The Astrophysical Journal
, 690:1585–1589, 2009 January 10
doi:
10.1088/0004-637X/690/2/1585
c
2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
ANOMALOUS MICROWAVE EMISSION FROM THE H
ii
REGION RCW175
C. Dickinson
1
, R. D. Davies
2
,J.R.Allison
3
,J.R.Bond
4
, S. Casassus
5
, K. Cleary
6
,R.J.Davis
2
,M.E.Jones
3
,
B. S. Mason
7
, S. T. Myers
8
, T. J. Pearson
6
, A. C. S. Readhead
6
, J. L. Sievers
4
, A. C. Taylor
3
, M. Todorovi
́
c
2
,G.J.White
9
,
and P. N. Wilkinson
2
1
Infrared Processing and Analysis Center, California Institute of Technology, M
/
S 220-6, 1200 E. California Blvd., Pasadena, CA 91125, USA
2
Jodrell Bank Observatory, University of Manchester, Lower Withington, Macclesfield, Cheshire, SK11 9DL, UK
3
Oxford Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
4
Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, Canada
5
Departamento de Astronom
́
ıa, Universidad de Chile, Casilla 36-D, Santiago, Chile
6
Chajnantor Observatory, California Institute of Technology, M
/
S 105-24, Pasadena, CA 91125, USA
7
National Radio Astronomy Observatory, Green Bank, WV, USA
8
National Radio Astronomy Observatory, Socorro, NM, USA
9
Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK
Received 2008 July 24; accepted 2008 September 11; published 2008 December 22
ABSTRACT
We present evidence for anomalous microwave emission in the RCW175 H
ii
region. Motivated by 33 GHz
13
resolution data from the Very Small Array (VSA), we observed RCW175 at 31 GHz with the Cosmic
Background Imager (CBI) at a resolution of 4
. The region consists of two distinct components, G29.0-0.6
and G29.1-0.7, which are detected at high signal-to-noise ratio. The integrated flux density is 5
.
97
±
0
.
30 Jy
at 31 GHz, in good agreement with the VSA. The 31 GHz flux density is 3
.
28
±
0
.
38 Jy (8
.
6
σ
) above the
expected value from optically thin free–free emission based on lower frequency radio data and thermal dust
constrained by
IRAS
and
WMAP
data. Conventional emission mechanisms such as optically thick emission
from ultracompact H
ii
regions cannot easily account for this excess. We interpret the excess as evidence
for electric dipole emission from small spinning dust grains, which does provide an adequate fit to the data.
Key words:
ISM: individual (RCW175) – radiation mechanisms: general – radio continuum: ISM
1. INTRODUCTION
In recent years there has been mounting observational ev-
idence for a new diffuse component emitting at frequencies
≈
10–60 GHz. The anomalous microwave emission was first
detected at 14 and 32 GHz by Leitch et al. (
1997
). Since then,
a similar picture has emerged both at high latitudes (Banday
et al.
2003
; de Oliveira-Costa et al.
2004
; Fern
́
andez-
Cerezo et al.
2006
; Hildebrandt et al.
2007
; Bonaldi
et al.
2007
) and from individual Galactic sources (Casassus
et al.
2004
,
2006
,
2007
;Watsonetal.
2005
; Scaife et al.
2007
;
Dickinson et al.
2007
), although negative detections have also
been reported (Dickinson et al.
2006
; Scaife et al.
2008
). The
spectral index between 20 and 40 GHz is
α
≈−
1
.
1 (Davies
et al.
2006
) with some evidence of flattening at
∼
10–15 GHz
(Leitch et al.
1997
; de Oliveira-Costa et al.
2004
; Hildebrandt
et al.
2007
). The emission appears to be very closely corre-
lated with far-IR data suggesting a dust origin. Various emission
mechanisms have been suggested, including hot (
T
∼
10
6
K)
free–free (Leitch et al.
1997
), flat spectrum synchrotron (Bennett
et al.
2003
), spinning dust (Draine & Lazarian
1998a
,
1998b
),
and magnetic dust (Draine & Lazarian
1999
). The overall pic-
ture is still very unclear and new data covering the range
10–60 GHz are urgently needed.
