arXiv:1102.0590v1 [astro-ph.SR] 2 Feb 2011
The Arecibo Methanol Maser Galactic Plane Survey–IV:
Accurate Astrometry and Source Morphologies
J. D. Pandian1, E. Momjian2, Y. Xu3, K. M. Menten4and P. F. Goldsmith5
We present accurate absolute astrometry of 6.7 GHz methanol masers de-
tected in the Arecibo Methanol Maser Galactic Plane Survey using MERLIN
and the Expanded Very Large Array (EVLA). We estimate the absolute astrom-
etry to be accurate to better than 15 and 80 milliarcseconds for the MERLIN
and EVLA observations respectively. We also derive the morphologies of the
maser emission distributions for sources stronger than ∼ 1 Jy.
spatial extent along the major axis of the regions showing maser emission is
∼ 775 AU. We find a majority of methanol maser morphologies to be complex
with some sources previously determined to have regular morphologies in fact
being embedded within larger structures. This suggests that some maser spots
do not have a compact core, which leads them being resolved in high angular
resolution observations. This also casts doubt on interpretations of the origin of
methanol maser emission solely based on source morphologies. We also investi-
gate the association of methanol masers with mid-infrared emission and find very
close correspondence between methanol masers and 24 µm point sources. This
adds further credence to theoretical models that predict methanol masers to be
pumped by warm dust emission and firmly reinforces the finding that Class II
methanol masers are unambiguous tracers of embedded high-mass protostars.
Subject headings: masers — instrumentation: high angular resolution — astrom-
etry — stars: formation
1Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96822; jpan-
2National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801
3Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
4Max-Planck-Institut f¨ ur Radioastronomie, Auf dem H¨ ugel 69, 53121 Bonn, Germany
5Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109
– 2 –
The 6668.519 MHz JK = 50− 61A+transition is the strongest and most widespread
Class II methanol maser line (Menten 1991). There is strong evidence that 6.7 GHz methanol
masers trace very early evolutionary phases of high-mass star formation.
multi-wavelength studies of 6.7 GHz methanol masers have found their environment to be
characteristic of massive protoclusters (Minier et al. 2005). Moreover, recent work mod-
eling the spectral energy distributions of the masers’ host sources have found them to be
consistent with that of massive young stellar objects (MYSOs) undergoing rapid accretion
(Pandian et al. 2010).
However, the question where the maser emission originates relative to the MYSO is
still the subject of considerable debate. Early studies (e.g. Norris et al. 1993; Phillips et al.
1998) found a number of methanol masers to have linear or arched morphologies with mono-
tonic velocity gradients across the spatial features. These were interpreted to be indicative
of the masers originating in an edge-on circumstellar disk. However, the minimum mass
for the central object derived from the disk hypothesis was found to be unrealistically high
in some sources (Walsh et al. 1998). In contrast, assuming that the linear maser distribu-
tions trace the full extent of such a disk, the enclosed mass was found to be much less
than 1 M⊙ in a number of sources (Minier et al. 2000). This led to suggestions that the
masers only trace partial disks around massive stars, or are associated with outflows or
shock fronts. Dodson et al. (2004) suggested that methanol masers originated behind a pla-
nar shock propagating through a star forming core in order to explain the observation of
velocity gradients within individual spot clusters that were perpendicular to the main large
scale velocity gradient across the linear morphology.
High spatial resolution mid-infrared studies suggest that many methanol masers, includ-
ing some with linear morphologies, are associated with outflows rather than being located
in disks. While G35.20–0.74 is the best example of masers occurring along the walls of
an outflow cavity (De Buizer 2006), several other masers were found to have their linear
morphologies aligned parallel to outflows (De Buizer 2003). However, there are also sources
such as NGC 7538 IRS1 (Pestalozzi et al. 2009) where there is strong evidence that masers
originate in a disk. Furthermore, in the recent study of the proper motions of 12.2 GHz
methanol masers (which are the second strongest Class II methanol masers) in W3(OH),
Moscadelli et al. (2010) find that the linear distribution of the masers can be well fit with
a flat rotating disk that is seen almost edge-on. Since there is very good spatial correlation
between the locations of 6.7 and 12.2 GHz methanol masers, this can be taken as evidence
for a disk origin of some 6.7 GHz methanol masers.
Recent high resolution imaging of 6.7 GHz methanol masers with the European VLBI
– 3 –
network (EVN) revealed a number of masers to have ring shaped morphologies (Bartkiewicz et al.
