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Stellar Population Synthesis of NGC 2336 and NGC2841 Galaxies

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Stellar Population Synthesis of NGC 2336 and NGC2841 Galaxies Il Cielo come Laboratorio A.S. 2010-2011 Stellar population synthesis of NGC 2336 and NGC2841 galaxies Gastaldello Niccolo´1, Saggin Filippo1, Zanini Valentina2 1Liceo G.B. Quadri, Vicenza 2Liceo Don G. Fogazzaro, Vicenza Abstract. We examined the spectra of galaxies NGC 2336 and NGC 2841 in order to determine a model of the stellar population for different parts of the galaxies. The two–dimensional spectra were divided into various parts and the obtained monodimensional spectra were compared with a suitable linear combination of standard star spectra, from O type to M type. Also the influence of internal reddening was examined, considering one of the many spectra analysed. 1. Introduction C spectrograph, 300 tr./mm grating and 200 microns slit width. Telescope scale: 10.78 arcsec/mm ; ccd scale: The observed spectrum of a given galaxy is an integra- 0.63 arcsec/pixel. Exposure time and slit position angle tion of the spectra of the stars dust and gas that compose p.a. are reported in the following table: it and, obviously, it depends on the percentage of the stars of different type. Stellar population synthesis con- sists in reproducing the observed spectrum of a galaxy Object exp. time (s) P.A.(0) with a linear combination of stellar spectra of various NGC 2336 1800 75 types. This studies are crucial in understanding galax- NGC 2841 900 60 ies structure even in relation with the galaxy morpho- logical type, and in understanding stellar formation and evolution phenomena. A galaxy has not an uniform composition of stars: the amount of young and old stars changes in its sub- structure. In the center of the galaxy (bulge) there are older and colder stars than in the disk and arms, where the stars are younger and hotter. Moreover, in the outer regions of spiral galaxies the presence of gases is greater than in the bulge. The gases and the interstellar dust are also responsible of a modification of spectra, with an effect called galactic extinction. It consists in an absorption and a scattering of the light, depending NGC 2336 on the wavelength. From the moment that the size of the dust granules is similar to the blue light wavelength ( 400 nm ), the intensity of the blue light is more atten- uated than the red light, resulting in a spectrum which is redder than expected (reddening). On the stellar pop- ulation this phenomenon causes an underestimation of blue stars. 2. Observational Data We analysed the spectra of the two galaxies NGC 2336 and NGC 2841, taken with 1.22 m Galileo telescope at Colle Pennar Observatory in Asiago, equipped with B& NGC 2841 Gastaldello, Saggin, Zanini: Stellar population synthesis of NGC 2336 and NGC2841 1 The main features of the galaxies are reported in the v ∆λ following table. Data source: NED archive z = = c λ0 The following table summarizes the z measurements for Object NGC 2336 NGC 2841 R.A. 07h27m04.05s 09h22m02.634s the selected spectral lines: Dec. +80d10m41.1s +50d58m35.47s Morph. Type SAB(r)bc SA(r)b redshift z (10−3) mag. (V) 10.43 9.22 lines NGC 2336 NGC 2841 d (Mpc) 33.617 17.823 Hβ – 2,50 −3 z (10 ) 7.35 2.128 [NII] 7.22 1.97 Na 7.42 – CaK 7.45 – 3. Work description CaH 7.22 – z 7.33 2.23 Using the software IRAF (Image Reduction and Analysis Facility – NOAO), we started extracting a number of subimages (monodimensional spectra) from Our measurements are in good agreement with the two-dimensional spectra. We selected the subim- those found in the Nasa/Ipac extragalactic database. ages examining the Hα emission line, obtaining a num- In order to compare standard stellar spectra with the ber of intensity profiles from the center to the edge of ours, we normalized them so that at 5500 Å the inten- our galaxies. sity value was 1 (arbitrary units). We smoothed each spectrum (key s of splot; pixel number: 5.) so reducing the noise. The last operation was to convert the spec- tra from fits format to ASCII tables, in order to employ Topcat for the next step analysis (task wspectext). 