RCW175 (Rodgers et al.
1960
) is a diffuse H
ii
region, which
consists of a “medium brightness” optical filament (G29.1-
0.7, S65)
∼
7
×
5
in extent, and a nearby compact source
(G29.0-0.6), which is heavily obscured by dust. Although the
filament is clearly seen in high-resolution data, the compact
counterpart is considerably brighter. The ionization is thought
to be provided by a single B-1 II type star, which forms part of a
five-star cluster (Forbes
1989
; Sharpless
1959
) at a distance of
3.6 kpc.
Observations made with the Very Small Array (VSA) at
33 GHz (Watson et al.
2003
; Dickinson et al.
2004
) as part
of a Galactic plane survey (M. Todorovi
́
c et al. 2009, in
preparation) indicate that RCW175 is anomalously bright by a
factor of
≈
2, when compared with lower frequency data. In this
paper, we present accurate Cosmic Background Imager (CBI)
31 GHz observations of RCW175 and make a comparison with
ancillary radio/far-infrared (FIR) data. We find that the emission
at 31 GHz is significantly above what is expected from a simple
model of free–free and vibrational dust emissions.
2. DATA
2.1. 31 GHz: Cosmic Background Imager
The CBI is a 26–36 GHz 13-element comounted interferome-
ter, operated at the Chajnantor Observatory, Chile. The original
CBI used 0.9 m dishes to provide high temperature sensitivity
measurements on angular scales
∼
30
–6
(Padin et al.
2002
;
Readhead et al.
2004
). Recently, it has been upgraded with
1.4 m dishes (CBI2) to give increased temperature sensitivity
on angular scales
≈
4
–15
(A. C. Taylor et al. 2009, in
preparation).
We observed RCW175 at R.A.
=
18
h
46
m
40
s
, decl.
=
−
03
d
46
m
00
s
(J2000), on four nights in May 2007, with a total
integration time of 6 hr with CBI2. Boresight rotations of the
array were used to improve the
u, v
-coverage. Ground spillover
was removed by subtracting observations of a comparison field,
8 minutes later in right ascension, observed at the same hour an-
gles. Jupiter was the primary amplitude
/
phase calibrator with
absolute calibration tied to a Jupiter temperature of 146
.
6
±
0
.
75 K at 33 GHz (Hill et al.
2008
). Observations of sec-
ondary calibrators showed that the pointing was good to better
than 1
.
1585
1586
DICKINSON ET AL.
Vol. 690
0.0
0.5
1.0
DECLINATION (J2000)
RIGHT ASCENSION (J2000)
18 48 30
00
47 30
00
46 30
00
45 30
00
44 30
-03 20
30
40
50
-04 00
10
RCW175
Figure 1.
CBI 31 GHz CLEANed map of the RCW175 region. Contours are at
10, 20, 30, 40, 50, 60, 70, 80, 90% of the peak brightness, 1.04 Jy beam
−
1
.The
CBI primary beam (FWHM) is shown as a dashed line.
A uniform-weighted, CLEANed map is shown in
Figure
1
. The synthesized beam is 4
.
3
×
4
.
0 and the primary
beam is approximately Gaussian with FWHM 28
.
2 (30 GHz/
ν
).
Corrections for the primary beam were made directly to the
CLEAN components at each frequency; the bulk of the emis-
sion fits well within the extent of the primary beam. The total
flux density in the map is 6.4 Jy with a peak brightness of
1.04 Jy beam
−
1
and the noise level is 27 mJy beam
−
1
.
The CBI map detects and resolves both components in the
RCW175 region. G29.0-0.6 in the west is more compact and
is significantly brighter than the more diffuse G29.1-0.7 in the
east. Although there is some extended emission almost all (93%)
of the flux can be fitted by two Gaussian components. Note that
the entire extent of RCW175 is equivalent to
≈
13 beam areas;
thus the maximum CLEAN bias is
≈
0
.
