The ionized gas in the center and in the bar of the spiral galaxy NGC 6946

Roy, Jean-René; Belley, Julien

doi:10.1086/172420

Cited by 10 publications

(6 citation statements)

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“…These maps show a central emission peak, an extended disk, and some evidence for spiral 15 structure in the disk emission. Our higher resolution data set a limit of ≤24 ′′ × ≤34 ′′ (≤525 pc × ≤710 pc) FWHM on the size of the central 100 µm source, consistent with the size of the central CO (Ishizuki et al 1990) and Hα (Roy and Belley 1993) source. We find that 20% of the total 100 µm flux density of NGC 6946 comes from this central source.…”

Section: Ngc 6946supporting

confidence: 78%

High Spatial Resolution KAO Far-Infrared Observations of the Central Regions of Infrared-bright Galaxies

Smith

Harvey

1996

ApJ

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We present new high spatial resolution Kuiper Airborne Observatory (KAO) 50 µm and/or 100 µm data for 11 infrared-bright galaxies. The sizes of the central far-infrared emitting regions in three of these galaxies, NGC 660, NGC 891, and M 83, agree with those of the central star formation complexes. The Sb galaxy NGC 7331, which does not have observed star formation in its bulge, has a flat-topped 100 µm major axis profile which covers the bulge and inner spiral arms, without a bright central peak or pronounced depression. The remaining seven galaxies, all of which are known to have strong nuclear or circumnuclear star formation, are unresolved or marginally resolved with the KAO, with farinfrared source size limits consistent with the sizes of the central star formation complexes.Along with these new data, we have tabulated previously published KAO data for 11 other galaxies, and IRAS 60 and 100 µm data for the bulges of the large angular size galaxies M 31 and M 81. From the literature, we have compiled optical, near-infrared, and millimeter measurements for the central regions of the entire set of 24 galaxies. We have used this dataset to investigate dust heating and star formation in the central areas of galaxies. We find that L(FIR)/L(B) and L(FIR)/L(H) correlate with CO (1 − 0) intensity and 100 µm optical depth. Galaxies with optical or near-infrared signatures of OB star formation in their central regions have higher values of I(CO) and τ 100 than more quiescent galaxies, as well as higher far-infrared surface brightnesses and L(FIR)/L(B) and L(FIR)/L(H) ratios. The L(FIR)/L(Hα) ratio does not correlate strongly with CO and τ 100 . These data are consistent with a scenario in which OB stars dominate dust heating in the more active galaxies and older stars are important for the more quiescent bulges. Whether or not a galaxy bulge has strong star formation may be decided by a threshold effect; star forming galaxies have surface

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Section: Ngc 6946supporting

confidence: 78%

High Spatial Resolution KAO Far-Infrared Observations of the Central Regions of Infrared-bright Galaxies

Smith

Harvey

1996

ApJ

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“…Although NGC 6946 has only a small bar (Elmegreen et al 1998), Ishizuki et al (1990) present evidence for an efficient inflow of gas into the central 11 , fueling the activity observed there (Turner & Ho 1983;DeGioia-Eastwood 1985;Roy & Belley 1993;Elmegreen et al 1998) which has been interpreted as a mild or post starburst (Tacconi & Young 1990;Engelbracht et al 1996;Ptak et al 1999). Devereux & Young (1993) discuss the origin of the bright far-IR emission in NGC 6946 (Telesco & Harper 1980;Engargiola 1991) which, at 5 × 10 10 L , is amongst the highest measured in a nearby spiral galaxy (Devereux & Young 1993).…”

