THE 1.1 mm CONTINUUM SURVEY OF THE SMALL MAGELLANIC CLOUD: PHYSICAL PROPERTIES AND EVOLUTION OF THE DUST-SELECTED CLOUDS^*

Continuum observations at 1.1 mm toward the SMC were conducted with the AzTEC instrument (Wilson et al. 2008a) installed on the ASTE telescope (Ezawa et al. 2004, 2008) in the Atacama desert, Chile. AzTEC is a 144-pixel bolometer camera with a bandwidth of 49 GHz and a center frequency of 270 GHz. The angular resolution is 28'' at FWHM. The observations covered a total of a 4.5 deg² field of the SMC by connecting four patches of scans that are denominated as southwest (SW), northeast (NE), Wing, and N88, as shown in Figure 1. Table 1 shows the summary of each observing field. Each scan was performed by orthogonal raster scans of a 11 × 11 field (position angle of 20°). Observations were performed from 2008 October 7 to December 26. The total observing time is 42 hr with an on-source time of ∼30 hr. The N88 field was observed under bad weather conditions (τ_{220 GHz} ≳ 0.1), whereas the remaining fields were observed with good atmospheric transparency (τ_{220 GHz} ≲ 0.1). Quasars J2355-534 and J2326-502 were observed every two hours to measure the pointing offsets during the observations. The resulting pointing accuracy after applying the pointing correction was better than 3'' at the 1σ confidence level (Wilson et al. 2008b). Uranus was observed once every night for the purpose of absolute flux calibration and point-spread function measurement of each bolometer pixel. Flux calibration has an uncertainty of 8% (Wilson et al. 2008b; Liu et al. 2010).

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Table 1.  Observed Fields of AzTEC/ASTE SMC Wide Observations

Field	SW	NE	Wing	N88
R.A. (J2000)	${00}^{{\rm{h}}}{50}^{{\rm{m}}}{00}^{{\rm{s}}}$	${01}^{{\rm{h}}}{01}^{{\rm{m}}}{00}^{{\rm{s}}}$	${01}^{{\rm{h}}}{13}^{{\rm{m}}}{00}^{{\rm{s}}}$	${01}^{{\rm{h}}}{24}^{{\rm{m}}}{00}^{{\rm{s}}}$
Decl. (J2000)	−73°10'00''	−72°20'00''	−73°00'00''	−73°15'00''
Position angle	20°
Field size	11 × 11
Scan method	Orthogonal raster scan
1σ noise (mJy beam−1)	5.1	6.0	6.4	11.6

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The correlated noise removal was performed using a principal component analysis (PCA) cleaning (Laurent et al. 2005). The time fluctuation of the bolometer signal is dominated by atmospheric emission, which is spatially extended on the sky and has a strong correlation among the bolometer pixels. On the other hand, point-source emission and detector noise do not have a correlation among the bolometer pixels. This means that the principal components that have large eigenvalues can be removed as atmospheric emission. Therefore, PCA cleaning is an effective way to remove the atmospheric emission and to retrieve faint point-source signals. The detailed treatment of PCA cleaning in the AzTEC IDL routine, which we use, is described in Scott et al. (2008) and Downes et al. (2012).

The PCA cleaning method is only sensitive to point sources; as extended astronomical objects have a correlation among bolometer pixels in the same manner as the atmosphere emission. FRUIT (Liu et al. 2010) is a method to recover extended components by performing the PCA cleaning iteratively. In the FRUIT procedure, the signals retrieved by the PCA cleaning are used as an astronomical source model. Time-stream data that have the source models subtracted are analyzed by the PCA cleaning, and astronomical signals that are retrieved are added to the source models. This process continues iteratively until no significant signals are found. The sky maps are constructed by adding the final source models to the source-free maps.

As a result of the FRUIT imaging, noise levels of 5–12 mJy beam⁻¹ are achieved (see also Table 1 for each field). The point response function (PRF), after applying FRUIT, gives a Gaussian FWHM of 40'', which corresponds to a physical scale of 12 pc at the distance of the SMC.

Figure 2 shows the 1.1 mm continuum flux map of the whole SMC reduced using the FRUIT analysis. Most of the representative star-forming regions in the SMC, such as N27, N66, N81, N83, N84, and N88, were detected with sufficient S/N ratios of >10 at the peak positions. The PCA map is also shown in the Appendix.

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Extended sources were identified by contouring significant emission (∼5σ) that is brighter than 30 mJy beam⁻¹ in the SW, NE, and Wing fields, and 60 mJy beam⁻¹ in the N88 field. In addition, sources smaller than the effective resolution of 40'' were eliminated from the catalog. Two slightly significant sources (5.8σ in the Wing field and 6.5σ in the N88 field) that have no object corresponding to the 24–160 μm range and the 3.5σ PCA objects were excluded from the catalog because they would be false detections.

The extended source catalog of each field is shown in Table 2. We have identified a total of 44 sources, including 24, 14, 5, and 1 sources in the SW, NE, Wing, and N88 fields, respectively. The errors of the peak fluxes were given by the map noise level at the source positions and were not considered as systematic uncertainty in the calibration. The errors of the total flux were propagated normally from the map noise level and the calibration error. The source radii were estimated under the assumption that the structure is spherically symmetrical, and the deconvolved radii were calculated by assuming that the PRF is the Gaussian profile with 40'' FWHM.

