Up: A radio continuum study
In previously published work, 192 radio sources towards the LMC and 93
towards the SMC have been classified as either Hii regions, SNRs or
Of the 192 sources towards the LMC, 188 were detected in recent Parkes
surveys (see Table 5). Of these 188 sources, 53 appear to be Hii
regions, 34 are confirmed SNRs, 24 are possible SNRs and 71 are listed as background
Six sources are listed as ambiguous in Paper VI.
Four confirmed LMC SNRs listed in Mathewson et al. (1983a),
Mills et al. (1984b), Tuohy et al. (1982) and
Chu et al. (1997) could not be detected (B0505-679,
B0509-675, B0543-689 and J0540-697) in any of our six surveys. We have
discussed these four SNRs in Paper VI. Gurwell & Hodge (1990),
mainly using optical identifications, have reported a number of galaxies
behind the LMC, but only a few of these have detectable radio emission.
The remaining 295 (483-188) sources listed in Table 5 provide new
candidates for Hii regions, SNRs and background galaxies or
quasars. For 229 of these there is a fitted spectral index.
In the SMC, 102 sources were known and classified from previous radio
investigations. Out of these, 92 were detected in the Parkes surveys
(Table 6). Many of the SMC sources were discussed in Ye
(1988). Of these 92 sources, 14 appear to be Hii regions, 10
are confirmed SNRs, 2 are possible SNRs and 66 are listed as background
Six confirmed SMC SNRs (SNR B0044-7324, B0045-7322, B0046-7335,
B0056-7227, B0056-7223 and B0104-7221) and three SNR candidates
(B0051-7259, B0056-7250 and B0057-7308) in the SMC could not be
detected in any Parkes surveys because of their small angular size, weak
radio emission and confusion. For the same reasons a number of background
galaxies detected by Ye (1988) were missed.
The remaining 131 (224-92) unclassified sources (of which 73 have a fitted
spectral index) towards the SMC are investigated to determine their nature.
We have used the following criteria to classify the discrete radio sources
listed in Tables 5 and 6:
Because the MCs are transparent to radio continuum, background radio
galaxies and quasars are seen throughout the MCs. It is very difficult to
distinguish radio galaxies and quasars just on a basis of radio spectra and
therefore, for this work, these sources will be treated together.
Lists of background sources towards the LMC have been published in Gurwell
& Hodge (1990), Dickey et al. (1994) and Ye
(1988). From their lists, 65 sources (marked in Table 5, Col. 17 as
BG) are detected in our radio surveys as background, and an additional six
as very probable background sources (marked in Table 5, Col. 17 as BG?).
All of these 71 sources obey the criteria from Sect. 4.1. and will be
treated as definite background sources.
- For Background source candidates -
background sources are either radio galaxies with generally steep radio spectra
() or quasars which have flat spectra (). Objects well removed from the stellar/nebular component of the MC
are expected to be clear of the MCs objects and, by default, to be populated by
background objects. Background objects are defined using following criteria.
- All radio sources outside the defined area of the LMC or
the SMC are background sources. The area of the LMC is defined between
Dec(B1950)=-7230: the area of the SMC as defined
between RA(B1950)=0030 to
- Radio sources with very steep spectra ( -0.8) are more likely to be background objects. However, there is a wide
range of radio spectral index for background sources.
- There is no H or IR detection (however some
background sources can be detected in one of these frequencies but this is
not likely) (Paper VIII).
- For SNR and SNR candidates -
supernova remnants (SNRs) in the MCs have been the subject of numerous studies,
both as individual SNRs and as a statistical sample. A series of criteria in
several spectral ranges have been defined to identify SNRs in the MCs. Following
Mathewson et al. (1983a and reference therein) and Rosado
(1993b) the candidate must satisfy at least two of the following
conditions. It should:
- be an extended soft X-ray source appearing either on
Einstein surveys (Long et al. 1981; Wang et al.
