Hubble results on Hanny's Voorwerp – the whole story

We just submitted the journal paper describing the Hubble results on Hanny’s Voorwerp, to the Astronomical Journal. It’s not on arxiv.org yet – you can get a PDF here. Here’s a “brief” summary.

Data: In this paper we look at a whole collection of new data, obtained since the original discovery paper. These include:

Hubble, of course:
– WFC3 (Wide-Field camera 3) images in the near-ultraviolet, deep red, and near-infrared. These filters were designed to exclude most of the light from the gas, so we could look for star clusters, especially in Voorwerp and its surroundings, and with the IR image, look deeper into the dust around the nucleus of IC 2497.
– ACS (Advanced Camera for Surveys) images tuned to the wavelengths of [O III] and Hα emission. These were intended for fine structure in the gas and its ionization. As it turned out, we saw streamers, fine details, embedded star formation, and local interaction with a jet from IC 2497.
– STIS (Space Telescope Imaging Spectrograph) red and blue spectra across the galaxy nucleus. This let us isolate structure near the nucleus much better than from the ground, so we would look for any gas that has a line of sight to a brighter nucleus than we see directly (perhaps because of foreground dust).

GALEX (the Galaxy Evolution Explorer satellite): wide-field ultraviolet spectra and images, which help us put the Hubble UV data in context.

Kitt Peak 2.1m telescope – redshift of the companion galaxy, and the nature of star-forming regions just southwest of the nucleus of IC 2497

3.5m WIYN telescope BVI images – the best non-HST data, which we used to confirm techniques for reducing effects of cosmic-ray impacts in ACS images.

In no particular order, our major science results are:

1 – We found no evidence that the quasar in IC 2497 is still bright. The Hubble spectra show no highly-ionized gas near the center, as we might expect if it were bright but blocked by dust clouds from our point of view. In fact, with HST data we can refine our estimates of the radiation intensity seen by gas near the core and out in the Voorwerp, and these estimates only widen the shortfall. That is, we can constrain the quasar’s (former) brightness to have been brighter than our earlier lower limits from the ground-based data (because of the sharper Hubble images, we can tell better how close gas is to the nucleus of IC 2497 and how bright the brightest peaks are in the Voorwerp). To quote a famous 1960s television character, “It’s dead, Jim.”

2 – As we told everyone at the AAS, we found regions of star formation in the Voorwerp. This is part of a broader picture of a directed flow of gas out from IC 2497 in a fairly narrow jet or cone. The small radio jet seen with VLBI radio techniques (the Rampadarath et al. paper, using data from the UK MERLIN network at the European VBI network) points within about 10 degrees of the direction where we see a “small” area of star formation in Hanny’s Voorwerp (no more than 5000 light-years across), and this is precisely aligned with the one area where we see tendrils of gas pointed away from IC 2497 (the area I once called Kermit’s Fingers). We see these areas of star formation in two ways – in the images filtered to minimize the gas contribution, we see the light from young star clusters themselves, even into the ultraviolet. And in the images which isolate the ionized gas, we see its ionization state (and emission-line ratios) shift in local regions around the star clusters, to the ratios that we see when the gas is ionized by young stars and not AGN. In the color images, that shows up a a shift from green (where [O III] is much stronger) to red (where Hα is the stronger line). However, compared to some other active galaxies whose jets impact surrounding gas, the effects are modest in Hanny’s Voorwerp; the jet or outflow has compressed gas and triggered star formation, but at only a quarter the rate of the similarly-sized Minkowski’s Object, which sits right in the path of a more powerful jet from the radio galaxy NGC 541. The balance tells us something about the amount of material that can be in the outflow, in order not to have pulled any more gas out into filaments , and form any more stars, than we see. This also suggests something to look for in the future – star clusters in the middle of nowhere that were formed by outflows from now-faded active galactic nuclei.

