OIII arc in Gemini

An arc-shaped emission nebula (referred to as the arc in the rest of the text) in the constellation Gemini with dominant OIII lines and an apparent length of about 14° is presented on this page. The object has already been recorded in far UV surveys. What is new, however, is that this object can be seen in visible light and that it is a strong OIII emitter. This topic is discussed in the section “Classification and discussion of the superstructure”.

Click on the image for a full scale version.

OIII arc in Gemini in OHS

This image is a false color composite where [OIII] (505.7 nm) is mapped to red, Hα is mapped to green, and [SII] (671.7 nm and 673.0 nm) is mapped to blue. All three color channels contain some continuum light, which appears magenta in the image due to the choice of the color factors. Stars are partially subtracted (with different factors for each color so that the average star color is white).

The southern (lower) part of the large arc-shaped object that extends across the entire image is only visible in [OIII] light (red). A smaller part, about 3° in diameter, is only bright in Hα and [SII] light (cyan). Some fainter filaments (red and bluish) on the left side are visible in all three wavelengths and are probably related to the large arc.

The bright object at the lower right is SH2-274. Also note that there are a lot of weakly ionized background nebulae (HII and [SII], green to cyan).

Follow this link for an annotated presentation of the mentioned objects.

Intensity ratios

In the following images, the three wavelengths are shown separately to facilitate comparison.
OIII arc in Gemini in [OIII] light
OIII arc in Gemini in H-alpha light
OIII arc in Gemini in [SII] light
Regions for intensity measurements
Regions for luminance measurements
The monochrome images show the three color channels separately. Stars are partially subtracted. The color image marks the regions for which intensity measurements were performed (see below).

The following table contains the results of intensity measurements for the regions marked in the upper image. The average intensity in Rayleighs is given in each case. The region labeled “bg” was used as a background reference.

Region [OIII] [SII]
K1a 0.02 2.04 0.06
K1a' 1.68 1.53 1.14
K1b 2.20 0.63 2.27
K1c 0.24 0.72 1.54

For the determination of the intensity, some parameters had to be estimated. The transmission ratio of the optics and filters was assumed to be 75% for [OIII] and Hα and 70% for the [SII] doublet. The atmospheric extinction under best conditions was assumed to be 20%. The errors resulting from these estimations only distort the absolute measured values by a factor, but have no influence on the intensity ratios between the wavelengths. More relevant are the errors caused by the background calibration, as they distort the intensity ratios between the individual emission lines. The accuracy of the background reference (the two regions marked “bg” in the image above) is limited by the fact that the entire region is full of faint background nebulae.

Classification and discussion of the superstructure

The arc is probably part of a larger supernova remnant. To better assess the situation, an image with a larger field of view has been created from very early data collected by the Northern Sky Narrowband Survey.
OIII arc in Gemini in OHS: wide field view
The color composition is the same as above: [OIII] (505.7 nm) is mapped to red, Hα is mapped to green, and [SII] (671.7 nm and 673.0 nm) is mapped to blue. All three color channels contain some continuum light, which appears magenta in the image due to the choice of the color factors. Stars are partially subtracted (with different factors for each color so that the average star color is white).

The image is centered on the pulsar PSR B0656+14, the suspected origin of the Monogem ring. The arc could belong to that Supernova remnant, as discussed below.

Far UV observations

The arc can also be seen in Galex far UV (134nm to 179 nm) images as the thin filament. Far UV spectroscopy was performed by Kim et al.. In the low-resolution images of that work, the arc is visible and denoted as region “R1”. Interestingly, the spectroscopic study shows only CIII and CIV lines, no OIII lines. In visible light, OIII lines are dominant. Kim et al. interpret this structure as “the blast wave [of the Monogem SNR, see below] with an isolated cloud”.

The arc as a part of the Monogem ring?

The arc could be a part of the Monogem ring, a supernova remnant (SNR) probably with the same progenitor as the Pulsar PSR B0656+14 (Thorsett et al.). In X-rays, the Monogem SNR appears as a primary ring with a diameter of 18.4° and an eastern blowout, resulting in a size of about 25°. A possible explanation for the distorted morphology is a higher density of the interstellar medium (ISM) near the galactic plane resulting in a slower expansion (Knies et al., 2018). The primary ring corresponds approximately with [OIII] filaments near the galactic plane. The arc lies near the outer boundary of the blowout. If the arc is completed to a ring, its diameter would be about 50°, but its center lies in a similar direction as PSR B0656+14. Thus, if the arc is a part of the Monogem ring, its 3D morphology could be described as egg-shaped, where the narrow end near the galactic plane is formed by the primary ring and the wider end is determined by the arc.

