Gravitationally-lensed Twin Quasar

[Martin Meredith]

Here's a shot from last night's session of the first gravitationally-lensed object to be found (back in 1979). It is a twin-image of a redshift 1.42 (8.7 billion light years distant) quasar in Ursa Major, with the lensing caused by a foreground galaxy at a mere 3.7 billion light years. There's a good entry on Wikipedia in which I was intrigued to learn that there is a 400+ day lag between the two images. I'm not sure how such a precise estimate could be made unless there are variations in light output that are out of sync? More research needed...

The two images of the one object are about 6 arc sec apart -- about 3 pixels in my setup. I was surprised to get them well separated as seeing (or perhaps heat currents from neighbours' fires) was very poor at times. Also in shot is NGC 3079, a mag 11.4 type SBcd galaxy seen almost edge on at a distance of 64 million LY.

I've oriented the image so that N is up. This is a stack of 4 x 30s.

A couple more objects from last night in the same part of the sky: NGC 2403 in Cam, a bright, large, M33-like type SABc galaxy, and NGC 4605, a mag 10.8 SBc in Uma. The latter from this angle is a little reminiscent of M82. I used it as a jumping off point to explore the nearby Hubble Deep Field.

 

Thanks for looking

Martin

 

[Martin Meredith]

I've just been reading more on the way the time difference was estimated in this paper: http://arxiv.org/pdf/astro-ph/9610162v2.pdf

Indeed, it is the case that variations in brightness were used, allowing a prediction of when a particular sharp change would occur in the delayed image. (I didn't mention magnitudes in the above post but they hover around the 16.8-17.1 region in the r any g bands.)

What's interesting also is that these estimates have allowed a direct single step estimate of Hubble's constant.

Martin

 

[Astrojedi]

Very interesting... The double quasar is definitely intriguing...  The paper estimates distance to +/- 10% error range which while very large in absolute terms is actually pretty constrained given the typical distance measurements.

P.S. I like how clean and dark the background is in these images. I can only dream of skies like that here.

 

[Paul81]

Awesome stuff Martin, and very intriguing observation - you can definitely see the separation!

[Stu]

That is an amazing image Martin.

A question..... Is the foreground Galaxy just too dim to be seen? Presumably it is located between the two quasar images?

[Martin Meredith]

Thanks everyone for the comments and likes.

Stu, I've been looking into the lensing galaxy. It is known as YGKOW G1 (redshift 0.39) which is sitting almost in the line of sight of the lower (southern) member of the pair as seen in the shot. The only magnitude I've been able to find so far is in the near-infrared H-band, of around 15.14. I don't know how that would translate into a visual magnitude.

There's an image of the lensing galaxy showing its relationship to the quasar images in this paper: http://www.iac.es/proyecto/quasar/arts/2000c.pdf

[edit] Fig 2 in this paper shows the lensing galaxy's V-band magnitude as a function of radius, peaking at V=20. Not the same as an integrated magnitude though (which would be brighter), as far as I can work out.

Martin