James Webb Space Telescope Feed Post

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

Left: Key structural features around the starburst ring, including two streams of bar-driven gas inflow from large radii and the two “contact points” where the inflow gas collides with the ring material. Markers and background image are the same as the left panel in Figure 9. Right: Distributions and properties of the ALMA YMCs and HST clusters, as a function of the deprojected azimuthal angle (increasing counter-clockwise, with zero-point defined by the galaxy position angle). The blue shaded regions indicate azimuthal angle ranges for the Northern/Southern contact points highlighted in the left panel. There is tentative evidence for a progression in YMC evolutionary stage (from Type 1 to Type 4) and gradual decrease in YMC gas to stellar mass ratio along the Eastern side of the ring orbit (i.e., from -180? to 0? in azimuthal angle), but the trends are less obvious for the other side. No clear correspondence or variation is seen in the azimuthal distribution of the HST clusters either. 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.