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Posts: 131433
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Caldwell 23
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ngc891



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RE: NGC 891
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Title: A resolved analysis of cold dust and gas in the nearby edge-on spiral NGC 891
Author: T. M. Hughes, M. Baes, J. Fritz, M. W. L. Smith, T. J. Parkin, G. Gentile, G. J. Bendo, C. D. Wilson, F. Allaert, S. Bianchi, I. De Looze, J. Verstappen, S. Viaene, M. Boquien, A. Boselli, D. L. Clements, J. I. Davies, M. Galametz, S. C. Madden, A. Remy-Ruyer, L. Spinoglio

We investigate the connection between dust and gas in the nearby edge-on spiral galaxy NGC 891. High resolution Herschel PACS and SPIRE 70, 100, 160, 250, 350, and 500 m images are combined with JCMT SCUBA 850 m observations to trace the far-infrared/submillimetre spectral energy distribution (SED). Maps of the HI 21 cm line and CO(J=3-2) emission trace the atomic and molecular hydrogen gas, respectively. We fit one-component modified blackbody models to the integrated SED, finding a global dust mass of 8.5x107 solar masses and an average temperature of 23±2 K. We also fit the pixel-by-pixel SEDs to produce maps of the dust mass and temperature. The dust mass distribution correlates with the total stellar population as traced by the 3.6 m emission. The derived dust temperature, which ranges from approximately 17 to 24 K, is found to correlate with the 24 m emission. Allowing the dust emissivity index to vary, we find an average value of = 1.9±0.3. We confirm an inverse relation between the dust emissivity spectral index and dust temperature, but do not observe any variation of this relationship with vertical height from the mid-plane of the disk. A comparison of the dust properties with the gaseous components of the ISM reveals strong spatial correlations between the surface mass densities of dust and the molecular hydrogen and total gas surface densities. Observed asymmetries in the dust temperature, and the H2-to-dust and total gas-to-dust ratios hint that an enhancement in the star formation rate may be the result of larger quantities of molecular gas available to fuel star formation in the NE compared to the SW. Whilst the asymmetry likely arises from dust obscuration due to the geometry of the line-of-sight projection of the spiral arms, we cannot exclude an enhancement in the star formation rate in the NE side of the disk.

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Posts: 131433
Date:
PGC 9031
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NGC 891 (also Caldwell 23, IRAS 02195+4209, MCG 7-5-46, UGC 1831 and PGC 9031) is a magnitude +9.9 edge-on spiral galaxy located 24 million light-years away in the constellation Andromeda.

The galaxy hosted Supernova 1986J

The galaxy was discovered by German-British astronomer William Herschel using a 47.5 cm (18.7 Inch) speculum reflecting telescope at Datchet on October 6, 1784

 RA  02h 22m 33.0s, Dec +42° 20' 50"

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RE: NGC 891
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Hubble Spies Edge-on Beauty

closeup of ngc 891 showing dark filaments escaping the galactic halo
Credit: ESA/Hubble & NASA 

Visible in the constellation of Andromeda, NGC 891 is located approximately 30 million light-years away from Earth. The NASA/ESA Hubble Space Telescope turned its powerful wide field Advanced Camera for Surveys towards this spiral galaxy and took this close-up of its northern half. The galaxy's central bulge is just out of the image on the bottom left.

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Title: The 3-Dimensional Structure of NGC 891 and M51
Authors: Andrew Schechtman-Rook, Matthew A. Bershady, Kenneth Wood, Thomas P. Robitaille

We investigate the three-dimensional structure of the nearby edge-on spiral galaxy NGC 891 using 3D Monte Carlo radiative transfer models, with realistic spiral structure and fractally clumped dust. Using the spiral and clumpiness parameters found from recently completed scattered light models we produce lower resolution SED models which reproduce the global UV-to-FIR SED of NGC 891. Our models contain a colour gradient across the major axis of the galaxy - similar to what is seen in images of the NGC 891. With minor adjustment our SED models are able to match the majority of M51's SED, a similar galaxy at a near face-on different inclination.

