Galaxy Metallicity: What Oxygen Tells Us About The Lifecycles of Galaxies Designed by Prof Jess Werk, modified by Marie Wingyee Lau

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1 Introduction Galaxy Metallicity: What Oxygen Tells Us About The Lifecycles of Galaxies Designed by Prof Jess Werk, modified by Marie Wingyee Lau Before stars and galaxies came into existence, the very young universe was made only of the lightest, simplest elements (i.e. hydrogen, helium). The first massive stars to form over 10 billion years ago were the first to generate heavier elements. Astronomers call all elements heaver than lithium, including oxygen, metals. In galaxies near and far, all so-called metals have been fused in the cores of stars or formed during explosive events as stars end their lives. Thus, when we measure the metal content of gas in galaxies, we are measuring the amount of gas that has been cycled through a previous generation of stars. Understanding the metal content of galaxies and how it varies with different stellar populations and other galaxy properties is of fundamental importance to extragalactic astronomers. In this project, you will measure the metal content of 63 star-forming galaxies by analyzing their spectra with Excel or Numbers. In astronomy, we call the metal content of a galaxy metallicity. Galaxy metallicity has many proxies, but one of the most common is the number density of oxygen atoms relative to that of hydrogen atoms, sometimes also called oxygen abundance. In galaxies that are actively forming stars, you can measure the total intensity of the emission lines in their spectra to determine how much oxygen they have relative to hydrogen. You will also use hydrogen emission lines (Halpha) to measure the rate at which the galaxies are forming stars (star formation rate, in mass per year, aka SFR). Once you have measured galaxy metallicities, you will see if it relates at all to the stellar content ( stellar mass ) of a galaxy and/or its star formation rates. You will be asked to draw conclusions about how metallicity relates to the properties of galaxies, and what it tells you about how galaxies process metals throughout their lives. Getting Started The file called COSMOS_galaxyinfo.list is a simple text file with 6 columns that has some helpful information about each galaxy you will be studying. The first column is the galaxy name, the second and third are the right ascension and declination of the galaxies (their precise coordinates, like longitude and latitude, only in the sky), the fourth and fifth are their apparent brightnesses (called apparent magnitude in astronomy) in two different color filters called gband and r-band from the SDSS galaxy survey, and the last column contains the logarithm (Base 10) of the total stellar mass of the galaxy in units of solar masses. One solar mass the the mass of our sun, or kg. These are stellar masses of galaxies, so they will have anywhere betwen 10 8 and solar masses of stars in them (i.e. billions of suns). The instructions you are now reading are also contained in the directory in pdf format: COSMOSgalaxymetallcity_instructions_MarieLau.pdf. There is another file for instructions written by the original author of this project, Prof Jess Werk. That set of instructions requires installation of a software package named IRAF, which is more fun but also more complicated. My set of instructions requires knowledge of spreadsheet software only. You are free to use Python or other languages and packages.

2 For every galaxy in the list above, there should be at least one.fits spectrum (some have separate red and blue depending on the spectrograph used to take them) and one.pdf image of the spectrum. All of the galaxies should have prominent emission line features! As an aid to you, all of these emission lines are labled in the pdf files. You cannot plot a.fits spectrum with a spreadsheet software, so please go to the asciifiles folder to find the spectra. Their content are identical to the fits files. Open one of the ascii files of spectra using your spreadsheet software and inspect, such as, J _67_24_blueasciispec.txt. If you use Excel, you may open the plain text file directly. If you use LibreOffice, you have to start a blank spreadsheet first before you open the file. If you use Google sheet, rename the file extension from.txt to.csv first, and then under File click Import, then under Import action select Insert new sheet(s). The first column is wavelength in units of angstroms, which is m). The second column is flux, a.k.a. brightness or intensity, in units of erg s -1 cm -2 angstrom -1. If you use Excel or Google sheet, plot as a broken-line graph with the data symbol set to none. If you use LibreOffice, plot as an X-Y scatter plot with no symbols and line set to continuous. The x-axis should be wavelength and the y-axis should be flux. You might want to label the axes. If your computer becomes very slow, try to plot only the first 1000 data points first, and then the next 1000 data points. The galaxies that have blue and red spectra separated were taken with the LRIS spectrograph on the Keck telescope in Hawaii. The galaxies whose spectra have blue and red (short and long wavelength) all in one file were observed as part of the SDSS survey. All galaxies have SDSS images available. If you would like to see images of the galaxies whose spectra you will examine, go to the website:

3 here, you can make color finder charts for all of the galaxies by entering their RA and dec into the left-hand parameter tool. The easiest way to do this is to copy and past the RA and dec into the appropriate boxes (the current format will convert to decimal degrees) and hit Get Image. You can zoom in and out, and highlight certain objects. Some of the galaxies are faint little smudges, and some are nice spirals. In general, you can assume for now that the faint smudges are farther away, so they don t look as pretty. Part One: Emission line Measurements You will be using spreadsheet to measure the emission line fluxes of several emission lines in each galaxy. the emission lines of the galaxies as seen in their spectra are redshifted according ot how fast each galaxy is moving away from us. 1. Start with the J _67_24 galaxy. Open the.pdf spectrum to see which emission lines are present in the spectrum and where to look for them. The top x-axis shows you the wavelengths of the line in the galaxy rest-frame (i.e. their unredshifted values). The bottom x- axis shows you the wavelength as we observe it (redshifted values). Labeled are one [OII] emission line at rest-frame 3727 angstroms, the Hbeta emission line at rest-frame 4683 angstroms, two [OIII] emission lines at 4960 and 5008 angstroms, the Hbeta emission line at rest-frame 6563 angstroms, and one [NII] emission line at rest-frame 6583 angstroms. 2. Plot the spectrum. Start with the blue side. For the J _67_24 galaxy, there is one emission line in the blue spectrum, and it is from ionized oxygen [OII], a forbidden transition of oxygen at 3727 angstroms in its rest-frame. Since this galaxy is redshifted, the [OII] emission line appears at about 4550 angstroms.to zoom in on an emission line, plot +/- tens of angstroms around that emission line. 3. Once you are zoomed in on an emission line, calculate the total flux in the line above the galaxy continuum. You can visually estimate where the continuum level is around +/- tens of angstroms around that emission line, excluding the emission line. Sutract each data point on that emission line by the continuum level. For each data point, multiply the flux value by the

4 separation between it and the next data point on the wavelength axis, which is roughly 1.1 angstroms for the example galaxy. Then, sum up these values. 4. Then measure the central wavelength of that emission line. To do this, calculate a weighted mean of the wavelength values of those data points on the emission line, weighted by their flux values. 5. You will want to record the following values in a file for each galaxy. You can make a simple text file or word file table, or you can enter the values into another spreadsheet like so: Galaxy Name Emission line Obs Center Wave Flux J _67_24 [OII] Call this file, for instance, J _67_24_linemeasurements (with a file extension depending on whatever you choose to use). Measure the flux and central wavelengths of the emission lines in all 63 galaxies. Keep good records of the values you measure, since you will use them later. Here is a list of the emission lines you will want to measure, and their rest-frame wavelengths. You can ignore some of them. [OII] at 3727 Angstroms Hbeta at 4861 Angstroms [OIII] at 5008 Angstroms Halpha at 6563 Angstroms [NII] at 6583 Angstroms If a galaxy is missing any one of these emission lines (i.e. it is not detected), make a note of it, and enter xxx and xxx in the Obs Center wave and Flux columns of your line measurements file. Part Two: Calculate the Redshift of Each Galaxy For this calculation, you will compare the rest-frame wavelengths of the emission lines and the observed wavelengths of the emission lines, and obtain the redshift of the galaxy. This redshift will translate into a distance using the Hubble Law (look it up). Galaxies further from us are moving away from us faster. Redshift = z = (Obs Wavelength - Rest Wavelength) / Rest Wavelength. Thus, in the example, using only the [OII] emission line, I would calculate z to be ( ) / 3727, or You ll want to do this for ALL five (or however many lines you measure) of the emission lines and take an average to get the redshift for each galaxy. Each emission line should give fairly consistent results (within 0.03 or so). If it does not, it s a BIG hint that you measured the wrong line! Then, use Hubble s Law to get the distance to each galaxy using the average redshift you calculated:

