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Measuring distances in the Universe: SNIa This exercise was initially produced by J.-C. Mauduit & P. Delva, inspired by a work of N. Ysard, N. Bavouzet & M. Vincendon in France. It is based on 2 images obtained from professional archives (Digital Sky Survey, photographic plates on a 48-inch (~1.2m) Schmidt telescope) and by Marc Serreau (a French amateur astronomer with a C8 (8-inch ~ 20cm) telescope. The quality of the images is moderate, but it is possible to perform astronomical measurements. Supernovae of type Ia can be used as standard candles to measure the distance D of galaxies. It is one method among others (see also the exercise on Cepheid method) to scale the Universe. This method enables to study very distant objects because supernovae are very bright. The luminosity of supernovae of type Ia (SNIa later on) at maximum is the same for all SNIa. SNIa are called “standard candles” and are thus good distance indicators. The absolute luminosity of a SNIa is L = (1,4±0,4)* 1036 Watts (Allen's Astrophysical Quantities). The more distant is the SNIa, the weaker is its apparent luminosity. It is thus possible to determine the distance of a SNIa by comparing its observed flux with its (absolute) luminosity. More precisely, at a distance D from the star, the luminosity is distributed on a sphere of radius D and of surface S = 4*π*D². By definition, its flux F (in W/m²) at the distance D is F = L/S where L is the luminosity in W. One can derive the distance D of the galaxy: D = √ (L / 4πF) In 2005, a SNIa has exploded in the NGC691 galaxy. Its name is SN2005W. We are going to measure the distance of NGC691 with the help of this SN. 1. Open the image file images_snia/NGC691_13111953.fits with SalsaJ. On this image, taken in November 1953 (i.e. before the explosion of the SN in 2005), one can note a bright star A on the left hand side of the galaxy. 2. Identify the star A and the reference star, as indicated on the figure above with yellow arrows. 3. Measure the intensity of the star A and the reference star with the tool Photometry Check that the radius of the star and that the sky background obtained is coherent with the pixel values you can see on the image. 4. The flux of the reference star is known: Fref = 5.24×10-14 W/m². Estimate the flux of the star A: FA = SN2005W has exploded next to star A and those two stars are superimposed on the image. It is not possible to measure their intensities separately. 5. Open the image file images_snia/NGC691_07022 005.fits. This image has been taken after the explosion of SN2005W (located very close to the star A). 6. Identify the objects in the 2 images In order to identify the various objects in the field adjust the contrast with ImageAdjustBrightness/Contrast If you are not sure of the identification of the object, you can measure distances and angles between different point sources. For this, select an image, click on Analyse/Tools/ROI manager, a ROI manager window opens, draw a straight line selection and click on add [it], then repeat the operation. Once you have drawn several lines you click on Measure and you will have a measure window displaying the positions, lengths and angles of all the lines. This enables to compare the features between the 2 images. 7. Measure the total intensity of SN2005W and the star A. Check the sky background obtained is coherent with the pixel values you can see on the image. 8. As before, derive the combined flux of SN2005W and of the star A. Ftot = FSN2005W + FA = using the measurement of the intensity of the reference star and the combined stellar image (S20005W and A). 9. Estimate FSN2005W = 10. The absolute luminosity L of the SNIa is given in the introduction, estimate the distance1 to NGC691: DNGC691 = ............ m = ............. Mpc. 11. As you know the error on L, estimate the error on DNGC691. 12. You can compare your results with the professional measurements obtained for this galaxy, available on the NASA/IPAC Extragalactic Database (NED) http://nedwww.ipac.caltech.edu/forms/byname.html Star A Reference star & supernova position th Epoch 1 (13 November 1953) Intensity measurement2 A = Ref = Flux (Wm-2) F A= FRef =5.24×10-14 Supernova not detected Epoch 2 (7th February 2005) 1 2 Intensity measurement2 A + SN = Ref = Flux (Wm-2) FA+ FSN = FRef =5.24×10-14 FSN = Luminosity (W) LSNmax = 1.4±0.4 1036 Distance D= √ (LSN / 4πFSN)= 1 parsec = 3.086 1016m; 1 light year = 9.461.1015m Instrumental unit History Saul Perlmutter Supernova Cosmology Project Brian Schmidt Adam Riess High- z Supernova Search Team In 1998, both teams started to study the positions of SNIa in the far Universe. The challenge was difficult: there is only one SNIa occurring per galaxy per millennium! Fortunately, there are a lot of galaxies in the Universe. By studying thousands of galaxies, they managed to found enough SNIa, and began to realize something: the furthest ones appear less bright than expected! At the same time, the two teams realized that only a Universe in accelerated expansion can explain this effect, and published their results in scientific papers. Their discovery implied that the Universe was filled by an energy that “pulls” the galaxies apart, called the Dark Energy. It opened so many new perspectives in cosmology that in 2011, both teams were awarded a Nobel prize. Basics on stellar photometry In SalsaJ, we can perform photometry of stars. The idea is to compute the instrumental flux3 (or intensity) of one star. As they are very distant, stars are (even from space) point sources for the observer. However, our instruments (refractor or telescope) have a transfer function: it includes the diffraction limit and the imperfections of the mirrors (quality of the surface, etc.), and the atmosphere (turbulence) further enlarges it. The photometry tool computes the sum of the pixel flux (or intensity) and subtracts the contribution of the background (moon and various parasite lights). The column entitled Star’s intensity provides this quantity. You have to check the other parameters to be sure that your measurements are correct. The sky intensity can be checked with the pixel values of your images (by scrolling the mouse on the image in SalsaJ), while you can check what is inside the sky’s radius if there are unexpected results. In principle, you should use the same Star’s radius for all your measurements on a single image, as the resolution (and the transfer function) is in principle the same all over the image (unless there are some distortions). Checking the value of the Sky’s intensity is important in crowded fields or if there is a strong background (e.g. a gradient). You can check the resolution of your image with a slice on one star: for one image the full width half maximum should be the same for all stars. Plot profile of one star: this displays the transfer function. The Full Width Half Maximum (FWHM) is about 4-5 pixels, and is the same all over the image. 3 We measure a quantity proportional to the number of photons that reaches the instrument of observation (camera CCD mounted in a telescope). SalsaJ determines the photometric parameters automatically. In some cases, it is necessary to further tune or fix them. To change the star’s radius, you should use the Photometry Settings, and click on Forced Star Radius. It is then possible to change it and check it interactively on the image. Similarly, you can modify the other parameters: this is usually necessary for special cases (if there are perturbations close to your star). It is possible to save the content of the Photometry window as an excel file 4 This project has been funded with support from the European Commission. This publication [communication] reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein.