7 APR 2020
Hi everyone,
It’s Tereza again and in this blog I would like to share with you my first observing experience at the GRANTECAN done in the visitor mode. Visitor mode means that the observing program has some special constraints so it cannot be observed in the normal service mode by only a support astronomer, but the principal investigator of the observing proposal actually needs to be present at the telescope. I bet now you are thinking: It must be so amazing to visit a telescope, likely many programs are in the visitor mode! Surprisingly, the opposite is the case. It’s not that astronomers do not want to visit telescopes, but the visitor mode presents one major possible obstacle and that is the (almost) unpredictable weather. I will come back to this again in a few paragraphs, but let me at first introduce our visitor and their super interesting observing program.
It was my great pleasure to spend three observing nights with the astronomer Montserrat Villar Martin from Centro de Astrobiologia in Madrid. Montserrat is a principal investigator of a proposal aiming to study a special class of objects called quasars. Under normal circumstances I would neither be allowed to speak nor write about these observations. However, Montserrat, who is very keen and active in astronomy outreach, agreed that I can disclose some information from the observations in this blog. Let me explain the science goal of the observations in a bit more detail, with a small warning – I am no expert in this field, and thus some information mentioned can be very simplified and not as clear as if it would be explained by an expert.
At first what is a quasar? The name is a shortcut for a quasi-stellar object, and, as many names and definitions in astronomy, this name has a historical context. Quasars were first observed as sort of mysterious sources of strong radio emission which however looked star-like (that is quasi-stellar) in photographic images. Later it has been shown that quasars are actually located at the centers of distant galaxies that host super massive central black holes accreting surrounding gas and dust. The accretion of the matter on the central black hole is responsible for the immense brightness of the central parts of these galaxies, making them visible and star-like, at visible wavelengths, despite being far away. The accretion of the matter is also responsible for the emission in the radio wavelengths, since the matter is extremely hot (even hotter than inside of the sun) it’s in a so-called ionized state. Such a matter is called a plasma, which basically means it is composed of electrically charged particles (electrons and protons). These are changing their velocity (and are thus accelerating) on curved trajectories due to a magnetic field and consequently they radiate synchrotron radiation and thus also radio waves. Fun fact: the same physical effect, that is radiation emitted by charged particles, is used in TV and radio antennas! Now, we have these strange objects called quasars and more researchers noticed that quasars can be divided into two groups, radio loud and radio quiet. This basically means that radio loud quasars emit way more in radio in comparison to their optical brightness, for radio quiet quasars it is the other way around. This, together with a number of other properties of quasars, still presents open questions that ask to be answered. One way to understand quasars more is to study their effects on the host galaxies and their surroundings. It is expected that the radio loud quasars will impact the host galaxy on a large scale (larger than the galaxy itself), possibly shutting down ongoing star-formation activity and ionizing the gas in the galaxy. In the case of radio quiet quasars, this seems to be more complicated.
Some studies suggest that radio quiet quasars should exhibit a negligible effect on the host galaxy and their activity was expected to be confined to the close vicinity of the galactic center, while others argue the opposite. To contribute to this discussion, Montserrat’s team discovered a radio quiet quasar that actually shows extended (galaxy-sized) emission in radio wavelengths (using VLA 1.4GHz) and quite surprisingly also emission in the Halpha line (GTC-OSIRIS). Emission in the Halpha line suggests that there is gas which is ionized by very young massive stars, and so this emission most likely implies a site of new stars being born, that exhibit a correlated morphology with the radio emission. This offers a very promising new clue on how the energetic jets originating in the galactic center interact with the circum-galactic medium. Thus, Montserrat successfully applied for more time at the VLA radio telescope and at GRANTECAN to study the interaction of the radio jets with the circum-galactic medium for radio quiet quasars. The VLA data confirmed the presence of an extended galaxy-scale radio structure for a number of galaxies, each hosting a radio quiet quasar. Now it’s the turn for the GRANTECAN to show what these quasars look like in Halpha emission (remember, Halpha is a spectral line closely associated with ongoing star-formation).
This is no easy task for any telescope, these objects are quite far away and the Halpha features are going to be faint and thus a large telescope is needed to collect enough photons. Indeed, GRANTECAN is currently the biggest optical telescope on Earth. But, not only that these structures are faint, if they are present, they might be very large and thus an appropriate field of view is needed as well. This is a problem for the instrument MEGARA. MEGARA is an integral-field spectrograph -- basically this means that the instrument takes an image, but each pixel is also a spectrum (in the third dimension). This would present a perfect instrument for the study by Montserrat, as the Halpha line shifts it’s measured wavelength based on the redshift at which the quasar is, and with MEGARA one would only need to extract that region of the spectrum. However, the field of view of MEGARA is, by necessity, small; it basically only covers the central parts of the galaxies in the sample. One could do a mosaic of many images, but that would require a tremendous amount of time and time is very precious (and expensive) at large telescopes, such as the GRANTECAN. So, what are other options? Luckily, the legendary multi-use spectrograph OSIRIS mounted on the GRANTECAN can offer a solution in this situation. OSIRIS has a so-called tunable filter which can be tuned to take an image in a certain wavelength range (also around the Halpha line, as needed) and has a large field of view. The tunable filter is, in essence, a low resolution Fabry-Perot interferometer, and the tuning to a particular wavelength range is set by the separation between the optical plates of the filter.
Because of the sensitive nature of this observation, it was important that Montserrat was on-site, so she could decide what object to observe, if the conditions are good, if the set-up is accurate and then perform a rapid data reduction, basically in real-time, to decide if it is worth to continue observing that target. Together with Montserrat, the first two nights Antonio Cabrera Lavers was also present. He is the science director of the GRANTECAN, and an expert on this instrument and was therewith able to provide correct technical support if needed.
However, as I mentioned at the beginning, weather is hard to predict especially on the long terms and thus, during the first two nights out of three in total, the conditions were not good enough to observe quasars. One of the issues was so called calima: dust in the atmosphere that is brought here from the quite nearby Sahara. In addition to that it was partially cloudy and the seeing, that is the local turbulence in the atmosphere, which blurs the night sky, was too large.
Luckily, on the third night the conditions were just right and thus we managed to observed all the high priority targets! I can provide you with a small spoiler – the preliminary results are more than encouraging and have Halpha detection for basically all the quasars. I am sure the results will be published soon and will contribute to the general understating of the fascinating radio quiet quasars.
Tereza Jeřábková, Astronomical Institute of Czech Academy of Sciences, Ondrejov