The PdBI Arcsecond Whirlpool Survey (PAWS): The Role of Spiral Arms in Cloud and Star Formation

Abstract

The process that leads to the formation of the bright star-forming sites observed along prominent spiral arms remains elusive. We present results of a multi-wavelength study of a spiral arm segment in the nearby grand-design spiral galaxy M51 that belongs to a spiral density wave and exhibits nine gas spurs. The combined observations of the (ionized, atomic, molecular, dusty) interstellar medium with star formation tracers (H II regions, young <10 Myr stellar clusters) suggest (1) no variation in giant molecular cloud (GMC) properties between arm and gas spurs, (2) gas spurs and extinction feathers arising from the same structure with a close spatial relation between gas spurs and ongoing/recent star formation (despite higher gas surface densities in the spiral arm), (3) no trend in star formation age either along the arm or along a spur, (4) evidence for strong star formation feedback in gas spurs, (5) tentative evidence for star formation triggered by stellar feedback for one spur, and (6) GMC associations being not special entities but the result of blending of gas arm/spur cross sections in lower resolution observations. We conclude that there is no evidence for a coherent star formation onset mechanism that can be solely associated with the presence of the spiral density wave. This suggests that other (more localized) mechanisms are important to delay star formation such that it occurs in spurs. The evidence of star formation proceeding over several million years within individual spurs implies that the mechanism that leads to star formation acts or is sustained over a longer timescale.

Young stars as tracers of a barred-spiral Milky Way

“…For instance, at around (4, −2)kpc and there are clear spurs pointing near-perpendicular to the arms. Such star forming spurs are seen in other disc galaxies, most noticeably in M51, where Schinnerer et al (2017) show that multiple tracers of star formation are concentrated in patches in the spurs of the arms.…”

The impact of galactic disc environment on star-forming clouds

“…Keto & Myers 1986;Heyer et al 2004;Oka et al 2001;Rosolowsky & Blitz 2005;Gratier et al 2012;Wong et al 2011) also suggest common properties between GMCs in molecular-rich environments. Even the effect of the spiral potential is debated, with Schinnerer et al (2017) suggesting that the GMAs observed in M51 are actually blended gas spurs and can be separated into structures of a similar mass to inter-arm clouds. This matches simulations by Baba et al (2017) who found no evolution sequence of GMC properties across a spiral arm, although notes that the clouds are still affected by the external pressure determined by galactic-scale features.…”

Cloud-scale ISM Structure and Star Formation in M51

“…Despite our averaging over moderately large (370 pc) areas, timescale effects may also be at play. The τ mol Dep map in Figure 9 shows significant azimuthal structure, and as shown by Schinnerer et al (2017), star formation tends to occur in spur-like structures downstream of the arms. We refer the reader to extensive discussions in Meidt et al (2013), Colombo et al (2014a), and Querejeta et al (2016), Schinnerer et al (2017), and references therein, for more discussion.…”

“…Galactic dynamics relate to molecular gas structure and star formation in M51 (e.g., see Koda et al 2009;Hughes et al 2013a;Colombo et al 2014a;Meidt et al 2015;Schinnerer et al 2017). With this in mind, we separate our correlation analysis by dynamical regions (Section 3.4).…”

“…This could reflect evolutionary effects on scales larger than our averaging beam. For example, Schinnerer et al (2017) show the formation of stars along spurs displaced downstream from the arms. Or it could be driven additional suppression of star formation in the arms by dynamical effects not captured by b 40pc .…”