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Issue
A&A
Volume 567, July 2014
Article Number A59
Number of page(s) 5
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201424128
Published online 10 July 2014

© ESO, 2014

1. Introduction

There is a large discrepancy between the theoretically predicted numbers of the Galactic supernova remnants (SNRs; e.g., Berkhuijsen 1984; Li et al. 1991; Tammann et al. 1994) and the observational detections (Green 2009; Ferrand & Safi-Harb 2012), which is often attributed to the limitations of the current observations on the angular resolutions to detect distant small SNRs, the sensitivities to reveal faint SNRs, and the insufficient sky coverage for the completeness of detections. In addition, strong confusion from the diffuse Galactic emission makes discoveries of SNRs in the inner Galaxy even more difficult.

Previously, the big steps in the discovery of the Galactic SNRs relied on the radio surveys carried out by powerful telescopes, e.g., Shaver & Goss (1970) and Clark et al. (1975) on the 408 MHz and 5 GHz surveys by the Parkes and Molonglo telescopes. Reich et al. (1988) found many SNRs using the λ21 cm and λ11 cm surveys made by the Effelsberg 100-m telescope. With the Molonglo telescope again, Whiteoak & Green (1996) identified SNRs with the 843 MHz survey data. The most recent significant increase in the number of the Galactic SNRs was made by Brogan et al. (2006), using the high angular resolution of the VLA to detect 35 new SNRs in the inner Galaxy. There are also some individual discoveries, although not many. The Sino-German λ6 cm polarization survey of the Galactic plane1 (Sun et al. 2007, 2011a; Gao et al. 2010; Xiao et al. 2011) conducted by the Urumqi 25-m radio telescope provides a good hunting ground for discovering new Galactic SNRs. Gao et al. (2011) identified two new faint SNRs G25.1−2.3 and G178.2−4.2. Supplemented by the Urumqi λ6 cm data, Foster et al. (2013) identified another two new SNRs G152.4−2.1 and G190.9−2.2 from the Canadian Galactic Plane Survey (CGPS) data. With the Giant Meterwave Radio Telescope, Roy & Pal (2013) have recently discovered a young SNR G354.4+0.0 in the central region of the Galaxy. The CGPS has also contributed several other detections, e.g. G85.4+0.7 and G85.9−0.6 by Kothes et al. (2001), G107.5−1.5 by Kothes (2003), G96.0+2.0 and G113.0+0.2 by Kothes et al. (2005), G108.2−0.6 by Tian et al. (2007), and G141.2+5.0 by Kothes et al. (2014). However, most of the radio continuum surveys of the Galactic plane are limited in a latitude range, e.g. \hbox{$|b| \leqslant 5\degr$}. This is not sufficient to discover and study the SNRs off the plane, such as the Cygnus Loop (G74.0−8.5; e.g., Harris & Roberts 1960; Sun et al. 2006) and the CTA 1 (G119.5+10.2; e.g., Walsh & Brown 1955; Sun et al. 2011b).

Following the Urumqi λ6 cm plane survey, a survey extending the latitude range from b = 5° to b = 20° has been carried out since 2012 (Gao et al., in prep.). It currently collects the data of λ6 cm total intensity and linear polarization from the longitude range from ℓ = 90° to 160°. Here we report the discovery of a new SNR G150.3+4.5 by combing the new data with the λ6 cm plane survey data. We introduce the data in Sect. 2. The radio continuum emission, the spectrum, and the evidence collected in other bands are discussed in Sect. 3. We summarize our work in Sect. 4.