2009). The ring morphologies were modeled as an inclined disk or torus around a MYSO
having expanding or infalling kinematics. In addition, a number of masers were found to be
associated with sources that had excess emission measured in the 4.5 µm IRAC band with the
Spitzer space telescope (see Cyganowski et al. 2009 for a detailed discussion of these sources
called Extended Green Objects or EGOs). Taken together, this was interpreted as evidence
for the origin of methanol maser emission in the interface region between a disk/torus and
One of the potential problems in very high angular resolution studies is that a significant
fraction of the maser flux is resolved out. For example, Minier et al. (2002) found that a
number of maser spots had a compact core and a more diffuse halo, while some spectral
features had no detectable compact core. Hence, lower angular resolution observations are
required to determine the full extent and morphology of maser emission. It is also of interest
to carry out this work towards a large and homogeneous sample from a blind survey.
The most sensitive blind survey to date for 6.7 GHz methanol masers is the Arecibo
Methanol maser Galactic Plane Survey (AMGPS; Pandian et al. 2007, hereafter Paper I).
AMGPS detected 86 6.7 GHz methanol masers, of which 48 were new detections. Taking
into account the pointing accuracy of the Arecibo telescope, the uncertainty in the position
of the AMGPS masers is ∼ 18′′at the 95% confidence level (Pandian & Goldsmith 2007,
Paper II hereafter). Paper II also presented an analysis of the properties of the masers
including association with mid-infrared sources, a number of methanol masers were found to
not have mid-infrared counterparts in the Galactic plane survey using the Midcourse Source
Experiment (MSX; Price et al. 2001). Pandian et al. (2009, Paper III hereafter) presented
the systemic velocities and distances to the sources (all the distances used in this paper are
taken from Paper III) leading to the determination of the luminosity function of 6.7 GHz
methanol masers. In this paper, we present accurate astrometry for the AMGPS sources
using MERLIN1and the Expanded Very Large Array2(EVLA). We also derive morphologies
of the maser emission for the strong sources (peak flux densities ? 1 Jy), and examine the
association of methanol masers with mid-infrared sources found in recent sensitive surveys
using the Spitzer space telescope.
1Based on observations made with MERLIN, a National Facility operated by the University of Manchester
at Jodrell Bank Observatory on behalf of STFC.
2The National Radio Astronomy Observatory is a facility of the National Science Foundation operated
under cooperative agreement by Associated Universities, Inc.
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2. Observations and data reduction
2.1. MERLIN observations
The MERLIN observations of the AMGPS masers (W51 main and G41.87–0.10 ex-
cluded) were carried out between 2007 February and June using six antennas except for
observations between March 5 and March 11 which employed only five antennas. Two corre-
lator configurations were used for the observations. In the wide-band mode, two polarizations
were observed with a total bandwidth of 16 MHz split into 32 channels with a channel width
of 0.5 MHz. In the narrow-band mode, two polarizations were observed with a bandwidth
of 1 MHz split into 256 channels yielding a channel spacing of 3.9 kHz. The primary and
bandpass calibrator, 3C84 was observed in both wide and narrow-band modes. The phase
calibrators (see Table 1) were only observed in wide-band mode to achieve adequate signal
to noise ratio, while the target sources were only observed in narrow-band mode. Phase
referencing was used with each scan on a target source being preceded and followed by a
scan on a phase calibrator. To achieve good uv-coverage, the target observing sequence was
cycled through a group of sources with similar systemic velocities. No Doppler tracking was
used, and each group of targets was observed at a fixed frequency. This observing sequence
was repeated several times over the course of each observing run which typically lasted for
∼ 10 hours. The final integration time was roughly 45 minutes per source, with the resulting
1σ channel noise being ∼ 45 mJy beam−1. At the adopted rest frequency of 6668.519 MHz
for the maser transition, the velocity resolution in the narrow-band mode is 0.18 km s−1.
The data were initially reduced using local MERLIN software (Diamond et al. 2003).
After initial editing and flux calibration on 3C84, the data were converted into FITS for-
mat with subsequent processing being carried out using the Astronomical Image Processing
System (AIPS) package of NRAO. The flux calibration was carried out assuming the flux
density of 3C84 to be 14.5 Jy in 2007 February, 15.0 Jy in 2007 March, 15.5 Jy in 2007
April, 16.0 Jy in 2007 May, and 16.5 Jy in 2007 June. The phases of 3C84 were calibrated
in both the wide-band and narrow-band data, with the derived phase offset between the two
data sets being used to transfer the calibration from the phase reference source (which is
only observed in the wide-band mode) to the target sources (which are only observed in the
narrow-band mode). After calibration, the task “CVEL” was used to shift the spectra to
the systemic velocity with respect to the local standard of rest (LSR) and correct for the
effects of Earth’s motion. The target spectra were then inspected followed by imaging the
brightest channel to obtain the target astrometry. The typical beamsize was 60 × 35 mas
with a position angle of 20◦.
