Molecular gas in the barred spiral M 100. I. The IRAM 30m map.

Details Molecular gas in the barred spiral M 100. I. The IRAM 30m map. Molecular gas in the barred spiral M 100. I. The IRAM 30m map.

Sep 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...325..769s&link_type=abstract

Astronomy and Astrophysics, v.325, p.769-781

Astronomy and Astrophysics

Astrophysics

18

Galaxies: Kinematics And Dynamics Of, Galaxies: Spiral, Galaxies: Individual, Radio Lines

Scientific paper

We present a study on the distribution, kinematics and evolution of the molecular gas disk in the barred SBbc spiral M100(NGC4321). The J=2-1 and J=1-0 lines of ^12^CO have been mapped with the IRAM 30m telescope, with resolutions HPBW=12" and HPBW=21", within the inner 8kpc of the disk (1"=82pc, assuming D_M100_=17.1Mpc). Complementary ^13^CO(1-0) observations of the nucleus, spiral and interarm regions are used to study the nature of H_2_ gas. CO emission traces a markedly asymmetric two spiral-arm structure stretching out from a molecular gas bar. The CO bar has a diameter D~90" and it is aligned with the stellar bar seen in I and K band images (PA=110^o^). Arm I (II) springs off the western (eastern) side of the bar at r~50". Arm II is split up in two armlets (at r~40-50"), it disappears and finally shows up at a position angle close to the major axis northern crossing (PA=330^o^). The splitting starts near the inner 3::1 resonance identified by Elmegreen et al (1992) in the enhanced optical images of M100. Molecular gas in the bar is strongly concentrated in a nuclear disk of r_nucl_~30" and M(H_2_)_nucl_=2.8x10^9^Msun_. Arms I-II display different arm-interarm contrasts (on average, R_a/ia_(CO) increases with radius from 2.5 (r~30") up to 6.5 (r~110"), being higher for arm I than for arm II. Also arms I-II relate differently to other spiral arm tracers, underlying the asymmetry in the disk. Comparison between CO, HI and Halpha_ maps show that there is no coherent sequence in the relative location of the star formation tracers along the spiral arms. Evidences of triggering of star formation along spiral arms are poor: R_a/ia_(Halpha_) is only ~2R_a/ia_(CO) and systematic offsets between Halpha_ and CO ridges (expected to lie downstream and upstream the spiral potential minimum, respectively, assuming trailing arms inside corotation) are hard to find and at places they are absent or even inverted. CO reveals as the best tracer of gas kinematics in the inner disk. The CO rotation curve (V_rot_) is steeper than the curves derived from the HI and Halpha_ data. V_rot_ reaches 200km/s in less than 1kpc. The signature of the m=3 instability has been also identified in the CO derived velocity field. The magnitude and the sign of streaming motions, associated with the spiral arms and the bar, are consistent with the CO disk to be inside corotation (r_cor_=110"). A secondary wave compression develops in the eastern side where arm II is split. Massive star formation (MSF) is inhibited along the gaseous bar, indicating that M100 is an evolved barred system (Friedli and Benz, 1995). Star formation rate (measured as the ratio SFR=F(Halpha_)/I_CO_) is lower for the nuclear disk than for the disk itself. However we suspect the measurement of SFR to be subjected to major uncertainties: the X=N(H_2_)/I_CO_ conversion factor might be 3 times lower than implicitly assumed and to vary within the disk (lower for the interarm than for the arms and nuclear region) . Moreover extinction affects F(Halpha_) mostly in the nucleus where the classical Schmidt law breaks down paradoxically (SFR_ND_~N_gas_alpha^, α~1). MSF is set on at a distance r_c_~12kpc where N(H_2_) approaches the Toomre (1964) gravitational instability threshold (N_c_~7Msun_). Also for r

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