2. Data

2.1. Urumqi λ6 cm data

The λ6 cm total intensity I and the linear polarization U and Q data used in this paper come from a portion of the Sino-German λ6 cm polarization survey of the Galactic plane (Gao et al. 2010) and the extended survey with the latitude range from b = 5° to b = 20°. Observations of both surveys were made with the same λ6 cm system mounted on the Urumqi 25-m radio telescope, Xinjiang Astronomical Observatories, Chinese Academy of Sciences. The observation strategy, data reduction, and calibration follow the same procedures, which have been described in detail in Sun et al. (2007) and Gao et al. (2010). The angular resolution of the λ6 cm map is 9\hbox{$\farcm$}5. The sensitivity is about 1.0 mK Tb for total intensity and about 0.3 mK Tb for polarization.

2.2. Effelbserg λ11 cm data

The λ11 cm total intensity map is a part of the radio continuum survey of the Galactic plane made by the Effelsberg 100-m radio telescope (Fürst et al. 1990). We extracted the data from the survey sampler of the Max-Planck-Institut für Radioastronomie2. The map has an angular resolution of 4\hbox{$\farcm$}3. We measured 6.7 mK Tb as the rms (1σ) for the λ11 cm total intensity data.

2.3. Effelsberg λ21 cm data

The λ21 cm radio continuum data was observed by the Effelsberg telescope for two parts: the data above the latitude of b = 4° come from the Effelsberg Medium Latitude Survey (EMLS; Uyanıker et al. 1998; Reich et al. 2004), while the data below b = 4° come from the Effelsberg λ21 cm radio continuum plane survey (Reich et al. 1997). The angular resolution is about 9\hbox{$\farcm$}4, comparable to that of the λ6 cm data. The rms of the map is about 20.0 mK Tb.

2.4. Canadian λ21 cm & λ73 cm data

Both of the Canadian λ21 cm (1420 MHz) and λ73.5 cm (408 MHz) data are from the Canadian Galactic Plane Survey (CGPS) conducted by the synthesis telescopes of the Dominion Radio Astronomical Observatory (DRAO; Taylor et al. 2003; Landecker et al. 2010). The data were extracted from the the Canadian astronomy data center3. The λ21 cm (1420 MHz) data has an angular resolution of 60′′ × 49′′ in the area of \hbox{$\ell = 150\fdg3, b = 4\fdg5$}, while the λ73.5 cm (408 MHz) data has a beam of 3\hbox{$\farcm5 \times 2\farcm$}8. In this paper, we use the CGPS maps for qualitative study and for showing fine structures on small scales.

3. Results

We present the Urumqi λ6 cm, the Effelsberg λ11 cm, and the Effelsberg λ21 cm total intensity images of a \hbox{$3\fdg5 \times 4\fdg35$} area around the target in the top panels of Fig. 1. The loop structure seen from the λ6 cm image is named G150.3+4.5, according to its geometric center. To study the faint and extended emission of this object, prominent point-like sources (S1.4 GHz> 20 mJy) within the central field of 3° are subtracted based on the NVSS source catalog (Condon et al. 1998). For point-like sources that have determined radio flux density spectral indices (Sν = να) (Vollmer et al. 2005), we simply extrapolate the flux densities from 1.4 GHz to the other two frequencies. For sources with unknown spectral indices, a spectral index of α = −0.9 is used for extrapolation. We show the total intensity images in the middle panels of Fig. 1 for the λ6 cm, λ11 cm and the Effelsberg λ21 cm bands after the point-like sources are removed. The λ6 cm polarization image and the high resolution total intensity images of the DRAO λ21 cm and λ73.5 cm are shown in the bottom panels of Fig. 1. The currently available radio continuum observations of the Effelsberg λ11 cm data ends up at b = 5°, while the DRAO λ21 cm and λ73.5 cm data extend to about \hbox{$b = 5\fdg5$}. The angular resolutions of the λ6 cm, the Effelsberg λ21 cm, the DRAO λ21 cm, and the DRAO λ73.5 cm images are 9\hbox{$\farcm$}5, 9\hbox{$\farcm$}4, 60′′ × 49′′ and \hbox{$3\farcm5 \times 2\farcm8$}, respectively. To increase the signal-to-noise ratio, the Effelsberg λ11 cm total intensity image was convolved to an angular resolution of 6′.

