A spatially resolved study of ionized regions in galaxies at different ...

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146 4 • Long-slit spectrophotometry of multiple knots of Hii galaxies ID R eq F(Hα) L(Hα) Q(H o ) M ion M(HII) SFR (arcsec) (erg s −1 cm −2 ) (erg s −1 ) (s −1 ) (M ⊙) (M ⊙) M ⊙/yr IIZw71 A 1.64 1.25×10 −14 4.9×10 38 3.6×10 50 5.6×10 4 7.3×10 3 0.004 IIZw71 B 2.94 4.83×10 −14 1.9×10 39 1.4×10 51 2.4×10 5 3.3×10 4 0.015 IIZw71 C 2.65 3.96×10 −14 1.6×10 39 1.1×10 51 4.4×10 5 2.3×10 4 0.012 IIZw71 D 1.43 1.10×10 −14 4.3×10 38 3.2×10 50 5.3×10 4 6.4×10 3 0.003 J1657 A · · · 1.75×10 −14 5.4×10 40 3.9×10 52 2.2×10 6 1.3×10 6 0.430 J1657 B · · · 7.32×10 −15 2.3×10 40 1.6×10 52 7.3×10 5 1.7×10 6 0.180 J1657 C · · · 9.49×10 −15 2.9×10 40 2.2×10 52 8.8×10 5 2.2×10 6 0.233 Table 4.10: Derived properties of the observed knots using the extinction-corrected Hα fluxes measured in the images from Gil de Paz et al. (2003) for IIZw71 and in the spectra for J1657. 4.4.4 Ionising stellar populations Some properties of the emission knots can be obtained from the measured Hα flux, such as Hα luminosity, number of ionizing photons, mass of ionizing stars and mass of ionized hydrogen (see Díaz et al., 2000). In order to obtain these quantities, for IIZw71 we analysed the Hα image of the galaxy, retrieved from the Palomar/Las Campanas atlas of blue compact dwarf galaxies (Gil de Paz et al., 2003). We defined elliptical apertures on this image for each of the four knots extracted in the spectroscopic observations and we measured all the flux inside the elliptical apertures up to the isophote corresponding to the 30% of the light in the brightest knot. These regions are shown in the right panel of Figure 4.3 and their sizes are given in the first column of Table 4.10, as the radius of a circular aperture of area equal to that encompassed by the refereed isophote. For J1657, due to the reasons given in the previous section, we have used the values obtained directly from the spectra, so no radius is given. The observed Hα flux was corrected in each knot for reddening using the values of the reddening constants, c(Hβ), given in Table 4.3 and 4.4. The derived values are given in Table 4.10. The derived values of the masses of the ionizing clusters can be compared with those provided by the STARLIGHT fit. For J1657 STARLIGHT gives slightly lower values than the ones we derive by a factor of 2. For IIZw71 STARLIGHT gives also lower values by a factor of 2 for knots A, C and, D, but by a factor of 4 for knot B. If we take the masses derived from the Hα fluxes for IIZw71 as an example, which are, in principle, lower limits since neither dust absorption nor a possible escape of photons are taken into account, and we use the calculated proportions of young to total mass given in Table 4.9 we obtain total masses between 7 × 10 7 and 4× 10 8 M ⊙ for knots A, B, and D and 5×10 9 M ⊙ for knot C. This high value points to a large contribution to the older population by the bulge of the host galaxy. This is consistent with the continuum light distribution shown in Figure 4.3
4.4. Discussion 147 which is wider than that associated with cluster C and peaks at a region displaced from that of the cluster by 1.6 arcsec (about 144 pc). On the other hand for knot B the cluster shows a significant continuum of the same spatial extent as the line emission and the maxima of both distributions coincide. Knots A and D show very little continua of their own, so their underlying population probably corresponds to the outer parts of the host galaxy. Since the differences in metallicity among the different knots of IIZw71 are not significant and the age distributions are also similar, this could indicate a common chemical evolution of these knots, probably related to the process of interaction with the companion galaxy IIZw70. In the case of J1657, since all the values of proportions of young to total mass are very similar, the same exercise yield a mass of about 3 × 10 8 M ⊙ for the three knots. The star formation rate (SFR) for each knot was derived from the Hα luminosity using the expression given by Kennicutt (1998): SF R = 7.9 × 10 −42 × L(Hα) The derived values are also given in Table 4.10. In the case of J1657, the values for SFR range from 0.180 M ⊙ yr −1 for knot B to 0.430 M ⊙ yr −1 for knot A, between one and two orders of magnitude larger than in IIZw71. For the polar ring, they vary from 0.003 M ⊙ yr −1 for knot D to 0.015 M ⊙ yr −1 for knot B and provide a total SFR for the ring of 0.035 M ⊙ yr −1 , about half of the values quoted by Kewley et al. (2005) for the integrated object, which is 0.066 M ⊙ yr −1 . Using the sizes of the emitting regions obtained from the Hα images and listed in Table 4.10, we obtain a rather constant SFR per unit area, of the order of 7×10 −8 M ⊙ yr −1 pc −2 . This is higher than the average value of individual Hii regions in polar rings given in Reshetnikov and Combes (1994) which is 3.2 ×10 −9 M ⊙ yr −1 pc −2 . It is worth to compare the luminosities and masses of H + of these two objects to those derived for the GEHRs in previous chapters. As it can be appreciated from Tables 2.4, 3.10 and 4.10, the values derived for knot A and B of NGC 5471 or knot A and C of NGC 6946 are of the same order of knot B of IIZw71, the most luminous one. Thus, one knot of this galaxy can be treated as a knot of a GEHR. Regarding J1657, the properties of one knot of this galaxy can be compared with those of a whole GEHR. It should be noted that Hii galaxies present a wide variety of luminosities, some of them presenting knots of the same order as individual knots of GEHRs. 4.4.5 Kinematics and dynamics of the polar ring We have analysed the differential radial velocity along the slit using the Hα and [Sii] 6717 Å emission lines. The heliocentric velocities are displayed in Figure 4.21 superimposed on the observed Hα profile. We find an asymmetric rotation, as it was already stated by Reshetnikov and Combes (1994), possibly affected by the expanding velocities of the bubbles of ionized
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Facultad de Ciencias Departamento d
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A Abigaíl, mi ut A mi familia, por
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La gente del grupo de Astrofísica
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Contents List of Figures List of Ta
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CONTENTS iii B •Physical conditio
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vi LIST OF FIGURES 3.12 Example of
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List of Tables 1.1 Integrated prope
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Chapter 1 Introduction 1.1 Overview
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1.1. Overview 3 Figure 1.1: Sample
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1.1. Overview 5 These massive star
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1.1. Overview 7 Figure 1.3: Color-m
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1.2. Aims and structure of this wor
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12 2 • The star formation history
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40 3 • IFS of a GEHR in NGC 6946
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β[H 84 3 • IFS of a GEHR in NGC
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Page 176 and 177: 156 Conclusiones Las abundancias to
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Page 180 and 181: 160 Appendix A where c=0.434C. This
Page 182 and 183: 162 Appendix B where R S2 = I(6717
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Page 191 and 192: C • Empirical calibrators 171 One
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Page 195 and 196: D • Stellar photometry results of
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Page 210 and 211: 190 REFERENCES Bertelli, G., Bressa
Page 212 and 213: 192 REFERENCES Girardi, L. & Bertel
Page 214 and 215: 194 REFERENCES Kunth, D. & Sargent,
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196 REFERENCES Pérez-Montero, E.,
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198 REFERENCES Terlevich, R., Melni
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