Archive for James Webb Space Telescope

Recalibration of Ultra-High-Redshift Galaxies

Posted in Astrohype, The Universe and Stuff with tags , , , , on August 10, 2022 by telescoper

Remember all the recent excitement about the extremely high redshift galaxies (such as this and this; the two examples shown above) “identified” in early-release JWST observations? Well, a new paper on the arXiv by Adams et al using post-launch calibration of the JWST photometry suggests that we should be cautious about the interpretation of these objects. The key message of this study is that the preliminary calibration that has been in widespread use for these studies is wrong by up to 30% and that can have a huge impact on inferred redshifts.

The new study does indeed identify some good candidates for ultra-high-redshift galaxies, but it also casts doubt on many of the previous claims. Here is a table of some previous estimates alongside those using the newly recalibrated data:

You will see that in most – but not all – cases the recalibration results in a substantial lowering of the estimated redshift; one example decreases from z>20 to 0.7! The two candidates mentioned at the start of this post are not included in this table but one should probably reserve judgement on them too.

The conclusive measurements for these objects will however include spectroscopy, and the identification of spectral lines, rather than photometry and model fits to the spectra energy distribution. Only with such data will we really know how many of these sources are actually at very high redshift. As the philosopher Hegel famously remarked

The Owl of Minerva only spreads its wings with the coming of spectroscopy.

Now a Galaxy at z>16?

Posted in The Universe and Stuff with tags , , , , , , on July 26, 2022 by telescoper

It’s less than a week since I posted an item about an object which is possibly the highest redshift galaxy ever observed (with z ~13) and now along comes a paper describing an object that may be of even higher redshift (with z~16.7). The abstract of the new paper – lead author of which is Callum Donnan of the University of Edinburgh – is here:

As with the previous object the redshift of this one is not obtained via spectroscopy (which usually involves the identification of spectral lines) but via fitting a spectral profile to photometric imaging data seen in different bands. The process for this galaxy is illustrated by this diagram from the paper:

There are 7 images along the top showing the source through various broad band filters. Suitably calibrated these can be converted to the flux measurements shown on the graph. Notice the first three images are significantly fainter than the others, so the first three points on the left of the graph are lower.

If this is a galaxy its spectrum is expected to possess a Lyman Break resulting from the fact that radiation of shorter wavelength than the Lyman Limit (912 Å) is absorbed by neutral gas surrounding the regions where stars are formed in the galaxy. In the rest frame of a galaxy this break is the ultraviolet region of the spectrum but because of the cosmological redshift it is observed in the infrared part of the spectrum for very distant galaxies. In this case the best fit is obtained if the break is positioned as shown, with the first three fainter points to the left of the break and the rest to the right. The break itself is straddled by two observational bands. Employing a number of different estimates the authors conclude that the redshift of this galaxy is z=16.7 or thereabouts.

There is no direct evidence for the sharp edge associated with the Lyman Break – and no spectral lines are observed either – so this all depends on the object being correctly identified as a high-redshift galaxy and not some other object at lower redshift. You have to assume this to get a redshift, but then all inferences are based on assumed models so there’s nothing unusual about this approach. The authors discuss other possibilities and conclude that there is no plausible alternative source. Take away the green template spectrum and you just see a spectrum that rises to a peak and falls again. The authors claim that there is no plausible low-redshift source with such a spectrum.

Anyway, here is a composite colour image of the source:

So is this now the earliest galaxy ever observed? And what object will I be asking this question about next week? One thing I can predict is that there are going to be many more such objects in the very near future!

The Earliest Galaxy we’ve seen?

Posted in Astrohype, The Universe and Stuff with tags , , , on July 20, 2022 by telescoper

The red smudge in the centre of this image is thought to be a galaxy with a redshift of around z=13, as seen by the NIRCam instrument on the James Webb Space Telescope. This redshift estimate is based on photometry so the object remains a candidate rather than a confirmed high-redshift galaxy, but if confirmed spectroscopically this would be the highest-redshift galaxy yet observed.

For more details on the observations and their implications see the preprint on arXiv here. It’s interesting (and challenging) that there are such bright galaxies at such an early stage of cosmic evolution, assuming of course that the redshift is correct. Photometric redshift estimates have been wrong before.

If we take the estimated redshift at face value and adopt the standard cosmological model, the lookback time to this galaxy (GLASS-z13) is about 97.6% of the current age of the Universe so we’re seeing it as it was just 330 million years after the Big Bang. It could therefore be the earliest galaxy we have seen. It isn’t very accurate to say that it is the oldest galaxy we’ve seen, as we are probably seeing it as it was when it was very young.

These observations come from JWST Early Science Release Programmes so are just a taster of what is to come. No doubt we’ll hear much more about high-redshift galaxies from JWST in future and there’s every chance that they will change our view of the high-redshift Universe in dramatic ways.

I’ll just mention here that I’m old enough to remember going to conferences where “high redshift” meant z=0.5! In those days the highest redshift objects were quasars, but they have long since been overtaken.