35Jy.Wefittedtwo
elliptical Gaussians plus a baseline offset using the aips task
jmfit. At 31 GHz we find that G29.0-0.6 has a deconvolved size
5
.
4
×
5
.
1 and integrated flux density,
S
i
=
2
.
20Jy. G29.1-0.7 is
10
.
0
×
8
.
2 and
S
i
=
3
.
76 Jy. Simulations showed that CBI flux
loss due to the limited
u, v
-coverage is
10% when fitting two
Gaussians to the bulk of the emission.
2.2. Ancillary Radio
/
FIR Data
Table
1
lists frequencies, angular resolutions, and references
for the data
10
used in this paper, and Figure
2
shows selected
maps centered on RCW175. The radio maps from 1.4 GHz to
14.35 GHz show enhanced emission at the same location as that
seen in the CBI image with the bright, more compact, hotspot
in the west (G29.0-0.6) and the fainter component to the east
(G29.1-0.7). For data with resolutions
∼
4
, the subcomponents
are not well separated and the bulk of the emission is confined
to a region
∼
10
×
5
. At these resolutions the region looks like
one extended object with G29.0-0.6 dominating at one end. The
10
Data were downloaded from the
Skyview
Web site
(
http://skyview.gsfc.nasa.gov
), the MPIfR Image Survey Sampler Web site
(
http://www.mpifr-bonn.mpg.de/survey.html
), the LAMBDA Web site
(
http://lambda.gsfc.nasa.gov/
), and the IRSA Web site
(
http://irsa.ipac.caltech.edu
).
higher resolution NVSS 1.4 GHz/
Spitzer
24
μ
m data show the
more compact G29.0-0.6 has an extent of
≈
2
. A filament of
emission to the east (G29.1-0.7) is coincident with the filament
seen in high-resolution optical
/
IR images. There is low-level
emission in the region in between, which is particularly evident
in the 24
μ
m image.
WMAP
5 yr total-intensity maps (Hinshaw et al.
2008
)
covering 23–94 GHz show no significant detection of emission
above the Galactic background at the location of RCW175.
The 94 GHz map, at
≈
12
.
6 resolution, is, however, useful for
placing an upper limit on the thermal dust contribution (see
Section
3.1
).
3. FLUX DENSITY SPECTRUM
3.1. Integrated Flux Density Spectrum
The integrated flux density in each map was calculated both
by fitting multiple Gaussians (with baseline offset) and by
integrating over a given aperture. For the interferometric data,
both methods gave roughly consistent results within the errors.
For total-power data, the aperture value could be overestimated
due to the background level. Moreover, a possible offset of
∼
0
.
5 in the CBI map could result in a bias depending on the
exact aperture location; an offset of
≈
0
.
5 does appear to be
visible when comparing CBI contours with the NVSS 1.4 GHz
map (Figure
2
). For these reasons, we chose the Gaussian fitting
method as described in Section
2.1
.
Integrated flux densities for RCW175 and the G29.0-0.6
component are given in Table
1
and plotted in Figure
3
. Absolute
calibration errors (assumed to be 10% if not known) were added
in quadrature to the fitting error reported by jmfit. We include
the 33 GHz value for RCW175 from the VSA, which is in good
agreement with the CBI value. The lower resolution (
≈
13
)
VSA data meant that a single Gaussian fit was adequate and that
flux loss was negligible. Since there is a correction factor of 1.53
for the primary beam at the position of RCW175, we assigned
a conservative 10% error to account for any small deviations
from the assumed Gaussian primary beam. An upper limit was
inferred from the
WMAP W
-band (94 GHz) map by fitting for
a parabolic baseline in the vicinity of RCW175 to account for
the Galactic background, and calculating the 3
σ
of the residual
map.
3.2. SED Fitting
For a classical H
ii
region, the radio
/
microwave spectrum
consists of free–free emission at radio wavelengths and vibra-
tional dust emission at infrared wavelengths. At the lowest ra-
dio frequencies (typically
1 GHz), the free–free emission is
optically thick and follows a
α
≈
+2 spectrum.
11
Above the
turnover frequency (typically
∼
1 GHz), the emission becomes
optically thin and follows a
α
≈−
0
.