4. Analysis and results ff NGC 2841 regions (yellow arrow: field star) Standard spectra of di erent star types (from O to M), were used to fit the model of the spectra of our galaxies. We considered a mean spectrum for any different main spectral type (O,B,A,F,G,K,M), taking the mean value of the various subclasses (e.g.: B1, B2, B3....B9). Using software Topcat we guessed a lot of linear combinations of standard star spectra in order to fit our galaxies spectra as good as possible. The better models of each galaxy region are reported in the following scheme: NGC 2336 regions NGC 2841 We examined 6 subimages for NGC 2841 and 6 for region stellar population synthesis NGC 2336. For each spectrum (subimage) we had to do a 55%G 40%K 5%M various corrections. b 50%G 45%K 5%M The first operation was to correct the reddening of c 30%G 70%K the spectrum due to the interstellar dust of our galaxy, d 55%G 45%K the Milky Way, using the task deredden of the pack- e 60%G 40%K age onedspec; we used the value A(V),taken from the f 80%F 15%G 5%K Nasa/Ipac extragalactic database, which was 0.052 mag for the NCG 2841 and 0.109 mag for the NGC 2336. A(V) is the total extinction at the visual band at 5550 Å. For one region of NGC 2336 we did also the reddening NGC 2336 correcton due to internal extinction. We guessed three region stellar population synthesis different A(V) value: 0.25 - 0.75 - 1.50. The next step a 10%B 60%F 30%G was to correct the redshift, operation done with the task b 70%F 10%G 20%K newredshift of the package tools. The redshift was mea- c 80%F 15%G 5%K sured on the central galaxy region, to avoid the galaxy d 15%F 25%G 55%K 5%M rotation effect. We calculated the redshift using the for- e 20%G 80%K mula: f 20%F 10%G 60%K 10%M 2 Gastaldello, Saggin, Zanini: Stellar population synthesis of NGC 2336 and NGC2841 NGC 2336 (a) – internal extintion guess NGC2841 – Residual A(V) mag stellar population synthesis region mean σ min max 0.11 10%B 60%F 30%M a 0.0235 0.1144 -0.2579 1.4896 0.25 25%B 50%F 25%G b 0.0292 0.0976 -0.2165 1.0238 0.75 40%B 30%F 30%G c 0.0044 0.0639 -0.2350 0.2018 1.50 80%B 10%F 10%G d 0.0436 0.1080 -0.4373 1.2786 e 0.0496 0.1474 -0.6732 1.8546 f 0.0718 0.4706 -2.1342 6.6196 The following images show some examples of the obatined fit: NGC2336 – Residual region mean σ min max a 0.0520 0.2206 -0.6962 2.3296 b 0.2542 0.1761 -0.5991 1.3066 c 0.0411 0.1975 -0.5005 2.0669 d 0.0446 0.1167 -0.5596 0.7848 e 0.0175 0.0679 -0.2593 0.2351 f 0.0499 0.1931 -1.2014 0.9319 NGC 2336 e region 5. Results As espected the galaxies show a different stellar distri- bution from the center to the edge. The bulge has more stars of the late spectral types while on the edge we find hot and young stars. In fact the disk and the spiral arms are the zone where the star formation is more active. For galaxy NGC 2336 the fit was more difficult and, at the end, the results were less satisfactory than the ones NGC 2336 e region – residual relative to te galaxy NGC 2841. We found a quite good simmetry in our models with respect to the galaxy cen- ter. At the end, only for region a of galaxy NGC 2336 we attempted to consider three different values of the internal extinction. We show that this parameter has a great influence in the stellar composition model: in fact, the hot to cold star percentage varies from 25% – 25% to 80% – 10% assuming A(V) = 0.25 or A(V) = 1.5 respectively. Unfortunately, the internal reddening for NGC 2841 d region a given galaxy is not known and indeed rigorous mod- els consider A(V) as a free parameter to be fitted by the model itself. NGC 2336 shows a minor cold stars content (few percentance of M star), compared with to NGC 2841 that seems dominated by G, K and M stars. References http://nedwww.ipac.nasa.gov NGC 2841 d region – residual We analyzed the residual (O–C) that is the differ- ence between the observed galaxy spectrum ad the syn- tetic one (the model). We compute the mean value, the standard deviation σ and the minimum and maximum values in order to evaluate the obtained fit. As you can see, apart the region “f” for NGC 2841 and region “a” for NGC 2336 these values support the reliability of our fit, although the fits themselves were made “by eye”..
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