David Martínez-Delgado

Aims. In this work we study the striking case of a narrow blue stream with a possible globular cluster-like progenitor around the NGC 7241 galaxy and its foreground dwarf companion. We want to figure out if the stream was generated by tidal interaction with NGC 7241 or if it first interacted with the foreground dwarf companion and later both fell together toward NGC 7241. Methods. We used four sets of observations, including a follow-up spectroscopic study of this stream based on data taken with the MEGARA instrument at the 10.4 m Gran Telescopio Canarias using the integral field spectroscopy mode, the Mount Lemmon 0.80 m telescope, the Telescopio Nazionale Galileo, the DESI Imaging Legacy surveys, and GALEX archival data. We also used high-resolution zoomed-in cosmological simulations. Results. Our data suggest that the compact object we detected in the stream is a foreground Milky Way halo star. Near this compact object we detect emission lines overlapping a less compact, bluer, and fainter blob of the stream that is clearly visible in both ultraviolet and optical deep images. From its heliocentric systemic radial velocity derived from the [O III]λ5007 Å lines (V_syst = 1548.58 ± 1.80 km s⁻¹) and new UV and optical broadband photometry, we conclude that this overdensity could be the actual core of the stream, with an absolute magnitude of M_g ∼ −10 and a g − r = 0.08 ± 0.11, consistent with a remnant of a low-mass dwarf satellite undergoing a current episode of star formation. From the width of the stream and assuming a circular orbit, we calculate that the progenitor mass can be typical of a dwarf galaxy, but it could also be substantially lower if the stream is on a very radial orbit or if it was created by tidal interaction with the companion dwarf instead of with NGC 7241. These estimates also suggest that this is one of the lowest mass streams detected to date beyond the Local Group. Finally, we find that blue stellar streams containing star formation regions are commonly predicted by high-resolution cosmological simulations of galaxies lighter than the Milky Way. This scenario is consistent with the processes explaining the bursty star formation history of some dwarf satellites, which are followed by a gas depletion and a fast quenching once they enter within the virial radius of their host galaxies for the first time. Thus, it is likely that the stream’s progenitor is undergoing a star formation burst comparable to those that have shaped the star formation history of several Local Group dwarfs in the last few gigayears.

In this work we study the striking case of a narrow blue stream around the NGC 7241 galaxy and its foreground dwarf companion. We want to figure out if the stream was generated by tidal interaction with NGC 7241 or it first interacted with the foreground dwarf companion and later both fell together towards NGC 7241. We use four sets of observations, including a follow-up spectroscopic study with the MEGARA instrument at the 10.4-m Gran Telescopio Canarias. Our data suggest that the compact object we detected in the stream is a foreground Milky Way halo star. Near this compact object we detect emission lines overlapping a bluer and fainter blob of the stream that is clearly visible in both ultra-violet and optical deep images. From its heliocentric systemic radial velocity (Vsyst= 1548.58+/-1.80 km s^-1) and new UV and optical broad-band photometry, we conclude that this over-density could be the actual core of the stream, with an absolute magnitude of M_g ~ -10 and a (g-r) = 0.08 +/- 0.11, consistent with a remnant of a low-mass dwarf satellite undergoing a current episode of star formation. From the width of the stream and assuming a circular orbit, we calculate that the progenitor mass can be the typical of a dwarf galaxy, but it could also be substantially lower if the stream is on a very radial orbit or it was created by tidal interaction with the companion dwarf instead of with NGC 7241. Finally, we find that blue stellar streams containing star formation regions are commonly predicted by high-resolution cosmological simulations of galaxies lighter than the Milky Way. This scenario is consistent with the processes explaining the bursty star formation history of some dwarf satellites, which are followed by a gas depletion and a fast quenching once they enter within the virial radius of their host galaxies for the first time.

Context.Mergers and tidal interactions between massive galaxies and their dwarf satellites are a fundamental prediction of the Lambda-cold dark matter cosmology. These events are thought to provide important observational diagnostics of non-linear structure formation. Stellar streams in the Milky Way and Andromeda are spectacular evidence for ongoing satellite disruption. However, constructing a statistically meaningful sample of tidal streams beyond the Local Group has proven a daunting observational challenge, and the full potential for deepening our understanding of galaxy assembly using stellar streams has yet to be realised.Aims.Here we introduce the Stellar Stream Legacy Survey, a systematic imaging survey of tidal features associated with dwarf galaxy accretion around a sample of ∼3100 nearby galaxies withinz ∼ 0.02, including about 940 Milky Way analogues.Methods.Our survey exploits public deep imaging data from the DESI Legacy Imaging Surveys, which reach surface brightness as faint as ∼29 mag arcsec⁻²in therband. As a proof of concept of our survey, we report the detection and broad-band photometry of 24 new stellar streams in the local Universe.Results.We discuss how these observations can yield new constraints on galaxy formation theory through comparison to mock observations from cosmological galaxy simulations. These tests will probe the present-day mass assembly rate of galaxies, the stellar populations and orbits of satellites, the growth of stellar halos, and the resilience of stellar disks to satellite bombardment.

