Star Formation and Asymmetry in the Spiral Arms of M51: Variable Star Formation Caused by More than One Spiral Density Wave

Abstract

In the inner 3 kpc of M51, we find that logarithmic spirals provide good fits to the peak intensities in molecular gas observed by the Berkeley-Illinois-Maryland Association array in the CO (J = 1-0) emission line along the spiral arms. However, we measure significant asymmetries between the location and density of the arms observed on one side of the galaxy compared with those on the opposite side. Between a radius of 1 and 2.2 kpc, the gas distribution traced in CO is lopsided with densities twice as large in one arm than the opposite one. In the same region, the spiral arms are offset by 20 from the position of the arm on the opposite side of the galaxy after a rotation of 180 . We use the ratio of CO emission to the emission in the Pa hydrogen recombination line to estimate the efficiency of star formation along each arm. Except for a narrow region at about 2 kpc in which star formation is surprisingly inefficient, the gas depletion time is approximately 200 million years despite large variations in the molecular gas density in the arms. We account for the deviations from two-armed bisymmetric structure with a simple model that includes an additional three-armed spiral density wave. This model accounts for the angular offset between the arm on one side compared with its opposite, the lopsided distribution in molecular gas, and interarm star formation to the northeast of the galaxy nucleus. Since the star formation efficiency is unaffected by the variations in gas density, and the variations in gas density can be accounted for by the presence of an additional three-armed density wave, we suggest that the star formation rate is variable and is highest where and when the maxima of the two spiral density waves coincide or constructively add. The inner region of M51 provides good evidence for the presence of more than one spiral density wave and a resulting variable rate of star formation.

The star cluster population of M 51

“…This trend is also reflected in the presence of large, isolated, and young star cluster complexes located in the west spiral arm, which appear to be absent on the east side of the galaxy. This is also consistent with the estimated star formation rates of Henry et al (2003). 5.…”

“…The higher cluster formation rate on the west side of the galaxy is also reflected in the presence of large, isolated and young star cluster complexes located in the western spiral arms, that appear to be largely absent in the eastern spiral arms . These complexes are typically less than ∼6 Myr old, contain 3 × 10 4 to 3 × 10 5 M , and are associated with large concentrations of CO (Henry et al 2003). Henry et al (2003) studied the present day star formation rate (SFR) and its relation to the molecular cloud content in the spiral arms of M 51, based on NICMOS (Paα) and BIMA-SONG (CO 1−0) observations.…”

“…These complexes are typically less than ∼6 Myr old, contain 3 × 10 4 to 3 × 10 5 M , and are associated with large concentrations of CO (Henry et al 2003). Henry et al (2003) studied the present day star formation rate (SFR) and its relation to the molecular cloud content in the spiral arms of M 51, based on NICMOS (Paα) and BIMA-SONG (CO 1−0) observations. They find that both the molecular cloud content and the SFR are higher in the NW and W spiral arm (regions 1a and 1b in their terminology) than in the SE and E arm (regions 2a and 2b).…”

The star cluster population of M 51

“…In reality, the cluster will also experience additional external perturbations, for example, the encounters with molecular clouds. The clusters in our sample are in the inner 5 kpc of the galaxy, where most of the giant molecular clouds reside (Henry et al 2003;Kuno et al 1995). The encounters with molecular clouds can speed up the disruption of clouds significantly (e.g.…”

Analysis of spiral arms using anisotropic wavelets: gas, dust and magnetic fields in M 51

“…Rand & Kulkarni (1990), Scoville et al (2001), and Tosaki et al (2002) noted that the Hα arms are located outside the CO arms. Henry et al (2003) found that the Paschen-α line emission is offset from the CO emission along part of the arms. All these shifts refer to the inner spiral system and are expected for arms triggered by density waves.…”