1 power law, with very
little dependence on physical conditions (Rybicki & Lightman
1979
; Dickinson et al.
2003
). At high frequencies (typically
>
100 GHz), vibrational dust emission becomes dominant.
Vibrational dust is well represented by a modified blackbody
function,
ν
β
+2
B
(
ν, T
dust
), where
β
is the dust emissivity index
and
B
(
ν, T
dust
) is the blackbody function for a given dust tem-
perature,
T
dust
.
The spectral energy distribution (SED) for RCW175 and
the G29.0-0.6 component is shown in Figure
3
.Thelow-
frequency (
<
15 GHz) data were fitted by an optically thin
11
Throughout this paper, we use the spectral index convention for flux density
given by
S
∝
ν
α
.
No. 2, 2009
ANOMALOUS MICROWAVE EMISSION FROM RCW175
1587
Table 1
Flux Densities for RCW175 and G29.0-0.6
Frequency
Telescope/
Angular Resolution
Reference
Flux Density
Flux Density
(GHz)
Survey
(arcmin)
For Data
RCW175 (Jy)
G29.0-0.6 (Jy)
1.4
Green Bank 300 ft
9
.
4
×
10
.
4
Altenhoff et al. (
1970
)6
.
0
±
1
.
8
Not resolved
2.7
Effelsberg 100 m
4.3
Reich et al. (
1990
)3
.
83
±
0
.
38
1
.
70
±
0
.
17
5
Parkes 64 m
4.1
Haynes et al. (
1978
)3
.
90
±
0
.
39
1
.
30
±
0
.
71
8.35
Green Bank 13.7 m
9.7
Langston et al. (
2000
)2
.
68
±
0
.
35
Not resolved
10
Nobeyama 45 m
3
Handa et al. (
1987
)
Not reported
1
.
39
±
0
.
14
14.35
Green Bank 13.7 m
6.6
Langston et al. (
2000
)3
.
21
±
1
.
83
Not resolved
31
CBI
4
.
3
×
4
.
0
This work
5
.
97
±
0
.
30
2
.
20
±
0
.
33
33
VSA
13
.
1
×
10
.
0
M. Todorovi
́
c et al. (2009, in preparation)
6
.
83
±
0
.
68
Not resolved
94
WMAP
≈
12
.
6
Hinshaw et al. (
2008
)
<
4
.
5(3
σ
)
<
4
.
5(3
σ
)
2997 (100
μ
m)
IRAS
≈
4
Beichman et al. (
1988
)
18320
±
10190
6970
±
2930
4995 (60
μ
m)
IRAS
≈
4
Beichman et al. (
1988
)
5864
±
1292
4131
±
548
12875 (24
μ
m)
Spitzer
MIPSGAL
0
.
033
http://irsa.ipac.caltech.edu/
Not fitted
Not fitted
DECLINATION (J2000)
RIGHT ASCENSION
(
J2000
)
18 48 30
00
47 30
00
46 30
00
45 30
00
44 30
-03 20
30
40
50
-04 00
10
DECLINATION (J2000)
RIGHT ASCENSION
(
J2000
)
18 48 30
00
47 30
00
46 30
00
45 30
00
44 30
-03 20
30
40
50
-04 00
10
DECLINATION (J2000)
RIGHT ASCENSION
(
J2000
)
18 48 30
00
47 30
00
46 30
00
45 30
00
44 30
-03 20
30
40
50
-04 00
10
DECLINATION (J2000)
RIGHT ASCENSION
(
J2000
)
18 48 30
00
47 30
00
46 30
00
45 30
00
44 30
-03 20
30
40
50
-04 00
10
24micron
1.4GHz
2.7GHz
94GHz
Figure 2.
Multifrequency maps of the RCW175 region. From left to right: NVSS 1.4 GHz, Effelsberg 2.7 GHz,
WMAP
94 GHz, and
Spitzer
MIPS 24
μ
m. Angular
resolutions and references are given in Table
1
. Contours are CBI 31 GHz at 20%, 40%, 60%, 80% of the peak brightness, 1.04 Jy beam
−
1
.
free–free power-law spectrum (
S
∝
ν
α
) fixed to the theoretical
value,
α
=−
0
.
