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Home All issues Volume 532 (August 2011) A&A, 532 (2011) A144 Full HTML
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Issue
A&A
Volume 532, August 2011
Article Number A144
Number of page(s) 7
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201117179
Published online 08 August 2011

© ESO, 2011

1. Introduction

Supernova remnants (SNRs) are the post-explosion relics of massive stars that have reached the end of their evolutionary life times. It has been predicted that the total number of Galactic SNRs is between 1000 and 10 000 (Berkhuijsen 1984; Li et al. 1991; Tammann et al. 1994). However, up to now just 274 SNRs have been identified (Green 2009). Among these, G192.8−1.1 has recently been disproved from being an SNR (Gao et al. 2011). This low SNR detection rate results from the insufficient sensitivity and resolution of available observations. Diffuse radio emission and discrete source complexes are not uniformly distributed along the Galactic disk, whose own emission confuses that from faint SNRs and hinders their identification.

A large number of SNRs have been identified from radio survey maps (e.g. Reich et al. 1988; Brogan et al. 2006). Shell-type SNRs can be recognized by their morphology, non-thermal spectra, and ordered polarization. Most known Galactic SNRs have a bright shell, with intrinsic magnetic fields running along the shell. The radio spectrum of an SNR is often described by a single power law, Sν ~ να. Here Sν represents the integrated flux density of an SNR at the observing frequency ν. In general, SNRs have a spectral index of α ~  −0.5, which is expected for SNRs in the adiabatic expansion phase with a compression factor of four (e.g. Reich 2002). Young SNRs can have steeper spectra (α ~  −0.6  to −0.8) and show radial magnetic fields (e.g. Reich 2002).

To discover new SNRs, observations of high sensitivity are needed, which resolve confusing structures. We have conducted the Sino-German λ6 cm polarization survey of the Galactic plane (Sun et al. 2007; Gao et al. 2010; Sun et al. 2011; Xiao et al. 2011) in the region of Galactic longitude 10° ≤ ℓ ≤ 230° and latitude |b| ≤ 5°, with an angular resolution of 9\hbox{$\farcm$}5. The sensitivity of the Urumqi survey, if extrapolated from 4.8 GHz to 1 GHz with a typical spectral index of α =  −0.5, is on average Σ1   GHz = 4.9 × 10-23   W   m-2   Hz-1   sr-1, lower than the surface brightness of the faintest Galactic SNR known to date, which is Σ1   GHz = 5.8 × 10-23   W   m-2   Hz-1   sr-1 for G156.2+5.7 (Reich et al. 1992; Xu et al. 2007). Therefore, it should be possible to detect new faint SNRs in the λ6 cm survey. Here, we report the discovery of two new SNRs, G178.2−4.2 and G25.1−2.3, and study their radio properties.

thumbnail Fig. 1

Radio images of G178.2−4.2 at λ6 cm from the Sino-German λ6 cm polarization survey in the top panels, at λ11 cm that we newly observed in the middle panels, and at λ21 cm from the Effelsberg Medium Latitude Survey (EMLS) (Reich et al. 2004) in the bottom panels. The angular resolutions of the λ6 cm, λ11 cm, and λ21 cm maps are 9.5′, 4.4′, and 9.4′, respectively. The left panels show the total-intensity (I) maps in color and in contours, with observed B-vectors overlaid (i.e. the observed E-vectors plus 90°) for polarization intensities PI > 2.4   mK  TB at λ6 cm and 32.0 mK  TB at λ11 cm. Vectors are not shown at λ21 cm. The vector length is proportional to PI. The I contours in the λ6 cm maps are 2n−12×1.6 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times1.6~(4\sigma)~{\rm mK}\ T_{\rm B} $}, (n = 1, 2, 3 ...), in the λ11 cm maps are 2n−12×12.8 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times12.8~(4\sigma)~{\rm mK}\ T_{\rm B}$} (n = 1, 2, 3 ...), and in the λ21 cm maps are 2n−12×44.0 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times44.0~(4\sigma)~{\rm mK}\ T_{\rm B} $} (n = 1, 2, 3 ...). The central panels also display the total-intensity maps, where strong point-like sources have been subtracted, to show the extended emission from the SNR more clearly. The right panels are the polarization intensity images with the contours for the total intensity maps with point-like sources subtracted. The rectangle in the λ6 cm image in the top central panel outlines the area used for the TT-plot spectral analysis displayed in Fig. 5.

