An Old Supernova Remnant within an HII Complex at l {\approx} 173{\circ}: FVW 172.8+1.5

Abstract

We present the results of HI 21-cm line observations to explore the nature of
the high-velocity (HV) HI gas at l ~ 173{\circ} . In low-resolution Hi surveys
this HV gas appears as faint, wing-like, HI emission that extends to velocities
beyond those allowed by Galactic rotation. We designate this feature FVW
(Forbidden Velocity Wing) 172.8+1.5. Our high-resolution (3.'4) Arecibo HI
observations show that FVW 172.8+1.5 is composed of knots, filaments, and
ring-like structures distributed over an area a few degrees in extent. These HV
HI emission features are confined within the limits of the HII complex
G173+1.5, which is composed of five Sharpless HII regions distributed along a
radio continuum loop of size 4{\circ}.4 {\times} 3{\circ}.4, or ~ 138 pc
{\times} 107 pc, at a distance of 1.8 kpc. G173+1.5 is one of the largest
star-forming regions in the outer Galaxy. We demonstrate that the HV HI gas is
well correlated with the radio continuum loop and that the two seem to trace an
expanding shell. The expansion velocity of the shell is large (55 km s-1)
suggesting that it represents a supernova-remnant (SNR). We derive physical
parameters for the shell and show these to be consistent with the object being
a SNR. We also detect hot X-ray emitting gas inside the HII complex by
analyzing the ROSAT all-sky X-ray background survey data. This also supports
the SNR interpretation. We conclude that the HV HI gas and the X-rays are most
likely the products of a supernova explosion(s) within the HII complex,
possibly in a cluster that triggered the formation of these HII regions.

Full text

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arXiv:1201.3692v1 [astro-ph.GA] 18 Jan 2012
An Old Supernova Remnant within an HII Complex at ℓ ≈ 173◦:
FVW 172.8+1.5
Ji-hyun Kang1,3,4, Bon-Chul Koo2, and Chris Salter1
1Arecibo Observatory, HC 3 Box 53995, Arecibo, PR 00612
2Department of Physics and Astronomy, Seoul National University, Gwanak 599
Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea
3Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea
4Korea Astronomy and Space Science Institute 776, Daedeokdae-ro, Yuseong-gu, Daejeon
305-348, Korea
jkang@kasi.re.kr, koo@astro.snu.ac.kr, csalter@naic.edu
ABSTRACT
We present the results of Hi 21-cm line observations to explore the nature of
the high-velocity (HV) Hi gas at ℓ ∼ 173◦. In low-resolution Hi surveys this
HV gas appears as faint, wing-like, Hi emission that extends to velocities beyond
those allowed by Galactic rotation. We designate this feature FVW (Forbidden
Velocity Wing) 172.8+1.5. Our high-resolution (3.′4) Arecibo Hi observations
show that FVW 172.8+1.5 is composed of knots, filaments, and ring-like struc-
tures distributed over an area a few degrees in extent. These HV Hi emission
features are confined within the limits of the HII complex G173+1.5, which is
composed of five Sharpless HII regions distributed along a radio continuum loop
of size 4◦.4 × 3◦.4, or ∼ 138 pc × 107 pc, at a distance of 1.8 kpc. G173+1.5
is one of the largest star-forming regions in the outer Galaxy. We demonstrate
that the HV Hi gas is well correlated with the radio continuum loop and that
the two seem to trace an expanding shell. The expansion velocity of the shell
is large (55 km s−1) suggesting that it represents a supernova-remnant (SNR).
We derive physical parameters for the shell and show these to be consistent with
the object being a SNR. We also detect hot X-ray emitting gas inside the HII
complex by analyzing the ROSAT all-sky X-ray background survey data. This
also supports the SNR interpretation. We conclude that the HV Hi gas and the
X-rays are most likely the products of a supernova explosion(s) within the HII
complex, possibly in a cluster that triggered the formation of these HII regions.
Subject headings: supernova remnants — ISM: individual (FVW172.8+1.5) —
radio lines: ISM — ISM: Hii regions – stars: formation

