A study of extended Lyman alpha halos around galaxies

Transcription

1 A study of extended Lyman alpha halos around galaxies Sourabh Chauhan 1 1 MIFA, UMN, Minnesota (Dated: July 5, 2016) PACS numbers: INTRODUCTION Lyman alpha emitters have been extensively studied[2] and various properties about Lyman alpha emitting galaxies have been identified[3]. Lyman alpha emitters have low metalicity, compact... Along with all these properties, it has been speculated that LAEs have extended Lyman alpha halos which are present far beyond UV extent of galaxy. A study of these halos can give us a better insight to how Lyman alpha photons transfer within galaxies and how they escape from galaxies. In this paper we study 9 green pea galaxies from the sample (Alina et. al. ). Our analysis shows that 8 of the 9 galaxies we studied have extended Lyman alpha halos. We quantify the spatial extent of these halos using our analysis (shown in ). Then we study the size of Lyman alpha halos as a function of various galaxy properties specified in (Alina et. al...). Where F(i) denotes the flux along as a function of spatial dimension, f(i,j) denotes the flux at a given pixel and N c is the total number of pixels in wavelength direction ( 400 pixels[5] for continuum) ˆ After this the flux is normalized with the maxima of flux and the points where the flux reaches half of it s maxima is found out on both side of maxima by linear interpolation. The summation of spatial pixel values at half of maximum value gives us full width half maxima (F W HM cont )for continuum profiles. ˆ Next we find Lyman alpha profiles after subtracting the continuum (continuum is not flat in our case). STEPS INVOLVED IN SPATIAL PROFILE ANALYSIS[1] ˆ First of all the four exposures of same galaxy[4] are stacked to get a combined 2D image of galaxy. Next we specify a rest wavelength range of Å to 1229 Å and choose the this image for analysis. ˆ Next the wavelength range for Lyman alpha and continuum are specified. We took region from Å to Å for Lyman alpha regime and Å to Å for UV continuum regime. A region for sky subtraction (of pixels) is also specified in the Lyman alpha wavelength regime but away from any influence from galaxies. ˆ Average value of sky flux is subtracted from all the pixel values. ˆ Next in the continuum regime the fluxes are added in dispersion direction to get flux as a function of spatial dimension. N c F (i) = f(i, j) j ˆ In this step the weighted average of flux values as a function of spatial direction is found out by convolving with Lyman alpha profile: N l F conv (i) = f(i, j) g(j) j Where index i species spatial dimension and j specifies wavelength direction. g(j) is the pixel count for Lyman alpha profile as a function of wavelength found in earlier step. f(i, j) denotes flux at a given pixel, N l is the total number of Lyman alpha wavelength regime pixel. ( 225 pixels ) ˆ After getting F conv (i), the profile is normalized with maxima and FWHM (F W HM Lyα ) is found out using the same method used for continuum. ˆ Errors are evaluated by assuming weights (w(i)) to be constant. Errors are evaluated using error = total flux over time/exposure time This underestimated errors calculated for continuum [6]...

2 2 TABLE I: Measured full width half maxima for each galaxy in continuum regime and Lyman alpha regime along with Lyman alpha to continuum ratio of FWHMs Galaxy redshift F W HM cont F W HM lym ratio G G G G G G G G G ˆ Errors in FWHM are calculates using Monte Carlo technique with 1000 realizations of profile around with in error bars. ˆ Finally the spatial profiles in both continuum and Lyman alpha regime are plotted along with the error bars for each galaxy??. CROSS CHECK WITH CENWAVE ANALYSIS In COS FUV external spectroscopic performance paper by Parviz Ghavamian [7], it is shown that one can get spatial extension of profiles because of different CEN- WAVE settings. Therefore if one does not take into account the extension of spatial profiles because of CEN- WAVE settings, one can misunderstand the spatial extension to be physical which is actually instrumental. Therefore, we performed similar analysis for the three CENWAVE settings 1600,1611,1623 which our galaxies are using. Fluxes along spatial direction are summed up and fourth order polynomial is fit to the profiles as a function of wavelength. Then the wavelength region for continuum and Lyman alpha corresponding to our galaxy were extracted from the full profile as shown in CENWAVE analysis figures 11,12,13. Average Lyman alpha region and continuum region is found out for each galaxy using polynomial fit and compared with FWHMs obtained for our galaxies in table. It turns out that the variations in spatial extension over this wavelength range of interest are very small in comparison to the differences in FWHMs of Lyman alpha and continuum we found out.(for one galaxy it is comparable to differences in FWHMs of Lyman alpha and continuum(ii. Therefore we conclude that the spatial extension observed in our galaxies is not due to any instrumental settings effect. It is something physical and can be explained by invoking Lyman alpha halos around galxies. UNDERSTANDING GALAXY PROPERTIES Several galaxy properties have already been derived for 9 green peas galaxies using in ALina et.al. In this section we study those properties as a function of full width half maxima ratio and see its implications. TABLE II: Cenwave analysis results for different CENWAVE settings. Average over Lyman alpha and continuum box (shown in fig 11,12,13) was found out using the values obtained by polynomial fit (column 2 and 3). Fourth column is difference between continuum and Lyman alpha. Fifth column shows the CENWAVE settings value. The last column shows the FWHM differences for our galaxies F W HM = F W HM Lyα F W HM cont. It is clear that the difference in FWHMs for our galaxies is larger than difference obtained just based on CENWAVE settings. Galaxy Ly avg cont avg difference CW (F W HM) G G G G G G G G β β denotes the FUV continuum slope F λ λ β. It works as a proxy for dust content in the galaxy. Higher the absolute value of this slope, lower will be the dust content in galaxy. Here an anti correlation between dust content and size of Lyman alpha halos is observed (except for one galaxy G0303 [8]). A straight line fit to the profiles give F W HM ratio = β [9]. This trend is in agreement with the previous studies ( ipac.caltech.edu/level5/sept15/hayes/paper.pdf[10]). It makes sense physically because less the dust content means less dust absorption letting Lyman alpha to escape to larger radii and form the extended Lyman alpha halos.