thumbnail Fig. 2

Same as Fig. 1, but for G25.1−2.3. Here the λ11 cm and λ21 cm maps were extracted from the Effelsberg λ11 cm survey (Reich et al. 1990a) and λ21 cm survey (Reich et al. 1990b), respectively. The contours in the λ6 cm total-intensity maps are 2n−12×12.8 (4σ) mKTB\hbox{$ 2^{\frac{n-1}{2}}\times12.8~(4\sigma)~{\rm mK}\ T_{\rm B} $} (n = 1, 2, 3 ...), in the λ11 cm total-intensity maps are 2n−12×72.0 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times72.0~(4\sigma)~{\rm mK}\ T_{\rm B} $} (n = 1, 2, 3 ...), and in the λ21 cm total-intensity maps are 2n−12×200.0 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times200.0~(4\sigma)~{\rm mK}\ T_{\rm B} $} (n = 1, 2, 3 ...). The polarization intensity threshold for the B-vectors is 12.0 mK  TB at λ6 cm and 75.0   mK  TB at λ11 cm. No polarization data at λ21 cm are presently available. The star-symbols in the λ6 cm maps (top left panel) indicate known pulsars in the field. The polygon in the λ6 cm image in top central panel outlines the area for the TT-plot spectral analysis shown in Fig. 5.

2. The λ6 cm maps of the two new SNRs

The Sino-German λ6 cm polarization survey of the Galactic plane was conducted with the Urumqi 25-m radio telescope. The λ6 cm maps are not only important for studies of the diffuse Galactic emission (Sun et al. 2011; Xiao et al. 2011), but also for studies of Galactic sources like SNRs (e.g. Sun et al. 2006; Xu et al. 2007; Shi et al. 2008; Xiao et al. 2008, 2009; Gao et al. 2011).

We have identified two extended shell-like sources, G178.2−4.2 and G25.1−2.3, in the survey maps. We used the “background filtering” technique (Sofue & Reich 1979) with a filter beam size larger than that of the objects to remove the diffuse large-scale Galactic radio emission. G178.2−4.2 has a size of 72′ × 62′, and G25.1−2.3 has a size of 80′ × 30′. Following Gao et al. (2011), we measured the mean values at the corners of the Stokes I, U, and Q maps in areas without obvious structures. A hyper plane defined by these corner mean values was subtracted to obtain the “intrinsic” total intensity (I) and polarization intensity (PI) images of the objects (see Figs. 1 and 2). The PI values were calculated as PI=(U2+Q2−1.2σU,Q2)1/2\hbox{$PI = (U^{2} + Q^{2} - 1.2 \sigma_{U,Q}^{2})^{1/2}$} following Wardle & Kronberg (1974). To study these objects and obtain their integrated flux densities, it is necessary to first subtract point-like sources in the field of the objects. Obvious discrete point-like radio sources were directly subtracted from the Urumqi λ6 cm, and also the Effelsberg λ11 cm and λ21 cm maps by Gaussian fitting, as we will use in the next section. Some unresolved sources were identified in the NVSS catalog (Condon et al. 1998). The flux densities of these sources in our observing bands were extrapolated from the NVSS flux density at 1.4 GHz, S1.4   GHz, using a spectral index either derived between the flux densities from the NVSS and the Effelsberg λ11 cm survey or quoted from Vollmer et al. (2005). Otherwise, the mean spectral index for a large sample of radio sources in the NVSS and WSRT surveys, α =  −0.9 (Zhang et al. 2003), was used if the spectral index of a source could not be determined. The total-intensity maps with point-like sources subtracted are shown in the central panels of Figs. 1 and 2.

2.1. G178.2−4.2

G178.2−4.2 is located in the anti-center region of the Galaxy and displays a circular morphology with a prominent shell in its north. Three strong radio sources, NVSS J052423+281232, NVSS J052427+281255, and NVSS J052432+281313 are located in the center of the SNR, which are the three components of the double-sided radio source 3C 139.2 (Leahy & Williams 1984; Liu & Zhang 2002) and not related to the SNR. Subtracting these and other point-like sources, the extended emission of the entire SNR G178.2−4.2 is clearly visible (see the central panels of Fig. 1). Strong polarized emission from the northern shell of G178.2−4.2 is detected at λ6 cm, with B-field directions orientated tangential to the shell.