thumbnail Fig. 1

Radio continuum images of the new SNR G150.3+4.5. Top panels: total intensity of G150.3+4.5 obtained from Urumqi λ6 cm (left), Effelsberg λ11 cm (central), and Effelsberg λ21 cm (right) observations. The contours run at 3.0 + (n − 1) × 3.0 mK (n = 1,2,...6) and 21.0 + (n − 7) × 21.0 mK (n = 7,8..) for the λ6 cm image, at 20.0 + (n − 1) × 13.4 mK (n = 1,2,...6) and 85.0 + (n − 7) × 212.0 mK (n = 7,8..) for the λ11 cm image, at 60.0 + (n − 1) × 40.0 mK (n = 1,2,...6) and 320.0 + (n − 7) × 600.0 mK (n = 7,8..) for the λ21 cm image. Middle panels: the same images as upper panels, but point-like sources are removed within the central 3° field as indicated by the circle in the left panel. Bottom panels: images for the λ6 cm polarization intensity (left), the CGPS λ21 cm total intensity (central) and λ73.5 cm total intensity (right). The contours on the λ6 cm polarization image is the same as for the λ6 cm total intensity image shown in the upper panels. The angular resolutions for observations of the Urumqi λ6 cm, Effelsberg λ21 cm, CGPS λ21 cm, and λ73.5 cm images are 9\hbox{$\farcm$}5, 9\hbox{$\farcm$}4, 60′′ × 49′′, and \hbox{$3\farcm5 \times 2\farcm8$}, respectively. The Effelsberg λ11 cm image was convolved to the resolution of 6′ to increase the signal-to-noise ratio. The rectangles A and B as indicated in the middle left panel are the areas for TT plots in Fig. 2. The dashed circle in the middle central panel indicates the newly discovered SNR G149.5+3.2 by Gerbrandt et al. (2014), which overlaps with the lower part of the western shell of G150.3+4.5. The two straight lines shown in the CGPS λ21 cm total intensity image indicate the emission, which does not seem to be connected to the eastern shell of G150.3+4.5.

3.1. Total intensity and the spectral index

The big loop G150.3+4.5 is clearly seen in the Urumqi λ6 cm and the Effelsberg λ21 cm images, extending to a higher latitude of more than b = 6°. It shows an enclosed oval shape at λ6 cm, which is about 2\hbox{$\fdg$}5 wide and 3° high. Three major shells of G150.3+4.5 can be identified in the λ6 cm image. The most prominent one is found in the east, curving to the center of the lower south, while a fainter shell can be identified in the west, also extending to the lower south. A much fainter and fragmented shell is found in the upper north. From the λ6 cm map after subtracting the point-like sources, we see the emission in that region is just around 3σ (~3.0 mK Tb) detection above the background.

Most of the structures seen at λ6 cm also appear in the Effelsberg λ21 cm total intensity map, but the loop is not closed, with a gap in the northwest. This part is detected at λ6 cm to be about 3.0 mK Tb, but below 3σ21 cm = 60 mK Tb at λ21 cm (see Fig. 1), which indicates that the brightness spectral index of this gap region is shallower than β = −2.46 (Sν = να, β = α − 2).

The λ11 cm data cover only the lower part of G150.3+4.5, but reveals more detailed structures with a 6′ angular resolution. The new SNR G149.5+3.2, as indicated in Fig. 1 has just been discovered by Gerbrandt et al. (2014). It overlaps with the lower part of the western shell of G150.3+4.5.