As indicated in Sect. 2.3.1 of Bartkiewicz et al. (2009), the astrometric accuracy is
– 5 –
limited by four factors – the accuracy of the phase calibrator position, accuracy in the
positions of the individual antennas, accuracy in the transfer of phase solutions from the
phase calibrator and targets (on account of the atmosphere), and the precision with which
the emission location can be determined given the beamsize. The phase calibrator positions
indicated in Table 1 have position uncertainties between 0.4 and 4.4 milliarcseconds (mas).
The antenna positions have uncertainties of 1–2 cm, which results in a position uncertainty of
∼ 10 mas. To determine the uncertainties from the phase transfer from the phase calibrator
to the targets, we inspected the phase change over a timescale corresponding to the separation
between the calibrator and target (e.g. an angular separation of 3◦corresponds to a 12 min
timescale). We estimate positional uncertainties to be less than 10 mas, with median values of
6 mas. The last uncertainty from formal fitting errors is usually ≪ 1 mas, and can usually be
ignored. Combining the various uncertainties, we estimate the absolute astrometric accuracy
to be better than 15 mas.
Among the 81 AMGPS masers targeted in the MERLIN observations, 30 sources were
either not detected, or had bad data. After verification of the peak flux densities of these
sources with the 100 m Effelsberg telescope to check for variability, we observed 27 sources
in 24 fields using the EVLA. The observations were carried out on 2008 October 16 and 18 in
the A configuration. A single polarization (RR) was recorded using the old VLA correlator
with a bandwidth of 1.56 MHz and 512 spectral channels. Since the lower 0.5 MHz of the
bandpass was unusable due to aliasing, this setup was required to achieve the highest possible
spectral resolution along with the required velocity coverage. The resulting channel spacing
was 3 kHz giving a velocity resolution of 0.14 km s−1at the rest frequency of the maser
Each target source was observed in a single 10 minute or two 15 minute scans depending
on its single dish flux density, with the resulting 1σ channel noise ranging from 20 to 35 mJy
beam−1. The sky frequencies were computed for each individual scan and placed within
the upper two-thirds of the 1.56 MHz bandwidth to ensure that no maser component would
fall in the lower 0.5 MHz of the bandpass. The source J1331+305 (3C286) was used to
set the absolute flux density scale, and the sources J1751+096 and J1925+211 as bandpass
calibrators. The phase calibrators, used to calibrate the complex gains were J1851+005,
J1856+061 and J1922+155 (Table 2), with each calibrator being used for a subset of the
target sources closest in angular distance.
The editing, calibration, and processing of the data were carried out using AIPS. After
– 6 –
the transfer of the complex gain calibration solutions to the target sources, the strongest
emission channel in each source was imaged and its position measured followed by self-
calibration (when sufficient signal to noise was available) in both phase and amplitude in
an iterative cycle. In several cases, multi-field imaging was necessary to account for strong
6.7 GHz methanol maser emission from other sources that fell within the primary beam of
the 25 m EVLA antennas. In some weak sources, multiple channels were smoothed in order
to detect the source.
The phase calibrators used in the EVLA observations have a position accuracy better
than 3 mas (Table 2). The EVLA antenna positions are known to an accuracy of ∼ 1 cm
implying a position uncertainty of 55 mas. The uncertainty in the astrometry resulting
from the transfer of phase solutions from the phase calibrator to the target (obtained from
examining the raw phases of the phase calibrator) is estimated to be less than 30 mas. In
addition, some sources are only detected with moderate to poor signal to noise ratio, and
consequently the formal fitting errors in determining the position of peak emission are as
high as ∼ 45 mas for these sources. Thus, we estimate the absolute astrometry to be accurate
to between 65 and 80 mas.
2.3.24 µm counterparts
To examine the relation between 6.7 GHz methanol masers and mid-infrared emission,
we looked for 24 µm counterparts in the Spitzer MIPSGAL survey (Carey et al. 2009). Since
the 24 µm point source catalog has not been released, we used the mosaiced images corrected
for artifacts for this purpose. The strong interstellar dust emission in the Galactic plane leads
to complex backgrounds in the images, which can significantly affect the measurement of the
positions of point sources. Hence, we used a 11 × 11 pixel median filter to subtract the
background, as suggested in the MIPSGAL data reduction cookbook3. The use of such
aggressive filtering reduces the source flux to some extent. While this can be accounted for,
we are primarily interested in the positions of the point sources in this work. Hence, we do
not derive flux densities of the 24 µm counterparts.
