WHAM Observations of Hα, [Sii], and [Nii] toward the Orion and Perseus Arms: Probing the Physical Conditions of the Warm Ionized Medium

Abstract

A large portion of the Galaxy (ℓ = 123 • to 164 • , b = −6 • to −35 • ), which samples regions of the Local (Orion) spiral arm and the more distant Perseus arm, has been mapped with the Wisconsin H-Alpha Mapper (WHAM) in the [S II] λ6716 and [N II] λ6583 lines. By comparing these data with the maps from the WHAM Hα Sky Survey, we begin an investigation of the global physical properties of the Warm Ionized Medium (WIM) in the Galaxy. Several trends noticed in emission-line investigations of diffuse gas in other galaxies are confirmed in the Milky Way and extended to much fainter emission. We find that the [S II]/Hα and [N II]/Hα ratios increase as absolute Hα intensities decrease. For the more distant Perseus arm emission, the increase in these ratios is a strong function of Galactic latitude, b, and thus, of height, z, above the Galactic plane, while the [S II]/[N II] ratio is relatively independent of Hα intensity. Scatter in this ratio appears to be physically significant, and maps of [S II]/[N II] suggest regions with similar ratios are spatially correlated. The Perseus arm [S II]/[N II] ratio is systematically lower than Local emission by 10%-20%. With [S II]/[N II] fairly constant over a large range of Hα intensities, the increase of [S II]/Hα and [N II]/Hα with |z| seems to reflect an increase in temperature. Such an interpretation allows us to estimate the temperature and ionization conditions in our large sample of observations. We find that WIM temperatures range from 6,000 K to 9,000 K with temperature increasing from bright to faint Hα emission (low to high [S II]/Hα and [N II]/Hα) respectively. Changes in [S II]/[N II] appear to reflect changes in the local ionization conditions (e.g. the S + /S ++ ratio). We also measure the electron scale height in the Perseus arm to be 1.0 ± 0.1 kpc, confirming earlier, less accurate determinations.

Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1

“…The distance to the Perseus arm is about 2:5 AE 0:5 kpc (see Reynolds et al 1995 and references therein). Haffner et al (1999) studied the H emission from the high-z extension of the Perseus arm and find that it has a scale height of about 1 kpc. Associated gas at b ¼ À45 would be at a distance of 3:6 AE 0:7 kpc and lie 2:5 AE 0:5 kpc below the Galactic plane.…”

Infrared Spectroscopy of the Diffuse Ionized Halo of NGC 891

“…Much information regarding the source(s) of ionization has come from optical emission line ratios in the Milky Way and external galaxies (e.g., Rand 1997Rand , 1998bGreenawalt et al 1997;Collins & Rand 2001;Otte et al 2002;Haffner et al 1999;Hoopes & Walterbos 2003;Madsen et al 2006). Studies of the spatial behavior of several diagnostic line ratios have supported the idea that photoionization by massive stars is the primary ionization mechanism, but crucial aspects of the data require a more complex picture.…”

“…The most characteristic result is that the ratios of [S ii] kk6716, 6731 and [N ii] kk6548, 6583 to H generally increase with distance from the suspected ionizing source, in the Milky Way and in external galaxy disks and halos. This was initially interpreted as a fall in the ionization parameter, U, with distance from the ionizing source (e.g., Domgorgen & Mathis 1994;Rand 1998b) but more recently has been attributed to an increase in the gas temperature in the DIG relative to the immediate star-forming environment due to a source of extra nonionizing heating (e.g., Haffner et al 1999;Otte et al 2002, using further information from [O ii] k3727/H in edge-on galaxies), while even more recently the importance of radiation field hardening during propagation (due to the longer mean free path for harder photons) has been stressed (Hoopes & Walterbos 2003;Wood & Mathis 2004, hereafter WM04).…”

“…It also incorporates radiation field hardening, nonionizing heating, and a range of possible input spectra and luminosities. WM04 find that spectral hardening reduces the need for the nonionizing heating source invoked by, e.g., Haffner et al (1999) by producing higher gas temperatures and ratios of forbidden lines to Balmer lines, while suppressing He i emission. However, it is still not possible to match all of the line ratio data for the Reynolds layer and the halo of NGC 891 without invoking additional heating or a secondary source of ionization.…”

Detection of highly-ionized diffuse gas in the Galactic plane

“…It has been shown that a large fraction of the Galaxy contains fully ionized gas of 8000 K with an electron density of approximately 0.2 cm −3 . Low ionization lines, such as [S ii] λ6716 and [N ii] λ6587, have been widely detected in the WIM and they have been used for the study of the properties of the WIM (e.g., Haffner et al 1999 and references therein), while a high ionization line of [O iii] λ5007 is quite weak and has been detected only at two positions in the Galactic disk (Reynolds 1985). Latest studies suggest the presence of external heat sources in the Galactic halo to explain the spatial distribution and strength of these emission lines (Reynolds 1997;Reynolds et al 1999).…”