Table 2.  AzTEC/ASTE 1.1 mm Extended Source Catalog of the SMC

ID		α	δ	Peak flux	S/N	1.1 mm total flux	R	${R}_{\mathrm{deconv}.}$
		(J2000)	(J2000)	(mJy/beam)		(mJy)	(pc)	(pc)
SW	1	00°48'248	−73d05m58s	149.4 ± 5.2	29.0	1272.8 ± 162.9	34.6	34.1
	2	00°48'088	−73d14m49s	133.9 ± 4.9	27.2	1194.5 ± 152.9	33.3	32.8
	3	00°45'204	−73d22m51s	112.6 ± 5.0	22.6	300.9 ± 38.6	17.1	16.0
	4	00°46'400	−73d06m08s	97.0 ± 5.0	19.3	260.7 ± 33.4	15.9	14.8
	5	00°48'558	−73d09m51s	66.9 ± 5.1	13.2	207.8 ± 26.7	16.1	15.0
	6	00°45'041	−73d16m41s	61.9 ± 4.9	12.5	89.3 ± 11.8	10.4	8.6
	7	00°45'277	−73d18m41s	55.6 ± 5.2	10.7	117.3 ± 15.2	12.1	10.6
	8	00°49'291	−73d26m27s	54.1 ± 5.0	10.9	49.3 ± 7.3	7.8	5.2
	9	00°46'336	−73d15m55s	53.9 ± 5.2	10.5	164.0 ± 21.1	14.7	13.5
	10	00°52'386	−73d26m32s	53.2 ± 4.9	10.9	78.8 ± 10.5	10.1	8.2
	11	00°45'224	−73d12m33s	50.9 ± 4.9	10.4	37.4 ± 6.3	6.9	3.7
	12	00°52'469	−73d17m51s	48.8 ± 5.1	9.7	177.0 ± 22.7	15.7	14.5
	13	00°47'058	−73d22m14s	47.5 ± 5.0	9.5	56.0 ± 7.9	8.8	6.6
	14	00°46'253	−73d22m11s	47.2 ± 5.1	9.2	26.5 ± 6.1	5.9	1.0
	15	00°56'065	−72d47m16s	43.6 ± 5.4	8.0	27.7 ± 6.3	6.1	1.7
	16	00°48'154	−73d10m32s	42.9 ± 5.1	8.5	25.2 ± 5.9	5.9	1.0
	17	00°52'267	−72d41m01s	40.8 ± 5.4	7.6	32.4 ± 6.2	6.8	3.4
	18	00°46'482	−73d32m04s	39.8 ± 5.0	8.0	23.7 ± 5.8	5.8	0.3
	19	00°45'297	−73d08m11s	39.6 ± 5.1	7.8	52.6 ± 7.5	8.8	6.6
	20	00°48'040	−73d23m06s	39.1 ± 5.0	7.8	44.3 ± 6.8	7.9	5.4
	21	00°50'458	−72d47m55s	36.8 ± 5.1	7.3	37.6 ± 6.2	7.6	4.8
	22	00°45'417	−73d16m38s	36.7 ± 5.3	6.9	64.7 ± 8.9	9.8	7.9
	23	00°48'201	−73d19m21s	35.7 ± 5.0	7.1	25.8 ± 5.7	6.2	2.2
	24	00°48'516	−73d07m26s	35.2 ± 4.9	7.2	24.5 ± 5.7	6.1	1.7
NE	1	00°59'093	−72d10m35s	112.2 ± 6.1	18.3	1682.6 ± 215.4	39.8	39.4
	2	01°05'058	−71d59m37s	87.5 ± 6.2	14.2	195.9 ± 25.2	14.4	13.2
	3	01°02'522	−71d53m46s	58.9 ± 6.2	9.5	88.6 ± 11.9	10.2	8.4
	4	01°03'482	−72d03m59s	55.8 ± 6.1	9.1	112.9 ± 14.8	11.9	10.4
	5	00°57'573	−72d39m24s	55.0 ± 6.0	9.2	42.8 ± 7.3	7.2	4.2
	6	01°03'078	−72d03m39s	52.2 ± 5.9	8.8	400.2 ± 51.2	23.0	22.3
	7	01°06'031	−72d03m35s	49.7 ± 6.0	8.3	29.1 ± 6.9	6.1	1.7
	8	01°02'311	−71d56m54s	46.6 ± 5.8	8.1	39.8 ± 6.9	7.2	4.2
	9	00°58'328	−72d14m54s	42.6 ± 6.0	7.1	36.6 ± 6.7	7.3	4.4
	10	01°03'489	−72d02m17s	42.4 ± 5.9	7.2	42.1 ± 7.0	7.8	5.1
	11	01°08'417	−71d59m12s	41.7 ± 6.9	6.0	44.5 ± 7.6	8.0	5.5
	12	01°05'354	−71d59m15s	41.7 ± 5.9	7.0	43.4 ± 7.1	7.8	5.1
	13	00°58'263	−72d18m00s	39.6 ± 6.0	6.6	26.6 ± 6.5	6.2	2.2
	14	01°03'280	−72d01m31s	38.3 ± 6.0	6.3	38.2 ± 6.8	7.4	4.5
Wing	1	01°14'473	−73d19m52s	75.5 ± 6.4	11.8	330.7 ± 42.4	19.1	18.2
	2	01°14'056	−73d17m04s	73.0 ± 6.6	11.1	342.2 ± 43.8	19.9	19.0
	3	01°09'138	−73d11m33s	71.3 ± 6.7	10.6	87.9 ± 11.9	10.1	8.3
	4	01°14'210	−73d15m41s	58.1 ± 6.5	8.9	67.6 ± 9.6	9.2	7.1
	5	01°14'251	−73d14m10s	55.8 ± 6.5	8.6	34.8 ± 7.4	6.5	2.8
N88	1	01°24'086	−73d09m00s	148.1 ± 11.2	13.2	130.1 ± 18.3	8.6	6.4

Note. The columns give (1) source ID, (2) R.A., (3) decl., (4) observed peak flux and noise level at 1.1 mm, (5) signal-to-noise ratio, (6) 1.1 mm total flux, and source radius (7) before and (8) after deconvolution.