5 Distance = v / H_0 H_0 is the Hubble constant, 70 km/s/mpc v is the velocity of your galaxy. You can get this from the redshift by noting that z ~ v/c for nearby galaxies, where c is the speed of light (3 x 10 5 km/s). You will get a v in km/s and the distance will be in units of Mpc! Start a file and keep track of all these measurements for each galaxy. You will add columns to this file as you continue the project. You will have one final file, called: GalaxyResults (do in excel if you can, because you will be making plots of these results!). Galaxy Redshift Distance (Mpc) J _67_ Part Three: Calculate the Absolute Magnitude of Each Galaxy in g and r bands In the galaxy info file, you have apparent brightnesses of each galaxy given to you. This is how bright the galaxies appear in the sky, and does not take into account the fact that galaxies that are farther away appear fainter, even though they may be intrinsically brighter. The quantity that measures the intrinsic brightness of a galaxy is called absolute magnitude. Now that we know the distance to each galaxy, we can calculate the absolute magnitude. We will do this in two filters using the distance modulus. M_absolute = m_apparent 5log(distance, in parsecs) + 5 Since you have the distance in Megaparsecs, you will convert it to parsecs by noting that 1 Mpc = 10 6 pc For the example galaxy, then, this would be: M_abs_gband = log(942*10 6 ) + 5, or in the g band. M_abs_rband = log(942*10 6 ) + 5, or in the r band. Record these values in your Galaxy Results file! Galaxy Redshift Distance (Mpc) M_g M_r J _67_ Absolute Magnitude is a weird quantity such that the more negative it gets, the brighter the object is intrinsically! For reference, our own Milky Way Galaxy has an absolute magnitude in the r band of about -21. Part Four: Calculate the Star Formation Rate If you were able to measure the Halpha emission line flux in Part One, then you will be able to calculate a star formation rate. Emission from Halpha (a Balmer Series hydrogen line) signals

6 the birth of young stars. Therefore the intensity of this line is directly proportional to the rate at which a galaxy is forming stars. To get the star formation rate, you first have to calculate an Halpha Luminosity. Flux is to luminosity as apparent magnitude is to absolute magnitude. That is, luminosity is intrinsic and may be compared across distances, whereas flux is apparent, and dependent upon distance. Luminosity = Flux * (4π*distance 2 ) Remember, your flux units are times ergs s -1 cm -2 This means you must put your distance into centimeters to use it in the above equation. In the case of J0401 (our example galaxy, using redspec now): d = 942 Mpc = 2.91x10 27 cm I measure the Halpha flux to be: 103.8x10-17 ergs s -1 cm -2 So, I plug into the equation to get Halpha luminosity of 1.1 x ergs s -1 Now, to go from Halpha Luminosity to star formation rate, we use an equation found by Robert Kennicutt: SFR (solar masses per year) = (7.9x10-42 ) * Halpha Luminosity Thus, it would appear this galaxy has a SFR of solar masses per year! Enter the SFR into your GalaxyResults table like so: Galaxy Redshift Distance (Mpc) M_g M_r SFR J _67_ Part Five: Calculate the Metallicity There are many ways to calculate the metallicity of a galaxy. In this case we are going to use a simple method developed my Pettini and Pagel in In this method, they take two emission line ratios to calculate the metallicity. First, calculate the flux ratio of [OIII] 5007 / Hbeta. In our example galaxy, I get: 37.2 / 33.1, or 1.1 HINT: to measure Hbeta, go way down to the bottom of the continuum, because you will find the emission line sits on top of a dent. Measure the other lines at the level of the continuum. Call this ratio O3HB. Record it in your results file. Next, calculate the flux ratio of [NII] 6583 to Halpha In the example, I get / 103.8, or 0.37 Call this N2 and record it in your Galaxy Results file. Then, you are going to take log (O3HB/N2), which we will call O3N2. In this case, it is log (1.1/0.37), or 0.47

7 Pettini and Pagel find that: Metallicity (12 + log O/H) = 8.73 (0.32 * O3N2) only if O3N2 is less than 1.9 AND Metallicity (12 + log O/H) = (0.57 * log N2) in cases where O3N2 is greater than 1.9 In this case, we use the first equation to calculate metallicity: (12 + log O/H) = 8.6 For reference, (O/H) of the sun is 8.7. In the results file, call Metallicity Z, and enter the final value as another column. Part Seven: Plotting Relations This is the fun part! We are going to investigate any correlations between the properties you have observed and calculated. So, make some graphs in spreadsheet using the data you have entered. 1. Plot with log (stellar mass) on the x-axis and metallicity on the y-axis. Do you see any trend? Do galaxies with more stellar mass tend to have a greater or smaller metallicity? 2. Plot with log (stellar mass) on the x-axis and log SFR on the y-axis. Again, do you see any trends here? 3. Plot with log SFR on the x-axis and metallicity on the y-axis. Comment on any trend or a lack of a trend. 4. Try absolute magnitude (r band first, then g band) on x-axis and stellar mass on y-axis. Any trend? Part Eight: Understanding Your Results For each of the plots you have made try to think about why you may have observed a trend. If, for instance, you find the absolute magnitude of a galaxy tends to be brighter with more stellar mass, say so, and say why. This may be obvious galaxies with more stars are going to be intrinsically brighter! If you find any noteworthy trends, look them up online some are very famous trends, and astronomers are still working to try to understand them!

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