The DRAO λ21 cm image has the sharpest resolution. The eastern shell of G150.3+4.5 can be traced clearly. The overlap of the lower part of the western shell with the new SNR G149.5+3.2 (Gerbrandt et al. 2014) is clearly seen. The upper part of the western shell, free of the contamination of G149.5+3.2, can be identified above b = 4°. The fragmented northern shell of G150.3+4.5 seen at λ6 cm cannot be identified. From the ~1′ resolution map, there is no continuous emission of the prominent eastern shell pointing to the north. Faint extended emission goes wider from \hbox{$\ell = 151\fdg3, b = 4\fdg1$} to \hbox{$\ell = 151\fdg6, b =4\fdg5$} and then turns to \hbox{$\ell = 150\fdg9, b = 5\fdg4$} (as indicated in Fig. 1). As for the Effelsberg λ21 cm map, the DRAO λ21 cm observations do not detect the emission in the northwest of the loop, either.

The DRAO λ73.5 cm image revealed the prominent eastern shell. The western shell of G150.3+4.5 appears to be fainter. The new SNR G149.5+3.2 can also be identified in the lower right of G150.3+4.5.

Gerbrandt et al. (2014) demonstrate the non-thermal synchrotron emission nature of the eastern shell of G150.3+4.5 and propose that it is a new SNR. We tested it with the TT-plot method (Turtle et al. 1962), using the Urumqi λ6 cm, the Effeslberg λ11 cm, and the Effelsberg λ21 cm data. We got the brightness temperature spectral index of β = −2.40 ± 0.17 between the λ6 cm and λ21 cm data, and β = −2.38 ± 0.42 between the λ6 cm and λ11 cm data, which are consistent with the result of Gerbrandt et al. (2014). The western shell of G150.3+4.5 is not mentioned in Gerbrandt et al. (2014). It can be identified from all total intensity images at various wavelengths as we showed. This shell and the eastern one both curve to the southern lower section, suggesting that they are from the same entity. For the emission that is free of the contamination by the new SNR G149.5+3.2 (see the rectangle area B as indicated in Fig. 1), we tested the spectrum of the upper part of the western shell of G150.3+4.5. TT plots give a spectral index of β = −2.69 ± 0.24 between the Urumqi λ6 cm and the Effelsberg λ21 cm data, and β = −2.44 ± 0.68 between the Urumqi λ6 cm and Effelsberg λ11 cm data, indicating its non-thermal nature.

thumbnail Fig. 2

TT plots between λ6 cm and λ21 cm and between the λ6 cm and λ11 cm for the eastern shell, i.e., region A (upper panels), and western shell, i.e., region B (lower panels) of G150.3+4.5, as indicated by the rectangles in Fig. 1.

thumbnail Fig. 3

60 μm infrared (left panel) and Hα emission (middle panel) in the area of G150.3+4.5. The contours are the same for the λ6 cm image shown in the upper panel of Fig. 1. Right panel: optical emission from the DSS2 red image in the field of G151.2+2.6. λ6 cm total intensity contours run at 6.0 mK Tb in steps of 6.0 mK Tb.

3.2. Other noticeable structures

Three noticeable structures other than the new SNR G150.3+4.5 are well resolved in the ~1′ resolution CGPS λ21 cm data. They are all located in the lower left hand corner of the maps in Fig. 1 and collected by Kerton et al. (2007) in the CGPS extended source catalog. G151.2+2.6 (CGPSE 169) has a circular shape with a size of about 40′, while G150.9+2.7 (CGPSE 168) and G151.4+3.0 (CGPSE 172) both have an arc shape with a length of about 20′. G151.2+2.6, however, is not detected at 408 MHz. Thus its nature is still unclear. Based on the steep non-thermal spectral indices found between 408 MHz and 1420 MHz CGPS data (α = −1.3 ± 0.3 for G150.9+2.7, α = −0.4 ± 0.2 for G151.4+3.0), Kerton et al. (2007) propose that the two arcs G150.9+2.7 and G151.4+3.0 form a new Galactic SNR G151.20+2.85.

In the Urumqi and Effelsberg maps with a ~10′ resolution, the circular G151.2+2.6 can be recognized in all three frequency maps. G150.9+2.7 is mixed with the upper part of G151.2+2.6 and cannot be distinguished. Instead of being an arc, G151.4+3.0 is seen to be an elongated point-like source.