A machine-readable version of the table is available.

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The detected 1.1 mm objects show a peak flux range of 35–149 mJy beam⁻¹, a total flux range of 24–1683 mJy, and a source radius range of 6–40 pc (03–23). SW-1, SW-2, and NE-1 have relatively large radii compared to the other detected objects. With the exception of these objects, the ranges of the peak flux, total flux, and radius are 35–148 mJy beam⁻¹, 24–400 mJy, and 6–23 pc (03–13), respectively.

The identification of the objects retrieved by PCA is summarized in the Appendix. Note that our PCA map is sensitive enough to serendipitously uncover distant gravitationally amplified galaxies (e.g., Takekoshi et al. 2013).

To evaluate the reproducibility performance of FRUIT for extended structures, we embedded 20 two-dimensional, axisymmetric Gaussian flux distribution sources with the combinations of four FWHMs of 1', 2', 3', and 4', and five peak fluxes of 30, 60, 90, 120 and 150 mJy, in a source-free region of the SMC Wing field. The peak flux and FWHM ranges were selected in order to simulate objects in the FRUIT maps. Peak fluxes of 30, 60, 90, 120, and 150 mJy ${\mathrm{beam}}^{-1}$ correspond to an S/N ratio of ∼5σ, 10σ, 15σ, 20σ, and 25σ, respectively, for the SW, NE, and Wing fields.

Figure 3 shows the result of the Gaussian fits before and after FRUIT was applied. We ascertained three facts through this analysis. First, about 80% of the FWHMs of Gaussian distribution were recovered but tended to have slightly lower recovery rates with larger input FWHMs. Second, the peak flux was recovered to approximately 90%–100% below the FWHM of 3'. Third, the recovery rate of the total flux decreased as the FWHM increased, and 60%–70% was recovered for 2'–3' FWHM objects. Considering the source size of the 1.1 mm objects, all the objects tended toward an ∼80% smaller FWHM value and recovered their peak flux correctly. On the other hand, the recovered total flux depends on the source size. All 1.1 mm objects showed FWHMs ≲3' from the peaks, suggesting that a total flux of ≳50% was recovered. In particular, the peak structure of SW-1, 2, and NE-1 showed an FWHM of 2'–3', which is expected to recover the total flux of 50%–70%. The other smaller objects also would recover a total flux of >60%. These results are also consistent with the previous FRUIT simulation studies by Shimajiri et al. (2011, 2015) and Komugi et al. (2011).

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As an alternative approach for investigating the missing flux of the extended emission, we estimated the missing flux of the 1.1 mm data after applying the FRUIT procedure using the Herschel maps (Meixner et al. 2013). Both 1.1 mm and Herschel 100–500 μm radiation are expected to arise from the cold dust component in the SMC, thus the Herschel maps should have a distribution similar to the 1.1 mm distribution. We constructed filtered Herschel maps, which have the diffuse emission filtered out in the same way as the 1.1 mm map using the following steps. First, appropriately scaled Herschel data were embedded into the 1.1 mm time-stream data that removed the astronomical signal component estimated by the FRUIT analysis of the 1.1 mm data. Here we used flux scaling factors of 0.036, 0.023, 0.047, 0.085, and 0.118 for 100, 160, 250, 350, and 500 μm, respectively, to make the peak flux comparable to the 1.1 mm images. Second, the time-stream data, in which the Herschel maps were embedded, are analyzed using the FRUIT procedure. Finally, the analyzed Herschel maps were rescaled by dividing by the scaling factor. These filtered Herschel maps were expected to have the extended flux filtered out in a similar way to that seen in the 1.1 mm images. Therefore, we can make a comparison with the 1.1 mm and the filtered Herschel data directly, and estimate the dust temperature of the detected objects without the bias of missing flux by using this data set.

Figure 4 shows the Herschel fluxes as estimated by the contours of the 1.1 mm objects before and after FRUIT analysis. The distribution of the raw and filtered fluxes are fit by

$$\begin{eqnarray*}&&{S}_{\lambda }^{\mathrm{FRUIT}}(\mathrm{Jy})={{aS}}_{\lambda }^{\mathrm{Raw}},\end{eqnarray*}$$

where a is 0.85, 0.76, 0.75, 0.72, and 0.69 for 100, 160, 250, 350, and 500 μm, respectively, with standard deviations of 0.09, 0.07 0.08, 0.08, and 0.07, which may arise from additional photometric errors caused by FRUIT. This result indicates that the total fluxes are systematically decreased in comparison to the intrinsic fluxes by applying the FRUIT procedure. Since the 1.1 mm fluxes are affected by the same filtering as the filtered Herschel data, it is possible to derive dust temperatures that are not biased due to the missing flux of the diffuse component when we use both the 1.1 mm and filtered Herschel data. We are not interested in the missing diffuse flux because this component is expected to be more diffuse than the size of the GMCs. Thus, we estimate the dust temperature and the lower limits of dust masses of the 1.1 mm objects using the 1.1 mm and Herschel data fluxes estimated from the filtered images.

