APOD and General Astronomy Discussion Forum
https://asterisk.apod.com/
On April 1th 1574 the Geuzen (The Spanish king said 'Ce sont que des gueux' , -they are just beggars- since then an honorary name) captured the city of Brielle in the south western parth of the Netherlands from the Spanish garrison. Local folklore remembering the capture of Brielle still exists. April 1th is since then 'pranksters day' in the Netherlands. 25th June is unknown here.Anthraquinone wrote:I am now wondering if, in other countries, there is an equivalent to 1st April on the 25th of June.
OTOH...since it was about 100,000 years ago:cgb wrote:No no no no no. And no!
You are looking at the first known photograph of the Grinch Nebula, overlooking all the Who's in the Whoville Galaxy.
From the STScI Digitized Sky Survey database, POSS2 data (red and blue plates, synthetic green channel):ta152h0 wrote:looks like a composite image . Need to examine the negative original. Or pencil in Hubble time. What does the Digital AllSky Survey show in that area ?
Close, but no cigar. The W is comparable to What, Which, Where,(Wolverhampton) Wanderers. The F in "four" is to strong, more like the V in Victory, in stead of Fiction. The Anglophone on BAUT most likely will live in Amsterdam, in that city the letter V is pronouced as an F.JohnD wrote:henk, how is the 'W' of VoorWerp pronounced? An Anglophone on BAUT suggest the whole word as "four-vairp". How close is that? J.
Festival of Fools, Ireland, May: http://www.foolsfestival.com/Anthraquinone wrote:I am now wondering if ... there is an equivalent to 1st April ...
Wonderful site! Thanks for the reality.JohnD wrote:Earlier reports showed it as blue, now its seen as green.
This site: http://www.astr.ua.edu/keel/research/voorwerp.html discusses frequencies in the emission spectrum, and comes down on the side of it being "more green [than blue] if seen with the eye's sensitivity." That equivocal statement doesn't match the intense green shown in the pics.
But we're shown lots of pics with 'synthetic colours' to represent wavelengths that are totally invisible the eye. These are usually wavelengths of extreme specificity, when the eye sees a wide range of frequencies as "green" or "blue".
No, they are completely different phenomena. The green flash is caused by simple atmospheric refraction- the air breaks up the Sun's light like a prism, and the flash is seen when the green part of the spectrum passes by the observer. Hanny's Voorwerp is mainly seen in the light of strong emission lines (not a continuum source). I don't think any of the published images show what could be called "accurate" color; it's debatable whether that term even means much. The true color of the nebula is actually gray, since that's the only way the eye could ever perceive it.dcrosby wrote:I got to thinking - the green in the picture on APOD reminds me of the "green flash" I saw a picture of some years back. It gave me the idea that it might be the product of a gravitational distortion of light somehow. I guess if it's because of a black hole, I wasn't too far off, but if it's really blue then I guess I was further off than I thought.
Hanny's Voorwerp: Evidence of AGN activity and a nuclear starburst in the central regions of IC 2497In 2007, a Dutch school teacher called Hanny van Arkel discovered an extraordinary object while combing though images for the Galaxy Zoo project to classify galaxies. The object appears as a bright green blob close to a spiral galaxy in the constellation of Leo Minor and soon became known as Hanny's Voorwerp (meaning Hanny's object in Dutch).
Hanny's Voorwerp is astounding because astronomers have never seen anything like it. Although galactic in scale, it is clearly not a galaxy because it does not contain any stars. Detailed spectrographic observations since its discovery suggest that it is a giant cloud of gas that is glowing an unusual green colour.
That raises an obvious question: what is causing the gas to glow?
Today, Hayden Rampadarath at the Joint Institute for VLBI in Europe, based in the the Netherlands and a multitude of friends, provide an answer. Their data comes from a new study of the nearby spiral galaxy IC 2497 using a couple of very long baseline interferometers to study the region at various wavelengths.