3 FIG. 1: Two dimensional stacked images of galaxies for four exposures with rest frame wavelength. For galaxy G0303 only two exposures were available in proper wavelength range. So only two exposures were stacked for galaxy G0303. Wavelength cuts for Lyman alpha region and continuum regions are also specified in images with two boxes. Left box corresponds to Lyman alpha regime and right box shows the continuum wavelengths chosen for analysis. 3

4 4 FIG. 2: Profiles showing the spatial extension of Lyman alpha region over UV regions. UV continuum spatial profiles are shown by blue and Lyman alpha spatial profiles are shown by green colors. It is visible that spatial profiles in UV are broader than Lyman alpha. Shaded region around the profiles denotes errors calculated using method specified in. FIG. 3:?? same as figure 2 FIG. 4: same as figure 2 f esc Larger the size of Lyman alpha halos, larger number of Lyman alpha photons should be escaping the galaxy. We expect that larger the escape fraction of halos larger should the FWHM ratio.

5 5 FIG. 5: caption same as figure 2 FIG. 7: caption same as figure 2 FIG. 6: caption same as figure 2 FIG. 8: caption same as figure 2 Metalicity 12+log(O/H) gives an estimate of metalicity of galaxies. What we should get :- larger the metalicity less Lyman alpha photons will be absorbed so larger Lyman alpha should be to escape to a larger radii. Therefore, we should get larger Lyman alpha halos for larger metalicity. What we get:- In the figure, we see that for smaller values of FWHM ratios i.e. smaller Lyman alpha halos there is smaller metalicity. But this trend is followed only until FWHM ratio of After 1.35, there is clear dip in metalicity of galaxies. In the metalicity range of 8.1 to 8.2 there is a dip

6 6 FIG. 9: caption same as figure 2 FIG. 11: Boxes specify Lyman alpha and continuum regions for galaxies G1244 and G1133 FIG. 10: caption same as figure 2 Extinction FIG. 12: Boxes specify Lyman alpha and continuum regions for galaxies G1054 and G0926 E(B-V) measures nebular gas extinction and it is directly related to H α /H β. In figure?? we observe that there is no proper trend in size of Lyman alpha halos and E(B V ).

7 7 FIG. 15: A figure caption. The figure captions are automatically numbered. FIG. 13: Boxes specify Lyman alpha and continuum regions for galaxies G1249, G1219, G1137, G0911. FIG. 14: A figure caption. The figure captions are automatically numbered. FIG. 16: A figure caption. The figure captions are automatically numbered.

8 8 FIG. 17: E(B-V) FIG. 19: RUV. FIG. 18: SFR SFR FIG. 20: MFUV. R UV M F UV ν r max ν b max ν r peak ν b peak

9 9 FIG. 21: WHa. FIG. 23: WHa. FIG. 22: WHa. FIG. 24: WHa. W Ly b α W Lyα W Lyar to get final profile as a function of all Galaxy properties. Based on Sept15/Hayes/paper.pdf one can assume that lyman alpha halo sizes is not directly dependent on steallar properties like stellar mass, nebular oxygen etc. Therefore we can choose other parameters for fitting the values. FITTING ANALYTIC FORMULA TO TREND... In this section we try to find out analytic forms of Lyman alpha to continuum size ratio as a function of different Galaxy properties. One can use multi variable fits and paramterize the fitting parameters further Fitted beta profile CONCLUSION Lyman alpha halos sizes are

10 10 [6] dnt know why [7] add ref CENWAVE paper [8] Why??? [9] we have disregarded galaxy G0303 for this fit. FIG. 25: WHa. FIG. 27: WHa. FIG. 26: WHa. [1] [2] add ref. [3] add ref. [4] Galaxy G00303 only two exposures were available in wavelength of interest [5] this number vary depending on the pixel resolution of the image. [10] add ref properly FIG. 28: WHa.