Table 1

Flux densities and the spectral indices of two SNRs.

2.2. G25.1−2.3

A well-pronounced half-shell structure of 80′ in length is visible at \hbox{$\ell = 25\fdg1$}, \hbox{$b = -2\fdg3$} after strong diffuse emission from the Galactic plane is filtered out (see Fig. 2). The background radio source centered at \hbox{$\ell = 24\fdg9, b = -1\fdg7$} is composed by NVSS J184245 − 075613 and NVSS J184249 − 075604 (Condon et al. 1998) and is subtracted. It is not clear whether the extended structure around \hbox{$\ell = 25\fdg65, b = -1\fdg95$} is a part of G25.1−2.3, because its spectral index from a TT-plot analysis remains inconclusive. In this paper we only discuss the radio properties of the shell region outlined in Fig. 2.

3. Radio properties of G178.2−4.2 and G25.1−2.3

The radio morphologies of G178.2−4.2 and G25.1−2.3 at λ6 cm, as well as the properties discussed below, suggest that both objects are SNRs. Using the radio maps at three bands, i.e. λ6 cm observed by using the Urumqi 25-m telescope as described above, λ11 cm and λ21 cm by the Effelsberg 100-m telescope, we hereby study the radio spectrum and polarization properties of G178.2−4.2 and G25.1−2.3.

We extracted the λ11 cm maps of G178.2−4.2 from the Effelsberg λ11 cm Galactic plane survey (Fürst et al. 1990; Reich et al. 1990a) and λ21 cm maps from an unpublished section of the “Effelsberg Medium Latitude Survey (EMLS)” (Reich et al. 2004). For G25.1−2.3, the λ11 cm and λ21 cm maps are from the Effelsberg λ11 cm (Fürst et al. 1990; Reich et al. 1990a) and λ21 cm Galactic plane surveys (Reich et al. 1990b, 1997). The angular resolution is 4\hbox{$\farcm$}4 for the λ11 cm Effelsberg observations and 9\hbox{$\farcm$}4 for the λ21 cm observations.

We noticed, however, that G178.2−4.2 was poorly traced in the λ11 cm survey map because of its limited sensitivity. The northern part of G178.2−4.2 is clearly detected near the edge of the recent Effelsberg λ11 cm map of S147 (Xiao et al. 2008) made with a new receiver. We therefore observed G178.2−4.2 again in March 2011 with the Effelsberg 100-m telescope for the new λ11 cm map. The 80 MHz bandwidth of the λ11 cm receiver was connected to an 8-channel polarimeter centered at 2639 MHz. The lowest 10 MHz channel was corrupted by interference and could not be used. The radio source, 3C 286, was used as the main calibrator assuming S11   cm = 10.4 Jy and 9.9% linear polarization and a polarization angle of 33°. We obtained six full coverages of the 2° × 2° field with in total 4 s integration time per 2′ pixel. We added the maps of seven channels without interference, and applied the standard data reduction and calibration procedures as already described by Xiao et al. (2008, 2009). The rms-noise measured in emission-free areas was 3.2 mK  TB in the total-intensity and 2.9 mK  TB in the polarization-intensity map. The λ11 cm maps of G178.2−4.2 shown in Fig. 1 are from the new observations.

thumbnail Fig. 3

Integrated flux densities and radio spectra of G178.2−4.2 (upper panel) and the outlined area of G25.1−2.3 (lower panel).

We measured the integrated flux densities of G178.2−4.2 and G25.1−2.3 from the radio maps at λ6 cm, λ11 cm, and λ21 cm. We got S6cm = 1.0 ± 0.1 Jy, S11cm = 1.6 ± 0.2 Jy, and S21cm = 1.8 ± 0.2 Jy for G178.2−4.2, and S6   cm = 3.7 ± 0.4 Jy, S11   cm = 4.7 ± 0.5 Jy, and S21   cm = 6.7 ± 0.7 Jy for the outlined area of G25.1−2.3 (see Fig. 2). The spectral indices fitted from these integrated flux densities (see Fig. 3) are α =  −0.48 ± 0.13 for G178.2−4.2 and α =  −0.49 ± 0.13 for the shell of G25.1−2.3, which indicate the non-thermal nature of radio emission of these two objects. These results are summarized in Table 1.

thumbnail Fig. 4

Spectral index maps for G178.2−4.2 and G25.1−2.3.

