Bachelor thesis project, VT-2019 Stellar populations in the Green Pea galaxy J1457+2232 Jan Malmgren University of Stockholm, Astronomy department, Sweden March 3, 2019 1 The galaxy swings around like a wheel of lighted smoke, and the smoke is made of stars. It is sunsmoke. For lack of other words we call it sunsmoke, do you see. I don’t feel languages are equal to what that vision comprehends. The riches of the languages we know, Xinombric, has three million words, but then the galaxy you’re gazing into now has more than ninety billion suns. Has there ever been a brain that mastered all the words in the Xinombric language? Not a one. Now you see. And do not see. ANIARA (poem 85), Harry Martinsson 2 Abstract In this report I present a study of possible age gradients in the Green Pea galaxy J145735.13+223201.8 to be able to conclude if there is an extended star forming history in such a galaxy. Data are coming from two different sources, highly resolved images in four different wavelengths of stars in the galaxy, and of nebular gas in a narrow band H Balmer line filter, from the Hubble Space Telescope1 (HST), as well as spectral line information from the Sloan Digital Sky Survey2 (SDSS). I compare the observations with stellar population models from two different libraries, Yggdrasil and Starburst99. Due to the highly resolved images from HST this is one of the first studies of spatially resolved stellar populations in a Green Pea galaxy. With the help from these spatially resolved images it was possible to study star clumps independently from each other. This would not be possible when using only data from SDSS. In this way it was possible to conclude an age difference between the centre of the galaxy and its outskirts. I found that the galaxy has an age gradient at a confidence level greater than 95%. 1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program #9368134 2 Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org. SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, Harvard-Smithsonian Center for Astrophysics, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatório Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University. 3 Content 1. Introduction .................................................................................................................... 6 2. The Data ......................................................................................................................... 7 2.1. The Data from HST ................................................................................................. 8 2.2. The Data from SDSS ............................................................................................... 8 3. Estimating galaxy properties from SDSS data ............................................................... 9 3.1. Estimating the metallicity ........................................................................................ 9 3.2. Estimating the reddening ....................................................................................... 10 3.3. Estimating the star formation rate (SFR) ............................................................... 11 4. Photometric analysis of the galaxy ............................................................................... 11 4.1. Identifying star clumps .......................................................................................... 11 4.2. Flux measurements ................................................................................................ 13 4.2.1. Basic considerations ....................................................................................... 13 4.2.2. Star clumps measurements ............................................................................. 13 4.2.3. Surface brightness .......................................................................................... 14 4.2.4. Compactness of the galaxy ............................................................................. 16 4.3. Magnitude measurements ...................................................................................... 16 4.3.1. Converting to magnitude ................................................................................ 16 4.3.2. Calculating error in magnitude difference ...................................................... 17 4.3.3. Colour maps of the galaxy .............................................................................. 17 4.3.4. Colour difference vs. colour difference .......................................................... 18 4.4. Suggestions for future improvements .................................................................... 19 5. Discussion ..................................................................................................................... 20 5.1. Age gradients in the galaxy ................................................................................... 20 5.1.1. Age by colour maps and surface brightness ................................................... 20 5.1.2. Age estimate by colour difference vs. colour difference analysis .................. 21 5.1.3. Age estimate by Hline strength .................................................................... 22 4 5.1.4. Age map based on Hline strength ................................................................ 23 5.1.5. Age estimate by colour surface plots.............................................................. 24 5.2. Is our galaxy really a Green Pea galaxy? .............................................................. 26 6. Conclusions .................................................................................................................. 27 7. Acknowledgement ........................................................................................................ 28 8. References .................................................................................................................... 30 5 1. Introduction The Universe contains an almost un-numerable amount of galaxies. Each galaxy contains anywhere from 106 to 1012 stars. The galaxies comes in many different types and shapes, even colours are different. As most of the light comes from the stars they will determine the colour of the galaxy. The most massive stars are the hottest and are therefore the bluest. Stars with lower masses will be cooler and have a redder light, even going into infrared for the smallest stars. As the most massive stars burn their nuclear fuel much faster than less massive stars they will have a shorter life, in the order of a few Myr, on the main sequence. They leave the main sequence when they have exhausted all hydrogen in the core and become red giants or supergiants. This means a bluer part in a galaxy contains young stars, or red parts of the galaxy is older due to the lack of young blue stars. Regarding types and shapes there are the elliptical ones, with very low star formation rate (SFR), spiral ones with spiral arms like our Milky Way with most of its star formation in the disk. In between elliptical and spiral ones we have the lenticular ones with a rotating disc and a bulge but with no spiral arms. Then there are irregular galaxies that lack a clear structure. In this report we will look at one type of these irregular galaxies, namely a Green Pea galaxy, with very high SFR. Green Pea galaxies are a sub-class family of star forming galaxies at redshift around 0.1 - 0.3. The characteristic green colour is a result of extremely bright nebular line emission in the 8.5 10 [OIII]5007 line. Green pea galaxies are characterized by low mass 10 -10 M⊙, high SFR ≳10 M⊙/yr, EW[OIII]5007 typically > 200Å, low metallicity 12+log(O/H) of 7.6 - 8.4 and low reddening E(B-V) < 0.26 (Cardamone, et al., 2009; Izotov, Guseva, & Thuan, 2011). For the reionization of the Universe it is believed compact (dwarf) galaxies can be a major contributor (Verhamme, et al., 2016). These compact