An additional complication arises from many regions being extremely bright and satu-
rated in the MIPSGAL data. While some regions are completely blanked and other sources
have the entire core of the point spread function (PSF) blanked, there are some sources
for which only the central few pixels are blanked. Although the latter are rejected in the
point source detection algorithm of MOPEX, there is sufficient information to measure the
– 7 –
positions of these sources. Hence, we measured the positions of all 24 µm counterparts by
Gaussian fitting of the PSF cores using the routine “JMFIT” in AIPS. Although the full
width at half maximum (FWHM) of the MIPS PSF at 24 µm is 5.9′′, the high signal to
noise ratio of the counterparts implies that the formal uncertainties in the Gaussian fits are
small, and that systematic errors dominate the uncertainties in the positions derived for the
We were able to measure the absolute positions of 57 sources using MERLIN (Table 3;
positions of masers in W51 main were obtained using the data of Xu et al. 2009) and an
additional 25 sources using the EVLA (Table 4). We also identified two masers in the
MERLIN data that were not in the original AMGPS catalog (one of these sources was
detected by Cyganowski et al. 2009). Both sources are located close to other sources, leading
them to be either missed or unresolved in the original survey. Thus, this work has determined
the absolute positions to 82 out of 88 AMGPS sources.
Point source counterparts at 24 µm were found for 60 out of 82 sources. 18 sources
were either fully saturated or in the middle of a blanked region. Another two sources was
found to be in the middle of extended emission, while no counterparts were found for two
sources. The positions of the 24 µm point source counterparts are tabulated in Table 5, and
comments about selected sources are included in Sect. 3.2.
3.1.Relative astrometry and spot morphology
Although the absolute astrometric accuracy is limited by the factors described in Sect.
2.1 and 2.2, the relative astrometry between the emission distributions in different velocity
channels can be determined to much higher precision. To this end, we imaged each source
over the full velocity range of its emission, and measured the positions of the maser spots
relative to the channel with peak emission. To improve the dynamic range in the data cubes,
we self-calibrated data of targets that were sufficiently strong (peak flux density ? 1 Jy).
The measurement of spot positions in the data cube was automated using the AIPS task
“SAD”. To obtain reliable fits, we imposed a 6σ threshold on both the peak and integrated
flux density to qualify as a maser spot.
Figs. 1 and 2 show the results of this work for the MERLIN and EVLA data respectively.
For each source, the top panel shows the integrated spectrum, while the bottom panel shows
– 8 –
the morphology of the maser spot emission as a function of both angular and physical
distance. The coordinates listed in Tables 3 and 4 serve as reference coordinates for the spot
maps. The LSR velocities of the maser spots are indicated next to the spots themselves.
When a group of spots form a morphology with a monotonic velocity gradient, only the initial
and final velocities are indicated at the ends of the feature. In cases where there is a compact
cluster of spots, or where the spots form a morphology without a monotonic velocity gradient,
the velocity range of the cluster is indicated next to the feature. Some sources observed with
MERLIN are detected in three spots or fewer, from which no morphological information can
be deduced. For these cases, only the integrated spectra are shown in Fig. 3. The EVLA data
have much poorer resolution compared to the MERLIN data, and thus spot morphologies can
be deduced only for the strong sources, or for sources that show a significant spatial extent
of spot emission. Hence, for all EVLA sources where the maser spots are concentrated in
the center within the fitting uncertainties, only integrated spectra are shown in Fig. 4. We
give a discussion of the spot morphologies for selected individual sources in Sect. 3.2.
In several sources with complex spectra, there were multiple maser spots for a given
velocity channel. When these spots are close enough to be blended together, SAD does not
provide a reliable fit. To overcome this problem, we inspected the fit results for each data
cube. When multiple spots were blended in the fit results, we repeated the fits manually
using the task “JMFIT”. This procedure is adequate to reliably determine the maser spot
morphologies for most sources. However, a small number of sources are too complex to
resolve using this technique. These sources are noted in the discussion in Sect. 3.2. While
Fig. 1 shows the spot maps determined from our data, higher angular resolution observations
are required to fully resolve individual spots in these sources and verify our results.
3.2. Selected individual sources
G35.03+0.35 – A complex emission structure is seen in this source as expected from its
line profile. There is an elongated though not perfectly linear structure seen between LSR
velocities of 41.9 and 44.9 km s−1with a linear velocity gradient across the structure. In ad-
dition, there are five other groups of spots covering the full velocity range of maser emission.