The objects mentioned above can be found in a JavaScript presentation.

The arc as a part of an other structure?

Although it is likely that the arc belongs to the Monogem Ring, this is not certain. There are a few other explanations:
  1. If the arc is completed to a ring, its perimeter would correspond with some Hα filaments in the Monoceros constellation, as seen in this JavaScript view. Thus, the arc could belong to a much larger object.
  2. Knies et al. (2024) discuss the possibility of a second SNR named G205.6+12.4 in a similar distance. Furthermore, a faint but almost complete Hα ring (detected some time ago) can be constructed, marked by a yellow circle in this JavaScript view. (Hint: Improve visibility by toggling the plot on and off with “2” key.) These two objects almost perfectly match the [OIII] filaments, especially the elongated southern filament. However, they do not explain the arc, which intersects the perimeter of G205.6+12.4 at a steep angle, as shown in this JavaScript presentation.

The research papers I found use data that cannot resolve the exact orientation of the arc, and I am not aware of any articles that used the high resolution GALEX data of the arc. A full-width view with [OIII] and [SII] data covering the entire ring constructed from the arc could provide further insights. However, complete coverage of the region is not expected before 2025.

A discussion about the superstructure of the arc can be found at Cloudynights forum.

Further observations

Image data

Images where captured with a camera array which is described on the instruments page.

Image data are:

View #1 View #2
Center position: RA: 7:44h, DEC: 18:30° RA: 7:59:48h, DEC: 14:14:19°
FOV: 8.75°×14.31° (RA×DEC, through center) 31°×31° (RA×DEC, through center)
Orientation: North is up North is up
Scale: 10 arcsec/pixel (in center at full resolution) 20 arcsec/pixel (in center at full resolution)
Projection type: Stereographic Stereographic
Exposure times (Sum of exposure times of all frames used to calculate the image):
Hα: 80 h
OIII: 55 h
SII: 52 h
Hα: 9.50d
OIII: 56 h
SII: 61 h

Image processing

All image processing steps are deterministic. There was no manual retouching or any other kind of non-reproducible adjustment. No “AI” was used; the images shown here are the results of deterministic calculations and not hallucinations of an “AI”.

Image processing steps where:

  1. Bias and dark current subtraction, flatfield correction, noise estimation
  2. Alignment and brightness calibration using stars
  3. Stacking with masking unlikely values and background correction
  4. Extracting stars from the emission line images using information from continuum images
  5. Denoising and deconvolution both components (stars and residual)
  6. RGB-composition
  7. Dynamic range compression using non-linear high-pass filter
  8. Tonal curve correction


S. E. Thorsett, R. A. Benjamin, Walter F. Brisken, A. Golden, and W. M. Goss. Pulsar psr b0656+14, the monogem ring, and the origin of the "knee" in the primary cosmic-ray spectrum. The Astrophysical Journal, 592(2):L71, jul 2003. [ DOI | http ]
I. J. Kim, K. W. Min, K. I. Seon, J. W. Park, W. Han, J. H. Park, U. W. Nam, J. Edelstein, R. Sankrit, and E. J. Korpela. Far-Ultraviolet Observations of the Monogem Ring. The Astrophysical Journal, 665(2):L139–L142, August 2007. [ DOI ]
Jonathan R Knies, Manami Sasaki, and Paul P Plucinsky. Suzaku observations of the Monogem Ring and the origin of the Gemini H α ring. Monthly Notices of the Royal Astronomical Society, 477(4):4414–4422, 04 2018. [ DOI | http ]
R. Weinberger, S. Temporin, and B. Stecklum. Detection of an optical filament in the Monogem Ring. Astronomy and Astrophysics, 448(3):1095–1100, March 2006. [ DOI | arXiv ]
Jonathan R. Knies, Manami Sasaki, Werner Becker, Teng Liu, Gabriele Ponti, and Paul P. Plucinsky. A new understanding of the gemini-monoceros x-ray enhancement from discoveries with erosita, 2024. [ arXiv | http ]

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