HST ACS image of Hanny's Voorwerp in oxygen III (green) and H-alpha (red)

Features in Hanny's Voorwerp

The outflow of gas we see toward the Voorwerp may roughly match, in age, another find from the Hubble data – an expanding ring of gas, 1500 light-years in size, heading out from the core in the opposite direction. This is another sign that the AGN has begun to affect its environment through mass motions rather than radiation alone. This was a serendipitous find – only because the spectrograph slit happened to cut across it could we spot this region only a half arcsecond from the core where the Doppler shifts and emission-like properties were quite distinct. With some extra processing, it turned up in the emission-line images as well, which is how we know it forms a loop. We suspect that putting it all together may show that the black hole’s accretion in IC 2497 hasn’t completely shut down, but has shifted from producing radiation to pumping more energy into motions of surrounding gas (as it’s called in the jargon, switched from quasar mode to radio mode). The speed of such a switch would inform theoretical understanding of these accretion disks, happening not on the periods of days that we see for black holes in our neighborhood with a few times the Sun’s mass, to a million years (or now maybe rather less) in a galactic nucleus.

HST ACS images of the core of IC 2497

3 – As we could sort of see from the SDSS images, IC 2497 is disturbed. Its spiral arms are twisted out of a flat plane, with dust lanes cutting in front of the central region. This fits with the idea that a tidal collision pulled out the massive tail of neutral hydrogen. On the other hand, we now see that the companion galaxy just to its east is a beautifully symmetric, undisturbed spiral (which we now know to have a precisely matching redshift, so they are almost certainly close together). One picture that would fit these data would be that IC 2497 is the product of a merger something like a billion years ago (more precisely, before the time when we see it), a merger which was either quite unequal in galaxy masses or unusual in leaving the disk of the galaxy in place although warped. There is a suggestion that the patch of star-forming areas just to the southwest of the center of IC 2497 might be all that remains of the other galaxy.

4 – This result may be a bit of an acquired taste, delving into emission-line physics. From the lack of a correlation between level of ionization and intensity of Hα emission, we can tell that, despite the amazing level of detail of blobs and strings we can see in the Hubble images, that there is fine structure on still smaller scales. The areas that are brighter are not, by and large, any denser than average (which would be the most natural way to have brighter H-alpha emission), they have more small blobs and filaments with about the same density. This could be general – if the outflow from IC 2497 has not reshaped the gas in the Voorwerp, most of the neutral hydrogen that’s not ionized by the galaxy nucleus would have the same kind of structure. That in turn would suggest that the common giant hydrogen tails around interacting galaxies are composed of masses of narrow threads of gas (maybe held together by magnetic fields), which is not the first thing we would guess from the limited-resolution radio data that are the only way we can see these tails unless they are ionized by a nearby AGN.

Several of these are results we will also look for in the Hubble images of selected Voorwerpjes – do we see star formation indicating there is an outflow from the AGN, and do we see the same evidence for fine structure in the gas?

From the spectrum, we have a pretty good idea what the chemical mix of elements is – by mass, around 77% hydrogen, 23% helium, and 0.25% of everything else (what astronomers like to call “metals”, although that mostly means carbon, nitrogen, and oxygen). This tells us a bit about where the gas didn’t come from – it was not blown out from deep inside IC 2497, because gas near the centers of big galaxies gets progressively enriched in heavy elements produced inside massive stars (then blown out in supernova explosions or less violent planetary nebulae). If the gas in the 21-cm hydrogen tail was pulled out from the outer regions of IC 2497 during an interaction with another galaxy, this would fit, since gas far out in spirals has been less affected by material produced in stars.

This is the first technique that has been able to look across times of many thousands of years, rather than the few decades that astronomers have been able to watch AGN. Getting a better handle on this was a big part of the search for voorwerpjes (so we get a better since of how unusual IC 2497 and the Voorwerp might be). From our initial sample of 19, we could make a crude estimate that AGN stay bright for roughly 20,000-200,000 years at a time. This comes from comparing the numbers of galaxies with clouds whose AGN are bright enough to account for them with the number where the AGN is too faint to light up the clouds (where iC 2497 is the strongest example). I have a project slowly getting started to look for even fainter examples (too dim to be picked up by the SDSS) around bright galaxies, so we might be able to look back even longer (up to a million years if very lucky). Geeky it might be, but I couldn’t resist calling this the TELPERION survey. As an acronym it’s forced, but Middle-Earth aficionados will see how appropriate the connotation is.

All in all, quite an adventure beginning with “What’s the blue stuff?”