Characterization of JWST science performance from commissioning

Posted in The Universe and Stuff with tags , , , on July 14, 2022 by telescoper

I don’t suppose it will take very long for science papers based on the first data from JWST to start appearing on arXiv but I haven’t seen any yet. There is however a very important with an uncountable number of authors, led by Jane Rigby, that describes the commissioning process. Uncountable by me, that is.

Here is the abstract:

This document characterizes the actual science performance of the James Webb Space Telescope (JWST), as known on 12 July 2022. Following six months of commissioning to prepare JWST for science operations, the observatory is now fully capable of achieving the discoveries for which it was built. A key part of commissioning activities was characterizing the on-orbit performance of the observatory. Here we summarize how the performance of the spacecraft, telescope, science instruments, and ground system differ from pre-launch expectations. Almost across the board, the science performance of JWST is better than expected. In most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.

Although it’s very long (60 pages) it’s well worth reading for an account of how meticulously the various calibrations etc were done. Various objects make cameo appearances, including Jupiter:

James Webb: the wrong name for a Space Telescope

Posted in History, LGBT, The Universe and Stuff with tags , , , on July 13, 2022 by telescoper

Following yesterday’s excitement about the new images from the James Webb Space Telescope I thought I’d share this video documentary that explains why the choice of name for this facility is highly inappropriate and should be changed. This is a matter I’ve blogged about previously, in fact, but the video is new.

Those First Results from JWST

Posted in The Universe and Stuff with tags , , , , , on July 12, 2022 by telescoper

As promised in my post earlier today, we gathered in a small lecture theatre in Maynooth to watch the “reveal” of various new images and other data from the James Webb Space Telescope. The images are indeed wonderful and spectacular, but the video stream was excruciatingly bad to watch, with more technical glitches than I’ve had hot dinners. It was like an astronomical version of Acorn Antiques!

Anyway, you can find them all the new results together with explanations and descriptions here so Ill just put up a gallery here:

Those are the four results released today. This image was previewed last night and appeared in my post earlier today:

I couldn’t resist, however, adding this spectrum of a faint reddish galaxy in the above image:

This spectrum is taken using the NIRSpec instrument on JWST. The observed wavelength along the horizontal axis is measured in microns. If I’ve got the line identifications correct I think this galaxy is at an amazing redshift of about z=8.5. Amazing. High redshift galaxy spectra obtained are usually a lot rattier than this. I think this demonstrates that JWST is going to revolutionize the field of galaxy formation.

The First Deep Field from JWST

Posted in Astronomy Lookalikes, The Universe and Stuff with tags , , , , , on July 12, 2022 by telescoper

I have to say that I didn’t stay up to watch the live stream of last night’s preview of this afternoon’s release of the first images from the James Webb Space Telescope. It started very late and I got sick of listening to the dreary music on the feed so went to bed. Nevertheless here is the first picture:

Credits: NASA, ESA, CSA, and STScI

This is a deep field image taken using JWST’s NIRCAM (Near-Infrared Camera). Note that the artifacts you see around some objects are diffraction spikes which occur around bright sources; their six-fold symmetry reflects the hexagonal structure built into the JWST’s mirror assembly. Sources sufficiently bright and compact enough to cause these spikes in deep field images are foreground stars: the extended, fainter objects are all much more distant galaxies.

The description from the NASA page is:

NASA’s James Webb Space Telescope has produced the deepest and sharpest infrared image of the distant universe to date. Known as Webb’s First Deep Field, this image of galaxy cluster SMACS 0723 is overflowing with detail.

Thousands of galaxies – including the faintest objects ever observed in the infrared – have appeared in Webb’s view for the first time. This slice of the vast universe is approximately the size of a grain of sand held at arm’s length by someone on the ground.

This deep field, taken by Webb’s Near-Infrared Camera (NIRCam), is a composite made from images at different wavelengths, totaling 12.5 hours – achieving depths at infrared wavelengths beyond the Hubble Space Telescope’s deepest fields, which took weeks. 

The image shows the galaxy cluster SMACS 0723 as it appeared 4.6 billion years ago. The combined mass of this galaxy cluster acts as a gravitational lens, magnifying much more distant galaxies behind it. Webb’s NIRCam has brought those distant galaxies into sharp focus – they have tiny, faint structures that have never been seen before, including star clusters and diffuse features. Researchers will soon begin to learn more about the galaxies’ masses, ages, histories, and compositions, as Webb seeks the earliest galaxies in the universe

Here is a close-up of one of the distorted galaxy images and othe features produced by gravitational lensing:

We’re having a special viewing in Maynooth this afternoon of the press conference which will unveil more new images from JWST – nice telescope, shame about the name. I may add comments on here if anything particularly exciting turns up. You can watch it here:

Let’s hope this one starts on time!

First Light at L2 for JWST

Posted in History, The Universe and Stuff with tags , , , on February 5, 2022 by telescoper

After a successful launch, subsequent deployment of its sunshield and mirrors, and arrival at its orbit around the Second Lagrange Point, the goal now for the James Webb Space Telescope is to align the optical components of the telescope to the required accuracy. This is not a simple task – each of the segments of the main mirror has to be aligned to within a fraction of a wavelength of the light it will observe (in the near-infrared part of the electromagnetic spectrum) – and it will take several months to complete. However, we did hear yesterday that the telescope has now seen “first light”, in the sense that the first photons have landed on its detectors. The first images to be formed will be blurry and distorted, but these will be used to adjust the components until they reach the required sharpness.

