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Discovery of Extensive [O iii] Emission Near M31

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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 H α and 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.
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Discovery of Extensive [O iii] Emission Near M31
To cite this article: Marcel Drechsler
et al
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, arenstein, Feldstraße 17, 09471 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 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, 1.5long 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 3.48×2.32FOV
with scale of 2.04′′ pixel1. 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
2
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
3
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 5.33×3.56FOV. 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, 1.5long [O III] filamentary emission arc1roughly 0.45wide,
tilted at position angles of 45to 55and centered 1.2southeast 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 ±2×1018 erg cm2s1arcsec2. 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′′ pixel1), 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
4
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Research Notes of the AAS ©2024. Published by the American Astronomical Society
... In an unexpected discovery, Drechsler et al. (2023) detected an extremely faint ;1°.5 long [O III] emission arc near M31 through the coaddition of hundreds of exposures obtained with small aperture telescopes (see Figure 1). This emission arc, named the Strottner-Drechsler-Sainty Object (SDSO), had not been reported previously in any deep broadband or Hα images of M31, and has no obvious emission counterpart in X-ray, UV, optical, infrared, or continuum radio surveys. ...
... The resulting images were circular with a FWHM resolution of ∼4 2 and ∼5 3 in the FUV and NUV bands, respectively. Figure 1 shows the original sum of 45.7 hr of [O III] images, revealing an emission nebula near M31 discovered by Drechsler et al. (2023) and hereafter referred to as either SDSO or simply as the M31 [O III] arc. The left panel shows the faintness of the arc, even using a narrow passband 30 Å filter and illustrates why the arc was missed in wide band images. ...
... The arc's small-scale details and coordinates can be seen in the enlarged Figure 2. Estimated to be ;1°.5 in length, ;0°.5 in width, and centered some 1°. 2 southeast of M31ʼs nucleus (Drechsler et al. 2023), this line-emission nebula appears composed of several broad and gently curved filaments aligned approximately northeast-southwest. Although displaying a filamentary appearance, the nebula's overall structure is mainly diffuse. ...
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... 3. RESULTS Figure 1 shows the [O III] emission nebula near M31 discovered by Drechsler et al. (2023) and hereafter re- arc. Estimated to be ≃ 1.5 • in length and ≃ 0.5 • in width and centered some 1.2 • southeast of M31's nucleus (Drechsler et al. 2023), this line-emission nebula appears composed of several broad and gently curved filaments aligned approximately NE-SW. ...
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The letter reports the discovery of a filamentary emission-line structure at right ascension 19 hr 31 min and declination +31 deg 10 min (1950) during an emission-line survey of the Milky Way. This structure is classified as a supernova remnant on the basis of its similarity to several other SNRs, but is shown not to be part of the Gamma Cygni complex. A physical diameter of about 70 pc is obtained for this SNR along with a distance of approximately 1200 pc, a shock velocity of at least 50 km/s, an initial density of about 0.5 per cu cm, and an age of approximately 300,000 yr.
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Context. During a search for previously unknown Galactic emission nebulae, we discovered a faint 36′ diameter H α emission nebula centered around the periodic variable YY Hya. Although this star has been classified as an RR-Lyr variable, such a classification is inconsistent with a Gaia distance of ≃450 pc. The GALEX image data also show YY Hya as having a strong UV excess, suggesting the existence of a hot and compact binary companion. Aims. We aim to clarify the nature of YY Hya and its nebula. Methods. In addition to our discovery image data, we obtained a 2.°5 × 2.°5 image mosaic of the whole region with CHILESCOPE facilities and time-series spectroscopy at MDM observatory. Also, we used data from various space missions to derive an exact orbital period and a spectral energy distribution. Results. We find that YY Hya is a compact binary system containing a K dwarf star that is strongly irradiated by a hot white dwarf companion. The spectral characteristics of the emission lines that are visible only during the maximum light of the perfectly sinusoidal optical light curve show signatures resembling those of members of the BE UMa variable family. These are post-common-envelope pre-cataclysmic variables. However, the companion star here is more massive than that found in other group members and, thus, the progenitor of the white dwarf must have been a star between 3 and 4 M ⊙ . The nebula appears to be an ejected common-envelope shell with a mass on the order of one M ⊙ and an age of 500 000 yr. This makes it the biggest such shell known thus far. The alignment of neighboring nebulosities some 45′ to the northeast and southwest of YY Hya suggests that the system has had strong bipolar outflows. We also briefly speculate that it might be related to the 1065 BP “guest-star” reported in ancient Chinese records.
