A test of arm-induced star formation in spiral galaxies from near-infrared and Hα imaging

ABSTRACT We have imaged a sample of 20 spiral galaxies in Hα and in the near–infrared K band(2.2µm), in order to determine the location and strength of star formation in theseobjects with respect to perturbations in the old stellar population. We have found thatstar formation rates are significantly enhanced in the vicinity of Kband arms. We havealso found that this enhancement in star formation rate in arm regions correlates wellwith a quantity that measures the relative strengths of shocks in arms. Assuming thatthe K band light is dominated by emission from the old stellar population, this showsthat density waves trigger star formation in the vicinity of spiral arms.Key words: galaxies: fundamental parameters – galaxies: ISM – galaxies: spiral –galaxies: stellar content – infrared: galaxies 1 INTRODUCTIONTwo of the leading theories of star formation in spiral galax-ies use the concept of a density wave, either as the actualtriggering mechanism, or as a means of organisation of star–forming material. The first of these theories has been termedthe large scale galactic shock scenario (Roberts1969; see alsoShu et al. 1972; Tosa 1973; Woodward 1975; Nelson & Mat-suda 1977). This model hypothesises that the gas settles intoa quasi–stationary state, with a velocity and density distri-bution that is driven by the gravitational field of the galaxy.The gas response can be non–linear to an imposed azimuthalsinusoidal potential, if relative motion between the densitywave and the cold interstellar medium (ISM) is supersonic(Binney & Tremaine 1987). This leads to the formation ofa shock near the trailing edge of spiral arms, assuming thatthe region is inside the corotation radius, which compressesthe gas to densities at which stars can form. In observationsof spiral galaxies the shock is thought to be characterisedby dust–lanes seen on the trailing edges of arms. The timedelay needed for the onset of star formation after the com-pression of the gas implies that star–forming regions shouldbe seen towards the leading edges of arms.Many of the recent advances in this area have arisenfrom studies of the atomic and molecular gas in nearby spiralgalaxies, through HI and CO line emission (e.g Nakai etal. 1994; Rand 1993, 1995). These studies reveal streamingvelocities of gas through the spiral arms and find offsetsbetween the peaks of the gas density and the old stellarpopulation, in agreement with the predictions of the largescale shock scenario.The alternative picture to this form of triggered starformation is stochastic star formation. Opik (1953) first hy-¨pothesised that a supernova explosion could trigger star for-mation. A model proposed and developed by several authors(e.g. Gerola & Seiden 1978; Seiden & Gerola 1982; Seiden1983; Jungwiert & Palous 1994; Sleath & Alexander 1995,1996) suggests that the dominant process for forming starsis stochastic self–propagating star formation, and not den-sity wave triggering. In this model, density waves are onlyresponsible for the organisation of the ISM and stars, and forconcentrating new HII regions along spiral arms (Elmegreen& Elmegreen 1986; Elmegreen 1993). Thus, star formationrate efficiency (i.e. normalised to unit mass of disc material)should be unaffected by location in arm or interarm regions,in this model. This is in clear distinction to the predictionsof the large–scale shock model.Previous Hα studies of spiral galaxies (Kennicutt 1989,1998a; Kennicutt, Tamblyn & Congdon 1994; and the re-view by Kennicutt 1998b) have mainly looked at globalstar–formation, particularly the form of the Schmidt law(Schmidt 1959, 1963). In this paper we are looking at lo-calised star formation in disc galaxies as well as global prop-erties of star formation, and comparing the distribution ofstar formation with the underlying old stellar population.