madbadgalaxyman
29-03-2011, 10:40 PM
NGC 4945 may not be "pretty", but for those of us who like enigmatic and mysterious objects that are hard to understand, this galaxy fits the bill.
This galaxy is fairly obviously two-armed, but the spiral arms and all other features are highly foreshortened and confused due to the near edge-on orientation of this system.
The two primary Spiral arms are more obvious in the near-infrared image from 2MASS, which I attach here:
91390
However, the overall structure of this galaxy is still highly uncertain, even at infrared wavelengths where the heavy dust distribution causes much less confusion.
The presence of heavy extinction from the dust in this galaxy is very obvious at visual wavelengths; see for instance the very fine LRGB images of NGC 4945 that are found at:
http://www.spiegelteam.de (http://www.spiegelteam.de/)
http://dg-imaging.astrodon.com (http://dg-imaging.astrodon.com/)
http://panther-observatory.com (http://panther-observatory.com/)
An excellent Amateur wide-field showing the environment of NGC 4945 is viewable as the APOD for January 9th, 2009.
An excellent “large Schmidt” image of N4945 and its environment can be found at www.eso.org (http://www.eso.org/) ( just search for the public image with this ID : eso0931b )
The interesting companion galaxies NGC 4976 (which is an S0/E or E/S0 galaxy that is near to the border between the Elliptical Galaxy morphological class and the S0 morphological class) and NGC 4945A are both at significantly higher redshift...... so it is unlikely that they are truly physically associated with NGC 4945.
( However there is a candidate streamer or tail of light emanating from NGC 4976........ but I have not confirmed its existence, as yet. )
These fine images of NGC 4945 provide evidence for the presence of a very actively star-forming disk component & spiral arms; note the dust streamers and chimneys and filaments that exit from one side of the major axis. In Malin & Sofue's high-resolution Optical study of the very heavy dust distribution seen within NGC 253, this sort of active disk - with its “smokelike” dust morphology - was referred to as a “boiling & steaming disk”.
Another interesting aspect of the morphology of NGC 4945 is the prevalence of semi-chaotic features and the fact that there is a significant degree of apparent asymmetry in the overall shape of this galaxy. The degree of asymmetry that is found in this galaxy would be somewhat too high if N4945 were an unperturbed galaxy of the Hubble type Sc, but the observed amount of asymmetry is about what we might expect for a galaxy that is somewhat later in the Hubble Sequence; many galaxies at about Hubble type Sd are very obviously entropic and semi-chaotic in their appearance.
Sd galaxies are nearly always less luminous systems than those Beautiful & Symmetric giant spiral galaxies that are favourite imaging targets; Gerard de Vaucouleurs referred to Sd galaxies as “subgiant” galaxies, which is a useful description that has never caught on.
Some further insight into the true structure of this enigmatic galaxy can be found in H-alpha images of it. I attach three Hydrogen alpha images of this galaxy.
The following H-alpha image was taken with the Magellan Baade telescope on June 8-10, 2006 :
91391
The second Ha image comes from the PhD thesis of Jorn Rossa
(2001)(Ruhr-Universitat Bochum) ;
this .jpg file has an R-band image at the top and an H-alpha (continuum subtracted)(H-alpha line only) image at the bottom :
91392
In Rossa's H-alpha image, note the “feathering” and streamers in the H-alpha emission on one side of the disk. This sort of appearance in Ha images is often due to ionized gas that is streaming out of a spiral galaxy that is very actively forming stars; the combined energies of numerous OB stars and multiple supernovae can push gas well out of the plane of a galaxy.
The following H-alpha image is taken from the reference (2004), ApJ, 606, 829 .
[ X-rays display as Blue, narrowband light from the Hydrogen-alpha line displays as Red, and broadband red (R-band) light displays as Green. ]
91393
In this composite three-colour image, the distribution of the H-alpha light, which is always closely associated with the spiral arms in a galaxy, hints that the spiral arms of NGC 4945 might actually be quite regular, despite the rather chaotic appearance of this galaxy that is created by the numerous heavy dust clouds.
And what of the fact that NGC 4945 seems to depart somewhat from its principal plane, at both ends of its major axis? Departures from the principal plane of a galaxy are very commonly seen in images of edge-on spiral galaxies ; these features are usually explained by hypothesizing the existence of either a warp (a genuine bend in the plane of a galaxy) or an outer spiral arm opening.
If you search through the professional literature of astronomy, you will find that there has been very little study of the overall structure of NGC 4945. For the most part, professional astronomers have been primarily interested in the extremely active central and circum-nuclear regions of this galaxy.
Remarkably little solid information exists about the large-scale structure of this galaxy!