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Title: A New Ultraluminous X-ray Source in the Nearby Edge-on Spiral NGC 891
Authors: Edmund J. Hodges-Kluck, Joel N. Bregman, Jon M. Miller, Eric W. Pellegrini

We report the discovery of a new candidate ultraluminous X-ray source (ULX) in the nearby edge-on spiral galaxy NGC 891. The source, which has an absorbed flux of F_X ~ 10^-12 erg/s/cm² (corresponding to L_X > 10^40 erg/s at 9 Mpc), must have begun its outburst in the past 5 years as it is not detected in prior X-ray observations between 1986 and 2006. We try empirical fits to the XMM-Newton spectrum, finding that the spectrum is fit very well as emission from a hot disk, a cool irradiated disk, or blurred reflection from the innermost region of the disk. The simplest physically motivated model with an excellent fit is a hot disk around a stellar-mass black hole (a super-Eddington outburst), but equally good fits are found for each model. We suggest several follow-up experiments that could falsify these models.

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Title: A Multi-Wavelength Infrared Study of NGC 891
Authors: C. Whaley, J. Irwin, S. Madden, F. Galliano, G. Bendo

We present a multi-wavelength infrared study of the nearby, edge-on, spiral galaxy NGC 891. We have examined 20 independent, spatially resolved IR images of this galaxy, 14 of which are newly reduced and/or previously unpublished images. These images span a wavelength regime from 1.2 microns in which the emission is dominated by cool stars, through the MIR, in which emission is dominated by PAHs, to 850 microns, in which emission is dominated by cold dust in thermal equilibrium with the radiation field. The changing morphology of the galaxy with wavelength illustrates the changing dominant components. We detect extra-planar dust emission in this galaxy, consistent with previously published results, but now show that PAH emission is also in the halo, to a vertical distance of z >= 2.5 kpc. We compare the vertical extents of various components and find that the PAHs (from 7.7 and 8 micron data) and warm dust (24 microns) extend to smaller z heights than the cool dust (450 microns). For six locations in the galaxy for which the S/N was sufficient, we present SEDs of the IR emission, including two in the halo - the first time a halo SED in an external galaxy has been presented. We have modelled these SEDs and find that the PAH fraction is similar to Galactic values (within a factor of two), with the lowest value at the galaxy's center, consistent with independent results of other galaxies. In the halo environment, the fraction of dust exposed to a colder radiation field, is of order unity, consistent with an environment in which there is no star formation. The source of excitation is likely from photons escaping from the disk.

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NGC 891 Globular Cluster Candidates
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Title: Globular Cluster Candidates in NGC 891
Authors: W. E. Harris, M. Mouhcine, M. Rejkuba, R. Ibata

We use deep images taken with the Advanced Camera for Surveys on board the Hubble Space Telescope of the disk galaxy NGC 891, to search for globular cluster candidates. This galaxy has long been considered to be a close analogue in size and structure to the Milky Way and is nearly edge-on, facilitating studies of its halo population. These extraplanar ACS images, originally intended to study the halo field-star populations, reach deep enough to reveal even the faintest globular clusters that would be similar to those in the Milky Way. From the three pointings we have identified a total of 43 candidates after culling by object morphology, magnitude, and colour. We present (V,I) photometry for all of these, along with measurements of their effective radius and ellipticity. The 16 highest-rank candidates within the whole sample are found to fall in very much the same regions of parameter space occupied by the classic Milky Way globular clusters. Our provisional conclusion from this survey is that the total globular cluster population in NGC 891 as a whole may be almost as large as that of the Milky Way.

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Posts: 131433
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NGC 891
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Title: Lopsided galaxies: the case of NGC 891
Authors: M. Mapelli (1), B. Moore (1), J. Bland-Hawthorn (2) ((1) University of Zürich, (2) University of Sydney)

It has been known for a long time that a large fraction of disc galaxies are lopsided. We simulate three different mechanisms that can induce lopsidedness: flyby interactions, gas accretion from cosmological filaments and ram pressure from the intergalactic medium. Comparing the morphologies, HI spectrum, kinematics and m=1 Fourier components, we find that all of these mechanisms can induce lopsidedness in galaxies, although in different degrees and with observable consequences. The timescale over which lopsidedness persists suggests that flybys can contribute to ~20 per cent of lopsided galaxies. We focus our detailed comparison on the case of NGC 891, a lopsided, edge-on galaxy with a nearby companion (UGC 1807). We find that the main properties of NGC 891 (morphology, HI spectrum, rotation curve, existence of a gaseous filament pointing towards UGC 1807) favour a flyby event for the origin of lopsidedness in this galaxy.

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