Archive for Lyman-alpha absorption

Weekly Update from the Open Journal of Astrophysics – 16/08/2025

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , , , , , , , , on August 16, 2025 by telescoper

It’s time once again for the usual update of papers published at the Open Journal of Astrophysics which I do every Saturday. Since the last update we have published two new papers, which brings the number in Volume 8 (2025) up to 116, and the total so far published by OJAp up to 351. The summer lull we always expected is now upon us, so this will be a shorter post than we have had of late.

The first paper to report this week is “The reflex instability: exponential growth of a large-scale mode in astrophysical discs” by Aurélien Crida (Université Côte d’Azur, France), Clément Baruteau (Université de Toulouse, France), Jean-François Gonzalez (Universite Claude Bernard Lyon 1, France), Frédéric Masset (Universidad Nacional Autonoma de México) and Paul Segretain, Philippine Griveaud, Héloïse Méheut & Elena Lega (Université Côte d’Azur).  This paper was published on Tuesday August 12th 2025 in the folder marked “Earth and Planetary Astrophysics“. It discusses a exponentially-growing instability in gas discs around stars caused by the motion of the central star in response to the disc.

The overlay – which you can make larger by clicking on it – is here:

You can find the officially accepted version on arXiv here.

The other paper this week, published in the folder Cosmology and Nongalactic Astrophysics, is “The galaxy-IGM connection in THESAN: the physics connecting the IGM Lyman-alpha opacity and galaxy density in the reionization epoch” by Enrico Garaldi (University of Tokyo, Japan), Verena Bellscheidt (Technical University of Munich, Germany), Aaron Smith (York University, Canada) and Rahul Kannan (University of Texas at Dallas, USA).  It presents a study of the relation between the Lyman-alpha effective optical depth of quasar sightlines and the distribution of galaxiesas as a probe of ionized regions around sources of photons. It was published on Wednesday August 13th 2025.

The overlay is here:

 

You can find the officially accepted version of the paper on arXiv here.

That concludes the papers for this week. I’ll do another update next weekend, though I expect things will remain relatively quiet until September.

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Absorbed in a Quasar Spectrum

Posted in The Universe and Stuff with tags , , , , , , on December 5, 2016 by telescoper

Many people seem to think that astronomers spend all their time looking at pretty pictures of stars and galaxies. Actually a large part of observational astronomy isn’t about making images of things but doing spectroscopy. In fact the rise of astronomical spectroscopy is what turned astronomy into astrophysics. But that’s not to say that spectra can’t be pretty either. Here is an example (from here) which shows the light from the quasar HE0940-1050 taken by the UVES instrument mounted on ESO’s Very Large Telescope in Chile.

This quasar is an interesting object, at a redshift of z= 3.0932 (which converts to a look-back time of about 11.6 billion years). The dark bands and lines you can see in the spectrum are caused by absorption of the light from the quasar by clouds of hydrogen gas between the quasar and the observer; the strength of the absorption indicates how much gas the light from the quasar has travelled through.  The absorption occurs at a particular wavelength corresponding to the Lyman-α transition but, because the clouds are all at different redshifts, each produces a line at a different observed wavelength in the quasar spectrum. There are many lines, which is why the collection of clouds responsible for them is often called the Lyman-α Forest. In effect the quasar sample is very much like a core sample, as if we were able to drill back in time to the quasar through the material that lies along the line of sight.

This spectrum is particularly remarkable because of the number of faint lines that can be seen: it’s like a detailed DNA Fingerprint of cosmic structure. It’s also very pretty.

 

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