Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...317..405s&link_type=abstract
Astronomy and Astrophysics, v.317, p.405-415
Astronomy and Astrophysics
Astrophysics
16
Galaxies: Kinematics And Dynamics, Galaxies: Spiral, Galaxies: Individual: Ngc 157, Galaxies: Sm, Radio Lines: Galaxies
Scientific paper
Numerical simulations of the interstellar medium under the action of a density wave provide an accurate method for determining the positions of the main resonances in grand-design spiral galaxies. Barred spiral galaxies are among the best candidates for a single and well defined wave mode, because bars are standing waves which may share the same pattern speed as the spiral structure. In line with our previous work on determination of the pattern speed in barred spiral galaxies (M 51, NGC 4321 and NGC 7479) by the method employed in this article, we have applied it to the grand-design spiral NGC 157 and compared the results with previous determinations of the position of the resonances in this galaxy. NGC 157 is an interesting case to test the theoretical predictions on star formation and nuclear activity and their relation with the existence and position of the main resonances induced by a spiral density wave: it is an isolated grand-design galaxy and possesses a weak bar, an inner and an outer pseudo-ring and a nuclear starburst. A pattern speed of {OMEGA}_p_=40km/s/kpc is derived from our numerical model and this places the corotation resonance at a radius of R_CR_~5kpc (~50"), in the middle of the disc and close to the point where the two inner arms suffer a bifurcation and broadening. This result differs slightly from the optical determination by Elmegreen et al. (1992ApJS...79...37E) and Elmegreen & Elmegreen (1995ApJ...445..591E), who identified the location of the corotation radius at the endpoint of the ridges of star formation, at a radius R_CR_=0.44R_25_ (~56"). With this pattern speed two inner Lindblad resonances are predicted at radii R_ILR1_=0.25kpc and R_ILR2_=0.75kpc respectively. The bar ends well inside the corotation limited by the disc scale length (~35") and a stellar nuclear oval misaligned with the major axis of the bar could be confined within the ILR_2_. The Hα image features and the starburst nucleus of NGC 157 are related to the global dynamics of the galaxy and therefore to the positions of the resonances. The Hα image shows a ringlike region of star formation located between the corotation and the outer Lindblad resonance (OLR). An inner pseudo-ring appears surrounding the main bar which is almost void of H II regions with the exception of three hot-spots that appear near the centre of the galaxy, one at the nucleus, and the two others at a radius of ~0.4kpc, between the ILR_1_ and the ILR_2_. These circumnuclear Hα features could be the signature of a patchy ring. NGC 157 has been classified as a late-type galaxy (SAB(rs)bc); its kinematical behaviour as well as the distribution of H II regions along the bar and its nuclear starburst are in agreement with recent surveys of star formation in bars (Arsenault, 1989) and classification of bar types in early and late type galaxies (Combes & Elmegreen 1993A&A...271..391C).
Rozas Maite
Sempere Maria J.
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