1991) or the ROSAT all-sky survey (Pietsch & Kahabka
1993; Pietsch et al. in preparation). A negative Hardness
Ratio 2(HR2) indicates that the source is a SNR (Paper VI);
- be a non-thermal radio source (=-0.4 on average but there is a
wide range and so spectral index is not itself conclusive) (Sect. 4);
- have optical emission with the following characteristics:
- [SII]/( H+[NII]) 0.4
Possibly the most sensitive diagnostic for identifying SNRs is narrow-band
optical CCD observations in the light of H and [SII]. The
ratio of the [SII] 6717, 6713 to H emission in
nebulae provides a good discriminator between Hii regions, which
typically show [SII] H of 0.1, and SNRs, which
usually have ratios >0.4. This is because sulphur is ionized almost
completely to the S++ state in Hii regions, while a wide range
of temperatures and ionization states in the cooling regions behind SNR
shocks gives rise to relatively strong [SII] emission;
- have H with looped or filamentary structure and
- an H flux density different from that expected for the
radio counterpart in the case of thermal source (in the case of finding an
Hii region coincident with radio sources) (Ye 1988).
- have weak or no IR emission. SNRs are not likely to appear in IRAS
surveys unless they are embedded in Hii regions (Paper VIII).
These criteria are biased toward radio properties and further kinematical
information is probably required. Furthermore, one must keep in mind that
several well-known SNRs in the MCs could not fulfil 2 of the 3 criteria given
above because of the following:
- some radio SNRs could be missed in X-ray surveys if their X-ray
emission is weak (or if it is attenuated by absorbing matter) (Paper VI);
- limitations in sensitivity of radio surveys. Some known MCs SNRs are
weak radio emitters and near the sensitivity limit of the Parkes telescope, seen
in only one radio frequency, and therefore have no radio spectral information.
One example is DEM L298;
- confusion with the surrounding Hii regions. Most of the LMC
SNRs are embedded in Hii regions.
- For Hii regions candidates -
Hii regions are expected to be within the different nebular regions of
the MCs. They are also expected:
- to be thermal radio sources (with =-0.15 on average) but
because of wide range of spectral indices this criterion is not itself
- Not to appear at X-ray frequencies - Hii regions are not
likely to be seen at X-ray frequencies (Paper VI);
- to have H, UV and/or IR positive detections (Paper VIII).
Outside the defined area of the LMC, 97 sources (marked in Table 5 Col. 17
as bg) have been found. These sources are very strong candidates for
background sources. Also, all of these sources obey the criteria
established in Sect. 4.1. Another group of 34 sources (marked in Table 5,
Col. 17 as bg?) satisfy some criteria of background sources and so are
treated as background-source candidates.
In total, at least 202 sources towards the LMC are classified as background
or background candidates, which is 42% of all sources found in the field
of the LMC. Most of the 75 unclassified sources are likely to be background
objects according to our source-density study (Sect. 4.2.3).
The most complete list of background sources towards the SMC can be found in
Ye (1988). From that list, 63 sources (marked in Table 6,
Col. 14 as BG) are detected in our radio surveys as background and an
additional three as very probable background sources (marked in Table 6,
Col. 14 as BG?). All of these 66 sources obey the criteria from Sect. 4.2.
and should be treated as definite background sources.
There are 108 sources outside the SMC area defined in Sect. 4.1. which have
not been previously classified (marked in Table 6, Col. 14 as bg). These
are very strong candidates for background sources as all of them satisfy
criteria established in Sect. 4.1.
In total, at least 174 sources towards the SMC are classified as background
or background candidates, which is 78% of all sources found in the field
of the SMC. This number could increase slightly as the 12 unclassified
sources are likely to be background according to the source-density study
The expected background integral source counts at our six survey frequencies
were obtained by interpolating the polynomial fits (Log N - Log
S study) of Wall (1994). These are the predicted number of
background sources in the observed fields of the LMC and the SMC at each
observing radio frequency (Tables 10 and 11).
Column 1 (Tables 10 and 11) lists the observing radio
frequency and Col. 2 gives the 5 cut-off level in mJy. Column 3
lists the total number of sources found at each observing frequency and the
area covered by each survey is given in Col. 4. The number of sources found
in our surveys to be stronger then 5 at each frequency is given in
Col. 5 and the total number of background sources stronger then 5as given in Col. 6. The number of sources estimated from Wall
(1994) for the given cut-off level (Col. 2) is given in Col. 7.
