A 9-Month Hubble Space Telescope Near-UV Survey of M87. I. Light and Color Curves of 94 Novae, and a Re-determination of the Nova Rate

TL;DR

This study presents a comprehensive 9-month near-ultraviolet and optical survey of M87 using Hubble, characterizing 94 novae's light curves, confirming nova recurrence limits, and estimating a high nova rate that suggests ground-based surveys miss many faint or fast events.

Contribution

The paper provides the largest extragalactic nova dataset to date, refines nova recurrence times, and accurately measures the nova rate in M87 using space-based observations.

Findings

Novae follow M87's K-band light closely near the nucleus.

NUV and optical nova light curves are highly heterogeneous.

Estimated nova rate in M87 is approximately 352 per year.

Abstract

M87 has been monitored with a cadence of 5 days over a 9 month-long span through the near-ultraviolet (NUV:F275W) and optical (F606W) filters of the Wide Field Camera 3 (WFC3) of the $\textit{Hubble Space Telescope}$. This unprecedented dataset yields the NUV and optical light and color curves of 94 M87 novae, characterizing the outburst and decline properties of the largest extragalactic nova dataset in the literature (after M31 and M81). We test and confirm nova modelers' prediction that recurrent novae cannot erupt more frequently that once every 45 days; show that there are zero rapidly recurring novae in the central $\sim$ 1/3 of M87 with recurrence times $ < $ 130 days; demonstrate that novae closely follow the K-band light of M87 to within a few arcsec of the galaxy nucleus; show that nova NUV light curves are as heterogeneous as their optical counterparts, and usually peak 5 to 30 days after visible light maximum; determine our observations' annual detection completeness to be 71 - 77\%; and measure the rate Rnova of nova eruptions in M87 as $352_{-37}^{+37}$/yr. The corresponding luminosity-specific classical nova rate for this galaxy is $7.91_{-1.20}^{+1.20}/yr/10^{10}L_\odot,_{K}$. These rates confirm that ground-based observations of extragalactic novae miss most faint, fast novae and those near the centers of galaxies. An annual M87 nova rate of 300 or more seems inescapable. A luminosity-specific nova rate of $\sim$ $7 - 10/yr/10^{10}L_\odot,_{K}$ in ${\it all}$ types of galaxies is indicated by the data available in 2023.