How can we discover objects we did not know existed within the large datasetsthat now abound in astronomy? We present an outlier detection algorithm that wedeveloped, based on an unsupervised Random Forest. We test the algorithm onmore than two million galaxy spectra from the Sloan Digital Sky Survey andexamine the 400 galaxies with the highest outlier score. We find objects whichhave extreme emission line ratios and abnormally strong absorption lines,objects with unusual continua, including extremely reddened galaxies. We findgalaxy-galaxy gravitational lenses, double-peaked emission line galaxies, andclose galaxy pairs. We find galaxies with high ionisation lines, galaxies whichhost supernovae, and galaxies with unusual gas kinematics. Only a fraction ofthe outliers we find were reported by previous studies that used specific andtailored algorithms to find a single class of unusual objects. Our algorithm isgeneral and detects all of these classes, and many more, regardless of whatmakes them peculiar. It can be executed on imaging, time-series, and otherspectroscopic data, operates well with thousands of features, is not sensitiveto missing values, and is easily parallelisable.

The extremely metal-poor (XMP) galaxies analyzed in a previous paper havelarge star-forming regions with a metallicity lower than the rest of thegalaxy. Such a chemical inhomogeneity reveals the external origin of themetal-poor gas fueling star formation, possibly indicating accretion from thecosmic web. This paper studies the kinematic properties of the ionized gas inthese galaxies. Most XMPs have rotation velocity around a few tens of km/s. Thestar-forming regions appear to move coherently. The velocity is constant withineach region, and the velocity dispersion sometimes increases within thestar-forming clump towards the galaxy midpoint, suggesting inspiral motiontoward the galaxy center. Other regions present a local maximum in velocitydispersion at their center, suggesting a moderate global expansion. The Halphaline wings show a number of faint emission features with amplitudes around afew percent of the main Halpha component, and wavelength shifts between 100 and400 km/s. The components are often paired, so that red and blue emissionfeatures with similar amplitudes and shifts appear simultaneously. Assuming thefaint emission to be produced by expanding shell-like structures, the inferredmass loading factor (mass loss rate divided by star formation rate) exceeds 10.Since the expansion velocity exceeds by far the rotational and turbulentvelocities, the gas may eventually escape from the galaxy disk. The observedmotions involve energies consistent with the kinetic energy released byindividual core-collapse supernovae. Alternative explanations for the faintemission have been considered and discarded.

We reexamine the systematic properties of local galaxy populations, usingpublished surveys of star formation, structure, and gas content. Afterrecalibrating star formation measures, we are able to reliably measure specificstar formation rates well below the "main sequence" of star formation vs mass.We find an unexpectedly large population of galaxies with star formation ratesintermediate between vigorously star-forming main sequence galaxies and passivegalaxies, and with gas content disproportionately high for their star formationrates. Several lines of evidence suggest that these quiescent galaxies form adistinct population rather than a low star formation tail of the main sequence.We demonstrate that a tight main sequence, evolving with epoch, is a naturaloutcome of most histories of star formation and has little astrophysicalsignificance, but that the quiescent population requires additionalastrophysics to explain its properties. Using a simple model for disk evolutionbased on the observed dependence of star formation on gas content in localgalaxies, and assuming simple histories of cold gas inflow, we show that theevolution of galaxies away from the main sequence can be attributed to thedepletion of gas due to star formation after a cutoff in gas inflow. Thequiescent population is composed of galaxies in which the density of disk gashas fallen below a threshold for disk stability. The evolution of galaxiesbeyond the quiescent state to gas exhaustion requires another process, probablywind-driven mass loss. The SSFR distribution of the quiescent and passiveimplies that the timescale of this process must be greater than a few Gyrs butless than a few tens of Gyrs. The environmental dependence of the galaxypopulations is consistent with recent theory suggesting that cold gas inflowsinto galaxies are truncated at earlier times in denser environments.

Multi-band images of the very isolated spiral galaxy NGC 5523 show a numberof unusual features consistent with NGC 5523 having experienced a significantmerger: (1) Near-infrared (NIR) images from the Spitzer Space Telescope (SST)and the WIYN 3.5-m telescope reveal a nucleated bulge-like structure embeddedin a spiral disk. (2) The bulge is offset by ~1.8 kpc from a brightness minimumat the center of the optically bright inner disk. (3) A tidal stream, possiblyassociated with an ongoing satellite interaction, extends from the nucleatedbulge along the disk. We interpret these properties as the results of one ormore non-disruptive mergers between NGC 5523 and companion galaxies orsatellites, raising the possibility that some galaxies become isolated becausethey have merged with former companions.