Gaia’s Third Data Release!

It seems like only yesterday that I blogged about the second release of data from the European Space Agency’s Gaia mission but today sees the release of the third data set, known to its friends as DR3. This completes the set after some initial data were released early as EDR3 back in 2020.

Gaia on the Launchpad at Kourou, French Guyana, on 13th December 2013

In case you weren’t aware, Gaia, launched way back in 2013, is an ambitious space mission to chart a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition, formation and evolution of the Galaxy. Gaia will provide unprecedented positional and radial velocity measurements with the accuracy needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group. This amounts to about 1 per cent of the Galactic stellar population.

Gaia is likely to operate until round about November 2024, so there’s a lot of data yet to come.

You can find a complete list of what is in DR3 here and if you want to go straight into the papers based on this dataset, go here. There is a nice promotional video here:

Follow @telescoper

Our own Galactic Black Hole

As I mentioned a while ago the Event Horizon Telescope team held a press conference this afternoon and to nobody’s surprise they used it announce an image of the (shadow of the event horizon around the) black hole at the centre of the Milky Way.

Here it is:

You can read the full press release here.

You may recall a great deal of excitement about three years ago concerning the imaging of the “shadow” of the event horizon of the black hole in the centre of the galaxy M87. The question I was asked most frequently back then is that there’s a much closer black hole in the centre of our own Galaxy, the Milky Way, so why wasn’t that imaged first?

It it true is that the black hole in the centre of M87 is ~10³ times further away from us than the black hole in the centre of the Milky Way – known to its friends as Sagittarius A* or SgrA* for short – but is also ~10³ times more massive, so its Schwarzschild radius is ~10³ times larger. In terms of angular resolution, therefore, the observational challenge of imaging the event horizon is similar in the two cases. However, in the the case of the Milky Way’s black hole the timescales involved are much shorter than in M87 and there is a greater level of obscuration along the line of sight. That’s why it took longer to produce the image.

It’s a very difficult observation of course and I’m not sure of the significance of the “lumps” you can see, but the dark region in the centre is what the image is really about and that seems to be exactly the predicted size. The resolution is about 20 microarcseconds. Congratulations to the Event Horizon Telescope team!

If you’re interested in learning more about how this image was made I recommend this short video:

Follow @telescoper

Astronomical Heads Up

You may recall a great deal of excitement about three years ago concerning the imaging of the “shadow” of the event horizon of the black hole in the centre of the galaxy M87. There was so much interest in this measurement that you could hardly move without seeing this picture somewhere or other:

The question I was asked most frequently back then is that there’s a much closer black hole in the centre of our own Galaxy, the Milky Way, so why wasn’t that imaged first? The answer is that the black hole in the centre of M87 is about 1000 times further away from us than the black hole in the centre of the Milky Way – known to its friends as Sagittarius A* or SgrA* for short – but is also about 1000 times more massive, so its Schwarzschild radius is 1000 times larger. In terms of angular resolution, therfore, the observational challenge of imaging the event horizon is similar in the two cases.

I mention this because the Event Horizon Telescope team who made the above image are holding a press conference next week at ESO on “groundbreaking Milky Way results from the Event Horizon Telescope Collaboration”.

I wonder what these “groundbreaking results” might be?

Follow @telescoper

The Complex Heart of the Milky Way

I couldn’t resist sharing this amazing radio image of the Galactic Centre made using the South African MeerKAT radio telescope:

Radio frequency electromagnetic radiation is able to penetrate the dust that permeates this region so can reveal what optical light can not. In particular you can see the very active region around the black hole at the centre of the Milky Way, bubbles caused by exploding stars and – most interesting of all – a number of magnetized filamentary structures.

It’s an extraordinarily beautiful picture made from a mosaic of 20 separate observations. In fact I like it so much I’ve cross-filed it in my “Art” folder. Those of us who work in astronomy or astrophysics are wont to say that there’s much more to it than pretty pictures, but when one like this comes along we’re all sure to geek out over it!

For more information about this image at the science behind it, see here.

Follow @telescoper

New Publication at the Open Journal of Astrophysics

Time to announce another publication in the Open Journal of Astrophysics. This one was actually published on Friday actually, but I didn’t get time to post about it until just now. It is the fourth paper in Volume 4 (2021) and the 35th paper in all.

The latest publication is entitled The local vertical density distribution of ultracool dwarfs M7 to L2.5 and their luminosity function and the ultracool authors are Steve Warren (Imperial College), Saad Ahmed (Open University) and Richard Laithwaite (Imperial College).

