In hierarchical models of galaxy formation, stellar tidal streams areexpected around most, if not all, galaxies. Although these features may provideuseful diagnostics of the $\Lambda$CDM model, their observational propertiesremain poorly constrained because they are challenging to detect and interpretand have been studied in detail for only a sparse sampling of galaxypopulation. More quantitative, systematic approaches are required. We advocatestatistical analysis of the counts and properties of such features in archivalwide-field imaging surveys for a direct comparison against results fromnumerical simulations. Thus, in this paper we aim to study systematically thefrequency of occurrence and other observational properties of tidal featuresaround nearby galaxies. The sample we construct will act as a foundationaldataset for statistical comparison with cosmological models of galaxyformation. Our approach is based on a visual classification of diffuse featuresaround a volume-limited sample of nearby galaxies, using a post-processing ofSloan Digital Sky Survey imaging optimized for the detection of stellarstructure with low surface brightness. At a limiting surface brightness of $28\\mathrm{mag~arcsec^{-2}}$, 14% of the galaxies in our sample exhibit evidenceof diffuse features likely to have arisen from minor merging events. Ourtechnique recovers all previously known streams in our sample and yields anumber of new candidates. Consistent with previous studies, coherent arc-likefeatures and shells are the most common type of tidal structures found in thisstudy. We conclude that although some detections are ambiguous and could becorroborated or refuted with deeper imaging, our technique provides a reliablefoundation for the statistical analysis of diffuse circumgalactic features inwide-area imaging surveys, and for the identification of targets for follow-upstudies.

We present the discovery of a vast cloud of ionized gas 13$^{\prime}$ (32kpc) north of the interacting system M51. We detected this cloud via deepnarrow-band imaging with the Burrell Schmidt Telescope, where it appears as anextended, diffuse H$\alpha$-emitting feature with no embedded compact regions.The Cloud spans $\sim$10$^{\prime}\times$3$^{\prime}$ (25$\times$7.5 kpc) insize and has no stellar counterpart; comparisons with our previous deepbroadband imaging show no detected continuum light to a limit of $\mu_{\rm lim,B} \sim$30 mag arcsec$^{-2}$. WIYN SparsePak observations confirm the cloud'skinematic association with M51, and the high NII/H$\alpha$, SII/H$\alpha$, andOI/H$\alpha$ line ratios we measure imply a hard ionization source such as AGNphotoionization or shock heating rather than photoionization due to youngstars. Given the strong NII emission, we infer roughly solar metallicity forthe cloud, ruling out an origin due to infall of primordial gas. Instead wefavor models where the gas has been expelled from the inner regions of the M51system due to tidal stripping or starburst/AGN winds and has been subsequentlyionized either by shocks or a fading AGN. This latter scenario raises theintriguing possibility that M51 may be the nearest example of an AGN fossilnebula or light echo, akin to the famous "Hanny's Voorwerp" in the IC 2497system.

CONTEXT. The maximum size of the Galactic stellar disk is not yet known. Somestudies have suggested an abrupt drop-off of the stellar density of the disk atGalactocentric distances $R\gtrsim 15$ kpc, which means that in practice nodisk stars or only very few of them should be found beyond this limit. However,stars in the Milky Way plane are detected at larger distances. In addition tothe halo component, star counts have placed the end of the disk beyond 20 kpc,although this has not been spectroscopically confirmed so far. AIMS. Here, we aim to spectroscopically confirm the presence of the diskstars up to much larger distances. METHODS. With data from the LAMOST and SDSS-APOGEE spectroscopic surveys, westatistically derived the maximum distance at which the metallicitydistribution of stars in the Galactic plane is distinct from that of the halopopulations. RESULTS. Our analysis reveals the presence of disk stars at R>26 kpc (99.7%C.L.) and even at R>31 kpc (95.4% C.L.).