12 for
T
e
≈
8000 K and frequencies around
∼
10 GHz. When fitting for the index we found
α
=−
0
.
32
±
0
.
15, which is consistent with this value. The infrared data
(60
/
100
μ
m) were fitted by a single temperature dust component
using a simple modified blackbody curve,
S
∝
ν
β
+2
B
(
ν, T
dust
).
We did not attempt to fit data at shorter wavelengths. Since there
are no data points in the submillimeter (
∼
100–1000 GHz), the
emissivity index was fixed at
β
=
+2
.
0, which is typical for H
ii
regions (e.g., Gordon
1988
). Flatter indices in the range
β
≈
1–
2 have been observed while the preferred value for
T
dust
≈
30 K
is
β
≈
+1
.
6 (Dupac et al.
2003
). However, the 94 GHz upper
limit does not allow flatter indices than
β
≈
2. The best-fitting
dust temperatures were
T
dust
=
30
.
1
±
5
.
7 K and
T
dust
=
37
.
6
±
6
.
4 K, for RCW175 and G29.0-0.6, respectively.
From Figure
3
it is clear that the
∼
30 GHz flux densities
are significantly higher than the simple model can allow for.
This model predicts a 31 GHz flux density for RCW175 of
2
.
69
±
0
.
24 Jy, compared to the measured value of 5
.
97
±
0
.
30 Jy. This corresponds to an excess over the free–free
emission of 3
.
28
±
0
.
38 Jy (8
.
6
σ
). Adopting the best-fit spectral
index at low frequencies results in a larger excess at 31 GHz,
although at a reduced significance level (6
.
4
σ
) due to the
increased error when fitting for an additional parameter. The
brighter component of G29.0-0.6 shows a similar level of excess
(Figure
3
).
4. DISCUSSION
A peaked (convex) spectrum is required to produce excess
emission at
∼
30 GHz without exceeding the upper limit at
94 GHz. The 3
σ
upper limit at 94 GHz strongly rules out
a cold thermal dust component, or a flatter dust emissivity.
Convex spectra can be produced by optically thick ultracompact
(UC) H
ii
regions, gigahertz-peaked spectrum (GPS) sources, or
some new mechanism such as electrodipole and magnetodipole
radiation.
UC H
ii
regions are self-absorbed up to higher frequencies
because of extremely high densities. Indeed it is possible to
fit the radio data including the VSA
/
CBI data by including a
homogenous compact H
ii
region with angular size
∼
1
and
emission measure of
∼
3
×
10
9
cm
−
6
pc. Such parameters are
within observed limits (Wood & Churchwell
1989
). However,
the additional component produces too much flux (
10 Jy) at
94 GHz. For the same reasons, magnetodipole radiation can also
be excluded as a major contributor.
GPS sources are high redshift radio sources in which the radio
jets have been highly confined and the synchrotron emission is
self-absorbed. A search through the NVSS catalog (Condon et
al.
1998
) revealed no bright (
100 mJy) compact radio sources
in this region and no bright (
20 mJy), point-like sources
appear in the NVSS 1.4 GHz map.
The anomalous emission could be due to electric dipole
radiation from small rapidly spinning dust grains. If such grains
have a residual dipole moment and spin at GHz frequencies, they
will emit over a narrow range of frequencies. Figure
3
shows
a typical spinning dust spectrum using the model of Draine &
Lazarian (
1998b
) for the Cold Neutral Medium (CNM), scaled
to fit the VSA
/
CBI data. The highly peaked spectrum allows
an adequate fit to the data both at
∼
10 GHz and yet consistent
with the 94 GHz upper limit. We note that the appropriate model
for Warm Ionized Medium (WIM) peaks at a slightly lower
frequency and predicts too much flux at
∼
10 GHz. Similarly,
the Molecular Cloud (MC) model peaks at too high a frequency
resulting in too much flux at 94 GHz. The peak intensity must
therefore lie close to 30 GHz. However, the models are fairly
1588
DICKINSON ET AL.
