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

Abstract

We use 0.1 observations from the Atacama Large Millimeter Array (ALMA), Hubble Space Telescope (HST),and JWST to study young massive clusters (YMCs) in their embedded“infant” phase across the central starburstring in NGC 3351. Our new ALMA data reveal 18 bright and compact (sub-)millimeter continuum sources, ofwhich 8 have counterparts in JWST images and only 6 have counterparts in HST images. Based on the ALMAcontinuum and molecular line data, as well as ancillary measurements for the HST and JWST counterparts, weidentify 14 sources as infant star clusters with high stellar and/or gas masses (∼105 Me), small radii ( 5 pc), largeescape velocities (6–10 km s−1), and short freefall times (0.5–1 Myr). Their multiwavelength properties motivateus to divide them into four categories, likely corresponding to four evolutionary stages from starless clumps toexposed H II region–cluster complexes. Leveraging age estimates for HST-identi ed clusters in the same region,we infer an evolutionary timeline, ranging from∼1–2 Myr before cluster formation as starless clumps, to∼4–6 Myr after as exposed H II region–cluster complexes. Finally, we show that the YMCs make up a substantialfraction of recent star formation across the ring, exhibit a nonuniform azimuthal distribution without a verycoherent evolutionary trend along the ring, and are capable of driving large-scale gas out ows