Spectral index maps for these two objects were derived from radio maps at three wavelengths, as shown in Fig. 4. The spectral indices vary in different areas. The spectral index of the northern shell of G178.2−4.2 is about α ~  −0.6, and that of the southern shell of G25.1−2.3 is about α ~  −0.5.

thumbnail Fig. 5

TT-plots for the shell of G178.2−4.2 outlined in Fig. 1 and the shell of G25.1−2.3 outlined in Fig. 2, using the Urumqi λ6 cm map and the Effelsberg λ11 cm and λ21 cm maps.

These spectral indices were verified by TT-plots (Turtle et al. 1962). From the brightness temperatures TB at two frequencies plotted against each other, the brightness spectral index, β, is defined as TB = νβ, so that α = β + 2. As shown in Fig. 5, we obtained β6   cm − 11   cm =  −2.53 ± 0.16 and β6   cm − 21   cm =  −2.63 ± 0.09 for the northern shell of G178.2−4.2, and β6   cm − 11   cm =  −2.45 ± 0.60 and β6   cm − 21   cm =  −2.59 ± 0.10 for the southern shell of G25.1−2.3. These values all agree with the results derived from the integrated flux densities and spectral index maps.

Using the total intensity and the radio spectral index, we calculated the surface brightness of both objects at 1 GHz (e.g. Kothes et al. 1998), obtaining Σ1   GHz = 1.505 × 10-19S1   GHz [Jy]/□ [arcmin2] = 7.2 × 10-23   Wm-2   Hz-1   sr-1 for G178.2−4.2, and Σ1   GHz = 5.0 × 10-22   Wm-2   Hz-1   sr-1 for the southern shell of G25.1−2.3. The surface brightness for G178.2−4.2 is to date the second lowest for a Galactic SNR, only slightly above that of the lowest, 5.8 × 10-23   Wm-2   Hz-1   sr-1 reported for SNR G156.2+5.7 (Reich et al. 1992).

Polarization observations of the G178.2−4.2 field are available at λ6 cm, λ11 cm, and λ21 cm. The diffuse extended polarized emission at λ21 cm (see the right bottom panel in Fig. 1) varies and has no obvious structural relation to G178.2−4.2. We therefore conclude that at λ21 cm, we most probably see polarized diffuse foreground or background Galactic emission and will not consider it in the following discussion.

At λ11 cm, the new sensitive Effelsberg observations of G178.2−4.2 not only show significant polarized emission in the northern shell but also two large patches of weak polarized regions (see the middle right panel in Fig. 1). The shell structure is better resolved at λ11 cm than at λ6 cm, and the detected polarized emission appears strong along the shell ridge. The small polarization patch near ℓ = 177°50′, b =  −4°10′ has a very different morphology when compared with the total-intensity map, and hence is most likely not the emission from the SNR. The large polarization patch between 178°40′ > ℓ > 178°10′ and  −3°50′ > b >  −4°20′ has a polarization percentage exceeding 100%. The total-intensity radio emission from the SNR is very weak and flocculent in this area, while the polarized emission is significant and continuous, which indicates that this polarized emission patch is likely also not physically related to the SNR, otherwise it should also appear at λ6 cm. However, neither of the λ11 cm polarization patches has a counterpart at λ6 cm, which strongly suggests that these polarized λ11 cm features originate from un-related Galactic emission within the interstellar medium. Polarization along the shell is clearly detected at both frequencies.