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The total 1σ photometric errors of the 1.1 mm object, given in Table 2, were estimated by taking into account the photometric calibration errors, the random error caused by FRUIT filtering, and the random error caused by the noise level of the FRUIT map. The photometric calibration errors are 8% at 1.1 mm (Wilson et al. 2008b), and 8% and 10% in the SPIRE (250, 350, and 500 μm) and PACS (100 and 160 μm) bands (Meixner et al. 2013; Gordon et al. 2014). The random error caused by FRUIT filtering is, at most, approximately 10%. As a result, the 1σ photometric error was estimated to be 12.8% in the 1.1 mm and SPIRE bands and 14.1% in the PACS bands. Subsequently, we added the random error caused by the map noise, which depends on the size of the objects and the noise level of the object area, to estimate the total 1σ photometric errors.

The flux densities of the identified 1.1 mm objects are shown in Table 3. The physical properties of the 1.1 mm objects were derived by the SED analysis under the assumption of single-temperature thermal dust emission using flux densities at the AzTEC and Herschel bands after removing the contribution of free–free emission. At first, the contributions of free–free emission to the 1.1 mm (and Herschel bands) fluxes were estimated by fitting the free–free spectra to the 4.8 and 8.64 GHz continuum data (Dickel et al. 2010, ATCA+Parkes) convolved to 40''. We estimated the photometric flux error of the 1.1 mm objects by using the image photometric errors of 1% and the image noise levels. The spectral index is flat for most objects, so we assume that synchrotron emission does not contribute to the radio data (Bot et al. 2010a). Assuming an electron temperature of 10⁴ K in the ionized regions, the flux density of the free–free emission is scaled by $1\mbox{--}0.16\mathrm{ln}(\nu /{10}^{11}\mathrm{Hz})$ (Reynolds 1992). Second, thermal emission from a cold dust component was estimated using the fluxes at 1100, 500, 350, 250, 160, and 100 μm after subtracting the free–free contribution. The total flux, S_λ, of thermal emission from the cold dust can be calculated using

$$\begin{eqnarray*}&&{S}_{\lambda }={\kappa }_{\mathrm{dust},\lambda }{B}_{\lambda }({T}_{\mathrm{dust}}){M}_{\mathrm{dust}}{D}^{-2},\end{eqnarray*}$$

where κ_dust,λ is the emissivity of dust, B_λ is the Planck function, M_dust is total dust mass, T_dust is the dust temperature, and D is the distance to the SMC. In the SED analysis, M_dust and T_dust are calculated from the 1.1 mm and Herschel fluxes. The total gas mass M_gas (including the H₂, H i, and He gas components) was estimated assuming a gas-to-dust mass ratio GDR:

$$\begin{eqnarray*}&&{M}_{\mathrm{gas}}=\mathrm{GDR}\times {M}_{\mathrm{dust}}.\end{eqnarray*}$$