Context. Tidal ring galaxies (TRGs) are rarely observed in the local universe due to their intrinsically transient nature. The tidal ring structures are the result of strong interactions between gas-rich stellar disks and smaller galactic systems, and do not last longer than ∼500 Myr. Therefore, these are perfect scenarios in which to look for the debris of recently accreted dwarf galactic systems. Aims. Our goal is to study the low surface brightness stellar structures around the TRG NGC 922 and to revise the hypothesis of its formation in light of these new data. Methods. We present new deep images of the TRG NGC 922 and its surroundings from the DESI Legacy survey data and from our observations with an amateur telescope. These observations are compared with results from high-resolution N-body simulations that were designed to reproduce an alternative formation scenario for this peculiar galaxy. Results. Our new observations unveil that the low surface brightness stellar tidal structures around NGC 922 are much more complex than reported in previous works. In particular, the formerly detected tidal spike-like structure at the north-east of the central galaxy disk is not connected with the dwarf companion galaxy PGC 3080368, which has been suggested as the intruder triggering the ring formation of NGC 922. The deep images reveal that this tidal structure mainly has a fainter giant umbrella-like shape, and thus it was formed from the tidal disruption of a different satellite. Using the broad-band g, r, and z DESI LS images, we measured the photometric properties of this stellar stream, estimating a total absolute magnitude in the r band of M_r = −17.0 ± 0.03 mag and a total stellar mass for the stream of between 6.9 and 8.5×10⁸ M_⊙. We performed a set of N-body simulations to reproduce the observed NGC922-intruder interaction, suggesting a new scenario for the formation of its tidal ring from the infall of a gas-rich satellite around 150 Myr ago. Finally, our deep images also reveal a tidal shell around the dwarf galaxy PGC 3080368, a possible fossil of a recent merger with a smaller satellite, which may suggest it is in its first infall towards NGC 922.

We report deep imaging observations with DOLoRes@TNG of an ultra-faint dwarf satellite candidate of the Triangulum galaxy (M33) found by visual inspection of the public imaging data release of the DESI Legacy Imaging Surveys. Pisces VII/Triangulum (Tri) III is found at a projected distance of |$72\, {\rm kpc}$| from M33, and using the tip of the red giant branch method, we estimate a distance of |$D=1.0^{+0.3}_{-0.2}\, {\rm Mpc}$|⁠, meaning the galaxy could either be an isolated ultra-faint or the second known satellite of M33. We estimate an absolute magnitude of M_V = −6.1 ± 0.2 if Pisces VII/Tri II is at the distance of M33, or as bright as M_V = −6.8 ± 0.2 if the galaxy is isolated. At the isolated distance, it has a physical half-light radius of |$r_{\rm h}=131\pm 61\, {\rm pc}$| consistent with similarly faint galaxies around the Milky Way. As the tip of the red giant branch is sparsely populated, constraining a precision distance is not possible, but if Pisces VII/Tri III can be confirmed as a true satellite of M33, it is a significant finding. With only one potential satellite detected around M33 previously (Andromeda XXII/Tri I), it lacks a significant satellite population in stark contrast to the similarly massive Large Magellanic Cloud. The detection of more satellites in the outskirts of M33 could help to better illuminate if this discrepancy between expectation and observations is due to a poor understanding of the galaxy formation process, or if it is due to the low luminosity and surface brightness of the M33 satellite population which has thus far fallen below the detection limits of previous surveys. If it is truly isolated, it would be the faintest known field dwarf detected to date.

Recent evidence of extremely metal-rich stars found in the Sombrero galaxy (M104) halo suggests that this galaxy has undergone a recent major merger with a relatively massive galaxy. In this paper, we present wide-field deep images of the M104 outskirts obtained with a 18-cm amateur telescope with the purpose of detecting any coherent tidal features from this possible major merger. Our new data, together with a model of the M104 inner halo and scattered light from stars around the field, allow us to trace for the first time the full path of the stream on both sides of the disc of the galaxy. We fully characterize the ring-like tidal structure and we confirm that this is the only observable coherent substructure in the inner halo region. This result is in agreement with the hypothesis that M104 was created by a wet major merger more than 3.5 Gyr ago that heated up the stellar population, blurring all old substructure. We generated a set of numerical models that reproduce the formation of the observed tidal structure. Our best-fitting model suggests the formation of this stream in the last 3 Gyr is independent of the wet major merger that created the M104 system. Therefore, the formation of the tidal stream can put a constraint on the time when the major merger occurred.