At λ6 cm, strong polarized emission is detected from the northern shell of G178.2−4.2, while weak polarized emission is spread over a large area both within and outside the SNR. These weak polarization patches are diffuse and extended and have a brightness temperature of a few mK TB. The lower and the central patches inside the SNR are clearly not related to the total-intensity emission and thus are probably un-related to the SNR. Therefore, the polarized emission reliably detected from G178.2−4.2 at both λ6 cm and λ11 cm is of the shell region. The polarization angles of the two frequencies differ by about 20°, corresponding to a rotation measure RM of about 36 + n × 350 rad m-2 (n =  ± 1,  ± 2 ...) by accounting for the nπ-ambiguity. Because of low RM values in the anti-center region (e.g. Spoelstra 1984; Sun et al. 2008), n = 0 is the most reasonable choice, so that RM = 36 rad m-2. This value is slightly higher but comparable to RM = 22.1 ± 2.2 rad m-2 for the source NVSS J052423+281232 at \hbox{$\ell = 178\fdg09, b = -4\fdg31$} within the SNR and RM = 11.8 ± 17.1 rad m-2 for NVSS J052734+285134 at \hbox{$\ell = 177\fdg94, b= -3\fdg37$} outside the SNR (Taylor et al. 2009) and the mean RM value of about 10 rad m-2 found for the nearby SNR S147 region (Xiao et al. 2008).

Polarization observations of the G25.1−2.3 field are available at λ6 cm, and λ11 cm. Some weak polarization patches have been detected at λ6 cm and λ11 cm (see Fig. 2, right panels). They have a very different morphology to the total-intensity emission, and seem not to be associated with the southern shell of G25.1−2.3. In general, intrinsic polarized emission of an SNR should be detectable at a higher frequency. This is not the case for G25.1−2.3. Comparing the morphologies of the λ6 cm and λ11 cm polarization map of G25.1−2.3, we conclude that the detected polarized radio emission at both wavelengths is not associated with the SNR. This is not unusual in this direction of the inner Galaxy, where the polarized emission originates from diffuse emission in a complex environment within the λ6 cm polarization horizon of about 3 kpc (see Sun et al. 2011).

thumbnail Fig. 6

Total-intensity contour maps of G178.2−4.2 at λ11 cm and G25.1−2.3 at λ6 cm with point-like sources subtracted overlaid onto the VTSS Hα image for G178.2−4.2 (upper panel) and the SHASSA Hα image for G25.1−2.3 (lower panel).

4. Signatures in other bands

We have overlaid the radio maps of these two SNRs onto images from other bands to see whether there is any spatial coincidence.

We superimposed the λ6 cm total-intensity contours of G178.2−4.2 and G25.1−2.3 onto the integrated CO image (Dame et al. 2001), the soft X-ray image in the 0.4–2.4 keV band and the super-soft X-ray image in the 0.1–0.4 keV band1. We could find no structural coincidence in these images with G178.2−4.2 and G25.1−2.3.

Using “The Virginia Tech Spectral-Line Survey (VTSS)2” and “The Southern H-Alpha Sky Survey Atlas (SHASSA)3” (Gaustad et al. 2001) for arcmin-resolution digital images of interstellar Hα emission, we have overlaid the total-intensity contour map of G178.2−4.2 at λ6 cm onto the VTSS Hα image and that of G25.1−2.3 at λ11 cm onto the SHASSA Hα image (Fig. 6). In the area of G178.2−4.2, a very weak broad band of Hα emission runs across the SNR area and is considered to be background or foreground emission. In the field of G25.1−2.3, a strong Hα emission patch is found to the north of the radio shell. The Hα emission appears to be weak within the shell areas of both G178.2−4.2 and G25.1−2.3. Therefore, we consider that the Hα emissions present in these directions are probably not physically associated with the SNRs.

thumbnail Fig. 7

Total-intensity contour map of G25.1−2.3 at λ6 cm overlaid onto the IRIS 60 μm infrared image.

thumbnail Fig. 8

H i survey images with red dash-dot contour lines in the three velocity ranges 32.0–36.1, 37.1–41.2, and 42.2–46.4 km s-1 in the area of G25.1−2.3. The H i cavity seems to have a morphological coincidence with the radio map of G25.1−2.3 (thick black contours).

We compared the radio maps of G178.2−4.2 and G25.1−2.3 with the IRIS4 60 μm infrared images (Miville-Deschênes & Lagache 2005). No coincidence was found for G178.2−4.2. Figure 7 shows G25.1−2.3, where the infrared patch positions have clear offsets to the radio shell. Therefore, we conclude that both G178.2−4.2 and G25.1−2.3 have no corresponding infrared emission.