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1.Introduction
In large-scale (ℓ,v) diagrams of Hi 21-cm line emission in the Galactic plane, there are
many small, faint, high-velocity “wing-like” features extending to velocities well beyond the
maximum or minimum permitted by Galactic rotation (Koo & Kang 2004). These “For-
bidden Velocity Wings (FVWs)” are likely due to energetic phenomena in the Galaxy, as
they are confined to small areas (? 2◦), and project smoothly beyond the general Galac-
tic emission to high velocities. Koo & Kang (2004) suggested that some of these FVWs
may represent the expanding shells of “missing” supernova remnants (SNRs), which are
not included in existing SNR catalogs. The basic idea is that old SNRs are faint in the
radio continuum, making it difficult to identify them because of both the confusion due to
the Galactic background emission, and observational limitations (e.g., Brogan et al. 2006).
Koo & Kang (2004) considered the fact that an old SNR should possess a fast-expanding
Hi shell that will still be present as a coherent entity after the remnant becomes too faint
to be visible in the radio continuum. If its expansion velocity is greater than the minimum
or maximum velocities permitted by Galactic rotation, then an old SNR shell, or part of it
(maybe the caps), could be detected as high-velocity Hi gas, e.g., FVWs. Indeed, Koo et al.
(2006) have carried out high-resolution Hi line observations toward one FVW and detected
a rapidly expanding (80 km s−1) Hi shell, the parameters of which are consistent with those
of the remnant of a SN that exploded some 0.3 Myr ago.
Recently, Kang & Koo (2007) identified 87 FVWs from the Leiden/Dwingeloo Hi sur-
vey (LDS; Hartmann & Burton 1997) and the Southern Galactic Plane Survey data (SGPS;
McClure-Griffiths et al. 2005). Among these, 6 FVWs are found to be coincident with SNRs,
4 with nearby galaxies, and 3 with high-velocity clouds. The rest (85%) are not associated
with any obvious objects that could be responsible for their high velocities. We have since
been making follow-up high-resolution Hi observations of these FVWs of unknown nature
using the Arecibo 305-m and Green Bank 100-m radio telescopes in order to identify their
natures and origins, and have now observed ∼ 30% of them. We find that about 40% of
the observed FVWs have shell-like morphologies. More than half of these are apparently
expanding at
The other 60% show irregular structures consisting of filaments and clumps. Most of these
show faint bumps in their line profiles indicating high-velocity Hi clouds. They might cor-
respond to the fast-moving, compact clouds in the disk, or in the disk-halo interface, that
have been recently detected in sensitive, high-resolution Hi surveys (Stanimirovi´ c et al. 2006;
Begum et al. 2010, and references therein). The full results from our survey will be presented
in a separate paper.
>∼50 km s−1, which supports the possibility of some FVWs being old SNRs.
Here, we report Arecibo Hi 21-cm line observations of FVW172.8+1.5. This particular

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FVW is unique in that it is associated with an HII complex in the outer Galaxy. We show
that FVW 172.8+1.5 is most likely an old SNR produced in this complex. The catalog of
Kang & Koo (2007) lists this feature as two objects, FVW173.0+0.0 and FVW173.0+3.0.
However, the Arecibo Hi image shows that these are likely to be parts of a single coherent
object, which we will call FVW172.8+1.5. In Section 2, we describe the Hi observations.
The Hi results are presented in Section 3. In Section 4, we present a multi-wavelength view
of the HII complex in this region and investigate its association with FVW 172.8+1.5. We
discuss the origin of FVW172.8+1.5 and the star-formation history in this area in Section
5, presenting a summary in Section 6.
2. The Hi 21-cm Line Observations
Hi 21-cm line observations of FVW172.8+1.5 were made in 2006 October with the
Arecibo 305-m telescope using the 7-beam, dual-polarization, Arecibo L-band Feed Array
(ALFA). The beam-to-beam spacing for the scanning pattern used was 1′.8. GALSPECT, a
dedicated spectrometer for Galactic Hi ALFA surveys, was used as the backend. Both wide-
and narrow-band spectra were acquired (see Stanimirovi´ c et al. 2006, for details). Here we
present data from the narrower 7.14-MHz band, which is analyzed into 8192 equally spaced
channels, giving an unsmoothed velocity resolution of 0.18 km s−1.
The field observed covers an area of 7◦.5×4◦.5 centered at (α,δ) = (5h37m,35◦40′) (J2000).
The observations employed the basket-weaving technique (Peek & Heiles 2008; Peek et al.
2007), which scans the sky with a zig-zag pattern by driving the telescope up and down on the
prime meridian at a speed of 1.5 arcmin/sec. For bandpass correction, a “Least-Squares Fre-
quency Switching” (LSFS; Heiles 2007) calibration was performed each day. Data reduction
used the IDL pipe-line codes developed by the Berkeley group (Peek & Heiles 2008). Using
these, the data are converted into a brightness temperature cube. The contamination due
to the coma sidelobes of the ALFA off-centre beams has been corrected (see Peek & Heiles
2008, for the details of data reduction). The final cube has a velocity resolution of 0.74 km
s−1, a spatial HPBW of 3.4 arcmin, and an rms of 0.13 K.
3. The Hi Results
Fig. 1 shows channel images of the Arecibo Hi data. The Hi emission associated with
FVW172.8+1.5 is visible for +19.5 km s−1? vLSR? +50.4 km s−1. At the highest velocities
(+40.1 km s−1? vLSR? +50.4 km s−1), the Hi appears to be largely separated into two con-

