are in the north. This coincides with the northern detections appearing to be qualitatively denser and broader than their southern counterparts. The maximum of the Galactic warp in the south is also probed in this survey. It is found that these fields do not resemble those in the Canis Major region suggesting that the warp does not change the shape of the colour–magnitude diagram as is witnessed around Canis Major. The origins and morphology of the Monoceros Ring are still elusive primarily due to its enormous extent on the sky. Continued probing of the Galactic Outer Disc is needed before a consensus can be reached on its nature. [ABSTRACT FROM AUTHOR]

We present the results of an Anglo-Australian Telescope (AAT) wide field camera survey of the stars in the Monoceros Ring (MRi) and purported Canis Major (CMa) overdensity in the Galactic longitudes of . Current numerical simulations suggest that both of these structures are the result of a single on-going accretion event, although an alternative solution is that the warped and flared disc of the Galaxy can explain the origin of both of these structures. Our results show that, with regards the MRi, the warped and flared disc is unable to reproduce the locations and strengths of the detections observed around the Galaxy. This supports a non-Galactic origin for this structure. We report eight new detections and two tentative detections of the MRi in this survey. The exact nature of the CMa overdensity is still unresolved, although this survey provides evidence that invoking the Galactic warp is not a sufficient solution when compared with observation. Several fields in this survey are highly inconsistent with the current Galactic disc models that include a warp and flare, to such an extent that explaining their origins with these structures is problematic. We also report that the blue plume stars previously invoked to support the dwarf galaxy hypothesis are unfounded, and associating them with an outer spiral arm is equally problematic. Standard Galactic models are unable to accommodate all the observations of these new structures, leading away from a warped/flared disc explanation for their origins and more toward a non-Galactic source. Additionally, evidence is presented in favour of a detection of the CMa dwarf stream away from the CMa region. As the outer reaches of the Galactic disc continue to be probed, the fascinating structures that are the MRi and CMa overdensity will no doubt continue to inform us of the unique structure and formation of the Milky Way. [ABSTRACT FROM AUTHOR]

An AAT/2dF Spectrograph Survey of low Galactic latitudes targeting the putative Canis Major dwarf galaxy and the (possibly) associated tidal debris of stars known as the Monoceros Ring, covering Galactic coordinates and , has revealed the presence of the Monoceros Ring in the background of the Canis Major dwarf galaxy. This detection resides at a Galactocentric distance of (13.5 ± 0.3 kpc heliocentric), exhibiting a velocity of with a dispersion of , both of these comparable to previous measurements of the Monoceros Ring in nearby fields. This detection highlights the increasing complexity of structure being revealed in recent surveys of the Milky Way thick disc and halo. [ABSTRACT FROM AUTHOR]

We present an atlas and follow-up spectroscopic observations of 87 thin stream-like structures detected with the STREAMFINDER algorithm in Gaia DR3, of which 28 are new discoveries. Here, we focus on using these streams to refine mass models of the Galaxy. Fits with a double-power-law halo with the outer power-law slope set to − β h = −3 yield an inner power-law slope of − γ h = - (0.97 − 0.21 + 0.17 ) , a scale radius of r 0 , h = 14.7 − 1.0 + 4.7 kpc , a halo density flattening q m, h = 0.75 ± 0.03, and a local dark matter density of ρ h,⊙ = 0.0114 ± 0.0007 M ⊙ pc−3. Freeing β yields β = 2.53 − 0.16 + 0.42 , but this value is heavily influenced by our chosen virial mass limit. The stellar disks are found to have a combined mass of 4.20 − 0.53 + 0.44 × 10 10 M ⊙ , with the thick disk contributing 12.4% ± 0.7% to the local stellar surface density. The scale lengths of the thin and thick disks are 2.17 − 0.08 + 0.18 and 1.62 − 0.13 + 0.72 kpc , respectively, while their scale heights are 0.347 − 0.010 + 0.007 and 0.86 − 0.02 + 0.03 kpc , respectively. The virial mass of the favored model is M 200 = 1.09 − 0.14 + 0.19 × 10 12 M ⊙ , while the mass inside of 50 kpc is M R <50 = 0.46 ± 0.03 × 1012 M ⊙. We introduce the Large Magellanic Cloud (LMC) into the derived potential models, and fit the Orphan stream therein, finding a mass for the LMC that is consistent with recent estimates. Some salient highlights include the nearby trailing arm of ω Cen, and a nearby very metal-poor stream that was once a satellite of the Sagittarius dwarf galaxy. Finally, we unambiguously detect a hot component around the GD-1 stream, consistent with it having been tidally preprocessed within its own dark matter subhalo. [ABSTRACT FROM AUTHOR]