We assemble the largest sample of oxygen rich Miras to date and highlight their importance for age-dating the components of the Galaxy. Using data from the Catalina Surveys and the All Sky Automated Survey for Supernovae, we extract a clean sample of ∼ 2, 400 O-Miras, stretching from the Galactic Bulge to the distant halo. Given that the period of O-Miras correlates with age, this offers a new way of determining age gradients throughout the Galaxy. We use our sample to show (i) disc O-Miras have periods increasing on moving outwards from ∼ 3 to 15 kpc, so the outer disc O-Miras are younger than the inner disc, (ii) the transition from younger disc to halo O-Miras occurs at r ∼ 15 kpc and is marked by a plummeting in period, (iii) there exists a population of young O-Miras likely kicked from the disc to heights of order of |Z | ∼ 10 kpc, (iv) great circle counts of old Miras show strong evidence for distant debris agglomeration associated with the Magellanic Clouds, (v) seven stars in our samples are located at distances between 200 and 500 kpc surpassing all previously established records, and, finally, (vi) O-Miras may be present in the Fornax, Sculptor, Sextans and Leo II Galactic dwarf spheroidals, as well as the distant globular cluster Pal 4. We spotlight the importance of O-Mira in the Era of Gaia as universal chronometers of the Galactic populations.

We assemble a catalog of Magellanic Cloud red giants from Data Release 2 of the Gaia mission and, utilizing machine-learning methods, obtain photometric metallicity estimates for them. In doing so, we are able to chemically map the entirety of the Magellanic System at once. Our maps reveal a plethora of substructure within our red giant sample, with the Large Magellanic Cloud (LMC) bar and spiral arm being readily apparent. We uncover a curious spiral-like feature in the southern portion of the LMC disk, hosting relatively metal-rich giants and likely a by-product of historic encounters with the Small Magellanic Cloud (SMC). Modeling the LMC as an inclined thin disk, we find a shallow metallicity gradient of −0.048 ± 0.001 dex kpc −1 out to ∼12°from the center of the dwarf. We see evidence that the SMC is disrupting, with its outer isodensity contours displaying the S-shape symptomatic of tidal stripping. On studying the proper motions of the SMC giants, we observe a population of them being violently dragged toward the larger Cloud. The perturbed stars predominantly lie in front of the SMC, and we interpret that they exist as a tidal tail of the dwarf, trailing in its motion and undergoing severe disruption from the LMC. We find the metallicity structure in the Magellanic Bridge region to be complex, with evidence for a composite nature in this stellar population, consisting of both LMC and SMC debris.Unified Astronomy Thesaurus concepts: Galaxies (573); Local Group (929); Large Magellanic Cloud (903); Small Magellanic Cloud (1468)

We exploit the extensive Gaia Data Release 2 set of Long Period Variables to select a sample of Oxygen-rich Miras throughout the Milky Way disk and bulge for study. Exploiting the relation between Mira pulsation period and stellar age/chemistry, we slice the stellar density of the Galactic disk and bulge as a function of period. We find the morphology of both components evolves as a function of stellar age/chemistry with the stellar disk being stubby at old ages, becoming progressively thinner and more radially extended at younger stellar ages, consistent with the picture of inside-out and upside-down formation of the Milky Way's disk. We see evidence of a perturbed disk, with large-scale stellar over-densities visible both in and away from the stellar plane. We find the bulge is well modelled by a triaxial boxy distribution with an axis ratio of ∼ [1 : 0.4 : 0.3]. The oldest of the Miras (∼ 9-10 Gyr) show little bar-like morphology, whilst the younger stars appear inclined at a viewing angle of ∼ 21 • to the Sun-Galactic Centre line. This suggests that bar formation and buckling took place 8-9 Gyr ago, with the older Miras being hot enough to avoid being trapped by the growing bar. We find the youngest Miras to exhibit a strong peanut morphology, bearing the characteristic X-shape of an inclined bar structure.

No abstract