The absolute position is consistent with that derived by Cyganowski et al. (2009) although
their poorer resolution fails to resolve individual spots in channels with multiple spots. The
24 µm MIPSGAL image is saturated in the central pixels, and the overall morphology gives
the appearance of two sources that are blended together. While Table 5 gives the results
of a single Gaussian fit, higher resolution data is required to determine the presence or ab-
sence of multiplicity. The source is associated with an extended green object (EGO) in the
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GLIMPSE data; a more detailed discussion can be found in Cyganowski et al. (2009).
G35.40+0.03 – This is one of three sources for which no reliable counterpart can be de-
termined at 24 µm. There is a point source 7′′away at 18h55m50s.87, 2◦12′26′′.4 which
is connected to extended emission towards the east suggestive of another point source at
roughly 18h55m51s.0, 2◦12′20′′(no Gaussian fit could be obtained). The latter is about
3.4′′from the maser position. It is not clear whether either of the two sources is associated
with the 6.7 GHz methanol maser.
G35.79–0.17 – This source was observed by Bartkiewicz et al. (2009) and observed to have
a linear morphology with a velocity gradient. While we detect this structure in our data,
there are three other groups of spots, one of which has a ring shaped morphology. Moreover,
the peak flux density detected in MERLIN is over 20 Jy, while Bartkiewicz et al. (2009)
detect a peak flux density of only 9.7 Jy. Hence, the EVN observations suffer from missing
flux, and the overall morphology of the maser emission is complex rather than linear. We
also note a significant position offset between the coordinates reported by Bartkiewicz et al.
(2009) and those in Table 3. However, the position of Bartkiewicz et al. (2009) is derived
from MERLIN single baseline data which has a much larger uncertainty. The position offset
in this source is consistent with the uncertainties quoted in Szymczak et al. (2007).
G36.70+0.09 – The spot morphology as seen by MERLIN is very similar to that derived
by Bartkiewicz et al. (2009) using the EVN. There are two main groups of maser spots in
this source. The southern group shows an inverted “S” structure, while the northern group
hosts a linear distribution with a velocity gradient between 61.9 and 62.4 km s−1. The EVN
observations, being more sensitive, also see more maser spots at high velocities (? 63 km s−1),
while we detect only a single spot at 63.0 km s−1.
G36.84–0.02 – There are two groups of maser spots in this source (Fig. 2) that are separated
by 1.2′′. However, both groups show spectral features within the overall velocity range
containing emission. This suggests that the two groups are discrete sources rather than
being kinematic features of a single source. However, given that the distance to the source
is only 3.7 kpc, this would imply a physical separation of only 0.02 pc (∼ 4500 AU). High
angular resolution mid-infrared and submillimeter observations are required to determine
whether the two groups of spots are indeed discrete, or whether they are excited by a single
G36.92+0.48 – The MIPS 24 µm image shows a cluster of at least 2-3 sources within 10′′in
this region. Taking into account the large distance to this source (15.8 kpc), the multiple
24 µm point sources probably reside in the same molecular cloud.
G37.02–0.03 and G37.04–0.04 – G37.04–0.04 is a new individual source detected in the
– 10 –
MERLIN data, and is located 49′′away from G37.02–0.03. The spectral features of the
source can be seen at a weak level in the published spectrum of G37.02+0.03 in Paper I,
and was most probably missed in the original survey due to its close proximity to G37.02–
0.03. The maser emission in G37.02+0.03 is confined to a small region, very similar to
that observed with EVN by Bartkiewicz et al. (2009). We note however that the peak flux
density as seen in MERLIN is much larger than that in EVN, and that maser spots outside
the central core appear to be resolved out in the EVN data. In contrast, the morphology of
G37.04–0.04 is more complex with more spectral features. There are two parallel emission
features seen, each with a linear morphology. While the northern feature shows a linear
velocity gradient, the southern feature does not show any coherent velocity structure and
appears to be a mere superposition of different spectral features.
G37.47–0.11 – This is a complex source with several spectral features. The emission features
between 60.8 and 63.1 km s−1form a linear structure with a velocity gradient. However,
the other spectral features are distributed randomly with a general tendency of features
at lower velocity to be located to the south. The overall morphology is similar to that
derived by Cyganowski et al. (2009) using the EVLA although we do not detect the eastern
spots seen in their map. Consequently, we do not see the double arc structure surmised by
Cyganowski et al. (2009). It should also be noted that some channels showed emission in
multiple spots which were unresolved by Cyganowski et al. (2009).
