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Discovery of Extensive [O iii] Emission Near M31
To cite this article: Marcel Drechsler
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2023
Res. Notes AAS
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Discovery of Extensive [O III] Emission Near M31
Marcel Drechsler ,1Xavier Strottner ,2Yann Sainty,3Robert A. Fesen ,4Stefan Kimeswenger ,5, 6
J. Michael Shull ,7Bray Falls,8Christophe Vergnes,9Nicolas Martino,9and Sean Walker10
1´
Equipe StDr, B¨arenstein, Feldstraße 17, 09471 B¨arenstein, Germany
2´
Equipe StDr, Montfraze, 01370 Saint Etienne Du Bois, France
354000 Nancy, Lorraine, France
46127 Wilder Lab, Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire, 03755, USA
5Institut f¨ur Astro- und Teilchenphysik, Universit¨at Innsbruck, Technikerstr. 25 8, 6020 Innsbruck, Austria
6Instituto de Astronom´ıa, Universidad Cat´olica del Norte, Av. Angamos 0610, Antofagasta, Chile
7Department of Astrophysical and Planetary Sciences and CASA, University of Colorado, 389-UCB, Boulder, CO 80309, USA
8Sierra Remote Observatories, 42120 Bald Mountain Road, Auberry, California, 93602, USA
9Various Amateur Observatory Sites, Lorraine, France
10MDW Sky Survey, New Mexico Skies Observatory, Mayhill, NM 88339, USA
ABSTRACT
We report the discovery of a broad, 1.5◦long filamentary [O III] emission nebulosity some 1.2◦
southeast of the M31 nucleus. This nebulosity is not detected in Hαand has no obvious emission
counterparts in X-ray, UV, optical, infrared, and radio surveys. To our knowledge, this emission
feature has not been previously reported in the literature. We briefly discuss its possible origin.
Keywords: Galaxies: individual (M31) - Galaxies: halo
INTRODUCTION
Optical emission-line sky surveys are especially useful for identifying various types of emission nebulae including
HII regions, planetary nebulae (PNe), supernova remnants (SNRs), stellar wind-blown bubbles, and stellar outflows.
The majority of such surveys concentrated on detecting Hαemission along the Galactic plane. However, with the
advent of affordable yet sensitive large-format CMOS detectors plus high transmission narrow passband filters, amateur
astronomers are playing an increasing role in detecting emission-line nebulae, both large and small, and not limited to
the Galactic plane.
The [O III] 4959, 5007 ˚
A emission lines are especially important for nebular studies. Here we present wide-field
[O III] images of the area around the M31 galaxy using small but fast telescope + camera imaging systems capable of
revealing faint and extended emission nebulae.
OBSERVATIONS
Wide-field exposures of M31 using [O III] 5007 ˚
A and Hαemission-line filters plus broadband RGB continuum filters
were obtained over 22 nights in August through October 2022 at various dark observing sites in Lorraine, France. These
observations employed a 106 mm refractor and a 6248×4176 pixel CMOS camera which provided a 3.48◦×2.32◦FOV
with scale of 2.04′′ pixel−1. In order to cut down on background light, narrow passband Hαand [O III] filters (30 ˚
A
FWHM) were used.
An initial series of 10 minute exposures of M31 were taken over multiple nights totaling 24.6 hr in [O III] (148×600s),
22.5 hr in Hα(135 ×600s), plus 2.5 hr in each of the RGB filters (30 ×300s). These images unexpectedly revealed
faint [O III] emission about a degree southeast of M31 which seemed to extend past the edge of the image.
In order to rule out filter reflections from bright stars, scattered light from M31 itself, or equipment artifacts,
additional images were taken with the same equipment but now centered at a different sky position. A long series of
exposures were again obtained totaling 24.2 hr in [O III] (145 ×600s), 19.5 hr in Hα(117 ×600s), plus 2 hr (24 ×300s)
in RGB filters. Faint extended [O III] emission nebulosity was again detected near M31.
Research Notes of the AAS ©2024. Published by the American Astronomical Society
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Figure 1. Top: Combined 48.6 hr [O III] 5007 ˚
A exposure image showing the discovery of broad emission nebula southeast
of M31. Bottom: Confirming 85.5 hr combined [O III] image. Note: Images are shown as obtained and unrotated. The bright
object above the [O III] filaments is the 4.5 mag B5 V star νAnd (HD 4727).