Very often SNRs show an associated H i cavity. The Leiden/Argentina/Bonn (LAB) H i survey data with a velocity resolution of 1.3 km s-1 (Hartmann & Burton 1997; Kalberla et al. 2005) was used to search for H i structures related to the SNRs in the velocity range from  − 450 to 400 km s-1, although the angular resolution of 36′ for the H i data is coarse. In the area of G178.2−4.2, there is a 3° × 2° H i shell centered at \hbox{$\ell = 178\fdg2, b =-4\fdg1$}, seen at a central radial velocity of  − 3.2 km s-1 over a velocity range of 17.5 km s-1 (Ehlerová & Palouš 2005). This is clearly larger than the SNR extent, and its relation to the SNR cannot be settled. In the area of G25.1−2.3, an H i cavity is found in the velocity range of 37.1 to 41.2 km s-1 (see Fig. 8), which has a very similar shape to G25.1−2.3 and hence might be related to the SNR. The kinetic distance for the central velocity of 39.2 km s-1 is 2.9 or 12.4 kpc according to the Galactic rotation model by Fich et al. (1989). These two values correspond to a span of 67 pc or 288 pc of the shell structure, respectively. Considering the physical size of an SNR shell, the nearer distance is preferred.

For some SNRs, the empirical relation between the surface brightness Σ and diameter D (e.g. Clark & Caswell 1976) is the only tool to estimate their distances, though with large uncertainties (e.g. Green 1984, 2004). We used the updated Σ − D relation of Case & Bhattacharya (1998) to estimate the distances of our two newly found objects. For G178.2−4.2, we found that the corresponding diameter of that surface brightness is 197 pc, so that its distance is 9.4 kpc, which places the object far outside the Galaxy, and seems not possible. For G25.1−2.3, we found that the diameter for the shell is 72 pc and the distance is 3.1 kpc, consistent with those derived from the HI data. Note, however, that the uncertainties of these estimates could be as large as 40%.

Several pulsars are known within the field of G25.1−2.3 (Fig. 2). They have distances of 4–6 kpc according to the pulsar dispersion measure and the electron distribution model of the Galaxy, NE2001 (Cordes & Lazio 2002). It is not possible to associate any of these pulsars unambiguously with the SNR.

5. Summary

We have found two shell-like objects, G178.2−4.2 and G25.1−2.3, from the radio map of the Sino-German λ6 cm polarization survey of the Galactic plane. In addition, using the Effelsberg λ11 cm and λ21 cm continuum and polarization maps, a shell-type morphology is confirmed for both. Their radio spectra are characteristic of non-thermal emission with spectral indices for the shells of α ~  −0.6 for G178.2−4.2 and α ~  −0.5 for G25.1−2.3. These values are typical for SNRs. An ordered magnetic field runs along the northern shell of G178.2−4.2. An H i cavity, likely at a distance of about 2.9 kpc, is probably associated with the SNR G25.1−2.3, which implies a diameter of up to 67 pc in size.

Acknowledgments

We would like to thank the anonymous referee for helpful comments. The Sino-German λ6 cm polarization survey was carried out with a receiver system constructed by Mr. Otmar Lochner at MPIfR mounted at the Nanshan 25-m telescope of the Urumqi Observatory of NAOC. The MPG and the NAOC/CAS supported the construction of the receiving system by special funds. We thank Mr. Maozheng Chen and Mr. Jun Ma for qualified maintenance of the receiving system for many years. The new λ11 cm map of G178.2−4.2 is based on observations with the 100-m telescope of the MPIfR at Effelsberg. The Chinese authors are supported by the National Natural Science foundation of China (10773016, 10821061, and 10833003), the National Key Basic Research Science Foundation of China (2007CB815403), and the Partner group of the MPIfR at NAOC in the frame of the exchange program between MPG and CAS for many bilateral visits. X.Y.G. thanks the joint doctoral training plan between CAS and MPG and the financial support from CAS and MPIfR. X.H.S. thanks the MPG and Prof. Michael Kramer for the financial support during his stay at the MPIfR. We used the Hα image for G178.2−4.2 from the Virginia Tech Spectral-Line Survey (VTSS), which is supported by the National Science Foundation. We thank Prof. Ernst Fürst for critically reading the manuscript.

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All Tables

Table 1

Flux densities and the spectral indices of two SNRs.