Article
Context. The Andromeda (M 31) galaxy subtends nearly 100 square degrees on the sky. Any study of its halo must therefore account for the severe contamination from the Milky Way halo stars whose surface density displays a steep gradient across the entire M 31 field of view. Aims. Our goal is to identify a population of stars firmly associated with the M 31 galaxy. Planetary nebulae (PNe) are one such population that are excellent tracers of light, chemistry, and motion in galaxies. We present a 16 square degree survey of the disc and inner halo of M 31 with the MegaCam wide-field imager at the CFHT to identify PNe, and characterise the luminosity-specific PN number and PN luminosity function (PNLF) in M 31. Methods. PNe were identified via automated detection techniques based on their bright [O III ] 5007 Å emission and absence of a continuum. Subsamples of the faint PNe were independently confirmed by matching with resolved Hubble Space Telescope sources from the Panchromatic Hubble Andromeda Treasury and spectroscopic follow-up observations with HectoSpec at the MMT. Results. The current survey reaches two magnitudes fainter than the previous most sensitive survey. We thus identify 4289 PNe, of which only 1099 were previously known. By comparing the PN number density with the surface brightness profile of M 31 out to ∼30 kpc along the minor axis, we find that the stellar population in the inner halo has a luminosity-specific PN number value that is seven times higher than that of the disc. We measure the luminosity function of the PN population and find a bright cut-off and a slope consistent with previous determinations. Interestingly, it shows a significant rise at the faint end, present in all radial bins covered by the survey. This rise in the M 31 PNLF is much steeper than that observed for the Magellanic clouds and Milky Way bulge. Conclusions. The significant radial variation of the PN specific frequency value indicates that the stellar population at deprojected minor-axis radii larger than ∼10 kpc is different from that in the disc of M 31. The rise at the faint end of the PNLF is a property of the late phases of the stellar population. M 31 shows two major episodes of star formation and the rise at the faint end of the PNLF is possibly associated with the older stellar population. It may also be a result of varying opacity of the PNe.
Article
The wide-field CCD camera at the CFH telescope was used to survey the giant stellar stream in the Andromeda galaxy, resolving stars down the red giant branch in M31 to I ~ 25, a magnitude deeper than our previous INT survey of this galaxy and extending 1 degree further out. The stream is seen to extend out to the south-east of M31 as far as we have surveyed (some 4.5 degrees, corresponding to a projected distance ~60 kpc). It is a linear structure in projection, and the eastern edge of the stream presents a sharp boundary in star counts suggesting that it remains a coherent structure. By analysing the luminosity function of the metal rich component of the stream we find that, at the furthest extent of our survey, the stream is 100 kpc further away along the line of sight than M31. It can then be traced to a point on the north-western side of the galaxy where it is some 30 kpc in front of M31, at which point the stream turns away from our survey area. Comment: 6 pages, 7 figures. Accepted for publication in MNRAS. A version with higher resolution figures and a larger figure 1 can be found at http://www.ast.cam.ac.uk/~alan/publications
  • G J Ferland
  • M Chatzikos
  • F Guzmán
Ferland, G. J., Chatzikos, M., Guzmán, F., et al. 2017, RMxAA, 53, 385. https://arxiv.org/abs/1705.10877