J/A+A/575/A17 NGC 891 Herschel PACS and SPIRE spectroscopy (Hughes+, 2015)
Insights into gas heating and cooling in the disc of NGC 891 from Herschel
far-infrared spectroscopy.
Hughes T.M., Foyle K., Schirm M.R.P., Parkin T.J., De Looze I., Wilson C.D.,
Bendo G.J., Baes M., Fritz J., Boselli A., Cooray A., Cormier D.,
Karczewski O.L., Lebouteiller V., Lu N., Madden S.C., Spinoglio L.,
Viaene S.
<Astron. Astrophys. 575, A17 (2015)>
=2015A&A...575A..17H 2015A&A...575A..17H
ADC_Keywords: Galaxies, nearby ; Galaxies, IR
Keywords: galaxies: individual: NGC 891 - galaxies: spiral - galaxies: ISM -
infrared: galaxies - ISM: lines and bands
Abstract:
We present Herschel PACS and SPIRE spectroscopy of the most important
far- infrared cooling lines in the nearby edge-on spiral galaxy, NGC
891: the [CII] 158, [NII] 122, 205, [OI] 63, 145, and [OIII] 88 micron
lines. We find that the photoelectric heating efficiency of the gas
varies from a mean of 3.5x10-3 in the centre up to 8x10-3 at
increasing radial and vertical distances in the disc. We compare the
observed flux of the FIR cooling lines and total IR emission with the
predicted flux from a PDR model to determine the gas density, surface
temperature and the strength of the incident far-ultraviolet (FUV)
radiation field, G0. Resolving details on physical scales of
∼0.6kpc, a pixel-by-pixel analysis reveals that the majority of the
PDRs in NGC 891's disc have hydrogen densities of 1<log(n/cm-3)<3.5
experiencing an incident FUV radiation field with strengths of
1.7<logG0<3.
Although these values we derive for most of the disc are consistent
with the gas properties found in PDRs in the spiral arms and inter-arm
regions of M51, observed radial trends in n and G0 appear to be
sensitive to varying optical thickness in the lines, demonstrating the
importance of accurately accounting for optical depth effects when
interpreting observations of high inclination systems. With an
empirical relationship between the MIPS 24 micron and [NII] 205 micron
emission, we estimate an enhancement of the FUV radiation field
strength in the far north-eastern side of the disc.
Description:
We present Herschel PACS spectroscopic observations of the [CII] 158,
[NII] 122, [OI] 63, 145, and [OIII] 88 micron fine-structure lines in
NGC 891 taken has part of the Very Nearby Galaxy Survey (P.I.: C.
Wilson). Observations were taken on the 28th February and 1st March,
2011, using the unchopped grating scan mode. They were processed from
Level 0 to Level 1 using the Herschel Interactive Processing
Environment (HIPE) v.9.2 with calibration files FM,41, following the
standard pipeline reduction steps for the unchopped observing mode.
Level 1 cubes were exported to PACSman v.3.5.2 (Lebouteiller,
2012A&A...548A..91L 2012A&A...548A..91L), where each individual spaxel's spectrum is fit
with a Gaussian function and second order polynomial for the line and
continuum baseline, respectively. Intensity maps of the integrated
flux are created from the individual rasters, also using PACSman, by
projecting the rasters onto a common, over-sampled grid with a 3.133
arcsec pixel size. Complete details on the data reduction may be found
in the paper.
Our data files are summarised in cubes.dat. Each file is a FITS cube
with 8 planes, where each plane contains the following maps: (1.)
integrated intensity map in units of W/m2/sr, (2.) integrated
intensity error map in units of W/m2/sr, (3.) velocity map in units
of km/s, (4.) velocity error map in units of km/s, (5.) full-width
at half-maximum map of the line fit in units of km/s, (6.)
full-width at half-maximum error map in units of km/s, (7.)
continuum map in Jy, and (8.) continuum error map in units of Jy.
Please note that the observations consist of raster maps and strips
for the [CII], [NII] and [OI] 63 micron lines, and only strips for the
remaining lines. The raw data can be retrieved from the Herschel
Science Archive (ESA) with the stated OBSIDs.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
cubes.dat 106 8 Summary of images
cubes/* . 8 Individual FITS cubes containing the reduced maps
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See also:
J/MNRAS/396/1231 : Astrophotometric catalogue of NGC 891 (Rejkuba+, 2009)
J/A+A/565/A4 : NGC 891 70-500um images (Hughes+, 2014)
Byte-by-byte Description of file: cubes.dat
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Bytes Format Units Label Explanations
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1- 33 A33 --- Filename Name of the FITS file, in subdirectory cubes
35- 40 A6 --- Line Fitted line
42- 48 F7.3 um lambda [63/158] Wavelength
50- 59 I10 --- OBSID Observation ID
61- 64 F4.1 arcsec FWHM PSF FWHM
66- 70 I5 s ExpTime Exposure time
72- 80 F9.5 deg RAdeg Right Ascension of center (J2000)
81- 89 F9.5 deg DEdeg Declination of center (J2000)
91- 94 F4.2 arcsec/pix Scale Scale of the image
96- 97 I2 --- Nx [41/60] Number of pixels along X-axis
99-100 I2 --- Ny [49/72] Number of pixels along Y-axis
102 I1 --- Nz [8/8] Number of slices
104-106 I3 Kibyte size [70/138] Size of FITS file
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Acknowledgements:
Thomas M. Hughes,
(End) Thomas M. Hughes [UGent, Belgium], Patricia Vannier [CDS] 27-Oct-2014