SIGNALS, the Star formation, Ionized Gas, and Nebular Abundances LegacySurvey, is a large observing program designed to investigate massive starformation and HII regions in a sample of local extended galaxies. The programwill use the imaging Fourier transform spectrograph SITELLE at theCanada-France-Hawaii Telescope. Over 355 hours (54.7 nights) have beenallocated beginning in fall 2018 for eight consecutive semesters. Oncecompleted, SIGNALS will provide a statistically reliable laboratory toinvestigate massive star formation, including over 50 000 resolved HII regions: the largest, most complete, and homogeneous database of spectroscopically andspatially resolved extragalactic HII regions ever assembled. For each fieldobserved, three datacubes covering the spectral bands of the filters SN1 (363-386 nm), SN2 (482 - 513 nm), and SN3 (647 - 685 nm) are gathered. The spectralresolution selected for each spectral band is 1000, 1000, and 5000,respectively. As defined, the project sample will facilitate the study ofsmall-scale nebular physics and many other phenomena linked to star formationat a mean spatial resolution of 20 pc. This survey also has considerable legacyvalue for additional topics including planetary nebulae, diffuse ionized gas,andsupernova remnants. The purpose of this paper is to present a generaloutlook of the survey, notably the observing strategy, galaxy sample, andscience requirements.

In $\Lambda$CDM cosmology, to first order, galaxies form out of the coolingof baryons within the virial radius of their dark matter halo. The fractions ofmass and angular momentum retained in the baryonic and stellar components ofdisc galaxies put strong constraints on our understanding of galaxy formation.In this work, we derive the fraction of angular momentum retained in thestellar component of spirals, $f_j$, the global star formation efficiency$f_M$, and the ratio of the asymptotic circular velocity ($V_{\rm flat}$) tothe virial velocity $f_V$, and their scatter, by fitting simultaneously theobserved stellar mass-velocity (Tully-Fisher), size-mass, and mass-angularmomentum (Fall) relations. We compare the goodness of fit of three models: (i)where the logarithm of $f_j$, $f_M$, and $f_V$ vary linearly with the logarithmof the observable $V_{\rm flat}$; (ii) where these values vary as a doublepower law; and (iii) where these values also vary as a double power law butwith a prior imposed on $f_M$ such that it follows the expectations from widelyused abundance matching models. We conclude that the scatter in these fractionsis particularly small ($\sim 0.07$ dex) and that the linear model is by farstatistically preferred to that with abundance matching priors. This indicatesthat the fundamental galaxy formation parameters are small-scatter single-slopemonotonic functions of mass, instead of being complicated non-monotonicfunctions. This incidentally confirms that the most massive spiral galaxiesshould have turned nearly all the baryons associated with their haloes intostars. We call this the failed feedback problem.

In this letter we construct a large sample of early-type galaxies withmeasured gas-phase metallicities from the Sloan Digital Sky Survey and GalaxyZoo in order to investigate the origin of the gas that fuels their residualstar formation. We use this sample to show that star forming ellipticalgalaxies have a substantially different gas-phase metallicity distribution fromspiral galaxies, with ~7.4% having a very low gas-phase metallicity for theirmass. These systems typically have fewer metals in the gas phase than they doin their stellar photospheres, which strongly suggests that the materialfuelling their recent star formation was accreted from an external source. Weuse a chemical evolution model to show that the enrichment timescale forlow-metallicity gas is very short, and thus that cosmological accretion andminor mergers are likely to supply the gas in >37% of star-forming ETGs, ingood agreement with estimates derived from other independent techniques.