G37.53–0.11 – There are two groups of maser spots in this source separated by 1.4′′. At
a distance of 9.9 kpc, this corresponds to a physical separation of 0.07 pc. High angular
resolution observations at infrared or submillimeter wavelengths are required to determine
whether the two groups are excited by a single or by two separate massive young stellar
objects. The MIPSGAL image of this source is completely saturated and hence the position
of the 24 µm point source cannot be measured.
G37.55+0.19 – There are two well separated groups of spectral features in this source which
are also spatially separated in the spot map (Fig. 3). The features at lower velocity (78.4 to
80.0 km s−1) are situated about 0.2′′to the west of the other spots. The eastern group of
spots is clustered in accordance with individual spectral features, but show a broad velocity
trend from the north-west to the south-east with increasing velocity. As in G37.53–0.11, the
position of the MIPSGAL counterpart cannot be determined due to saturation.
G37.60+0.42 – The intensity in our MERLIN spectrum is more than a factor of two higher
than the EVN spectrum of Bartkiewicz et al. (2009). The peak emission in the MERLIN
data occurs at 87.0 km s−1, while the EVN peak occurs at 85.8 km s−1. Since the masing spot
at 87.0 km s−1is about 30 mas to the east and 10 mas to the north of the spot at 85.8 km s−1,
the reference coordinate quoted in Table 3 is different from that of Bartkiewicz et al. (2009).
– 11 –
While the spot morphology determined from our data shows similarities to that derived
from EVN, some differences exist. The MERLIN spots are much more tightly concentrated
in declination between 84.9 and 89.0 km s−1, forming two linear structures with monotonic
velocity gradients. The two easternmost maser spots of Bartkiewicz et al. (2009) at veloc-
ities lower than 85 km s−1are not detected in our data. However, we detect several spots
at velocities greater than 91 km s−1which are not detected in the EVN data. Another
qualitative difference is that the spot distribution between 52 and 56 km s−1resembles a
ring rather than the linear structure seen in Bartkiewicz et al. (2009).
G37.74–0.12 – The MIPSGAL image of this source shows significant extended emission with
the background subtracted image revealing a bow-shock morphology. It is not clear whether
the morphology is intrinsic or whether it is due to a superposition of multiple sources. The
6.7 GHz methanol maser is coincident with the brightest emission region. The position
reported in Table 5 is the result of a Gaussian fit to the bright emission region.
G37.76–0.19 – The morphology of this source as seen with the EVLA shows a compact group
of spots between 54.5 and 55.1 km s−1(which is the strongest spectral feature in the source),
while the other spectral features are distributed over a larger area to the east. The latter
being weak, have relatively large fitting uncertainties, and so we do not attempt to deduce
any morphologies from the data.
G37.77–0.22 – This methanol maser is embedded in a large area of extended 24 µm emission,
and no point source can be discerned at the maser position.
G38.03–0.30 – As with G37.60, the MERLIN spectrum is much stronger than the EVN
spectrum of Bartkiewicz et al. (2009), and the velocity of peak emission in the MERLIN
data (58.2 km s−1) is different from that in the EVN data (55.7 km s−1). This leads to a
small offset between the position quoted in Table 3 and that of Bartkiewicz et al. (2009).
However, the spot morphology determined from MERLIN is consistent with (and essentially
identical to) that determined using EVN, except for the detection of spots at high LSR
velocities (62.8 and 63.7 km s−1) in the MERLIN data.
G38.12–0.24 – There are three groups of spots in this source. The velocity components
between 68.1 and 71.4 km s−1are tightly concentrated, while the two other groups of features,
76.7 – 77.7 km s−1and 79.1 – 79.3 km s−1, lie to the southeast and northeast respectively.
The weaker spectral feature around 74 km s−1, detected in only one channel, lies close to
the central cluster of spots.
G38.20–0.08 – This is a complex source with several strong spectral features. We measure a
peak flux density of 9.7 Jy at 84.3 km s−1, while the EVN data of Bartkiewicz et al. (2009)
detect a peak of only 0.83 Jy at 79.6 km s−1. Paper I lists a single dish peak flux density of
– 12 –
11.1 Jy at 79.6 km s−1. The MERLIN spectrum is very similar to that obtained at Arecibo
except that the 84 km s−1feature is much stronger in the MERLIN observation. While this
can be attributed to variability, the significant discrepancy between EVN and MERLIN is
unlikely to be due to variability, but suggests missing flux. The spot distribution is extremely
complex with some channels showing as many as 3 spots some of which are blended together.
Hence, the spot map shown in Fig. 1 should be treated with caution. While most of the spots
of Bartkiewicz et al. (2009) can be identified in our spot map, we also detect several features
that are not seen in EVN. We also do not detect any maser spot ∼ 180 mas north-west of
the central concentration of spots that is seen in the EVN data. The absolute position in
Table 3 is consistent with that of Bartkiewicz et al. (2009) when the different velocity of
peak emission is taken into account.