Table 3.  The Total Flux Densities of the 1.1 mm Extended Objects

ID		${S}_{4.8\mathrm{GHz}}^{\mathrm{Raw}}$	${S}_{8.64\mathrm{GHz}}^{\mathrm{Raw}}$	${S}_{1.1\mathrm{mm}}^{\mathrm{FRUIT}}$	${S}_{500\mu {\rm{m}}}^{\mathrm{FRUIT}}$	${S}_{350\mu {\rm{m}}}^{\mathrm{FRUIT}}$	${S}_{250\mu {\rm{m}}}^{\mathrm{FRUIT}}$	${S}_{160\mu {\rm{m}}}^{\mathrm{FRUIT}}$	${S}_{100\mu {\rm{m}}}^{\mathrm{FRUIT}}$	${S}_{70\mu {\rm{m}}}^{\mathrm{Raw}}$	${S}_{24\mu {\rm{m}}}^{\mathrm{Raw}}$	${S}_{8\mu {\rm{m}}}^{\mathrm{Raw}}$
		(mJy)	(mJy)	(mJy)	(Jy)	(Jy)	(Jy)	(Jy)	(Jy)	(Jy)	(mJy)	(mJy)
SW	1	80.6 ± 0.8	77.2 ± 0.8	1272.8 ± 162.9	12.88 ± 1.29	30.59 ± 3.06	61.27 ± 6.13	127.09 ± 12.71	137.64 ± 13.76	117.96 ± 5.90	4486.3 ± 179.5	1819.7 ± 91.0
	2	119.1 ± 1.2	117.6 ± 1.2	1194.5 ± 152.9	14.29 ± 1.43	33.85 ± 3.38	68.29 ± 6.83	137.38 ± 13.74	155.52 ± 15.55	127.36 ± 6.37	11938.1 ± 477.5	2690.2 ± 134.5
	3	35.8 ± 0.4	19.9 ± 0.2	300.9 ± 38.6	3.31 ± 0.33	8.00 ± 0.80	16.58 ± 1.66	34.19 ± 3.42	37.75 ± 3.78	23.69 ± 1.18	1864.1 ± 74.6	744.0 ± 37.2
	4	38.4 ± 0.4	31.3 ± 0.3	260.7 ± 33.4	2.63 ± 0.27	6.51 ± 0.65	13.74 ± 1.38	27.76 ± 2.78	29.71 ± 2.97	25.46 ± 1.27	1378.3 ± 55.1	867.9 ± 43.4
	5	20.7 ± 0.2	20.5 ± 0.2	207.8 ± 26.7	2.22 ± 0.23	5.18 ± 0.52	10.21 ± 1.02	20.52 ± 2.05	23.09 ± 2.31	20.46 ± 1.02	1031.0 ± 41.2	391.5 ± 19.6
	6	2.9 ± 0.1	4.3 ± 0.1	89.3 ± 11.8	0.74 ± 0.11	1.80 ± 0.20	4.04 ± 0.41	9.44 ± 0.95	12.19 ± 1.24	9.87 ± 0.49	874.3 ± 35.0	183.3 ± 9.2
	7	2.4 ± 0.1	−1.4 ± 0.1	117.3 ± 15.2	0.91 ± 0.11	2.06 ± 0.22	3.82 ± 0.39	6.62 ± 0.67	5.32 ± 0.57	5.37 ± 0.27	220.5 ± 8.8	145.1 ± 7.3
	8	4.4 ± 0.2	4.1 ± 0.2	49.3 ± 7.3	0.46 ± 0.11	1.13 ± 0.15	2.51 ± 0.27	5.73 ± 0.60	7.07 ± 0.77	5.90 ± 0.29	484.7 ± 19.4	143.6 ± 7.2
	9	5.4 ± 0.1	6.2 ± 0.1	164.0 ± 21.1	1.28 ± 0.14	3.05 ± 0.31	6.10 ± 0.61	10.77 ± 1.08	10.47 ± 1.06	9.00 ± 0.45	363.4 ± 14.5	288.9 ± 14.4
	10	1.5 ± 0.2	2.1 ± 0.1	78.8 ± 10.5	0.63 ± 0.10	1.42 ± 0.16	2.60 ± 0.27	4.52 ± 0.47	4.49 ± 0.51	3.91 ± 0.20	496.6 ± 19.9	79.1 ± 4.0
	11	0.9 ± 0.2	0.1 ± 0.2	37.4 ± 6.3	0.38 ± 0.12	0.93 ± 0.15	1.91 ± 0.22	3.58 ± 0.42	3.40 ± 0.48	2.43 ± 0.12	118.7 ± 4.7	86.1 ± 4.3
	12	3.1 ± 0.1	−0.7 ± 0.1	177.0 ± 22.7	0.90 ± 0.10	1.91 ± 0.20	3.38 ± 0.34	5.51 ± 0.56	4.48 ± 0.47	4.81 ± 0.24	124.4 ± 5.0	122.0 ± 6.1
	13	3.0 ± 0.2	3.3 ± 0.1	56.0 ± 7.9	0.43 ± 0.10	0.97 ± 0.13	1.69 ± 0.19	2.85 ± 0.33	2.57 ± 0.37	2.30 ± 0.12	84.9 ± 3.4	76.4 ± 3.8
	14	1.3 ± 0.3	1.4 ± 0.2	26.5 ± 6.1	0.34 ± 0.14	0.80 ± 0.16	1.55 ± 0.21	2.83 ± 0.38	2.77 ± 0.49	2.30 ± 0.11	145.5 ± 5.8	54.8 ± 2.7
	15	0.5 ± 0.3	−0.1 ± 0.2	27.7 ± 6.3	0.19 ± 0.15	0.44 ± 0.15	0.84 ± 0.17	1.60 ± 0.30	1.57 ± 0.42	1.28 ± 0.06	145.1 ± 5.8	38.5 ± 1.9
	16	2.6 ± 0.3	2.0 ± 0.2	25.2 ± 5.9	0.26 ± 0.14	0.57 ± 0.15	1.05 ± 0.17	1.83 ± 0.31	1.45 ± 0.43	1.74 ± 0.09	63.4 ± 2.5	39.0 ± 1.9
	17	1.5 ± 0.2	1.0 ± 0.2	32.4 ± 6.2	0.15 ± 0.13	0.34 ± 0.13	0.66 ± 0.15	1.31 ± 0.26	1.51 ± 0.38	1.61 ± 0.08	50.1 ± 2.0	19.0 ± 0.9
	18	1.3 ± 0.3	1.0 ± 0.2	23.7 ± 5.8	0.14 ± 0.14	0.32 ± 0.14	0.65 ± 0.15	1.24 ± 0.28	1.07 ± 0.42	0.95 ± 0.05	50.0 ± 2.0	22.4 ± 1.1
	19	2.6 ± 0.2	1.6 ± 0.1	52.6 ± 7.5	0.39 ± 0.10	0.87 ± 0.13	1.59 ± 0.18	2.76 ± 0.32	2.35 ± 0.36	2.33 ± 0.12	62.8 ± 2.5	58.7 ± 2.9
	20	1.5 ± 0.2	0.7 ± 0.2	44.3 ± 6.8	0.37 ± 0.11	0.81 ± 0.13	1.46 ± 0.18	2.12 ± 0.28	1.62 ± 0.34	1.68 ± 0.08	53.7 ± 2.1	58.9 ± 2.9