Section: Introductionmentioning

confidence: 92%

Molecular gas in NGC 6946

Walsh

Beck

Thuma

et al. 2002

A&A

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Abstract. We present imaging of molecular gas emission in the star-forming spiral galaxy NGC 6946. Our 12 CO(1-0) and 12 CO(3-2) images, made at 22 resolution with the IRAM 30-m and the Heinrich Hertz 10-m radio telescopes, are the most extensive observations of molecular gas in this galaxy and are among the most extensive observations of molecular gas in any spiral galaxy. The 12 CO(1-0) emission shows a central concentration aligned in a north-south ellipse, an extended diffuse component plus concentrations in the outer spiral arms.12 CO(1-0) is detected across most of the optical disk out to a projected radius of nearly 300 . The molecular component in NGC 6946 is unusually massive, with MH 2 /MHI = 0.57. The scale length of the disk in 12 CO(1-0) is the same, to within 5%, as the VRI, Hα, 15 µm, and FIR disks. The 12 CO(3-2) data shows broadly similar structure to the 12 CO(1-0) image out to the observed limit of 200 , although the arm-interarm contrast of 12 CO(3-2) is significantly larger than that of 12 CO(1-0) which suggests that molecules in the spiral arms are warmer or reside longer there. The rotation curve derived from the 12 CO(1-0) velocity field agrees, within the uncertainties, with the H i rotation curve. The velocity dispersion is 40 ± 10 km s −1 in the inner 2 kpc and 8 ± 3 km s −1 in the disk, without tendency to increase in the spiral arms. Spectra of the 13 CO(1-0), 13 CO(2-1), 12 C 18 O(1-0) isotopomers for several positions are used for line ratio studies of the state of the molecular ISM in NGC 6946. In the centre of NGC 6946 our LVG analysis suggests that the beam-averaged gas kinetic temperature is 40 ± 5 K, the molecular gas density is (3.3 ± 0.3) × 10 3 cm −3and that line opacities are high, with τ12 CO(3−2) ∼ 10. A star formation efficiency image for NGC 6946, made from the Hα image divided by the molecular gas image, ranges by over two orders of magnitude with highest values found in the northeastern spiral arm. The λ6 cm polarized emission image, which traces the regular part of the magnetic field, appears anti-correlated with the star formation efficiency. We present an analysis of the ISM in NGC 6946's disk by making 1-D and 2-D comparisons of images made in several wavebands. Using a point-by-point correlation technique, we investigate the distribution and kinematics of the molecular gas and its relation to the neutral and ionized gas, the mid-infrared-emitting dust, the radio continuum and the magnetic field, and find that the molecular gas is closely associated with the 7 µm-emitting dust. The highest correlation between any pair of tracers is found between the mid-infrared emission and the total radio continuum emission at λ6 cm. This cannot be due to dust heating and gas ionization in star-forming regions because the thermal radio emission is less correlated with the mid-infrared emission than the nonthermal emission. A coupling of magnetic fields to gas clouds is proposed as a possible scenario.

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“…Vila-Costas and Edmunds (1992) found that the global abundance gradients of barred spirals are in general shallower than that of non-barred spirals. For NGC 6946, the abundance gradient is apparently shallower in the bar region than in the outer region (Roy, Belley 1993). Furthermore, Martin and Roy (1994) found that the gradients are flatter when the length or the ellipticity of the bar increases.…”

Section: Correlation Between Central Concentration and Bar Strengthmentioning

confidence: 98%

Nobeyama CO Atlas of Nearby Spiral Galaxies: Distribution of Molecular Gas in Barred and Nonbarred Spiral Galaxies

Kuno¹,

Sato²,

Nakanishi³

et al. 2007

Publications of the Astronomical Society of Japan

187

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The data from a CO(1 -0) mapping survey of 40 nearby spiral galaxies performed with the Nobeyama 45-m telescope are presented. The criteria of the sample selection were (1) RC3 morphological type in the range Sa to Scd, (2) distance less than 25 Mpc, (3) inclination angle less than 79 • (RC3), (4) flux at 100 µm higher than ∼ 10 Jy, (5) spiral structure is not destroyed by interaction. The maps of CO cover most of the optical disk of the galaxies. We investigated the influence of bar on the distribution of molecular gas in spiral galaxies using these data. We confirmed that the degree of central concentration is higher in barred spirals than in non-barred spirals as shown by the previous works. Furthermore, we present an observational evidence that bars are efficient in driving molecular gas that lies within the bar length toward the center, while the role in bringing gas in from the outer parts of the disks is small. The transported gas accounts for about half of molecular gas within the central region in barred spiral galaxies. We found a correlation between the degree of central concentration and bar strength. Galaxies with stronger bars tend to have higher central concentration. The correlation implies that stronger bars accumulate molecular gas toward the center more efficiently. These results are consistent with long-lived bars.

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The ionized gas in the center and in the bar of the spiral galaxy NGC 6946

Cited by 10 publications

References 0 publications

High Spatial Resolution KAO Far-Infrared Observations of the Central Regions of Infrared-bright Galaxies

High Spatial Resolution KAO Far-Infrared Observations of the Central Regions of Infrared-bright Galaxies

Molecular gas in NGC 6946

Nobeyama CO Atlas of Nearby Spiral Galaxies: Distribution of Molecular Gas in Barred and Nonbarred Spiral Galaxies

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