Here is a screen grab of the overlay which includes the abstract:

You can click on the image to make it larger should you wish to do so. You can find the arXiv version of the paper here. This one is in the Astrophysics of Galaxies section but it also has overlap with Solar and Stellar Astrophysics.

Over the last few months I have noticed that it has taken a bit longer to get referee reports on papers and also for authors to complete their revisions. I think that’s probably a consequence of the pandemic and people being generally overworked. We do have a number of papers at various stages of the pipeline, so although we’re a bit behind where we were last year in terms of papers published I think may well catch up in the next month or two.

I’ll end with a reminder to prospective authors that the OJA  now has the facility to include supplementary files (e.g. code or data sets) along with the papers we publish. If any existing authors (i.e. of papers we have already published) would like us to add supplementary files retrospectively then please contact us with a request!

Follow @telescoper

Faraday Rotation in the Milky Way

Yesterday I came across a very interesting paper on the arXiv by Sebastian Hutschenreuter et al. entitled The Galactic Faraday rotation sky 2020 which contains this stunning map of Faraday Rotation across the sky (presented in Galactic coordinates, so the plane of the Milky Way appears across the middle of the map):

The abstract of the paper is here:

If you’ll pardon a short trip down memory lane, this reminds me of a little paper I did back in 2005 with a former PhD student of mine, Patrick Dineen (which is cited in the  Hutschenreuter et al. paper).

What we had back in 2005 was a collection of  Faraday Rotation measurements of extragalactic radio sources dotted around the sky. Their distribution is fairly uniform but I hasten to add that it was not a controlled sample so it would be not possible to take the sources as representative of anything for statistical purposes and there weren’t so many of them: we had three samples, with 540, 644 and 744 sources respectively.

Faraday rotation occurs because left and right-handed polarizations of electromagnetic radiation travel at different speeds along a magnetic field line. The effect of this is for the polarization vector to be rotated as light waves travel and the net rotation angle (which can be either positive or negative) is related to the line integral of the component of the magnetic field along the line of sight travelled by the waves. The picture below shows the distribution of sources, plotted in Galactic coordinates and coded black for negative and white for positive.

Some radio galaxies have enormously large Faraday rotation measures because light reaches us through regions of the source that have strong magnetic fields. However, for most sources in our sample the rotation measures are smaller and are thought to be determined largely by the propagation of light not through the emitting galaxy, near the start of its journey towards us, but through our own Galaxy, the Milky Way, which is near the end of its path.

If this is true then the distribution of rotation measures across the sky should contain information about the magnetic field distribution inside our own Galaxy. Looking at the above picture doesn’t give much of a hint of what this structure might be, however.

What Patrick and I decided to do was to try to make a map of the rotation measure distribution across the sky based only on the information given at the positions where we had radio sources. This is like looking at the sky through a mask full of little holes at the source positions. Using a nifty (but actually rather simple) trick of decomposing into spherical harmonics and transforming to a new set of functions that are orthogonal on the masked sky we obtained maps of the Faraday sky for the different samples. Here is one:

(The technical details are in the paper, if you’re interested.) You probably think it looks a bit ropey but, as far as I’m concerned, this turned out surprisingly well!

The most obvious features are a big blue blob to the left and a big red blob to the right, both in the Galactic plane. What you’re seeing in those regions is almost certainly the local spur (sometimes called the Orion Spur; see below), which is a small piece of spiral arm in which the local Galactic magnetic field is confined. The blobs show the field coming towards the observer on one side and receding on the other. The structure seen is relatively local, i.e. within a kiloparsec or so of the observer.

I was very pleased to see this come out so clearly from an apparently unpromising data set, although we had to confine ourselves to large-scale features because of instabilities in the reconstruction of high-frequency components.

Now, 15 years later we have the beautiful map produced by Hutschenreuter et al.

 

You’ll see the vastly bigger data set (almost a hundred times as many sources) and way more sophisticated analysis technique has produced much higher resolution and consequently more detail, especially near the Galactic plane, but we did at least do a fairly good job at capturing the large-scale distribution: the blue on the left and red on the right is clearly present in the new map.

There’s something very heartening about seeing scientific progress in action! This also illustrates how much astrophysics has changed over the last 15 years: from hundreds of data points to more than 50,000 and from two authors to 30!

Follow @telescoper

 

New Publication at the Open Journal of Astrophysics!