Research Notes of the AAS ©2024. Published by the American Astronomical Society
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Confirming images were sought and successfully obtained in late September through November 2022 using two
different telescope + camera systems at an observing site in California. The first series of images totaled 85.5 hr in
[O III] (513 ×600s) and 3.3 hr in RGB filters (40 ×300s) using a 106 mm telescope and a 9576 ×6388 pixel camera
with a 5.33◦×3.56◦FOV. A second series of [O III] exposures totaling 24.9 hr (299 ×300s) used a 135 mm telescope
and a 9576 ×6388 pixel CMOS camera. Both sets of images confirmed the presence of [O III] emission with the same
size and at the same location as seen in the earlier images.
Additional [O III] images were also taken taken in October and November 2022 using different telescopes and cameras
located in Lorraine, France and in southern New Mexico, USA. These images confirmed the presence of extensive faint
[O III] emission near M31.
RESULTS
Our wide-field imaging of M31 has revealed a faint, 1.5◦long [O III] filamentary emission arc1roughly 0.45◦wide,
tilted at position angles of 45◦to 55◦and centered ≃1.2◦southeast of M31’s nucleus (see Fig. 1). We estimate the
brightest portions of the arc to have an [O III] 5007 ˚
A surface brightness of 4 ±2×10−18 erg cm−2s−1arcsec−2. The
reality of this nebula is supported by images obtained using five different telescope + camera systems.
To our knowledge, this emission feature has not been previously reported in the literature. We find no appreciable
Hαemission coincident with the [O III] filaments, suggesting a flux ratio I([O III])/I(Hα)≥5. We also find no obvious
coincident emission in other on-line multi-wavelength image surveys: e.g., X-rays (ROSAT), UV (GALEX), infrared
(IRAS/IRIS, Planck), optical (DSS, SDSS), and radio (VLA FIRST, 408 MHz).
Why this [O III] emission feature had not been detected previously is the obvious question to address. Faint [O III]
emission nebula like we detected is virtually invisible in broadband filter images. We have considered and rejected
the [O III] emission as being an artifact caused by scattered light, internal reflections, image processing, or detector
amplifier glow. The absence of an earlier detection appears to be simply due to the combination of an extremely low
surface brightness line-emission nebula and its unusually large angular size. Many imaging systems are not suited for
detecting such a faint and large line-emission nebulosity.
For example, the CFHT MegaCam [O III] survey of M31’s halo PNe population (Bhattacharya et al. 2019) covered
the arc’s location but did not report any extended [O III] emission. Those images were taken with a relatively wide
[O III] 5007 ˚
A filter (∆λ= 102 ˚
A) with a small pixel scale (0.187′′ pixel−1), inappropriate for detecting faint, diffuse
and extended nebulae above background and detector noise. The combination of a wide FOV, pixel scales ≥2′′ and
narrow interference filters (FWHM ∼30 ˚
A) has been shown to be especially sensitive for detecting large, low surface
brightness nebulosities (Kimeswenger et al. 2021).
DISCUSSION
Curved, filamentary structures like this arc are seen in PNe, and the absence of of IR and UV signals could fit an
especially nearby PN scenario. However, CLOUDY models (Ferland et al. 2017) show that high [O III]/Hαratios with
subsolar metal abundances require electron temperatures above 60 000 K, and all potential white dwarfs seen in the
region are too cold to generate such a line ratio by photoionization. A filamentary nebula especially bright in [O III]
could also be a high-latitude Galactic SNR like G65.3+5.7 (Gull et al. 1977). However, the lack of coincident radio or
UV emissions is a problem for a SNR scenario.
The vector of M31’s proper motion measured by GAIA points roughly to the [O III] emission arc suggesting a
possible interaction of M31 with the Milky Way. But the arc seems much too close to M31 to fit that picture. More
likely, it lies within M31’s halo and is related to the numerous stellar streams, especially the Giant Stellar Stream
whose eastern edge lies close to the [O III] arc (McConnachie et al. 2003;Fardal et al. 2012).
A spectrum of the [O III] emission arc would offer radial velocity information which could establish an association
with M31 and its halo. A follow-up spectroscopic study of this emission arc is ongoing.
1Strottner-Drechsler-Sainty Object 1; https://planetarynebulae.net
Research Notes of the AAS ©2024. Published by the American Astronomical Society
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Research Notes of the AAS ©2024. Published by the American Astronomical Society