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Register a new userSpectroscopy of diffuse ionized gas in halos of selected edge-on galaxies
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In order to examine the excitation and ionization mechanism of extraplanar diffuse ionized gas (DIG) we have obtained optical longslit spectra of seven edge-on spiral galaxies. In four objects the brightest emission lines can be traced out to distances of typically 1.5 kpc above the disk. For NGC1963 and NGC3044 line ratios such as [NII]6583/Ha or [SII]6717/Ha as well as [OIII]5007/Hb could be measured for the halo DIG. This allows us to discuss the DIG in the halo of these objects in the framework of diagnostic diagrams. For these two objects, the line ratios of [OIII]5007/Hb decrease with increasing z, different from the recently reported trend in NGC891 (Rand 1998). We find that emission lines from the DIG in the disks are in good agreement with photoionization models using a dilute radiation field. However, with increasing z these models fail to predict the measured [OI]6300/Ha and HeI5876/Ha line ratios for NGC1963. Diagnostic diagrams reveal for NGC1963 the need for a second ionization mechanism of the halo DIG (besides photoionization). This additional source could be shock ionization. The same diagrams demonstrate an intermediate classification for NGC3044. Plots of [SII]/[NII] vs. emission measure reveal significant changes towards the halo and seem to trace local small scale density fluctuations of the extraplanar DIG.
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We investigate the prevalence, properties, and kinematics of extraplanar diffuse ionized gas (eDIG) in a sample of 25 edge-on galaxies selected from the CALIFA survey. We measure ionized gas scale heights from ${rm Halpha}$ and find that 90% have measurable scale heights with a median of $0.8^{+0.7}_{-0.4}$ kpc. From the ${rm Halpha}$ kinematics, we find that 60% of galaxies show a decrease in the rotation velocity as a function of height above the midplane. This lag is characteristic of eDIG, and we measure a median lag of 21 km s$^{-1}$ kpc$^{-1}$ which is comparable to lags measured in the literature. We also investigate variations in the lag with radius. $rm H{small I}$ lags have been reported to systematically decrease with galactocentric radius. We find both increasing and decreasing ionized gas lags with radius, as well as a large number of galaxies consistent with no radial lag variation, and investigate these results in the context of internal and external origins for the lagging ionized gas. We confirm that the ${rm [S{small II}]}$/${rm Halpha}$ and ${rm [N{small II}]}$/${rm Halpha}$ line ratios increase with height above the midplane as is characteristic of eDIG. The ionization of the eDIG is dominated by star-forming complexes (leaky ${rm H{small II}}$ regions). We conclude that the lagging ionized gas is turbulent ejected gas likely resulting from star formation activity in the disk as opposed to gas in the stellar thick disk or bulge. This is further evidence for the eDIG being a product of stellar feedback and for the pervasiveness of this WIM-like phase in many local star-forming galaxies.
In this second paper on the investigation of extraplanar diffuse ionized gas in nearby edge-on spiral galaxies we present the actual results of the individual galaxies of our H-alpha imaging survey. A grand total of 74 galaxies have been studied, including the 9 galaxies of a recently studied sub-sample (Rossa & Dettmar 2000). 40.5% of all studied galaxies reveal extraplanar diffuse ionized gas, whereas in 59.5% of the survey galaxies no extraplanar diffuse ionized gas could be detected. The average distances of this extended emission above the galactic midplane range from 1-2 kpc, while individual filaments in a few galaxies reach distances of up to |z| ~ 6 kpc. In several cases a pervasive layer of ionized gas was detected, similar to the Reynolds layer in our Milky Way, while other galaxies reveal only extended emission locally. The morphology of the diffuse ionized gas is discussed for each galaxy and is compared with observations of other important ISM constituents in the context of the disk-halo connection, in those cases where published results were available. Furthermore, we present the distribution of extraplanar dust in these galaxies, based on an analysis of the unsharp-masked R-band images. The results are compared with the distribution of the diffuse ionized gas.