The formation scenario of extended counter-rotating stellar disks in galaxiesis still debated. In this paper, we study the S0 galaxy IC 719 known to hosttwo large-scale counter-rotating stellar disks in order to investigate theirformation mechanism. We exploit the large field of view and wavelength coverageof the Multi Unit Spectroscopic Explorer (MUSE) spectrograph to derivetwo-dimensional (2D) maps of the various properties of the counter-rotatingstellar disks, such as age, metallicity, kinematics, spatial distribution, thekinematical and chemical properties of the ionized gas, and the dust map. Dueto the large wavelength range, and in particular to the presence of the CalciumTriplet 8498, 8542, 8662\AA (CaT hereafter), the spectroscopic analysis allowsus to separate the two stellar components in great detail. This permits precisemeasurement of both the velocity and velocity dispersion of the two componentsas well as their spatial distribution. We derived a 2D map of the age andmetallicity of the two stellar components as well as the star formation rateand gas-phase metallicity from the ionized gas emission maps. The main stellardisk of the galaxy is kinematically hotter, older, thicker and with largerscale-length than the secondary disk. There is no doubt that the latter isstrongly linked to the ionized gas component: they have the same kinematics andsimilar vertical and radial spatial distribution. This result is in favor of agas accretion scenario over a binary merger scenario to explain the origin ofcounter-rotation in IC 719. One source of gas that may have contributed to theaccretion process is the cloud that surrounds IC 719.

Disc truncations are the closest feature to an edge that galaxies have, butthe nature of this phenomena is not yet understood. In this paper, we explorethe truncations in two nearby (D ~15 Mpc) Milky Way-like galaxies: NGC 4565 andNGC 5907. We cover a wide wavelength range from the NUV and optical, to 3.6{\mu}m. We find that the radius of the truncation (26+/-0.5 kpc) is independentof wavelength. Surprisingly, we identify (at all wavelengths) the truncation ataltitudes as high as 3 kpc above the mid-plane, which implies that the thindisc in those outer regions has a width of at least this value. We find thecharacteristic U-shape radial colour profile associated with a star formationthreshold at the location of the truncation. Further supporting such an origin,the stellar mass density at the position of the truncation is ~1-2 M_sun/pc^2,in good agreement with the critical gas density for transforming gas intostars. Beyond the truncation, the stellar mass in the mid-plane of the discdrops to just 0.1-0.2% of the total stellar mass of the galaxies. The detectionof the truncation at high altitude in combination with the U-shape of theradial colour profile allows us to establish, for the first time, an upperlimit to the present-day growth rate of galactic discs. We find that, if thediscs of the galaxies are growing inside-out, their growth rate is less than0.6-0.9 kpc/Gyr.

We present stellar rotation curves and velocity dispersion profiles for 104quiescent galaxies at $z=0.6-1$ from the Large Early Galaxy Astrophysics Census(LEGA-C) spectroscopic survey. Rotation is typically probed across 10-20kpc, orto an average of 2.7${\rm R_e}$. Combined with central stellar velocitydispersions ($\sigma_0$) this provides the first determination of the dynamicalstate of a sample selected by a lack of star formation activity at largelookback time. The most massive galaxies($M_{\star}>2\times10^{11}\,M_{\odot}$) generally show no or little rotationmeasured at 5kpc ($|V_5|/\sigma_0<0.2$ in 8 of 10 cases), while ${\sim}64\%$ ofless massive galaxies show significant rotation. This is reminiscent of localfast- and slow-rotating ellipticals and implies that low- and high-redshiftquiescent galaxies have qualitatively similar dynamical structures. We compare$|V_5|/\sigma_0$ distributions at $z\sim0.8$ and the present day by re-binningand smoothing the kinematic maps of 91 low-redshift quiescent galaxies from theCALIFA survey and find evidence for a decrease in rotational support since$z\sim1$. This result is especially strong when galaxies are compared at fixedvelocity dispersion; if velocity dispersion does not evolve for individualgalaxies then the rotational velocity at 5kpc was an average of ${94\pm22\%}$higher in $z\sim0.8$ quiescent galaxies than today. Considering that the numberof quiescent galaxies grows with time and that new additions to the populationdescend from rotationally-supported star-forming galaxies, our results implythat quiescent galaxies must lose angular momentum between $z\sim1$ and thepresent, presumably through dissipationless merging, and/or that the mechanismthat transforms star-forming galaxies also reduces their rotational support.

Pressure supported systems modeled under MONDian extended gravity areexpected to show an outer flattening in their velocity dispersion profiles. Acharacteristic scaling between the amplitude of the asymptotic velocitydispersion and the radius at which the flattening occurs is also expected. Bycomprehensively analyzing the dynamical behavior of $\sim$300 extremely lowrotating elliptical galaxies from the MaNGA survey, we show this type ofpressure supported systems to be consistent with MONDian expectations, for arange of central velocity dispersion values of $60 km/s < \sigma_{central}< 280km/s $ and asymptotic velocity dispersion values of $28km/s <\sigma_{\infty}<250 km/s$. We find that a universal velocity dispersion profileaccurately describes the studied systems; the predicted kinematics of extendedgravity are verified for all well observed galaxies.