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The lopsided HI disk of UGC 11891

Submitter: George Heald
Description: UGC 11891 is the name of an inconspicuous dwarf spiral galaxy, located at a distance of about 30 million light years from Earth. The optical surface brightness of this galaxy is extremely low, and there is very little ongoing star formation. Therefore, the optical appearance of this galaxy is quite unremarkable. On the other hand, the neutral hydrogen (HI) disk is more photogenic. Deep WSRT observations of this galaxy have recently been carried out, in an attempt to carefully study the process of cold gas accretion -- HI falling onto the disk from intergalactic space, in the continuing formation of the galaxy.

Today's image shows the integrated HI map produced from the WSRT observations in orange, overlaid on the optical picture. The small, faint optical disk of the galaxy is reproduced in blue on top of that. The reason for the very high density of field stars is that UGC 11891 is located at a rather low galactic latitude. The total amount of HI measured in UGC 11891 is 3.1 billion solar masses.

The first feature that stands out clearly in the HI disk is the highly lopsided gas distribution. The optical emission is well centered in the brightest part of the HI disk, but the faint neutral gas extends much further to the north-west (upper right) than to the south-east. There is also a faint extension of the HI at the northernmost part of the disk. Since there are no obvious companion galaxies in the nearby field which could be interacting with UGC 11891, it may be the case that this HI tail is indicative of the accretion of cold gas. The lopsidedness of the HI disk may be related to this ongoing accretion process. The WSRT observations also provide velocity information about the HI (not shown in this image). This will help us to figure out how much accretion is taking place, and whether it is related to the lopsidedness of the HI disk.
Copyright: George Heald
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