	21	4.8 ± 0.2	5.2 ± 0.2	37.6 ± 6.2	0.11 ± 0.11	0.26 ± 0.11	0.58 ± 0.12	1.32 ± 0.24	1.70 ± 0.36	2.17 ± 0.11	201.4 ± 8.1	27.0 ± 1.4
	22	1.7 ± 0.2	1.7 ± 0.1	64.7 ± 8.9	0.45 ± 0.10	1.00 ± 0.13	1.95 ± 0.21	3.63 ± 0.39	3.80 ± 0.45	3.61 ± 0.18	114.2 ± 4.6	79.9 ± 4.0
	23	4.3 ± 0.2	2.9 ± 0.2	25.8 ± 5.7	0.25 ± 0.13	0.57 ± 0.14	1.08 ± 0.17	2.02 ± 0.31	1.80 ± 0.42	1.52 ± 0.08	63.9 ± 2.6	39.8 ± 2.0
	24	4.2 ± 0.3	3.0 ± 0.2	24.5 ± 5.7	0.23 ± 0.13	0.52 ± 0.14	1.01 ± 0.17	2.16 ± 0.32	2.42 ± 0.46	2.49 ± 0.12	92.8 ± 3.7	39.1 ± 2.0
NE	1	607.7 ± 6.1	589.9 ± 5.9	1682.6 ± 215.4	15.24 ± 1.52	37.25 ± 3.73	78.87 ± 7.89	179.40 ± 17.94	256.52 ± 25.65	191.39 ± 9.57	24577.0 ± 983.1	2733.6 ± 136.7
	2	37.4 ± 0.4	37.7 ± 0.4	195.9 ± 25.2	1.75 ± 0.19	4.57 ± 0.46	10.74 ± 1.08	29.07 ± 2.91	41.44 ± 4.15	32.89 ± 1.64	8303.7 ± 332.1	1088.0 ± 54.4
	3	7.4 ± 0.2	5.4 ± 0.1	88.6 ± 11.9	0.84 ± 0.13	2.14 ± 0.24	4.61 ± 0.47	9.61 ± 0.97	11.15 ± 1.14	8.10 ± 0.41	547.5 ± 21.9	169.6 ± 8.5
	4	22.3 ± 0.3	22.5 ± 0.2	112.9 ± 14.8	0.82 ± 0.12	1.95 ± 0.21	3.87 ± 0.40	8.98 ± 0.91	11.06 ± 1.12	7.87 ± 0.39	572.0 ± 22.9	114.5 ± 5.7
	5	2.2 ± 0.2	0.8 ± 0.2	42.8 ± 7.3	0.39 ± 0.14	0.96 ± 0.17	2.05 ± 0.25	4.27 ± 0.48	4.43 ± 0.56	3.29 ± 0.16	206.2 ± 8.2	65.0 ± 3.3
	6	39.0 ± 0.4	47.7 ± 0.5	400.2 ± 51.2	3.94 ± 0.40	9.11 ± 0.91	17.15 ± 1.72	31.26 ± 3.13	29.82 ± 2.98	21.73 ± 1.09	1264.0 ± 50.6	469.5 ± 23.5
	7	1.4 ± 0.3	0.3 ± 0.2	29.1 ± 6.9	0.23 ± 0.16	0.51 ± 0.17	0.93 ± 0.19	1.65 ± 0.31	1.55 ± 0.43	1.08 ± 0.05	62.5 ± 2.5	30.9 ± 1.5
	8	1.0 ± 0.2	1.0 ± 0.2	39.8 ± 6.9	0.25 ± 0.13	0.60 ± 0.14	1.21 ± 0.18	1.91 ± 0.29	1.61 ± 0.37	1.33 ± 0.07	54.3 ± 2.2	38.5 ± 1.9
	9	3.6 ± 0.2	3.4 ± 0.2	36.6 ± 6.7	0.30 ± 0.14	0.67 ± 0.15	1.16 ± 0.18	2.04 ± 0.30	1.74 ± 0.38	1.58 ± 0.08	45.5 ± 1.8	23.3 ± 1.2
	10	10.0 ± 0.2	9.2 ± 0.2	42.1 ± 7.0	0.32 ± 0.13	0.81 ± 0.15	1.63 ± 0.20	3.51 ± 0.40	4.38 ± 0.54	3.43 ± 0.17	180.5 ± 7.2	50.6 ± 2.5
	11	1.5 ± 0.2	1.7 ± 0.2	44.5 ± 7.6	0.17 ± 0.14	0.44 ± 0.15	0.87 ± 0.17	1.66 ± 0.27	1.53 ± 0.35	1.18 ± 0.06	69.2 ± 2.8	30.7 ± 1.5
	12	4.2 ± 0.2	6.9 ± 0.2	43.4 ± 7.1	0.22 ± 0.13	0.49 ± 0.13	0.98 ± 0.16	2.08 ± 0.29	2.40 ± 0.39	1.70 ± 0.08	73.2 ± 2.9	30.7 ± 1.5
	13	1.6 ± 0.2	0.5 ± 0.2	26.6 ± 6.5	0.07 ± 0.15	0.16 ± 0.16	0.31 ± 0.16	0.54 ± 0.26	0.60 ± 0.39	0.58 ± 0.03	14.0 ± 0.6	9.5 ± 0.5
	14	3.0 ± 0.2	5.1 ± 0.2	38.2 ± 6.8	0.36 ± 0.14	0.88 ± 0.16	1.67 ± 0.21	3.09 ± 0.38	2.87 ± 0.44	1.92 ± 0.10	77.9 ± 3.1	42.5 ± 2.1
Wing	1	42.2 ± 0.4	43.5 ± 0.4	330.7 ± 42.4	2.95 ± 0.30	7.44 ± 0.75	16.06 ± 1.61	37.21 ± 3.72	47.04 ± 4.71	38.72 ± 1.94	3383.6 ± 135.3	531.9 ± 26.6
	2	66.9 ± 0.7	60.2 ± 0.6	342.2 ± 43.8	4.20 ± 0.42	10.43 ± 1.04	22.64 ± 2.27	50.71 ± 5.07	63.72 ± 6.37	42.44 ± 2.12	4484.7 ± 179.4	864.5 ± 43.2
	3	29.0 ± 0.3	20.2 ± 0.2	87.9 ± 11.9	0.42 ± 0.11	1.10 ± 0.15	2.68 ± 0.29	8.40 ± 0.86	12.98 ± 1.32	11.16 ± 0.56	1707.6 ± 68.3	177.8 ± 8.9
	4	4.0 ± 0.2	2.8 ± 0.1	67.6 ± 9.6	0.74 ± 0.14	1.79 ± 0.21	3.83 ± 0.40	7.78 ± 0.80	7.48 ± 0.79	4.69 ± 0.23	261.2 ± 10.4	131.1 ± 6.6
	5	1.3 ± 0.2	−0.9 ± 0.2	34.8 ± 7.4	0.28 ± 0.16	0.68 ± 0.18	1.49 ± 0.22	2.96 ± 0.39	3.14 ± 0.49	2.10 ± 0.10	140.6 ± 5.6	41.1 ± 2.1
N88	1	51.5 ± 0.5	29.4 ± 0.3	130.1 ± 18.3	0.53 ± 0.22	1.48 ± 0.26	3.83 ± 0.44	12.40 ± 1.27	20.77 ± 2.10	19.01 ± 0.95	6525.8 ± 261.0	365.7 ± 18.3