In this work, we investigate the contribution of dust scattering to the diffuse H-alpha emission observed in nearby galaxies. As initial conditions for the spatial distribution of HII regions, gas, and dust, we take three Milky Way-like galaxies from state-of-the-art cosmological hydrodynamical simulations that implement different prescriptions for star formation, feedback, and chemical enrichment. Radiative transfer has been solved a posteriori, using the publicly-available Monte Carlo code Sunrise to take into account dust absorption and scattering of the H-alpha photons, originating exclusively from the HII regions. No contribution from recombinations in the diffuse ionized gas (DIG) component is explicitly or implicitly included in our model. Our main result is that the flux arising from scattered light is of the order of 1-2 per cent of the H-alpha flux coming directly from the HII regions. Building upon previous studies, we conclude that the DIG contributes lass than 50 per cent of the total H-alpha emission.
We present the first kinematic study of extraplanar diffuse ionized gas (eDIG) in the nearby, face-on disk galaxy M83 using optical emission-line spectroscopy from the Robert Stobie Spectrograph on the Southern African Large Telescope. We use a Markov Chain Monte Carlo method to decompose the [NII]$lambdalambda$6548, 6583, H$alpha$, and [SII]$lambdalambda$6717, 6731 emission lines into HII region and diffuse ionized gas emission. Extraplanar, diffuse gas is distinguished by its emission-line ratios ([NII]$lambda$6583/H$alpha gtrsim 1.0$) and its rotational velocity lag with respect to the disk ($Delta v = -24$ km/s in projection). With interesting implications for isotropy, the velocity dispersion of the diffuse gas, $sigma = 96$ km/s, is a factor of a few higher in M83 than in the Milky Way and nearby, edge-on disk galaxies. The turbulent pressure gradient is sufficient to support the eDIG layer in dynamical equilibrium at an electron scale height of $h_{z} = 1$ kpc. However, this dynamical equilibrium model must be finely tuned to reproduce the rotational velocity lag. There is evidence of local bulk flows near star-forming regions in the disk, suggesting that the dynamical state of the gas may be intermediate between a dynamical equilibrium and a galactic fountain flow. As one of the first efforts to study eDIG kinematics in a face-on galaxy, this study demonstrates the feasibility of characterizing the radial distribution, bulk velocities, and vertical velocity dispersions in low-inclination systems.
We investigate the impact of the diffuse ionized gas (DIG) on abundance determinations in star-forming (SF) galaxies. The DIG is characterised using the H$alpha$ equivalent width ($W_{text{H}alpha}$). From a set of 1,409 SF galaxies from the Mapping Nearby Galaxies at APO (MaNGA) survey, we calculate the fractional contribution of the DIG to several emission lines using high-$S/N$ data from SF spaxels (instead of using noisy emission-lines in DIG-dominated spaxels). Our method is applicable to spectra with observed $W_{text{H}alpha} gtrsim 10$ angstroms (which are not dominated by DIG emission). Since the DIG contribution depends on galactocentric distance, we provide DIG-correction formulae for both entire galaxies and single aperture spectra. Applying those to a sample of $,> 90,000$ SF galaxies from the Sloan Digital Sky Survey, we find the following. (1) The effect of the DIG on strong-line abundances depends on the index used. It is negligible for the ([O III]/H$beta$)/([N II]/H$alpha$) index, but reaches $sim 0.1$ dex at the high-metallicity end for [N II]/H$alpha$. (2) This result is based on the $sim$kpc MaNGA resolution, so the real effect of the DIG is likely greater. (3) We revisit the mass-metallicity-star formation rate (SFR) relation by correcting for the DIG contribution in both abundances and SFR. The effect of DIG removal is more prominent at higher stellar masses. Using the [N II]/H$alpha$ index, O/H increases with SFR at high stellar mass, contrary to previous claims.
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