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Oiiheating.Pdf THE ASTROPHYSICAL JOURNAL, 560:207È221, 2001 October 10 ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A. SEARCHING FOR ADDITIONAL HEATING: [O II] EMISSION IN THE DIFFUSE IONIZED GAS OF NGC 891,1 NGC 4631, AND NGC 3079 B. OTTE,2 R. J. REYNOLDS, AND J. S. GALLAGHER III2 Department of Astronomy, University of WisconsinÈMadison, 475 North Charter Street, Madison, WI 53706; otte=astro.wisc.edu, reynolds=astro.wisc.edu, jsg=astro.wisc.edu AND A. M. N. FERGUSON Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, Netherlands; ferguson=astro.rug.nl Received 2001 February 26; accepted 2001 June 7 ABSTRACT We present spectroscopic data of ionized gas in the disk-halo regions of three edge-on galaxies, NGC 891, NGC 4631, and NGC 3079, covering a wavelength range from [O II] 3727A to [S II] 6716.4 A . The inclusion of the [O II] emission provides new constraints on the properties of the di†use ionized gas, in particular the origin of the observed spatial variations in the line-intensity ratios. We used three di†er- ent methods to derive electron temperatures, abundances, and ionization fractions along the slit. The increase in the [O II]/Ha line ratio toward the halo in all three galaxies requires an increase in either electron temperature or oxygen abundance. Keeping the oxygen abundance constant yields the most rea- sonable results for temperature, abundances, and ionization fractions. Since a constant oxygen abun- dance seems to require an increase in temperature toward the halo, we conclude that gradients in the electron temperature play a signiÐcant role in the observed variations in the optical line ratios from extraplanar di†use ionized gas in these three spiral galaxies. Subject headings: galaxies: abundances È galaxies: general È galaxies: individual (NGC 3079, NGC 4631, NGC 891) È ISM: abundances È ISM: general 1. INTRODUCTION When examining ionized gas, it is common practice to & Cox 1993; Dove & Shull 1994), as well as the theory of distinguish between classical H II regions (Stro mgren photons created by neutrino decay (Sciama 1990), are spheres around OB stars) and di†use ionized gas (DIG), the further attempts to explain the ionization of eDIG. gas outside the boundaries of theStro mgren spheres. While Both Martin (1997) and Rand (1998) were able to explain II the run of several of the observed emission-line ratios from H regions are created by photoionization, the ionization II processes for the DIG are less well known. Many attempts H regions to the DIG with composite models. Their have been made to explain the DIG by photoionization models consisted of photoionization and one additional models (e.g.,Domgo rgen & Mathis 1994; Sokolowski 1992), ionization process (shock ionization, turbulent mixing while a few studies address the possibility of shock excita- layers). This additional process was needed to explain the III b tion (e.g., Sivan,Stasin ska, & Lequeux 1986). In recent rise in the [O ]/H line ratio with increasing distance from the disk. However, even with these composite models years, DIG was found not only in the disks of galaxies but II II also far above the stellar disks in the halo of the Milky Way it was not possible to explain the constant [S ]/[N ] line and in several edge-on galaxies at heights of more than 1 ratio, which was observed in NGC 891 (Rand 1998) as well kpc (e.g., Reynolds 1985; Rand, Kulkarni, & Hester 1990). as in the Milky Way (Ha†ner, Reynolds, & Tufte 1999) and Questions therefore arise about where this extraplanar DIG in other galaxies (Otte & Dettmar 1999). These data led (eDIG) comes from and how it is ionized. Ha†ner et al. (1999) to the conclusion that the electron Dynamical models of galaxies, such as ““ galactic temperature increases with increasing distance from the fountains ÏÏ (Shapiro & Field 1976) and ““ chimneys ÏÏ midplane of the galaxies. (Norman & Ikeuchi 1989), describe how gas can be trans- A rise in temperature can explain both the growing III b II II ported from the disk into the halo. Supernova explosions [O ]/H ratio and the constant [S ]/[N ] ratio with o z o that heat the gas in the disk and push it up into the halo are increasing galactic altitude without invoking an addi- o z o important for both the dynamics and the ionization of the tional ionization mechanism at high . Such a rise in electron temperature should also a†ect the [O II]/Ha line gas in the halo. Owing to the high velocities in this ejected II gas, shocks can arise and ionize the gas far above the disk. ratio. The [O ] 3727A emission line provides important The models of runaway O stars leaving the