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centrations centered at ∼ (5h37m,+36◦30′) and ∼ (5h27m,+34◦30′), These were previously
designated FVW173.0+3.0 and FVW173.0+0.0 respectively. The northern concentration
(FVW173.0+3.0) looks diffuse and clumpy, and appears to form a ring-like structure with
a diameter of ∼ 2◦. In contrast, the southern concentration (FVW173.0+0.0) appears to
be confined to a few small (? 30′) regions. At lower velocities (+16.6 km s−1? vLSR ?
+35.0 km s−1), a thin, filamentary feature that surrounds the two concentrations appears,
so that the entire structure appears to be a large shell with an extent of 4◦.4 × 3.4◦. The
crosses in Fig. 1 and the ellipse superposed on the +24.7 km s−1
approximate center and boundary of the shell structure. At +14.4 km s−1
locities, the Hi emission from the Galactic ISM dominates, so that no particular Hi feature
associated with the source is recognizable (see also § 4.1.1).
channel map mark the
and lower ve-
The various Hi features described above are also visible in Fig. 2, which is a three-color
image of the Hi gas integrated over different velocity intervals. In this figure, we super-
pose the Effelsberg 11-cm radio continuum contours (F¨ urst et al. 1990). It is seen that the
high-velocity Hi emission features lie essentially within the radio continuum filaments. The
northwestern high velocity Hi gas, represented by red and green colors, are confined within
thenorthern part of the continuum structure, while the Hi features at lower velocities
(colored blue) lie along the inner boundary of the outer radio continuum filaments. The
morphological relation between the Hi and the radio filaments strongly suggests their as-
sociation. In addition, there is little possibility of chance alignment of the two structures,
because confusion is low along the line of sight in the direction of ℓ ∼ 173◦. The increase of
the surface area and/or the size of the Hi -emitting region with decreasing velocity suggests
that we are looking at the receding portion of an expanding shell. The Hi morphology does
not exactly match that of a uniformly expanding spherical shell, but this could be due to a
non-uniform and inhomogeneous ambient interstellar medium. The radio continuum struc-
ture is part of the HII complex in this area, and we will investigate the association of the
two at other wavelengths in Section 4 and their physical properties and origin in Section 5.
The velocity structure of the high-velocity Hi features is shown in Fig. 3, which shows the
peak Hi brightness temperature in Galactic longitude as a function of Galactic latitude and
velocity. This position-velocity diagram shows the high-velocity gas over the entire field in a
single diagram and enhances the appearance of small-scale wing-like features. Fig. 3 shows
that FVW172.8+1.5 is composed of many small-scale, high-velocity wings spatially confined
to −1◦< b < +4◦, which suggests an inhomogeneus nature for the shell. These wings are
mostly straight but some have the shape of an extended ring, e.g., that at b = −0.2◦. This
indicates that the shell is composed of small clumps. Since only the high-velocity end portions
of the wings are visible, their central velocities are usually not accessible. However, a few line
profiles in the area near (5h34.5m,+36◦03′) show discrete spectral features at high velocities

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(Fig. 4). A Gaussian fit to these features results in a central velocity of +33 – +35 km s−1
and a velocity dispersion of 4.3 – 6.4 km s−1, corresponding to kinetic temperatures of 2,200
– 4,900 K if the broadening is entirely due to thermal motions.
4.The ℓ = 173◦region in Multi Wavebands
4.1. Radio Continuum
4.1.1.The HII Complex G173+1.5
The region of Auriga within which FVW172.8+1.5 lies is an HII complex composed of
Sharpless HII regions and OB associations in the Perseus arm (Fig. 5). The HII regions are
organized along a large (∼ 7◦×4◦) filamentary structure resembling a bow tie (Kerton et al.
2007). The Hii regions have been studied previously (e.g. Israel & Felli 1978), and they are
largely at two different distances (see Table 1). To the northeast of the structure, S231, S232,
S233, and S235 are known to be associated with a giant molecular cloud at a distance of
1.8 kpc (Evans & Blair 1981), with CO radial velocities of −18.1 to −23.0 km s−1(Blitz et al.
1982), in which active star formation is on-going (see § 4.3). The distances determined by
spectrophotometry of the exciting stars of individual HII regions range from 1.0 to 2.3 kpc,
but the distance to the associated molecular cloud (1.8 kpc) is often adopted as the distance
to the whole HII complex. To the south of the complex, two 1◦-sized HII regions S229 (IC
405) and S236 (IC 410) and the large (∼ 5◦) diffuse HII region S230 are present. S236 is
associated with a molecular cloud at −7.2 km s−1and might be at a considerably greater
distance because the estimated distance to its central star cluster (NGC 1893) ranges from
3.2 to 6 kpc (Sharma et al. 2007, and references therein). For the other two HII regions (S229
and S230), not much has been known, although Fich et al. (1990) measured Hα velocities
of +4.4 and 0.0 km s−1
toward S229 and S230, respectively. Toward S229, CO J = 1–0
emission at +6.7 km s−1has been detected, but it may not be related to the HII region
(Blitz et al. 1982). The small HII regions S234 (IC 417) and S237 in the middle of the “bow-
tie” have velocities of −13 and −4 km s−1, similar to the first and second groups respectively.
A recent photometric determination of the distance to the central cluster (Stock 8) of S234
also agrees with the distance to the northwestern group of HII regions, i.e., 2.05 ± 0.10 kpc
(Jose et al. 2008). We therefore consider that the Sharpless HII regions S231–S235 form the
active star formation complex G173+1.5 at a distance of 1.8 kpc at vLSR≈ −20 km s−1.
The other HII regions have considerably different velocities and distances, and they are not
considered to be associated with this complex.
The morphology of the region suggests an association between the large radio continuum