GALAXY
A Grand-Design Spiral in Eridanus
— NGC 1232 —
Processed by Nicolas Rolland, Paris. 26.5 hours gathered by Martin Pugh at El Sauce Observatory, Rio Hurtado, Chile.
Most of the mass in this picture is not in the picture.
Face-on, in Eridanus
NGC 1232 is an intermediate spiral in Eridanus, about 60 million light-years away, and William Herschel recorded it on 20 October 1784. It is turned almost exactly face-on to us, which is the only reason its structure can be read: the arms come out of the core and can be followed all the way to the edge of the disc without ever crossing each other. Two arms dominate and the rest branch off them, which is what earns a galaxy the label grand design — most spirals are patchier than this.
What holds it together is not what lights it
The arms are marked out by open clusters of hot blue stars, with dark lanes of dense interstellar dust running between them, and small knots of ionised hydrogen where the newest stars are still switching on. Those are the bright things, and they are not the massive things. Dim ordinary stars and interstellar gas, neither of which shows well in an image, carry enough mass between them to govern the motion of the inner galaxy. Further out even that is not enough: the speed at which the outer disc turns requires several times more matter than anything visible accounts for, and what that matter is remains unsettled.
Arms that bend the wrong way
There is one thing about this galaxy that a face-on view makes obvious: the arms do not curve smoothly. They are visibly kinked, bent out of the gentle logarithmic sweep a spiral usually follows, and a disc does not do that on its own. The usual explanation is gravitational — something passed close enough to pull on it, and the arms are still carrying the deformation. The small barred galaxy in the field, NGC 1232A, is the candidate that gets named. Whether it is the right one depends on how far away it actually is, and that is not settled.
Twenty-six hours, four and a half in hydrogen
A Planewave CDK17 at f/6.8 with adaptive optics ahead of the sensor, from Rio Hurtado in December 2019 — twenty-six and a half hours in all, seven of luminance and fifteen of colour at twenty minutes a frame, and four and a half of H-alpha at thirty. Those four and a half hours are what separate the HII knots from the general blue of the arms; without them the star-forming regions and the clusters they belong to blur into one another. The field runs 32 by 22 arcminutes and the galaxy covers about eight, which leaves most of the frame empty — deliberately, because the outer disc and whatever tidal material surrounds it extend well past the bright body, and framing tight would have cut off the part of the question this image can address. Martin Pugh gathered the data at El Sauce.