Vol. 690
(a)
(b)
Figure 3.
Integrated flux density spectrum for (a) both components combined
(RCW175) and (b) the brighter component (G29.0-0.6). The filled circles
represent data fitted by a power law with fixed spectral index (dotted line)
and a modified blackbody with fixed emissivity (dashed line). The Draine &
Lazarian (
1998b
) CNM spinning dust spectrum (dot-dashed line) has been fitted
to the 31
/
33 GHz data.
generic and allow considerable freedom in the dust parameters
(Finkbeiner
2004
). A similar situation is observed in the H
ii
region G159.6-18.5 (Watson et al.
2005
) and the dark cloud
LDN1622 (Casassus et al.
2006
).
The Draine & Lazarian (
1998b
) models are expressed in
terms of the intensity per hydrogen column density, in units
of Jy sr
−
1
cm
2
per H atom. We estimate an average col-
umn density of hydrogen for RCW175
N
(H)
≈
4
.
4
×
10
22
cm
−
22
using the canonical factor 2
.
13
×
10
24
Hcm
−
2
per unit
of
τ
100
μ
m
(Finkbeiner et al.
2004
). The best-fitting spinning
dust model in Figure
3
corresponds to
N
(H)
=
4
.
4
×
10
22
cm
−
22
. The observed levels are therefore consistent with the
emissivities of Draine & Lazarian (
1998b
). It is also remark-
able that the amplitude of the anomalous component is sim-
ilar to the emissivity relative to 100
μ
m observed at high
Galactic latitudes (Davies et al.
2006
), which is equivalent
to 1 Jy at 30 GHz per 6000 Jy at 100
μ
m. This does not
appear to be the case for all H
ii
regions, which have so
far been shown to have a reduced radio-to-dust emissivity
(Dickinson et al.
2007
; Scaife et al.
2007
). High-resolution,
multifrequency data in the range 10–100 GHz are needed to
confirm this result and to investigate the nature of anomalous
emission.
5. CONCLUSIONS
Using data from the VSA and CBI, we have observed excess
emission at
∼
30 GHz from the H
ii
region RCW175. The flux
density spectrum indicates that about half of the flux in this
region is from optically thin free–free emission leaving about
half unaccounted for. An upper limit at 94 GHz from
WMAP
data constrains the contribution of thermal dust emission. We
have discarded optically thick free–free emission from UC
H
ii
regions and GPS sources as possible candidates for this
excess; an upper limit at 94 GHz and high-resolution radio
data rule these out as the primary contributor at
∼
30 GHz.
We interpret the excess as electric dipole radiation from small
rapidly spinning dust grains as predicted by Draine & Lazarian
(
1998b
). These models provide a reasonable fit to the data that is
consistent both in terms of spectral shape and emissivity. High-
resolution, multifrequency data in the range 10–100 GHz are
needed to confirm this result and to investigate the nature of
anomalous emission.
We thank the anonymous referee for useful comments.
This work was supported by the Strategic Alliance for
the Implementation of New Technologies (SAINT; see
www.astro.caltech.edu/chajnantor/saint/index.html
), and we are
most grateful to the SAINT partners for their strong support. We
gratefully acknowledge support from the Kavli Operating Insti-
tute and thank B. Rawn and S. Rawn, Jr. The CBI was supported
by NSF grants 9802989, 0098734, and 0206416, and a Royal
Society Small Research Grant. We are particularly indebted to
the engineers who maintain and operate the CBI: C. Achermann,
R. Bustos, C. Jara, N. Oyarace, R. Reeves, M. Shepherd, and
C. Verdugo. C.D. thanks Roberta Paladini and Bill Reach for
useful discussions. C.D. acknowledges support from the U.S.
Planck
project, which is funded by the NASA Science Mis-
sion Directorate. S.C. acknowledges support from FONDECYT
grant 1030805, and from the Chilean Center for Astrophysics
FONDAP 15010003. A.T. acknowledges support from Royal
Society and STFC research fellowships.
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