Log N - Log S estimates for the LMC
Log N - Log S estimates for the SMC
Comparing these results (Cols. 6 and 7), the number of background sources
classified in this work (Sects. 4.2.1. and 4.2.2.) are very close to the
expected range of the Log N - Log S study. The agreement in
numbers, at the 10% level, is as close as can be expected, given the
sensitivity of integral counts to the flux-density calibration and flux
cut-off. These findings strengthen our confidence in the selection criteria
and source classification.
Figures 5 and 6 show the distribution of all classified
background sources towards the LMC and the SMC. These source distributions
are uniform, as expected.
Distribution of background sources towards the LMC|
Radio sources intrinsic to the MCs are considered to be either SNRs or
HII regions. SNRs are formed by the high-velocity and
high-temperature material ejected when a star ends its life with a violent
explosion. The notation "Hii region'' is usually reserved for
relatively dense regions of the interstellar gas which are nearly
completely ionized by the Lyman continuum radiation from massive and hot
stars (Mezger 1972). Hii regions in the MCs provide
the opportunity to study the ionized gas component of the interstellar
medium and star formation on a galactic scale, with a sensitivity and
spatial resolution which are comparable to most of the Galactic surveys.
Out of 62 SNRs (excluding the remnant of SN 1987A) and SNR candidates, 34
(marked in Table 5, Col. 17 as SNR) were confirmed to obey criteria from
Sect. 4.1. and therefore will be treated as certain SNRs. Four well-known
SNRs (see Sect. 4) could not be detected in any of our Parkes radio
surveys. The other 24 sources (marked in Table 5, Col. 17 as SNR?), remain
SNR candidates because of lack of confirmation. Thus only 12% (58 out of
483) are SNRs or SNR candidates. However, it is likely that more SNRs may
exist which are confused with Hii regions.
From previous studies, 53 radio sources (marked in Table 5, Col. 17 as
HII) are confirmed as Hii regions. They all obey the
criteria established in Sect. 4.1. and therefore they are classified as
certain HII regions. A further 41 previously unclassified sources
(marked in Table 5, Col. 17 as hII) have characteristics typical
for Hii regions and hence they will be treated as very good
candidates for Hii regions. Another 20 sources (marked in Table 5,
Col. 17 as hII?) obey at most two different criteria typical for
Hii regions and therefore they are suggested Hii region
candidates. Until further analysis one cannot be sure about the nature of
Distribution of background sources towards the SMC|
Of the 483 sources towards the LMC, 114 (24%) are thus listed as
Hii regions. This number could increase as 34 sources (marked in
Table 5, Col. 17 as ?) have some characteristics of MCs sources but one
cannot be sure of their intrinsic nature.
From Ye's (1988) list of 19 SMC SNRs and SNR candidates, 10
(marked in Table 6, Col. 14 as SNR) were confirmed by the criteria from
Sect. 4.1. and therefore they are certain SNRs. Six well-known SNRs and
three SNR candidates (see Sect. 4) could not be detected in any of our
Parkes radio surveys due to the flux density limits. The remaining two SNR
candidates (marked in Table 6, Col. 14 as SNR?) are still under study and,
at present, they will remain as SNR candidates.
Only 6% (12 out of 224) of sources towards the SMC shows characteristics of
SNRs. However, as for the LMC, it is likely that this number of SNRs could
be higher because of their confusion with Hii regions.
From previous studies, 14 radio sources listed in Paper V (and also marked
in Table 6, Col. 14 as Hii) are confirmed to be Hii
regions. They obey all criteria for Hii regions (Sect. 4.1.) and
therefore they are classified as certain Hii regions. Another ten
previously unclassified sources (marked in Table 6, Col. 14 as hII)
have all characteristics typical for Hii regions and hence they
will be treated as very good candidates for Hii regions.
Of the 224 sources towards the SMC, only 24 (11%) are listed as Hii
regions or Hii region candidates. Two other sources (marked in
Table 6 Col. 14 as ?) also have some characteristics of Hii regions
but their status also remains unclear.
Up: A radio continuum study
Copyright The European Southern Observatory (ESO)