Discovery of a large and faint nebula at the Triangulum galaxy

We report the discovery of a previously uncatalogued arch-shaped filamentary nebula at the outer part of the Triangulum galaxy (M33) centred at α = 1 h 34 m 25 s , δ = +30 ◦ 20 ′ 17 ′′ (ICRS). This discovery stems from meticulous observations employing deep exposures of M33, using both H α and [OIII] narrow-band filters. The nebula, designated as Roig1 Prades Sky , exhibits an H α surface brightness of 23 . 9 mag arcsec − 2 . Its sky projected location is 21 arcmin away from the M33 galactic centre towards the southeast direction with an extent of (120 × 440) ± 30 pc. Deep spectroscopic observations are required to unveil its real nature.


INTRODUCTION
Amateur astronomers have more access to telescope observation time in comparison to their professional counterparts, enabling them to conduct longer observations covering wider areas of the sky.Aleix Roig used this advantage to capture a very deep image of M33 covering an area of 2.4 • × 1.6 • around this galaxy.The primary objective was to seek for [OIII] and Hα emission regions in the outskirts of M33.
These observations took place at the Astrocat Observatory 1 , an amateur facility located in the dark skies of Prades (Spain), characterised by a sky brightness of µ V ∼ 21 mag arcsec −2 .The excepinfo@astrocat.info 1 https://astrocat.infotional conditions allowed Aleix Roig to set up this observatory back in 2014.He held other professional collaborations, such as the 100 hours image of the M101 galaxy presented at the 355 IAU Symposium 2 .

DATA
The images were acquired employing two small refractor telescopes, both synchronised by a MESU200 mount.Each telescope is equipped with its independent camera and filters, configured to observe the same region of the sky.The images were taken during two distinct periods: from September 22 to 26, and from October 5 to 11, 2023.Periods of high Moon illumination were carefully avoided.
The first telescope, a Takahashi FSQ85 (85 mm aperture, F/5.3), captured a total of 969 images, each lasting 300 seconds (total exposure time of 80.75 hours) using the [OIII] filter centred at 500.7 nm with a passband of 6.5 nm.We used a ZWO ASI294MM 4/3" camera with a pixel size of 2.1 arcsec/pixel and a field of view (FOV) of 2.44 • × 1.66 • .
The second telescope, a Takahashi FSQ106 (106 mm aperture, F/5.0), captured a total of 534 images, each lasting 300 seconds (total exposure time of 44.5 hours) using the Hα filter centred at 656.3 nm with a passband of 3 nm.The camera used was a ZWO ASI2600MM APS-C with a pixel size of 1.5 arcsec/pixel and a FOV of 2.54 • ×1.74 • .
The Takahashi FSQ106 was used to capture images with luminance (L), red (R), green (G), and blue (B) filters that were combined to obtain an LRGB-coloured image with a total exposure time of 28.5 hours.
The data processing was mainly done with PixInsight 3 to obtain the colour images.The use of J-PLUS (Cenarro et al. 2019) data, and more in particular its Hα (J0660) filter, allowed us to perform a crude photometric calibration and the estimation of surface brightness measurements by using Gnuastro (Akhlaghi & Ichikawa 2015).The depth of the combined Hα image is 24.9 mag arcsec −2 (3σ in 100 arcsec 2 ).Due to the lack of good-quality calibrated data, we could not obtain reliable brightness measurements in the [OIII] filter.

RESULTS
The resulting LRGB image is presented in the left panel of Figure 1.It shows the details in the Hα and [OIII] bands of the central part of M33.We found no emission at these wavelengths on the outskirts of the galaxy.Foreground Galactic cirrus can be distinguished with LRGB filters across those outer regions.We refer to the Astrocat Ob-3 https://pixinsight.com/servatory web4 to see different versions of the same data.
The top right panel of Figure 1 shows a zoomin of the discovered nebula.It has an Hα surface brightness of ∼ 23.9 mag arcsec −2 .The detection of the same feature using two independent telescopes and several nights rejects the possibility of being an instrumental artefact.
Furthermore, Roig1 Prades Sky is also visible in the deep image of M33 from Magrini et al. (2000b) 5 obtained with the Isaac Newton Telescope.The nebula is outlined as a bright [OIII] emitter at the top right corner of the second part of their image.This is a completely independent confirmation of our finding.The goal of these authors was obtaining a catalogue of planetary nebulae in M33 (Magrini et al. 2000a); they did not include our nebula among their candidates and it is not mentioned in the paper.Indeed, Roig1 Prades Sky is much more extended than the targets they studied.Galera-Rosillo et al. ( 2018) also searched for planetary nebulae at the outskirts of M33, but they do not cover our region of interest.
Roig1 Prades Sky might be the brightest part of a spherical shell.In that case, a lower limit of the diameter of this bubble would be ∼ 470 ± 30 pc.When including the two filaments extending southeast, this shell almost doubles its size.The angular distance between the nebula and the M33 centre is 21 arcmin.This translates to ∼ 5 kpc at the afore- (Gaia-DR3-303312480229584384).However, it does not seem to be at the centre of our nebula.Moreover, Roig1 Prades Sky is much bigger than previous WR nebulae found in M33 (typical diameters of a few tens of parsecs according to Drissen et al. 1991).These two properties are the reasons why we think that this structure might not be the nebula associated with the aforementioned WR.On the other hand, the high emission of excited oxygen makes us con-sider Roig1 Prades Sky as a possible supernova remnant.
The real nature of Roig1 Prades Sky is unknown.We plan to perform new observations and obtain deep spectroscopic data to determine its morphological, chemical and kinematic properties.

Figure 1 .
Figure 1.Left: LRGB image of M33.B and G represent [OIII], R represents Hα.The outskirts of M33 are mapped according to a classic LRGB composition with no [OIII] or Hα added.Roig1 Prades Sky is identified by two perpendicular lines.Top-right: Zoom-in of Roig1 Prades Sky from the left panel.The HII region [LHK2017] 318 is at the top right corner.Bottom-right: Same as above in grey scale with a box indicating the size measurement (0.48 × 1.80 arcmin).mentioned distance without considering the inclination of the M33 disc.The southern part of Roig1 Prades Sky coincides with the Wolf-Rayet (WR) star [NM2011]-J013425.11+301950.3(Gaia-DR3-303312480229584384).However, it does not seem to be at the centre of our nebula.Moreover, Roig1 Prades Sky is much bigger than previous WR nebulae found in M33 (typical diameters of a few tens of parsecs according toDrissen et al. 1991).These two properties are the reasons why we think that this structure might not be the nebula associated with the aforementioned WR.On the other hand, the high emission of excited oxygen makes us con-