Sthn Galaxies in UV -spectacular or weird -Part 1

[madbadgalaxyman (Robert)]

My fellow Galaxy Fanatics,

This post is the first of a series that displays very strange or very spectacular images of southern objects taken by the GALEX satellite.

Here is the "famous"(!????) galaxy ESO 104-044.
This little irregular has a low surface brightness at visual wavelengths, but, as is usual for irregular galaxies, it lights up like a flare at ultraviolet wavelengths.
This galaxy is probably physically associated with NGC 6744.
One might provocatively hypothesize that the elongation is due to a tidal effect!



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Here is a nice galaxy which is on the border between the irregular Hubble type and the spiral Hubble type.
This one is called NGC 1385.
Firstly, I attach a B band image of NGC 1385 which was obtained by OSUBSGS (displayed at a logarithmic scale), which shows the appearance of this galaxy in visual observation or visual wavelengths imaging:



Note how NGC 1385 has one very strong spiral arm, and one weak arm. Kenneth C. Freeman and Gerard de Vaucouleurs did some early modelling of galaxies from type Sd through to Irregular, and they found that arm asymmetries were actually inherent in barred galaxies that occupy this part of the Hubble Sequence. Also, Freeman, who is still at ANU and still one of the top 10 extragalactic astronomers in the world, found that the "one armed appearance" of many raggedy and chaotic spirals such as the LMC and NGC 4027 and NGC 1385, is most often due to perturbation by a lower mass companion.

Here is the Ultraviolet view of NGC 1385, from the GALEX satellite:



There are very bright knots of current star formation, extremely intense in far-ultraviolet wavelengths.
The high Star Formation Rate and respectable surface brightness of this galaxy should make it an appealing target for imaging


cheers,
madbadgalaxyman

P.S.
The very contrasty (but relatively low resolution) version of DSS2 that is found at wikisky shows a possible tidal arm in NGC 1385

[renormalised (Carl)]

Wonder what the population morphology is for 1385?? Wouldn't be surprised if it's similar to the LMC. 3-4 different levels of metallicity amongst the stars with a spread of about 6 billion years or so in the ages. Lots of gas and dust....lots of activity but not as evolved as the larger spirals.

[madbadgalaxyman (Robert)]

Mr Renormalised, you are probably right......

NGC 1385 may be as "normal" as the LMC.

Mind you, this definition of normality includes an interaction with a lower mass companion!

The asymmetry of NGC 1385 is not nearly so pronounced when it is seen in the near-infrared (which, I hasten to add, for the benefit of the "lesser brethren", usually traces the older stellar population that dominates the mass of a galaxy)

Here is an H-band image (1.6 microns)(near-infrared) of
NGC 1385, which I have displayed at a linear scale. This is taken from the OSUBSGS (survey).



As can be plainly seen here, the older stellar population of NGC 1385 is much more regularly arrayed than the very young stellar population traced by the blue channel in the GALEX ultraviolet image of this galaxy.
This, the asymmetry of this very young population may be a recent phenomenon, while the underlying mass of this galaxy may not be very perturbed.

[[ I note here, for those of you less accustomed to analyzing galaxy images, that just because the Hot & Young & Very Luminous stars within a galaxy are very chaotic or asymmetric in their distribution, it does not mean that the galaxy itself is actually perturbed. Young stars are only a small fraction of the total mass of a galaxy.
A good example of this is NGC 2903, which looks very messy at visual wavelengths, due to an overlay of dense gas clouds and very luminous stars.....but the mass of this galaxy is probably mildly perturbed (at most). ]]

Cheers,
madbadgalaxyman

One more important point is that NGC 1385 is not actually a low luminosity spiral galaxy like the LMC, if the redshift of N1385 is taken as a rough indicator of its distance.

This could mean either:

(1) This is an LMC-like galaxy that is experiencing a global starburst that is greatly increasing its luminosity

or

(2) This is a giant (non-dwarf) galaxy comparable to the Milky Way.

Possibility number 1 looks more likely to me at the moment, but I need to have a closer look at this issue before I can decide for sure.
NGC 1385, If I remember correctly, is probably a member of the "Eridanus Cloud" on the Fornax-Eridanus border; a vastly distended cloud of galaxies which resembles a vastly extended version of a galaxy cluster.......and this positioning would tend to support the idea that this is a galaxy of relatively high luminosity.
(there is incipient galaxy clustering in this area; very interesting, but here I digress!!)

P.S.
Another similar system, NGC 4027, albeit of even more asymmetric morphology, was once compared by Professor Kenneth C. Freeman (of ANU) to the LMC.

[renormalised (Carl)]

If I figured out the distance correctly (just over 21Mpc...68Mly), its long diameter (3.715') works out to 6.96Kpc, which is quite small...smaller than the LMC.

So, it's most likely (1).