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Hidden Gems on a Ring: Infant Massive Clusters and Their Formation Timeline Unveiled by ALMA, HST, and JWST in NGC 3351

ALMA CO (3–2) moment-0 map of the central 1×1 kpc (~20”×20”) of NGC 3351, tracing the distribution of the bulk molecular gas. This moment-0 map is made from the CO (3–2) cube with 0.''10 beam size and 5 km s-1 channel width (see Section 2.1.2). The dotted ellipse in the plot corresponds to a galactocentric radius of 500 pc, which encircles the region of interest in this study. Abstract: We study young massive clusters (YMCs) in their embedded "infant" phase with ~0.1" ALMA, HST, and JWST observations targeting the central starburst ring in NGC 3351, a nearby Milky Way analog galaxy. Our new ALMA data reveal 18 bright and compact (sub-)millimeter continuum sources, of which 11 have apparent counterparts in JWST images and only 6 have counterparts in HST images. Based on the ALMA continuum and molecular line data, as well as ancillary measurements for the HST and JWST counterparts, we identify 14 sources as infant star clusters with high stellar and/or gas masses (~105M?), small radii (?5pc), large escape velocities (6-10km/s), and short free-fall times (0.5-1Myr). Their multiwavelength properties motivate us to divide them into four categories, likely corresponding to four evolutionary stages from starless clumps to exposed HII region-cluster complexes. Leveraging age estimates for HST-identified clusters in the same region, we infer an evolutionary timeline going from 1-2Myr before cluster formation as starless clumps, to 4-6Myr after as exposed HII region-cluster complexes. Finally, we show that the YMCs make up a substantial fraction of recent star formation across the ring, exhibit an non-uniform azimuthal distribution without a very coherent evolutionary trend along the ring, and are capable of driving large-scale gas outflows.