disk, moving additional information about the ionization and heating into the halo (e.g., Gies 1987), and leaking ionizing photons processes in the DIG because of its high excitation energy. Below we present the results of observations of [O II], from the disk into the halo owing to low-density gas (Miller [O III], Hb,[NII], Ha, and [S II] emission from the eDIG 1 Based on observations made with the William Herschel Telescope, of three edge-on galaxies, NGC 891, NGC 4631, and NGC operated on the island of La Palma by the Isaac Newton Group in the 3079. These objects represent the Ðrst targets of a small Spanish Observatorio del Roque de los Muchachos of the Instituto de sample of edge-on galaxies that have been chosen for their AstroÐsica de Canarias. known eDIG emission. The analysis of the other galaxies in 2 Visiting Astronomer at the Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of our sample is still in progress. The results obtained from Universities for Research in Astronomy, (AURA) Inc., under cooperative NGC 891, NGC 4631, and NGC 3079 provide some evi- agreement with the National Science Foundation. dence for an increase in temperature with increasing height. 207 208 OTTE ET AL. Vol. 560 Additional information is obtained about variations in the bined nine spectra with a total integration time of 3 hr and ionization state and chemical abundances within the 20 minutes. Both slit positions were perpendicular to the gaseous halos. plane of the galaxies and are shown in Figures 2 and 3. OBSERVATIONS AND DATA REDUCTION During the observations of galaxies we also took a few 2. spectra of blank sky regions. The spectra of NGC 891 were obtained with the ISIS The spectra were reduced using standard procedures in spectrograph at the William Herschel 4.2 m Telescope on IRAF.3 For sky subtraction, we subtracted di†erently La Palma, Canary Islands, on 1999 September 10, under scaled sky spectra from the red and the blue parts of the photometric conditions and dark skies. Gratings R316R galaxy spectra of NGC 4631 and NGC 3079. This was and R300B were used for the red and blue arms, respec- necessary for the spectra taken during the Ðrst night, which tively. Blocking Ðlter GG495 was used for the red arm. The was partly cloudy. The sky spectra were smoothed along slit width was 1A. The blue arm was read out in a 2 ] 2 the slit by 15 pixels to increase the per-pixel signal-to-noise binned mode and yielded pixel scales of0A.40 pixel~1,or ratios. The resulting spectra are of very good quality, partic- 1.74A pixel~1. The covered wavelength range was from ularly because we appear to have successfully removed the about 3600 to 5400A . The red arm was rebinned after sky lines that can be confused with [O II] emission. For sky reduction and calibration to match the spatial pixel scale of subtraction in our NGC 891 spectra, we averaged over the blue arm. The wavelength dispersion for the red arm about 100 rows in the galaxy spectra that did not show any was 1.47A pixel~1, with a wavelength range from about galactic emission and used this average spectrum as sky. 5700 to 7200A . We combined two 30 minute and one 20 The spectra were then calibrated in wavelength and cor- minute exposures in each arm. The slit position is the same rected for distortion using standard star exposures at di†er- as in Rand (1998) and is shown in Figure 1. ent positions along the slit. After the Ñux calibration, we The spectra of NGC 4631 and NGC 3079 were obtained combined the spectra at each slit position by carefully with the GoldCam spectrograph at the 2.1 m telescope on examining the positions of emission lines in the wavelength Kitt Peak, Arizona, from 2000 February 29 to March 6. We direction and the position of Ñux features in the spatial used grating 9 with decker 4 and a slit width of2A.5. This direction. Individual spectra were shifted by integer pixels, if yielded a pixel scale of0A.80 pixel~1, or 2.44 A pixel~1, and a necessary, to overlap features and thus avoid broadening of wavelength range from about 3500 to 7400A . We used the lines and features during the combining procedure. The Ðlter WG345 to remove possible overlaps between orders. routine for cosmic-ray removal during combining Exposure times varied from 20 to 30 minutes depending on (averaging) of the spectra was insufficient. We therefore did the weather. For NGC 4631 we combined 11 spectra with a not remove cosmic rays during image combining, but we total integration time of 4.5 hr. For NGC 3079 we com- later cleared the areas around emission lines of cosmic rays by hand before measuring emission lines.
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