To know their properties, we determined the spectra first. We subtracted the point-like sources from the Urumqi λ6 cm, the Effelsberg λ11 cm and the Effelsberg λ21 cm data based on the NVSS catalog. For the circular G151.2+2.6, to avoid the contamination from G150.9+2.7, the TT plot is only made for the region below \hbox{$b = 2\fdg6$}. We obtained the brightness temperature spectral index of β = −2.03 ± 0.32 between λ6 cm and λ11 cm, and β = −2.18 ± 0.08 between λ6 cm and λ21 cm. The flat radio continuum spectrum implies that G151.2+2.6 may have a thermal origin.

For G150.9+2.7, we cannot see it owing to the overlap with G151.2+2.6. For the source G151.4+3.0, a non-thermal spectral index is found to be β = −2.62 ± 0.15 between the λ6 cm and λ21 cm data, supporting the idea of Kerton et al. (2007) that G151.4+3.0 is a (part of) SNR.

3.3. Hints in other bands

We search for coincidence of the objects we discussed in infrared, Hα, and DSS2 red maps (McLean et al. 2000). From the ancillary 60 μm infrared map downloaded from the CGPS website, except for the western part of G151.2+2.6, we do not see any strong correlations between the infrared and the radio continuum emission toward any extended objects in Fig. 1 (see Fig. 3, left panel). Kerton et al. (2007) suggest that there is related infrared emission in the eastern shell of G150.3+4.5. Together with the flat spectral index they derived, the eastern shell was suggested to be an H ii region. We agree with Gerbrandt et al. (2014) that the strong overall infrared emission does not appear to be correlated with the eastern shell of the new SNR G150.3+4.5.

We extracted the Hα image from the Wisconsin Hα mapper northern sky survey (Haffner et al. 2003). Again we do not see any firm correlation with any structures except for some parts of G151.2+2.6 (Fig. 3, middle panel). Strong Hα emission is seen in the eastern part of the image, and it overlaps with the eastern shell of the new SNR G150.3+4.5. This could possibly explain the non-detection of the polarized emission at λ6 cm as a result of depolarization, if the magneto-ionized medium appears as the foreground.

From the DSS2 red image, Gerbrandt et al. (2014) show that an optical filamentary structure is coincident with the lower part of the eastern shell of G150.3+4.5. For the other parts of G150.3+4.5, we do not see this coincidence. A ring-shaped optical counterpart is found for G151.2+2.6 (see Fig. 3, right panel). This, together with the flat radio continuum spectrum, the Hα, and infrared emission, strongly suggest that it is an H ii region.

4. Summary

By combining the Urumqi λ6 cm Galactic plane survey and the new medium latitude survey, we discovered an enclosed oval-shaped object G150.3+4.5. It contains three shells in the east, west, and north, respectively. Effelsberg λ11 cm and λ21 cm radio continuum data are used for the spectral index analysis, and the high angular resolution CGPS λ21 cm and λ73.5 cm data are used for its fine structure. The eastern and western shells of G150.3+4.5 can be firmly confirmed, while the faint northern shell seen at λ6 cm has not been detected in other radio frequencies.

The TT plots between λ6 cm, λ11 cm, and λ21 cm are employed for analyzing the spectra of the eastern and western shells of the newly discovered loop of G150.3+4.5. We found non-thermal radio spectra for the shells on both sides. The spectral index for the eastern shell is about β ~ −2.4, and about β ~ −2.7 for the western shell. From the properties of this half loop and the optical filamentary structrure that is related to the eastern radio shell (Gerbrandt et al. 2014), G150.3+4.5 should be a SNR, though the polarized emission from G150.3+4.5 is not detected, which might be due to the sensitivity limit or depolarization effect in the eastern shell.