A machine-readable version of the table is available.

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We used plausible dust parameters of the SMC obtained by far-infrared to millimeter wavelength surveys, including Spitzer, Planck, and Herschel data (e.g., Leroy et al. 2007; Planck Collaboration et al. 2011; Gordon et al. 2014). The emissivity of the dust, ${\kappa }_{\mathrm{dust},\lambda }=12.5\times {(160\mu {\rm{m}}/\lambda )}^{\beta }{\mathrm{cm}}^{2}\ {{\rm{g}}}^{-1}$, is assumed (Draine & Li 2007; Draine et al. 2014). Recent data at millimeter and submillimeter wavelengths have pointed out the shallow index of the emissivity, β = 1.2 ± 0.3 (Aguirre et al. 2003; Leroy et al. 2007; Bot et al. 2010b; Israel et al. 2010; Planck Collaboration et al. 2011). Here we assumed the index β = 1.2, for simplicity. We also assumed that the GDR = 1000, which is a typical value in the SMC (Leroy et al. 2007; Planck Collaboration et al. 2011; Gordon et al. 2014), to estimate the total gas mass, though these studies showed an uncertainty of a factor of two.

The number density and column density of molecular hydrogen (H₂) were estimated by assuming that the 1.1 mm objects are dominated by the molecular component. This assumption usually causes overestimates of the molecular component. However, the use of the spatially filtered maps for this analysis, in which most of the diffuse components (corresponds to H i gas) are removed, supports the validity of the assumption above. This assumption is also consistent with the robust spatial correlation between the 1.1 mm objects and CO emission (described in detail in Section 6.2.1). We estimated the amount of ${{\rm{H}}}_{2}$ molecules using the total-to-H₂ molecular gas mass ratio of 1.36, by taking into account the relative abundance of helium and heavier elements (Muller et al. 2010). The number density was estimated by assuming spherically symmetric clouds with radii of R.

Computing the SED fits for each object naturally leads to computations of the dust mass, dust temperature, and other supplemental physical parameters. The SED of SW-1 is shown in Figure 6 as an example, and that of the others in an online figure set. Free–free contributions were subtracted to some extent from the 1.1 mm flux densities. The AzTEC and Herschel fluxes correspond to a single graybody component of the 30 K dust, which is the dominant dust mass component. The 24 and 70 μm fluxes show the excess from the cold dust component spectrum that would originate from warm dust components, such as very small grains (e.g., Desert et al. 1990).

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The derived physical parameters from the SED fits are shown in Table 4. Most of the 1.1 mm objects have free–free contaminations smaller than 10%, but seven sources (including N88-1) have a free–free contribution of over 10% for the 1.1 mm fluxes. We noted that NE-1, which is associated with the active star-forming region, N66, has a free–free contamination of over 20% for the 1.1 mm flux. The dust temperatures displayed a range of 17–45 K, and an average and standard deviation of 28.7 ± 4.4 K,¹⁸ which is higher than the global dust temperature of 21.6 ± 1.9 K, estimated over the whole SMC, with an emissivity index of β = 1.2 (Planck Collaboration et al. 2011). Lee et al. (2015) derived the spatially resolved (∼45'') dust temperature distribution using Herschel data and assuming a fixed emissivity index β = 1.5. The derived temperature range was 21–35 K, which is similar to the temperature estimated using our data set and assuming β = 1.5 (25.0 ± 3.6 K). The gas masses are distributed in the range of 4.1 × 10³–3.4 × 10⁵ M_⊙. The massive 1.1 mm objects, SW-1, SW-2, and NE-1, that have gas masses of >10⁵ M_⊙, correspond to the famous star-forming regions Lirs 49, SMCB-2 N, and N66, respectively. Bot et al. (2010a) estimated the gas masses of 3.1 × 10⁵ M_⊙ for Lirs 49 and 4.6 × 10⁵ M_⊙ for SMCB-2 N, which are comparable to the result of our SED fits, considering the differences in the dust temperature and GDR.