Well, Maynooth University may well be still (partially) closed as a result of the Covid-19 pandemic but the The Open Journal of Astrophysics is definitely fully open.

In fact we have just published another paper! This one is in the Astrophysics of Galaxies section and is entitled A Bayesian Approach to the Vertical Structure of the Disk of the Milky Way. The authors are Phillip S Dobbie and Stephen J Warren of Imperial College, London.

Here is a screen grab of the overlay:

 

You can find the arXiv version of the paper here.

I’d like to take this opportunity to thank the Editorial team and various referees for their efforts in keeping the Open Journal of Astrophysics going in these difficult times.

Follow @telescoper

The Gaia Sausage

I had to undertake a top secret mission on Friday, which turned out to be much less exciting than I’d hoped, but at least it gave me an excuse to catch some of the Royal Astronomical Society Open Meeting followed by dinner at the RAS Club. I actually sat next to the Club Guest Michael Duff, the eminent theoretical physicist Michael Duff who gave a nice after-dinner speech.

An artist’s impression of the Gaia Sausage. The Gaia fork has not yet been proved to exist.

The last talk at the RAS Meeting was by Neil Wyn Evans of Cambridge University in the Midlands on the subject of the `Gaia Sausage‘ (which, as you can see, has its own Wikipedia page). The Gaia Sausage is so named because it is consists of a marked anisotropy of the velocity distribution of stars in Milk Way, which is elongated in the radial direction (like a sausage) indicating that many stars are on near-radial (i.e. low angular momentum orbits). This feature has been revealed by studying the second data release from Gaia.

The work Wyn described in his talk is covered by a nice press release from Cambridge University which links to no fewer than five articles on it and related topics, which can all be found on the arXiv here, here, here, here and here.

The most plausible explanation of the Gaia Sausage is that it is a consequence of a major collision between the Milky Way with a smaller galaxy containing about 10⁹ stars about 8-10 billion years ago, as illustrated in this simulation.

I vote that this explanation of the velocity structure of the Milky Way should henceforth be called the Big Banger Theory.

Geddit?

I’ll get my coat.

Follow @telescoper

Gravitational Redshift around the Black Hole at the Centre of the Milky Way

I’ve just been catching up on the arXiv, and found this very exciting paper by the GRAVITY collaboration (see herefor background on the relevant instrumentation). The abstract of the new paper reads:

The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A* is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU, ~1400 Schwarzschild radii, the star has an orbital speed of ~7650 km/s, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z ~ 200 km/s / c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f, with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 +/- 0.09 (stat) +\- 0.15 (sys). The S2 data are inconsistent with pure Newtonian dynamics.

Note the sentence beginning `Over the past 26 years…’!. Anyway, this remarkable study seems to have demonstrated that, although the star S2 has a perihelion over a thousand times the Schwarzschild radius of the central black hole, the extremely accurate measurements demonstrate departures from Newtonian gravity.

The European Southern Observatory has called a press conference at 14.00 CEST (13.00 in Ireland and UK) today to discuss this result.

Follow @telescoper

A Ghost of a Jet?

Last week an article in Nature News caught my eye. Ghostly jets seen streaming from Milky Way’s core was the headline. It’s based on a paper by Su & Finkbeiner recently submitted to the arXiv. There’s even a picture showing the jets in glorious technicolour:

Wow! Impressive stuff. If the jets look like that it’s amazing nobody ever saw them before!

Oh, hang on. The picture is an “artist’s conception”. In other words, it’s what the jets might look like if they actually existed, as imagined by a bloke with a box of crayons.

And how strong is the evidence that they do exist? Here’s the last paragraph of the Nature article (my emphasis):

Although the emissions are dim and the observations don’t have the statistical significance that astronomers require for proof, Baganoff says that several properties make them compelling evidence of jets. They are brighter at higher γ-ray energies and also brighter than the surrounding interstellar medium. They also seem to be long and thin, as would be expected of jets. “Taking all of the evidence together, it appears highly plausible that the features are jets emanating from the Galactic Centre,“ he says.

If they “don’t have the statistical significance that astronomers require for proof” then one wonders why they’re being given so much publicity. In any case the “ghostly jets seen streaming from the Milky Way’s core” can’t be said to have really been “seen” for certain. But they are “highly plausible”. In other words, the authors would like them to be there.

All I can say is that it must have been a slow news day at Nature.

Still, nice drawing.

Follow @telescoper