Hubble type
-
I would have classed this as Hubble type E7 Elliptical galaxy, but it could very well be a side view of a spiral galaxy.
Posted
-
by
JeanTate
in response to Melanie_Milsted's comment.
E7 galaxies are very rare, and all are dwarfs.
I think it's far more likely that this is a nearly-edge-on disk galaxy, possibly a spiral (not all disk galaxies have spiral arms).
There are ways to check, to see which is more likely, using data from SDSS; would you like to learn more?
Posted
-
I sometimes use DECaLS Residuals for reference, seems to indicate 2-arm spiral edge-on:
http://imagine.legacysurvey.org/?ra=354.8801&dec=0.2017&zoom=16&layer=decals-resid-dr1j
Posted
-
by
JeanTate
in response to Ghost_Sheep_SWR's comment.
That's very cool, Ghost_Sheep_SWR, I didn't even know about that! Thanks.
There are ways to check, to see which is more likely, using data from SDSS
If you click through to the SDSS image, then click on the PhotoObj link at the left (this is for DR9; it's a bit different in DR12), you'll see a long 2-column table. Parameter names are on the left, values the right. These are all derived from the photometric data (plus some meta-data), and no spectroscopic data was used (for many objects, there is no spectroscopic data).
The parameters with "AB" in their names refer to estimates of how 'out of round' the object is (technically it's the ratio of the minor to the major axis lengths, from various fitted ellipses). For example, expAB_r (fitting ellipses using an exponential luminosity profile, in the r band) is 0.222668±8.616772E-3 (the 'error' is given in expABErr_r). The "ellipticity" of a galaxy, of the elliptical kind, is defined as 10*(1-b/a), rounded to an integer somehow (I don't know how). So this object would be an E8 (or perhaps an E7) elliptical, if it were one. Most unlikely to be an elliptical!
The five fracDeV parameters (one for each band) may also help, but they can be very misleading. Technically, this is an estimate of the fractional likelihood that the luminosity profile is a deVaucouleurs one , rather than an exponential or PSF one (stars have a PSF profile, ellipticals usually have a profile that is close to a deVaucouleurs one, and the disks of spirals generally close to an exponential one). For this object, all five values (for the five bands) are 0 (well, the z-band one is 0.066737), consistent with this being not an elliptical.
That's just two metrics; there is at least one other you can use, based solely on SDSS PhotoObj parameters (holler if you're interested).
Ain't astronomy wonderful? 😃
Posted
-
It might help, but its only the 'leftover' after DECaLS Models is substracted from the real image, so not as accurate / scientific as method described above... Wonderfull AND difficult 😃
DECaLS Model / Residuals does have merit though, when searching for asteroids for instance they usually stand out quit obvious in Models, more than the real image.
Posted
