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تسجيل مستخدم جديدA dust component 2 kpc above the plane in NGC 891
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The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in H$alpha$ emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time. We suggest that NGC 891 is intrinsically almost symmetric and the large asymmetry in H$alpha$ emission is mostly due to dust attenuation. We will quantify the additional optical depth needed to cause the observed asymmetry in this model. By comparing large strips on the North East side of the galaxy with strips covering the same area in the South West we can quantify and analyze the asymmetry in the different wavelengths. From the 24 $mu$m emission we find that the intrinsic asymmetry in star formation in NGC 891 is small i.e., $sim 30%$. The additional asymmetry in H$alpha$ is modeled as additional symmetric dust attenuation which extends up to $sim$ 40arcsec (1.9 kpc) above the plane of the galaxy with a mid-plane value of $tau$=0.8 and a scale height of 0.5 kpc
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We compare 850um SCUBA images of NGC 891 with the corresponding V-band optical depth predicted from radiation transfer simulations. These two tracers of dust show a very similar distribution along the minor axis and a reasonable agreement along the m
ajor axis. Assuming that the grains responsible for optical extinction are also the source of 850um emission we derive a submillimeter emissivity (emission efficiency) for dust in the NGC 891 disk. This quantity is found to be a factor of 2-3 higher than the generally-accepted (but highly uncertain) values adopted for the Milky Way. It should be stated, however, that if a substantial fraction of dust in NGC 891 is clumped, the emissivity in the two galaxies may be quite similar. We use our newly-acquired emissivity to convert our 850um images into detailed maps of dust mass and, utilizing 21cm and CO-emission data for NGC 891, derive the gas-to-dust ratio along the disk. We compute an average ratio of 260 -- a value consistent with the Milky Way and external spirals within the uncertainties in deriving both the dust mass and the quantity of molecular gas. The bulk of dust in NGC 891 appears to be closely associated with the molecular gas phase although it may start to follow the distribution of atomic hydrogen at radii >9 kpc (i.e. >0.5 R_25). Using the optical depth of the NGC 891 disk, we quantify how light emitted at high redshift is attenuated by dust residing in foreground spirals. For B-band observations of galaxies typically found in the Hubble Deep Field, the amount of light lost is expected to be small (~ 5%). This value depends critically on the maximum radial extent of cold dust in spiral disks (which is poorly known). It may also represent a lower limit if galaxies expel dust over time into the intergalactic medium.
We analyse Herschel/SPIRE images of the edge-on spiral galaxy NGC 891 at 250, 350 and 500 micron. Using a 3D radiative transfer model we confirm that the dust has a radial fall-off similar to the stellar disk. The dust disk shows a break at about 12
kpc from the center, where the profile becomes steeper. Beyond this break, emission can be traced up to 90% of the optical disk in the NE side. On the SW, we confirm dust emission associated with the extended, asymmetric HI disk, previously detected by the Infrared Space Observatory (ISO). This emission is marginally consistent with the large diffuse dust disk inferred from radiative transfer fits to optical images. No excess emission is found above the plane beyond that of the thin, unresolved, disk.
We present deep far-infrared observations of the nearby edge-on galaxy NGC 891 obtained with the Herschel Space Observatory and the Spitzer Space Telescope. The maps confirm the detection of thermal emission from the inner circumgalactic medium (halo
) and spatially resolve a dusty superbubble and a dust spur (filament). The dust temperature of the halo component is lower than that of the disk but increases across a region of diameter ~8.0 kpc extending at least 7.7 kpc vertically from one side of the disk, a region we call a superbubble because of its association with thermal X-ray emission and a minimum in the synchrotron scaleheight. This outflow is breaking through the thick disk and developing into a galactic wind, which is of particular interest because NGC 891 is not considered a starburst galaxy; the star formation rate surface density, 0.03 Msun/year per square kiloparsec, and gas fraction, just 10% in the inner disk, indicate the threshold for wind formation is lower than previous work has suggested. We conclude that the star formation surface density is sufficient for superbubble blowout into the halo, but the cosmic ray electrons may play a critical role in determining whether this outflow develops into a fountain or escapes from the gravitational potential. The high dust-to-gas ratio in the dust spur suggests the material was pulled out of NGC 891 through the collision of a minihalo with the disk of NGC 891. We conclude that NGC 891 offers an example of both feedback and satellite interactions transporting dust into the halo of a typical galaxy.
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