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Register a new userA SOFIA Survey of [CII] in the galaxy M51 II. [CII] and CO kinematics across spiral arms
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We present the first complete, velocity-resolved [CII] 158um image of the M51 grand-design spiral galaxy, observed with the upGREAT instrument on SOFIA. [CII] is an important tracer of various phases of the interstellar medium (ISM), including ionized gas, neutral atomic, and diffuse molecular regions. We combine the [CII] data with HI, CO, 24um dust continuum, FUV, and near-infrared K-band observations to study the evolution of the ISM across M51s spiral arms in both position-position, and position-velocity space. Our data show strong velocity gradients in HI, 12CO, and [CII] at the locations of stellar arms (traced by K--band data) with a clear offset in position-velocity space between upstream molecular gas (traced by 12CO) and downstream star formation (traced by [CII]). We compare the observed position--velocity maps across spiral arms with synthetic observations from numerical simulations of galaxies with both dynamical and quasi-stationary steady spiral arms that predict both tangential and radial velocities at the location of spiral arms. We find that our observations, based on the observed velocity gradients and associated offset between CO and [CII], are consistent with the presence of shocks in spiral arms in the inner parts of M51 and in the arm connecting the companion galaxy, M51b, in the outer parts of M51.
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We present a [CII] 158um map of the entire M51 (including M51b) grand--design spiral galaxy observed with the FIFI-LS instrument on SOFIA. We compare the [CII] emission with the total far--infrared (TIR) intensity and star formation rate(SFR) surface density maps (derived using H_alpha and 24um emission) to study the relationship between [CII] and the star formation activity in a variety of environments within M51 on scales of 16 corresponding to ~660 pc. We find that [CII] and the SFR surface density are well correlated in the central, spiral arm, and inter-arm regions. The correlation is in good agreement with that found for a larger sample of nearby galaxies at kpc scales. We find that the SFR, and [CII] and TIR luminosities in M51 are dominated by the extended emission in M51s disk. The companion galaxy M51b, however, shows a deficit of [CII] emission compared with the TIR emission and SFR surface density, with [CII] emission detected only in the S-W part of this galaxy. The [CII] deficit is associated with an enhanced dust temperature in this galaxy. We interpret the faint [CII] emission in M51b to be a result of suppressed star formation in this galaxy, while the bright mid- and far-infrared emission, which drive the TIR and SFR values, are powered by other mechanisms. A similar but less pronounced effect is seen at the location of the black hole in M51s center. The observed [CII] deficit in M51b suggests that this galaxy is a valuable laboratory to study the origin of the apparent [CII] deficit observed in ultra-luminous galaxies.
We present SOFIA/FIFI-LS observations of the [CII] 158${mu}$m cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO, and H I data to compare [CII] emission to dust properties, star formation rate (SFR), H$_2$, and HI at 560pc scales via stacking by environment (spiral arms, interarm, and center), radial profiles, and individual, beam-sized measurements. We attribute $73%$ of the [CII] luminosity to arms, and $19%$ and $8%$ to the center and interarm region, respectively. [CII]/TIR, [CII]/CO, and [CII]/PAH radial profiles are largely constant, but rise at large radii ($gtrsim$8kpc) and drop in the center ([CII] deficit). This increase at large radii and the observed decline with the 70${mu}$m/100${mu}$m dust color are likely driven by radiation field hardness. We find a near proportional [CII]-SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. [CII] also becomes increasingly luminous relative to CO at low SFR (interarm or large radii), likely indicating more efficient photodissociation of CO and emphasizing the importance of [CII] as an H$_2$ and SFR tracer in such regimes. Finally, based on the observed [CII] and CO radial profiles and different models, we find ${alpha}_{CO}$ to increase with radius, in line with the observed metallicity gradient. The low ${alpha}_{CO}$ (galaxy average $lesssim2,M_{sun},pc^{-2},(K,km,s^{-1})^{-1}$) and low [CII]/CO ratios ($sim$400 on average) imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.
We present new SOFIA [CII] and ALMA CO(J=1-0) observations of the nearby asymmetric barred spiral galaxy NGC 7479. The data, which cover the whole bar of the galaxy and the counter-arms visible in the radio continuum, are analyzed in conjunction with a wealth of existing visible, infrared, radio, and X-ray data. As in most normal galaxies, the [CII] emission is generally consistent with emission from cooling gas excited by photoelectric heating in photo-dissociation regions. However, anomalously high [CII]/CO ratios are seen at the two ends of the counter-arms. Both ends show shell-like structures, possibly bubbles, in H-alpha emission. In addition, the southern end has [CII] to infrared emission ratios inconsistent with normal star formation. Because there is little HI emission at this location, the [CII] emission probably originates in warm shocked molecular gas heated by the interaction of the radio jet forming the counter-arms with the interstellar medium in the galaxy. At two other locations, the high [CII]/CO ratios provide evidence for the existence of patches of CO-dark molecular gas. The [CII] and CO observations also reveal resolved velocity components along the bar. In particular, the CO emission can be separated into two components associated to gas along the leading edge of the bar and gas trailing the bar. The trailing gas component that amounts to approximately 40% of the gas around the bar region may be related to a minor merger.
(Abridged) We use new multi-wavelength radio observations, made with the VLA and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51 on scales from $200pc$ to several $kpc$. Interferometric and single dish data are combined to obtain new maps at wwav{3}{6} in total and polarized emission, and earlier wav{20} data are re-reduced. We compare the spatial distribution of the radio emission with observations of the neutral gas, derive radio spectral index and Faraday depolarization maps, and model the large-scale variation in Faraday rotation in order to deduce the structure of the regular magnetic field. We find that the wav{20} emission from the disc is severely depolarized and that a dominating fraction of the observed polarized emission at wav{6} must be due to anisotropic small-scale magnetic fields. Taking this into account, we derive two components for the regular magnetic field in this galaxy: the disc is dominated by a combination of azimuthal modes, $m=0+2$, but in the halo only an $m=1$ mode is required to fit the observations. We disuss how the observed arm-interarm contrast in radio intensities can be reconciled with evidence for strong gas compression in the spiral shocks. The average arm--interam contrast, representative of the radii $r>2kpc$ where the spiral arms are broader, is not compatible with straightforward compression: lower arm--interarm contrasts than expected may be due to resolution effects and emph{decompression} of the magnetic field as it leaves the arms. We suggest a simple method to estimate the turbulent scale in the magneto-ionic medium from the dependence of the standard deviation of the observed Faraday rotation measure on resolution. We thus obtain an estimate of $50pc$ for the size of the turbulent eddies.
Theoretical studies on the response of interstellar gas to a gravitational potential disc with a quasi-stationary spiral arm pattern suggest that the gas experiences a sudden compression due to standing shock waves at spiral arms. This mechanism, called a galactic shock wave, predicts that gas spiral arms move from downstream to upstream of stellar arms with increasing radius inside a corotation radius. In order to investigate if this mechanism is at work in the grand-design spiral galaxy M51, we have measured azimuthal offsets between the peaks of stellar mass and gas mass distributions in its two spiral arms. The stellar mass distribution is created by the spatially resolved spectral energy distribution fitting to optical and near infrared images, while the gas mass distribution is obtained by high-resolution CO and HI data. For the inner region (r < 150), we find that one arm is consistent with the galactic shock while the other is not. For the outer region, results are less certain due to the narrower range of offset values, the weakness of stellar arms, and the smaller number of successful offset measurements. The results suggest that the nature of two inner spiral arms are different, which is likely due to an interaction with the companion galaxy.
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