What is Hoag's Object?

Perhaps one of the most striking looking galaxies I know of is "Hoag's Object" (seen below by the Hubble Space Telescope; NED information, Wikipedia article, lookUP information). The wikipedia article talks of it as an object which fascinates both amateur and professional astronomers.

 I am really curious to know how it might look through an amateur telescope, and I'd love to see it for myself some day (RA=15 17 14, Dec=+21 35 08 in the Serpens constellation), Astrometry.net can help give an idea - see Hoag's Object images on Flickr found by the service.

HST Image of Hoag's Object. Credit: NASA.

Through HST as you can see the object appears to be made up of a red spheroidal core, surrounded by a blue ring of star formation (with a gap between the two). The ring shows some spiral structure. In my opinion, one of the most fun things about the object is the more distance ring galaxy which can be seen through the gap (just to the right of 12 o'clock). This to me demonstrates the sheer size of the universe. To find such a rare object behind such a rare object seems quite extraordinary. 

So why am writing about Hoag's Object today, well appearing on the arXiV this morning is a paper addressing the formation scenarios for Hoag's Object (Finkelman et al. 2011, MNRAS in press), which struck my interest so I thought I'd write about it. It puts forward a new scenario for the formation of this unusual object as well as talking about the two previously suggested models. In addition they present some new data for our consideration.

 The three models discussed are:

1. Ring formed as a collisional ring.

In this model another galaxy would have passed through the centre of Hoag's object, and what is observed is the merger remnant. The Cartwheel Galaxy is perhaps the most famous of this class of objects. It's shown below in a HST image, and illustrates the most obvious problem with interpreting Hoag's Object in this way.

Cartwheel Galaxy. Credit: HST, NASA

You can see quite clearly in the above image the culprits in the galactic collision. No such neighbours exist for Hoag's Object. In addition the ring appears to be at rest with respect to the central spheroid - which would be unlikely if the ring were collisional.

2. Ring formed through a bar instability which has since dissolved. 

 In this model at an earlier time there would have been a strong bar, and material would have flowed out along it to form the ring. The main objection to this theory appears to be the lack of evidence for any residual bar in the central spheroid. I should perhaps point out that this was a theory previously put forward by one of the co-authors of today's paper (Noah Brosch), so presumably his co-authorship on this new paper is an indication that he no longer believes this to be the best model.

3. Gas Accretion. 

This is the new theory put forward (although I should say it seems rather similar to me to one discussed by Schwiezer et al. 1987). In this model the object has a very low density HI disk which accreted at early times onto the spheroid and is only dense enough to form stars in the ring.

In fact the kinematic data does seem to suggest that Hoag's object (other than having its only visible disk light in a ring of course) is a normal disk galaxy, with the spheroidal component playing the role of a central classical bulge. At the risk of getting too technical in a blog, check out the Halpha velocity map and HI line profile below.

Ignoring the gap in the Halpha velocity map these two observations look to me basically identical to what you would expect from a normal nearly face-on disk galaxy. The classic "double horned" HI profile is usual interpreted as a coming from a rotating disk of HI with a central gap. HI (atomic hydrogen in its ground state) emits (due to hyperfine spitting of the ground state) at a single frequency of 1420 Mz (21cm) - the broadening of the line is caused by Doppler shifting of the emission (indicated along the x-axis is the velocity of the Doppler shifter HI line) and the peaks at the maximum velocity are interpreted as a pile up of HI in the flat part of a galaxy rotation curve (see global HI profiles, or galaxy rotation curves for more information on this).

HI in Hoag's Object from Schwiezer et al. 1987

The Halpha velocity map is showing a similar thing. Halpha is a spectral line emitted by excited hydrogen. Again this emits at a single frequency (656.28 nm), which is shifted due to the Doppler effect. The map shows the velocity of this line colour coded such that red indicates a greater velocity than the mean for the galaxy and blue a smaller. This is again interpreted as a rotating disk of hydrogen with the upper left moving away from use and the lower right towards us. The velocities are very consistent with what's seen in the HI profile, suggesting the HI is indeed coming from the ring.

Halpha velocity field in Hoag's Object from Finkelman et al. 2011

 What is needed to complete this picture is a HI map of Hoag's Object. I think this could be fairly easilly done with the EVLA, and I will be interested to see it when it is.