Hi Steve,
I've been trying to get together a multi-wavelength collection of NGC 7552 images for some time.
The morphology of this galaxy is often compared with that of NGC 7582, yet it seems to me there are undeniable peculiarities in N7552.
This is one of those galaxies for which the old-style photographic process image from the Carnegie Atlas of Galaxies is a good one:
Because this atlas image is reproduced from a blue-sensitive photographic plate, the dust lanes are heavier than they look in most CCD exposures, due to the presence of more extinction (within N7552) than in red light.
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Here's an image at a much longer wavelength, an Near-infrared H-band image (1.6 micrometres) from the OSUBSGS survey, reproduced at a linear scale:
Much of the dust extinction is gone in this image, as most 1.6 micron photons
pass through all but the heaviest dust lanes, and furthermore this image emphasizes an older stellar population. It is remarkable how intense the central feature is in this galaxy.
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The intensely bright central region is also well shown in the following near-infrared (J+H+K bands) image from 2MASS:
Also, it can be seen in this image that, despite the confusing and dusty appearance of this galaxy in the optical regime, its bar, as defined by the old mass-dominant stellar population, looks normal..... so the overall distribution of stars within this galaxy is probably
not highly disturbed (indeed, the NIR isophotes of the bar are nothing unusual).
Yet, referring back to the Blue-sensitive image of NGC 7552 in the de Vaucouleurs Atlas of Galaxies (see the link in my previous post in this thread), the distribution of dust and Population I material is unusual and possibly disturbed. Young material (e.g. OB stars and dust and molecular gas) within disk galaxies is “kinematically cold” (i.e. low random velocities and high co-ordinated rotation), so it is easily disrupted; and it is always of small mass relative to the total luminous mass of a spiral galaxy.
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NGC 7552 does look interesting in (continuum subtracted) Hydrogen alpha line images.
(( For the uninitiated, I here explain that a galaxy image made with a Hydrogen alpha filter includes not only nebular light, but
also some light from the
stars of a galaxy (this is called the stellar continuum). But, using a mathematical model, it is possible to turn a raw H-alpha image into a “continuum subtracted” H-alpha image of a galaxy, and such an image includes the H-alpha line only......in other words, an image which shows
only nebular light from HII regions ))
So here is an H-alpha (continuum subtracted) image of NGC 7552 by Salman Hameed of Hampshire College (references: 1999, AJ,
118, 730 and 2005, AJ,
129, 2597) :
Despite considerable dimming from dust extinction, the central region of this galaxy is still intense in H-alpha light, undoubtedly due to its circumnuclear ring of OB stars and H-alpha regions. (this is a "circumnuclear starburst")
Here is Hameed's comparison of his H-alpha image (right panel) with a broadband optical image in the left panel :
The H-alpha emission seen along the bar follows the two
leading edges of the bar, where models indicate that the interstellar gas within a barred spiral galaxy is shocked into compression (this also causes the characteristic
leading edge dust lanes in barred spirals) ;
gas compression here must have formed young and massive stars, which have ionized the interstellar medium in order to make the HII regions that are seen along the bar.
However, the
apparent (as observed)
asymmetry of this galaxy is considerable, in Hydrogen alpha light.
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I now compare the distribution of H-alpha light (which mainly comes from HII regions and which is a therefore an excellent tracer of where the most massive & hot & luminous stars are), with another good tracer of the distribution of OB stars within a galaxy; far-ultraviolet (FUV) light.
Here is the FUV image of NGC 7552 taken by GALEX, displayed firstly at a Square Root scale and secondly at a linear scale:
To be honest, I don't know exactly what to make of the morphology of this galaxy in FUV light....but extinction will be very high at this wavelength, so many of the regions with OB stars may not be visible from our line-of-sight.
Certainly, any morphological interpretation of NGC 7552 will be tentative, if it is based on visible or ultraviolet wavelength images; there is a lot of obscuring dust evident in this galaxy that can easily hide structures existing behind it.
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best regards, Robert