Their conclusion is that, like many galaxies, IC 2497 contains a massive black hole at its centre. The infall of matter into the black hole generates a cone of radiation emitted in a specific direction. The great cloud of gas that is Hanny's Voorwerp just happens to be in the firing line. The black hole radiation is ionising the gas, causing it to glow green.
What has confused the issue is that another cloud of dust and gas sits between us on Earth and IC 2497 and this prevents us from directly seeing the black hole (or the active galactic nucleus as astronomers call it).
That seems a sensible interpretation. And certainly more likely than another idea put forward last year. This proposed that some 10,000 years ago, IC 2497 suddenly underwent a dramatic outburst of quasar-like radiation and then became quiet. What we see today from this cloud of gas some 10,000 light years from IC 2497, is simply a reflection of this outburst. In other words, Hanny's Voorwerp is a quasar light echo.
That was unsatisfactory because it resolved one mystery merely by posing another: what could cause an entire galaxy to flare up briefly and suddenly?
The evidence that IC 2497 is still active today puts to rest this idea. It also explains why Voorwerps are so rare: the radiation cones from active galactic nuclei are highly directional so only occasionally do unlucky gas clouds get caught in the cross fire.
Radio Observations Provide New Explanation for Hanny's VoorwerpA group of researchers, led by Professor Michael Garrett, General Director of the Netherlands Institute for Radio Astronomy, ASTRON, have made new, high resolution radio observations of the region of space around Hanny's Voorwerp, the mysterious, greenish gas cloud discovered by Dutch school teacher Hanny van Arkel.
The astronomers undertook an observational campaign at radio wavelengths using the European Very Long Baseline Interferometry Network (EVN) and the UK's Multi-Element Radio Linked Interferometer Network (MERLIN), pointing their telescopes at the centre of the neighbouring galaxy IC 2497. In these measurements, several radio telescopes across Europe and the UK were linked together in real-time, in order to gain a detailed picture of the very central region of the galaxy. They observed a field a few arcseconds (the 3600th part of a degree) across, with a spatial resolution of about 70 milliarcseconds (0.07 arcseconds).
According to Garrett: "the observations show two bright and very compact sources with broadband spectra that argue strongly for the existence of an active galactic nucleus (AGN) at the centre of the galaxy, IC2497. One of the sources appears to be identified with the base of the super-massive blackhole at the centre of the AGN itself, while the other one is likely to be the result of an energetic jet expelled by the black-hole and now interacting with the dense gas that surrounds IC2497". The radiation output from the AGN is believed to heat Hanny's Voorwerp to a temperature above 10000 degrees.
But this is only part of the story. It also appears that surrounding the AGN, a lot of extended radio emission is also observed. The researchers argue that this is radio emission associated with a nuclear starburst. "We knew that IC 2497 is forming stars, but we were susprised to find that the star formation seems to be concentrated in a very small central region, only 3000 light years across", Hayden Rampadarath (University of Machester) explains. Garrett: "It is fairly unusual to find both vigorous star formation and AGN radio activity in the same system and on similar scales. It seems that IC2497 swings both ways".
The radio observations show that in this small region, IC 2497 is producing stars with a total mass of 70 suns every year. This star formation rate is pretty large, especially in the local Universe - it is about 6 times higher than in the nearby 'Starburst-Galaxy' poster-child, M82." "We know of only a few hundred of these types of galaxy, Luminous Infrared Galaxies, LIRGs, in the local universe", Gyula Józsa, support scientist at ASTRON, adds, "but they must have been much more frequent in the past". At half the universe's age, most stars have been formed in LIRGs. It is also typical that for this kind of galaxy large amounts of dust obscure optical and ultraviolet light towards the observer.
The observations support the group's earlier hypothesis that a hidden AGN in the centre of IC2497 is ionising a distinct region of gas that surrounds the giant galaxy. That distinct region is what we know as Hanny's Voorwerp. Such phenomenae must be rare in the local Universe, as they depend on a specific geometry of the observer, galaxy and gas, plus the interaction of several galaxies in the field in order to fuel the AGN and the starburst, and to create the gas reservoir that forms part of the Voorwerp.