In the text

All Figures

thumbnail Fig. 1

Radio images of G178.2−4.2 at λ6 cm from the Sino-German λ6 cm polarization survey in the top panels, at λ11 cm that we newly observed in the middle panels, and at λ21 cm from the Effelsberg Medium Latitude Survey (EMLS) (Reich et al. 2004) in the bottom panels. The angular resolutions of the λ6 cm, λ11 cm, and λ21 cm maps are 9.5′, 4.4′, and 9.4′, respectively. The left panels show the total-intensity (I) maps in color and in contours, with observed B-vectors overlaid (i.e. the observed E-vectors plus 90°) for polarization intensities PI > 2.4   mK  TB at λ6 cm and 32.0 mK  TB at λ11 cm. Vectors are not shown at λ21 cm. The vector length is proportional to PI. The I contours in the λ6 cm maps are 2n−12×1.6 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times1.6~(4\sigma)~{\rm mK}\ T_{\rm B} $}, (n = 1, 2, 3 ...), in the λ11 cm maps are 2n−12×12.8 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times12.8~(4\sigma)~{\rm mK}\ T_{\rm B}$} (n = 1, 2, 3 ...), and in the λ21 cm maps are 2n−12×44.0 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times44.0~(4\sigma)~{\rm mK}\ T_{\rm B} $} (n = 1, 2, 3 ...). The central panels also display the total-intensity maps, where strong point-like sources have been subtracted, to show the extended emission from the SNR more clearly. The right panels are the polarization intensity images with the contours for the total intensity maps with point-like sources subtracted. The rectangle in the λ6 cm image in the top central panel outlines the area used for the TT-plot spectral analysis displayed in Fig. 5.
In the text
thumbnail Fig. 2

Same as Fig. 1, but for G25.1−2.3. Here the λ11 cm and λ21 cm maps were extracted from the Effelsberg λ11 cm survey (Reich et al. 1990a) and λ21 cm survey (Reich et al. 1990b), respectively. The contours in the λ6 cm total-intensity maps are 2n−12×12.8 (4σ) mKTB\hbox{$ 2^{\frac{n-1}{2}}\times12.8~(4\sigma)~{\rm mK}\ T_{\rm B} $} (n = 1, 2, 3 ...), in the λ11 cm total-intensity maps are 2n−12×72.0 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times72.0~(4\sigma)~{\rm mK}\ T_{\rm B} $} (n = 1, 2, 3 ...), and in the λ21 cm total-intensity maps are 2n−12×200.0 (4σ) mKTB\hbox{$2^{\frac{n-1}{2}}\times200.0~(4\sigma)~{\rm mK}\ T_{\rm B} $} (n = 1, 2, 3 ...). The polarization intensity threshold for the B-vectors is 12.0 mK  TB at λ6 cm and 75.0   mK  TB at λ11 cm. No polarization data at λ21 cm are presently available. The star-symbols in the λ6 cm maps (top left panel) indicate known pulsars in the field. The polygon in the λ6 cm image in top central panel outlines the area for the TT-plot spectral analysis shown in Fig. 5.
In the text
thumbnail Fig. 3

Integrated flux densities and radio spectra of G178.2−4.2 (upper panel) and the outlined area of G25.1−2.3 (lower panel).
In the text
thumbnail Fig. 4

Spectral index maps for G178.2−4.2 and G25.1−2.3.
In the text
thumbnail Fig. 5

TT-plots for the shell of G178.2−4.2 outlined in Fig. 1 and the shell of G25.1−2.3 outlined in Fig. 2, using the Urumqi λ6 cm map and the Effelsberg λ11 cm and λ21 cm maps.
In the text
thumbnail Fig. 6

Total-intensity contour maps of G178.2−4.2 at λ11 cm and G25.1−2.3 at λ6 cm with point-like sources subtracted overlaid onto the VTSS Hα image for G178.2−4.2 (upper panel) and the SHASSA Hα image for G25.1−2.3 (lower panel).
In the text
thumbnail Fig. 7

Total-intensity contour map of G25.1−2.3 at λ6 cm overlaid onto the IRIS 60 μm infrared image.
In the text
thumbnail Fig. 8

H i survey images with red dash-dot contour lines in the three velocity ranges 32.0–36.1, 37.1–41.2, and 42.2–46.4 km s-1 in the area of G25.1−2.3. The H i cavity seems to have a morphological coincidence with the radio map of G25.1−2.3 (thick black contours).
In the text

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