For more details of this process see here.

Incidentally, it is worth saying a little bit about L2, the second Lagrange point of the Earth-Sun system. As the diagram below shows, this orbits the Sun at a greater distance from the Sun than the Earth. According to Kepler’s Laws, and ignoring the Earth’s gravitation, a test particle placed in a circular orbit at this radius would move more slowly than the Earth and would not therefore hold a fixed position relative to the Earth and Sun as it went around. The effect of the Earth’s gravity however is to supply an extra force to speed it up a bit, so it can keep up and thus remain in a fixed configuration relative to both Earth and Sun.

The opposite applies to L1: an object placed here would, in the absence of Earth’s gravity, move more quickly and thus pull ahead of the Earth. Having the Earth there holds it back by just the right amount to maintain a fixed position in the rotating frame.

The interesting thing about L1 & L2 is that while they are both equilibrium points, they are both unstable to radial perturbations. An object placed at either of these points would move away if disturbed slightly. JWST does not therefore just sit passively at L2 – it moves in a so-called halo orbit around L2 a process which requires some fuel. It’s not that there’s an actual mass at L2 for it to orbit around, but that its motion produces a Coriolis Force that keeps it from moving away. It’s very clever, but does require a bit of energy to keep it in this orbit.

Unlike L1 & L2, the Lagrange Points L4 & L5 are stable and therefore attract all kinds of space junk, such as asteroids, cometary debris, and preprints by Avi Loeb.

Another interesting Lagrange Point is that Joseph-Louis Lagrange was born in 1736 in Turin, but that does not mean that he was Italian. At that time Italy did not exist as a political entity; in 1736 Turin was part of the Kingdom of Sardinia. Although born in the part of the world now known as Italy, he was never an Italian citizen. In fact he lived most of his life in Berlin and Paris and died in 1813, long before the Kingdom of Italy was founded (in 1861).

Webb Deployment

Posted in Biographical, The Universe and Stuff with tags on January 8, 2022 by telescoper

I’ve been busy all afternoon supervising an online examination and I’m now about to cook my dinner, which means I haven’t got time to post anything much but I will pass on the very welcome news that the James Webb Space Telescope has now completed the lengthy sequence of operations that were required to deploy a large sunshield, the primary and secondary mirrors and other bits and bobs needed to turn it into a proper observatory rather than just a basket of deployables. The whole thing should now look something like this:

Artists impression of Webb (with, by the look of it, a bit of the Cosmic Web)

There were over 300 potential single-point failures in this sequence so these last couple of weeks have been as stressful as the launch. Now commissioning work can begin.

This all gives me the excuse to have a celebratory drink just like I did on Christmas Day. Cheers!

On Nominal

Posted in The Universe and Stuff with tags , on December 26, 2021 by telescoper

Yesterday’s launch of the James Webb Space Telescope reminded me of something I blogged about many years ago. At regular intervals during the launch we heard staff at Mission Control in Kourou saying that everything was “nominal”. As always when wondering about the meaning of words, I searched in the One True Chambers Dictionary, where I found:

nominal, adj relating to or of the nature of a name or noun; of names; by name; only in name; so-called, but not in reality; inconsiderable, small, minor, in comparison with the real value, hardly more than a matter of form…

So was the so-called launch of JWST only in name, but not in reality? Was it faked? Is the telescope real?

That reminds me that years and years ago I had an idea for a crime novel with a plot that revolves around the murder of a prominent cosmologist just as some important scientific discovery is about to be announced. Suspicion gathers that the whole thing is an enormous hoax and the discovery bogus. But the experiment is shrouded in secrecy, and was so expensive that it can’t easily be repeated, so  who can tell, and how?

I wouldn’t put it past some conspiracy theorist to argue that the data from JWST (assuming there is some) is manufactured.

It’s very difficult to know for sure whether any scientific discoveries are genuine or not, even if the data and analysis procedures are made public. There’s always the possibility that everything might have been fabricated or simulated, but in most cases the experiment can be repeated at a later date and the fraud eventually exposed, such as in the Schön Scandal.  In Big Science, this may not be practicable. However, Big Science requires big teams of people and the chances are someone would blow the whistle, or try to…

Anyway, I know that there are people out there who take everything I write on this blog absurdly literally so I’ll spell it out that I am in no way suggesting that the James Webb Space Telescope is a fraud. Or predicting that there’ll be a murder just before the first observations are released. Any similarity purely coincidental and all that. And I’ve never had time to write the book anyway – perhaps a publisher might read this and offer me an advance as an incentive?

Moreover, going back to the Chambers Dictionary, I note the final definition given there and omitted above i

…according to plan (space flight)

So that’s that. Nothing sinister. I’m not sure how “nominal” acquired that meaning, mind you, but that’s another story…