Research ArticleASTRONOMY

Discovery of diffuse optical emission lines from the inner Galaxy: Evidence for LI(N)ER-like gas

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Science Advances  03 Jul 2020:
Vol. 6, no. 27, eaay9711
DOI: 10.1126/sciadv.aay9711
  • Fig. 1 Observations of the Tilted Disk.

    Integrated velocity channel map of HI 21-cm emission over inner Galaxy forbidden velocities for positive longitudes (−110 km s−1vLSR ≤ −50 km s−1; bluescale) and negative longitudes (+50 km s−1vLSR ≤ +110 km s−1; redscale), showing the tilted distribution of this gas. Hα contours integrated over the same velocity range show evidence for an ionized counterpart to the neutral gas. A projection of a tilted elliptical HI disk model (23) integrated over all velocities is shown with black and gray contours, while the blue-to-white and red-to-white contours show the emission predicted by the model in the same velocity range as the data with contour values of 0.1, 10, 75, and 100 K km−1 s−1. The purple line shows the projected outline of the Fermi Bubble (38) extrapolated into Galactic Center. The dotted circles and arrows show the location of two 1° WHAM beams and their corresponding optical emission line spectra for Hα, Hβ, [NII], [SII], and [OIII]. A rescaled (1/20) HI spectrum averaged over the WHAM beam is also shown to demonstrate the kinematic agreement between the neutral and ionized gas observations. The bright emission around vLSR = 0 km s−1 is from local emission in the solar neighborhood.

  • Fig. 2 BPT diagram with the Milky Way.

    BPT diagram of the [OIII]/Hβ versus [NII]/Hα line ratios for SDSS DR7 galaxies (grayscale) and the Milky Way (colored data), as observed with WHAM. Classification lines in blue, cyan, and orange separate regions by their modeled primary excitation mechanism (79). Orange-shaded filled circles are WHAM observations of the Tilted Disk structure we have modeled. Green plus symbols are extinction-corrected WHAM observations of the Scutum direction (24) and cover a range of 4 kpc < RG < 7 kpc. Pink lines and shaded regions show additional observations of [NII]/Hα of the Tilted Disk where either [OIII] or Hβ are not detected. Error bars are either 1σ errors or upper limits (arrows).

  • Fig. 3 [NII]/Hα versus Galactocentric radius.

    [NII]/Hα line ratio toward the Tilted Disk as a function of Galactocentric radius (RG). Points with orange error bars correspond to the orange points in the BPT diagram of Fig. 2, while points with pink error bars correspond to the pink vertical bars in the BPT diagram. An additional offset along both axes of 0.01 is added to the point with the largest values on both axes to differentiate the two points with very similar values.

Supplementary Materials

  • Supplementary Materials

    Discovery of diffuse optical emission lines from the inner Galaxy: Evidence for LI(N)ER-like gas

    D. Krishnarao, R. A. Benjamin, L. M. Haffner

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