Milky Way from Perseus to Monoceros: Orion-Eridanus Superbubble, California Nebula region and dense molecular clouds

A 80°×70° view of several large HII regions, including the Orion-Eridanus Superbubble and the California Nebula. The latter appears to be part of a much larger region, mostly obscured by dense molecular clouds in the constellations Taurus and Perseus.

Click on the image to load a high resolution (780 MP) version using a JavaScript viewer.

Milky Way from Perseus to Monoceros: Orion-Eridanus Superbubble, California Nebula region and dense molecular clouds in H-alpha, blue continuum and red continuum — False color composite where Hα (without continuum) is mapped to red, blue continuum (including [OIII] and Hβ emissions) is mapped to green, and red continuum (without Hα but with [SII] emissions) is mapped to blue. Emission nebulae are reddish while reflection nebulae are green to blue. Stars are partially subtracted in order to make the faint nebulae visible.
The large red structure on the left side, which looks like a skull, is the Orion-Eridanus Superbubble. The left “eye” of the “skull” is the Barnard's Loop, while the right one is the λ Ori Nebula, ionized by λ Ori cluster located in a distance of about 400 pc (Gaia DR3). There is a debate about the morphology of the Orion-Eridanus Superbubble and whether the λ Ori Nebula is part of it. Some of these theories are compared in the next section.
Some filaments of the reflection nebulae (green to blue) are parallel to the HII filaments (red) of the Orion-Eridanus Superbubble, indicating that they are clearly related. However, most of the reflection nebulae belong to the molecular clouds in Taurus and Perseus, including the Plejades (green in the image because blue is mapped to green). Most of these structures are located within a distance of 130 pc to 300 pc.
Behind these molecular clouds lies another large HII region, visible only through gaps in the dense clouds. The brightest part of this structure is the California nebula, located at a distance of approximately 400 pc.

Milky Way from Perseus to Monoceros: Orion-Eridanus Superbubble, California Nebula region and dense molecular clouds in H-alpha — Pseudo-color composite where the color depends on Hα intensity (without continuum), as shown in the legend with units Rayleighs. The Brightest stars are added in white to visualize the regions where Hα intensity is uncertain due to contamination by starlight. (Unless pixels are saturated, adding white stars does not destruct the intensity information contained in the color, because the color map was chosen so that one color component (red, green, or blue) is always zero. Thus, the color can be reconstructed by subtracting the value of the smallest component from the other two components.)
Same view as above, but here only showing Hα, revealing more details of the emission nebulae.

Legend for Milky Way from Perseus to Monoceros: Orion-Eridanus Superbubble, California Nebula region and dense molecular clouds in H-alpha — Pseudo-color composite where the color depends on Hα intensity (without continuum), as shown in the legend with units Rayleighs. The Brightest stars are added in white to visualize the regions where Hα intensity is uncertain due to contamination by starlight. (Unless pixels are saturated, adding white stars does not destruct the intensity information contained in the color, because the color map was chosen so that one color component (red, green, or blue) is always zero. Thus, the color can be reconstructed by subtracting the value of the smallest component from the other two components.)
Same view as above, but here only showing Hα, revealing more details of the emission nebulae.

Remarks

This section provides additional information about the objects mentioned in the image caption.

Orion-Eridanus Superbubble

There are at least three theories about the morphology of the Orion-Eridanus Superbubble:

Wilson et al. (2005) consider Barnard's Loop to be the nearest edge of the Orion-Eridanus Superbubble, located in a distance of about 120 pc. In this model, the λ Ori Nebula lies outside the superbubble.
Pon et al. (2014) proposed that Barnard's Loop is a distant edge of the Orion-Eridanus Superbubble, located 400 pc away. The λ Ori Nebula would lie outside the superbubble.
Ochsendorf et al. (2015) suggest that the eastern edge of the Orion-Eridanus Superbubble lies close to the galactic plane. Furthermore, they show that Barnard's Loop, the λ Ori Nebula, and smaller shells are independent structures within the superbubble. Pon et al. (2016) adapted their model to this theory while maintaining a distance of 400 pc for Barnard's loop. (Sketches of the morphology for the older and newer theories can be found in Figure 3 of their work.) The concept of nested bubbles is further supported by the likely existence of another large and faint but nearly complete HII ring (Visibility can be improved yb toggling the plot on and off using then ‘2’.)

Click on the links below for visualizations of the approximate morphology of the Orion-Eridanus Superbubble according to the theories provided above. The presentations also include the two known large bubbles in that region and the possible new one.

It should be mentioned that the bubble would appear more or less egg-shaped due to perspective, which is not accurately represented in the JavaScript viewer. Following Theory 1 (Wilson et al., 2005), the wider end of the egg (being nearer) would be the Barnard's Loop. This seems most unlikely to me. Theory 2 (Pon et al., 2014) appears more plausible, as Barnard's Loop would align with the narrow end. However, based on the images, Theory 3 (Ochsendorf et al., 2015) seems the most convincing.

Image data

This image was calculated using data from the Northern Sky Narrowband Survey, DR0.1. Click the link for detailed information or visit the instruments page for information about the equipment. Here is some additional image-specific information:

Center position:	RA: 5:16h, DEC: 18.5°
FOV:	80°×70° (RA×DEC, through center)
Orientation:	JavaScript viewer: North is up Above: North is right
Scale:	10 arcsec/pixel (in center at full resolution)
Projection type:	Stereographic

References

: Andy Pon, Bram B. Ochsendorf, João Alves, John Bally, Shantanu Basu, and Alexander G. G. M. Tielens. Kompaneets Model Fitting of the Orion-Eridanus Superbubble. II. Thinking Outside of Barnards Loop. ApJ, 827(1):42, August 2016. [ DOI | arXiv | http ]
: Bram B. Ochsendorf, Anthony G. A. Brown, John Bally, and Alexander G. G. M. Tielens. Nested Shells Reveal the Rejuvenation of the Orion-Eridanus Superbubble. ApJ, 808(2):111, August 2015. [ DOI | arXiv | http ]
: Andy Pon, Doug Johnstone, John Bally, and Carl Heiles. Kompaneets model fitting of the Orion-Eridanus superbubble. MNRAS, 444(4):3657–3669, November 2014. [ DOI | arXiv | http ]
: B. A. Wilson, T. M. Dame, M. R. W. Masheder, and P. Thaddeus. A uniform CO survey of the molecular clouds in Orion and Monoceros. A&P, 430:523–539, February 2005. [ DOI | arXiv ]

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