For the other prominent structures in the observed area, we confirm that the arc structure G151.4+3.0 has a non-thermal spectrum, based on the Urumqi λ6 cm and the Effelsberg λ21 cm data. This agrees with the result of Kerton et al. (2007) that it has a SNR origin. But from the morphology, it is still not clear G150.9+2.7, and G151.4+3.0 form one SNR or two separated SNRs. We found a flat radio continuum spectrum for the circular G151.2+2.6 and the related Hα and infrared emission in its western part, and also the related optical emission in the DSS2 red image, which all suggest that G151.2+2.6 is an H ii region.

Acknowledgments

We thank the referee for helpful comments that improved our paper. The authors are supported by the National Natural Science Foundation of China (11303035) and by the Strategic Priority Research Program “The Emergence of Cosmological Structures” of the Chinese Academy of Sciences, Grant No. XDB09010200. X.Y.G. is also supported by the Young Researcher Grant of National Astronomical Observatories, Chinese Academy of Sciences. We acknowledge Mr Otmar Lochner for construction of the excellent λ6 cm receiver and Mr Maozheng Chen and Jun Ma for their skillful maintenance.

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All Figures

thumbnail Fig. 1

Radio continuum images of the new SNR G150.3+4.5. Top panels: total intensity of G150.3+4.5 obtained from Urumqi λ6 cm (left), Effelsberg λ11 cm (central), and Effelsberg λ21 cm (right) observations. The contours run at 3.0 + (n − 1) × 3.0 mK (n = 1,2,...6) and 21.0 + (n − 7) × 21.0 mK (n = 7,8..) for the λ6 cm image, at 20.0 + (n − 1) × 13.4 mK (n = 1,2,...6) and 85.0 + (n − 7) × 212.0 mK (n = 7,8..) for the λ11 cm image, at 60.0 + (n − 1) × 40.0 mK (n = 1,2,...6) and 320.0 + (n − 7) × 600.0 mK (n = 7,8..) for the λ21 cm image. Middle panels: the same images as upper panels, but point-like sources are removed within the central 3° field as indicated by the circle in the left panel. Bottom panels: images for the λ6 cm polarization intensity (left), the CGPS λ21 cm total intensity (central) and λ73.5 cm total intensity (right). The contours on the λ6 cm polarization image is the same as for the λ6 cm total intensity image shown in the upper panels. The angular resolutions for observations of the Urumqi λ6 cm, Effelsberg λ21 cm, CGPS λ21 cm, and λ73.5 cm images are 9\hbox{$\farcm$}5, 9\hbox{$\farcm$}4, 60′′ × 49′′, and \hbox{$3\farcm5 \times 2\farcm8$}, respectively. The Effelsberg λ11 cm image was convolved to the resolution of 6′ to increase the signal-to-noise ratio. The rectangles A and B as indicated in the middle left panel are the areas for TT plots in Fig. 2. The dashed circle in the middle central panel indicates the newly discovered SNR G149.5+3.2 by Gerbrandt et al. (2014), which overlaps with the lower part of the western shell of G150.3+4.5. The two straight lines shown in the CGPS λ21 cm total intensity image indicate the emission, which does not seem to be connected to the eastern shell of G150.3+4.5.
In the text
thumbnail Fig. 2

TT plots between λ6 cm and λ21 cm and between the λ6 cm and λ11 cm for the eastern shell, i.e., region A (upper panels), and western shell, i.e., region B (lower panels) of G150.3+4.5, as indicated by the rectangles in Fig. 1.
In the text
thumbnail Fig. 3

60 μm infrared (left panel) and Hα emission (middle panel) in the area of G150.3+4.5. The contours are the same for the λ6 cm image shown in the upper panel of Fig. 1. Right panel: optical emission from the DSS2 red image in the field of G151.2+2.6. λ6 cm total intensity contours run at 6.0 mK Tb in steps of 6.0 mK Tb.
In the text

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