The 24 µm MIPSGAL image shows a strong point source at 19h01m18s.86, 4◦39′26′′.7
with the methanol maser being coincident with a weak source which lies on the Airy diffrac-
tion ring of the strong source. The weak source, which cannot be fit by a Gaussian, has an
approximate position of 19h01m18s.7, 4◦39′37′′by visual inspection. Further work including
PSF subtraction of the strong source is required to determine the position of the counterpart
accurately. The position of the MIPS counterpart reported in Table 5 is obtained by visual
G38.26–0.08 – This source (observed with EVLA) has three groups of spectral features
which are spatially resolved. The features 6.3 – 7.6 km s−1and 11.7 – 12.9 km s−1populate
the eastern and south-eastern part of the spot map respectively, while the strong spectral
features between 14.2 and 15.5 km s−1are more compact and lie to the west.
G40.28–0.22 – This is another complex source with an unusually large number of spectral
features. The frequency used to observe this source led to the loss of features at velocities
greater than ∼ 79 km s−1. The overall shape of the spectrum obtained using MERLIN is
very similar to that in Paper I although variability is seen in some spectral features – the
feature around 65 km s−1is stronger while the peak flux density is lower in the MERLIN
data compared to Paper I. The spot map shows about 8 groups of spots, at least two of which
are extended linearly with a monotonic velocity gradient. However, the overall distribution
of spots is complex with no velocity structure. The 24 µm MIPSGAL image is saturated in
G40.62–0.14 – There are four spectral features in this source that are clearly resolved spa-
tially. However, the overall morphology should be classified as “multiple” if not complex.
The MIPSGAL 24 µm counterpart is fully saturated and hence its position could not be
– 13 –
G41.12–0.22 – There are two spectral features in this source with the stronger feature showing
a linear morphology with a velocity gradient.
G41.23–0.20 – There are four groups of spots in this source. While three groups show
linear morphologies, two out of three groups do not show any clear velocity gradients. The
MIPSGAL 24 µm image shows extended emission at the maser location. The background
subtracted image shows a few point sources along with some residual extended emission.
Hence there is some uncertainty in the position of the counterpart reported in Table 5.
G41.34–0.14 – The maser spots in this source fall into two broad groups. The spots at
velocities less than 10 km s−1lie to the southeast, while those at velocities greater than
10 km s−1lie to the northwest. No distinct morphology can be discerned in the northwest,
while the southeast group could possibly be fit with a ring morphology.
G42.03+0.19 – This is a complex source. The emission between 7.2 and 11.0 km s−1are
distributed in four clusters with an overall north-south trend with decreasing velocity. The
other spots are distributed in a complex morphology though it is conceivable to fit the
spots between 7.2 and 14.6 km s−1with a ring distribution. We also detect a maser spot at
17.0 km s−1although the MERLIN spectrum does not show a spectral feature at this velocity
(this spectral feature is clearly seen in the Arecibo spectrum in Paper I). The position of
the 24 µm counterpart has some uncertainty since a significant fraction of the PSF core is
saturated and blanked.
G42.43–0.26 – The position of the 24 µm counterpart of this maser cannot be measured on
account of it being completely saturated.
G42.70–0.15 – There are five spectral features in this source each of which are spatially
distinct. While most individual features display a linear or arched morphology, there is no
overall velocity structure in this source. It is possible that the different maser spots can be
fit with a ring morphology.
G43.04–0.46 – There are three regions of emission in this source, two in the southeast and
one in the northwest. The northwest group is offset from the other two groups by ∼ 2.5′′,
which corresponds to a physical separation of 0.1 pc. While it is possible that the northwest
group of spots is a distinct maser excited by a different YSO than that pumping the southeast
group, complementary data at high angular resolution is required to confirm this scenario.
The MIPSGAL counterpart is close to the southeast group of spots.
W49N region – Using the MERLIN data, we are able to obtain accurate astrometry for all
five methanol masers identified in Paper I. We do not identify any new emission regions to
a 1σ limit of 60 mJy beam−1. Four out of five sources have previously published positions
– 14 –
accurate to 0.4′′using the Australia Telescope Compact Array (ATCA; Caswell 2009). The
two weak sources, G43.17–0.00 and G43.18–0.01 show a simple compact morphology or are
detected in only one maser spot. The other three sources are discussed in more detail
below. The entire region is saturated in the MIPSGAL 24 µm image and hence no 24 µm
counterparts can be determined for any of these sources.
