Novae in the Whirlpool: a sharp boost in M51 nova rate from HST data

Novae are luminous outbursts that occur when the hydrogen-rich envelope of an accreting white dwarf ignites in a thermonuclear runaway. Despite their importance in determining stellar distributions in external galaxies — novae are our only means of studying the underlying CV populations (and indeed most binaries) in galaxies beyond the Local Group — and their significance in the realm of cosmology as “standard candle” type Ia supernovae (SNIa) progenitors, a lack of consensus on their rates in different galaxies has long persisted. In the past few years, dense-cadence Hubble Space Telescope (HST)-based surveys, and infrared Galactic surveys, have begun to overcome the limitations inherent in ground-based surveys of external galaxies and dust extinction in the Milky Way; these limitations had previously hampered the determination of nova rates and how they differ between different galaxy types. Several recent studies have consequently shown sharply increased nova rates relative to those previously claimed, triggering a robust debate on the merits and implications of some of the newer findings. We present 14 nova candidates that were serendipitously observed during a year-long HST survey of the massive spiral galaxy M51 (the “Whirlpool Galaxy”). We show that realistic simulations based on well-observed nova light curves indicate an intrinsic nova rate of 172 novae/yr, an order of magnitude higher than previously estimated. Our findings strongly suggest that the conventionally accepted universal low nova rates in all types of galaxies may be artifacts of the constraints of ground-based (and often low-cadence) surveys. They demonstrate that, contrary to theoretical models, the luminosity-specific nova rates (LSNR) in a giant elliptical galaxy (M87) and a giant spiral galaxy (M51), both determined from HST surveys, may not significantly differ.