भारतीय खगोलभौतिकी संस्थान
Indian Institute of Astrophysics
Exploring the Universe since 1786

Galaxy interactions in groups can lead to intense starbursts and the activation of active galactic nuclei (AGNs). The stripped gas from the outer disk can lead to star-forming clumps along the tidal tails or sometimes tidal dwarf galaxies. We investigate the impact of interaction on various galaxy properties, including morphology, star formation rates, and chemical composition in the galaxy group AM 1054-325 using Multi Unit Spectroscopic Explorer (MUSE) data. We conduct a comprehensive spatially and spectrally resolved investigation of the star formation rate, star formation histories, metallicity, and AGN activity. The galaxy subgroup AM 1054-325A shows multiple star-forming clumps in Hα emission along the western tidal tail, which are formed due to tidal stripping. These clumps also have higher metallicities. AM 1054-325B is quenched and shows disturbed gas kinematics and the signature of gas accretion in the Hα map. The specific star formation along the tidal tail is higher, contributing to the galaxy’s overall stellar mass growth.

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A detailed study of stellar populations in Milky Way (MW) satellite galaxies remains an observational challenge due to their faintness and fewer spectroscopically confirmed member stars. We use unsupervised machine learning methods to identify new members for nine nearby MW satellite galaxies using Gaia data release-3 (Gaia DR3) astrometry, the Dark Energy Survey (DES) and the DECam Local Volume Exploration Survey (DELVE) photometry. Two density-based clustering algorithms, DBSCAN and HDBSCAN, have been used in the four-dimensional astrometric parameter space (α2016, δ2016, μα cos δ, μδ) to identify member stars belonging to MW satellite galaxies. Our results indicate that we can recover more than 80% of the known spectroscopically confirmed members in most satellite galaxies and also reject 95–100% of spectroscopic nonmembers. We have also added many new members using this method. We compare our results with previous studies using photometric and astrometric data and discuss the suitability of density-based clustering methods for MW satellite galaxies.

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The unluckiest star orbits a supermassive black hole elliptically. Every time it reaches the pericenter, it shallowly enters the tidal radius and gets partially tidally disrupted, producing a series of flares. Confirmation of a repeated partial tidal disruption event (pTDE) requires not only evidence to rule out other types of transients but also proof that only one star is involved, as TDEs from multiple stars can also produce similar flares. In this Letter, we report the discovery of a repeated pTDE, AT 2022dbl. In a quiescent galaxy at z = 0.0284, two separate optical/UV flares have been observed in 2022 and 2024 with no bright X-ray, radio, or mid-infrared counterparts. Compared to the first flare, the second flare has a similar blackbody temperature of ∼26,000 K, slightly lower peak luminosity, and slower rise and fall phases. Compared to the Zwicky Transient Facility TDEs, their blackbody parameters and light-curve shapes are all similar. The spectra taken during the second flare show a steeper continuum than the late-time spectra of the previous flare, consistent with a newly risen flare. More importantly, the possibility of two independent TDEs can be largely ruled out because the optical spectra taken around the peak of the two flares exhibit highly similar broad Balmer, N iii, and possible He ii emission lines, especially the extreme ∼4100 Å emission lines. This represents the first robust spectroscopic evidence for a repeated pTDE, which can soon be verified by observing the third flare, given its short orbital period.

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