G43.15+0.02 – Although the spectrum in Fig. 1 is affected by negative sidelobes of G43.16+0.02,
the spot map was verified to not suffer from these effects. The spot map shows a simple
linear morphology with an overall velocity gradient.
G43.16+0.02 – The spectral features between 7.7 and 8.9 km s−1, and 9.1 and 10.2 km s−1
form two roughly linear features with monotonic velocity gradients (though a sub-feature
around 8.9 km s−1is an outlier) that are perpendicular to each other. The emission between
15 and 20 km s−1traces a linear and ring shaped morphology in addition to a few relatively
G43.17+0.01 – There are at least five morphological features in this source. The emission
between 18.8 and 19.3 km s−1, and 20.0 and 20.5 km s−1traces two linear structures in the
northwest, while the 20.2 – 20.7 km s−1and 21.2 – 22.1 km s−1features form two clusters
to the southeast. The emission between 20.2 and 21.1 km s−1traces another linear feature
which connects to the cluster between 21.2 and 22.1 km s−1. In addition, a number of spots
between 19.1 and 20.0 km s−1form a relatively random distribution.
G43.80–0.13 – This is a complex source with several maser spots being blended in our
MERLIN data. While the spot distribution in Fig. 1 shows two linear features between 39
and 41 km s−1, higher angular resolution data is required to confirm this. We also detect
four other clusters of spots in this source. The spectrum in Fig. 1 is Hanning smoothed to
suppress ringing arising from the Gibbs phenomenon. The peak flux density is 53.9 Jy in the
unsmoothed spectrum which reduces to 40.8 Jy after Hanning smoothing. Considering the
velocity resolution of the MERLIN data, this source is consistent with no variability seen
between the epochs of the Arecibo and MERLIN observations. The position of the maser’s
24 µm counterpart cannot be measured on account of saturation.
G44.31+0.04 – The 24 µm counterpart of this maser is saturated in the central pixels.
G45.07+0.13 – Even though the peak flux density is over 60 Jy in this source, there are
only two spectral features with the strong feature displaying a linear morphology with a
monotonic velocity gradient. The MIPSGAL 24 µm image is fully saturated at the location
of the maser, and so no point source counterpart could be identified.
G45.44+0.07 – This source is in a bright region with extended emission at 24 µm that is
– 15 – Download full-text
saturated close to the maser position. Although no point source is seen after background
subtraction, this could be due to the close proximity to blanked region.
G45.47+0.13 – There are four emission regions in this source. Of particular interest is the
double emission structure at 59.6 and 59.7 km s−1(i.e. each velocity channel has emission
in two spots) in the northeast. As with G45.44+0.07, this maser is located at the edge of a
saturated region at 24 µm, and hence no 24 µm can be identified for the source.
G45.47+0.05 – While there are multiple spectral features in this source, the maser spots
between 55.6 and 57.6 km s−1form a single morphological feature although the velocity
gradient is not monotonic across the spots between 56.5 and 57.6 km s−1. A second linear
feature between 57.7 and 58.3 km s−1is located about 10 mas to the east.
G45.49+0.13 – The maser spots in this source are concentrated in a compact region. The
spot morphology is linear with a velocity gradient, except for the spot at 58.0 km s−1.
G45.81–0.36 – There are five emission regions in this source. The spectral features between
velocities of 59.7 and 66.4 km s−1, which include the strongest peak, each display a simple
morphology. In contrast, the features between 67.4 and 70.3 km s−1show a more complex
distribution with no overall velocity structure.
G46.12+0.38 – This source displays three spectral features, two of which have linear mor-
phologies. The third feature is identified in only two spots, and hence no morphological
information can be deduced for this feature.
G48.90–0.27 – While the overall morphology in this source is linear, there is no velocity
structure across the feature.
G48.99–0.30 – This maser, as with a number of other sources in the W51 region is in a region
that is saturated at 24 µm and hence no point source counterpart can be identified at this
G49.27+0.31 – There are two emission regions in this source, one to the northeast and the
other to the southwest. The southwest region shows a single extended morphology between
velocities of –6.1 and –1.9 km s−1, and a monotonic velocity gradient is seen between –5.6
and –3.0 km s−1.
G49.35+0.41 – The emission in this maser is relatively compact with the spots between
velocities of 66.9 and 69.1 km s−1forming a reversed “S” shaped morphology.
G49.41+0.33 & G49.42+0.32 – The single dish spectrum shows two main groups of spec-
tral features, one between –27 and –23.5 km s−1, and a second group between -16 and
–9.5 km s−1, although Paper I also shows weak emission around –19.5 km s−1. As indicated