Deep and high-resolution near-infrared imaging would probably reveal the true structure of this puzzling object; has anyone ever seen a really good NIR image of NGC 4945??
This galaxy is fairly obviously two-armed, but the spiral arms and all other features are highly foreshortened and confused due to the near edge-on orientation of this system.
The two primary Spiral arms are more obvious in the near-infrared image from 2MASS, which I attach here:
91390
However, the overall structure of this galaxy is still highly uncertain, even at infrared wavelengths where the heavy dust distribution causes much less confusion.
The presence of heavy extinction from the dust in this galaxy is very obvious at visual wavelengths; see for instance the very fine LRGB images of NGC 4945 that are found at:
http://www.spiegelteam.de (http://www.spiegelteam.de/)
http://dg-imaging.astrodon.com (http://dg-imaging.astrodon.com/)
http://panther-observatory.com (http://panther-observatory.com/)
An excellent Amateur wide-field showing the environment of NGC 4945 is viewable as the APOD for January 9th, 2009.
An excellent “large Schmidt” image of N4945 and its environment can be found at www.eso.org (http://www.eso.org/) ( just search for the public image with this ID : eso0931b )
The interesting companion galaxies NGC 4976 (which is an S0/E or E/S0 galaxy that is near to the border between the Elliptical Galaxy morphological class and the S0 morphological class) and NGC 4945A are both at significantly higher redshift...... so it is unlikely that they are truly physically associated with NGC 4945.
( However there is a candidate streamer or tail of light emanating from NGC 4976........ but I have not confirmed its existence, as yet. )
These fine images of NGC 4945 provide evidence for the presence of a very actively star-forming disk component & spiral arms; note the dust streamers and chimneys and filaments that exit from one side of the major axis. In Malin & Sofue's high-resolution Optical study of the very heavy dust distribution seen within NGC 253, this sort of active disk - with its “smokelike” dust morphology - was referred to as a “boiling & steaming disk”.
Another interesting aspect of the morphology of NGC 4945 is the prevalence of semi-chaotic features and the fact that there is a significant degree of apparent asymmetry in the overall shape of this galaxy. The degree of asymmetry that is found in this galaxy would be somewhat too high if N4945 were an unperturbed galaxy of the Hubble type Sc, but the observed amount of asymmetry is about what we might expect for a galaxy that is somewhat later in the Hubble Sequence; many galaxies at about Hubble type Sd are very obviously entropic and semi-chaotic in their appearance.
Sd galaxies are nearly always less luminous systems than those Beautiful & Symmetric giant spiral galaxies that are favourite imaging targets; Gerard de Vaucouleurs referred to Sd galaxies as “subgiant” galaxies, which is a useful description that has never caught on.
Some further insight into the true structure of this enigmatic galaxy can be found in H-alpha images of it. I attach three Hydrogen alpha images of this galaxy.
The following H-alpha image was taken with the Magellan Baade telescope on June 8-10, 2006 :
91391
The second Ha image comes from the PhD thesis of Jorn Rossa
(2001)(Ruhr-Universitat Bochum) ;
this .jpg file has an R-band image at the top and an H-alpha (continuum subtracted)(H-alpha line only) image at the bottom :
91392
In Rossa's H-alpha image, note the “feathering” and streamers in the H-alpha emission on one side of the disk. This sort of appearance in Ha images is often due to ionized gas that is streaming out of a spiral galaxy that is very actively forming stars; the combined energies of numerous OB stars and multiple supernovae can push gas well out of the plane of a galaxy.
The following H-alpha image is taken from the reference (2004), ApJ, 606, 829 .
[ X-rays display as Blue, narrowband light from the Hydrogen-alpha line displays as Red, and broadband red (R-band) light displays as Green. ]
91393
In this composite three-colour image, the distribution of the H-alpha light, which is always closely associated with the spiral arms in a galaxy, hints that the spiral arms of NGC 4945 might actually be quite regular, despite the rather chaotic appearance of this galaxy that is created by the numerous heavy dust clouds.
And what of the fact that NGC 4945 seems to depart somewhat from its principal plane, at both ends of its major axis? Departures from the principal plane of a galaxy are very commonly seen in images of edge-on spiral galaxies ; these features are usually explained by hypothesizing the existence of either a warp (a genuine bend in the plane of a galaxy) or an outer spiral arm opening.
If you search through the professional literature of astronomy, you will find that there has been very little study of the overall structure of NGC 4945. For the most part, professional astronomers have been primarily interested in the extremely active central and circum-nuclear regions of this galaxy.
Remarkably little solid information exists about the large-scale structure of this galaxy!
Deep and high-resolution near-infrared imaging would probably reveal the true structure of this puzzling object; has anyone ever seen a really good NIR image of NGC 4945??