Table 4.  Physical Properties of the 1.1 mm Extended Objects

ID		Tdust	Mdust	Mgas	${n}_{{{\rm{H}}}_{2}}$	${N}_{{{\rm{H}}}_{2}}$	1.1 mm free–free
		(K)	(${M}_{\odot }$)	($\times {10}^{3}\,{M}_{\odot }$)	(${{\rm{H}}}_{2}\,{\mathrm{cm}}^{-3}$)	($\times {10}^{21}\,{{\rm{H}}}_{2}\,{\mathrm{cm}}^{-2}$)
SW	1	30.0 ± 1.0	332.1 ± 32.7	332.1 ± 32.7	28.8 ± 2.8	4.1 ± 0.4	4.0%
	2	30.7 ± 1.1	336.1 ± 33.2	336.1 ± 33.2	32.4 ± 3.2	4.4 ± 0.4	6.4%
	3	30.8 ± 1.1	80.8 ± 8.0	80.8 ± 8.0	58.3 ± 5.8	4.1 ± 0.4	5.3%
	4	30.5 ± 1.0	67.6 ± 6.7	67.6 ± 6.7	60.4 ± 6.0	3.9 ± 0.4	8.5%
	5	29.9 ± 1.0	55.3 ± 5.5	55.3 ± 5.5	47.2 ± 4.7	3.1 ± 0.3	6.4%
	6	33.6 ± 1.4	17.0 ± 2.0	17.0 ± 2.0	53.8 ± 6.2	2.3 ± 0.3	2.6%
	7	25.0 ± 0.8	32.9 ± 3.4	32.9 ± 3.4	66.5 ± 6.8	3.3 ± 0.3	0.1%
	8	33.3 ± 1.5	10.6 ± 1.4	10.6 ± 1.4	79.1 ± 10.6	2.5 ± 0.3	5.6%
	9	27.1 ± 0.9	41.4 ± 4.1	41.4 ± 4.1	46.9 ± 4.7	2.8 ± 0.3	2.3%
	10	26.2 ± 0.9	20.2 ± 2.4	20.2 ± 2.4	70.5 ± 8.2	2.9 ± 0.3	1.5%
	11	28.4 ± 1.3	11.1 ± 1.6	11.1 ± 1.6	121.2 ± 17.7	3.4 ± 0.5	0.8%
	12	23.9 ± 0.7	34.5 ± 3.6	34.5 ± 3.6	32.2 ± 3.4	2.1 ± 0.2	0.3%
	13	25.0 ± 1.0	15.0 ± 2.1	15.0 ± 2.1	78.7 ± 10.9	2.9 ± 0.4	3.7%
	14	28.8 ± 1.7	8.5 ± 1.6	8.5 ± 1.6	147.3 ± 27.2	3.6 ± 0.7	3.4%
	15	26.3 ± 2.2	6.7 ± 1.8	6.7 ± 1.8	107.2 ± 28.8	2.7 ± 0.7	0.3%
	16	25.8 ± 1.9	7.9 ± 1.9	7.9 ± 1.9	137.1 ± 32.1	3.3 ± 0.8	5.8%
	17	26.1 ± 2.3	5.9 ± 1.7	5.9 ± 1.7	68.3 ± 19.8	1.9 ± 0.5	2.5%
	18	25.2 ± 2.6	5.9 ± 1.9	5.9 ± 1.9	106.2 ± 34.4	2.5 ± 0.8	3.1%
	19	25.0 ± 1.1	14.0 ± 2.0	14.0 ± 2.0	73.2 ± 10.5	2.7 ± 0.4	2.5%
	20	23.2 ± 1.1	14.9 ± 2.3	14.9 ± 2.3	106.3 ± 16.8	3.5 ± 0.5	1.5%
	21	29.0 ± 2.6	4.1 ± 1.2	4.1 ± 1.2	33.6 ± 9.7	1.0 ± 0.3	8.7%
	22	27.2 ± 1.1	13.9 ± 1.8	13.9 ± 1.8	52.2 ± 6.9	2.1 ± 0.3	1.7%
	23	27.3 ± 1.9	7.2 ± 1.6	7.2 ± 1.6	106.1 ± 23.3	2.7 ± 0.6	8.8%
	24	30.3 ± 2.2	5.5 ± 1.3	5.5 ± 1.3	87.2 ± 20.0	2.2 ± 0.5	9.4%
NE	1	34.8 ± 1.5	305.9 ± 33.7	305.9 ± 33.7	17.3 ± 1.9	2.8 ± 0.3	23.0%
	2	39.1 ± 1.8	32.3 ± 3.7	32.3 ± 3.7	38.7 ± 4.4	2.3 ± 0.3	12.4%
	3	31.5 ± 1.2	21.3 ± 2.4	21.3 ± 2.4	71.9 ± 8.1	3.0 ± 0.3	4.6%
	4	31.8 ± 1.3	19.4 ± 2.3	19.4 ± 2.3	40.8 ± 4.8	2.0 ± 0.2	12.8%
	5	30.0 ± 1.4	10.8 ± 1.6	10.8 ± 1.6	105.3 ± 15.9	3.1 ± 0.5	2.1%
	6	27.2 ± 0.8	116.6 ± 11.2	116.6 ± 11.2	34.2 ± 3.3	3.2 ± 0.3	6.8%
	7	25.6 ± 2.1	7.7 ± 2.1	7.7 ± 2.1	123.0 ± 33.0	3.1 ± 0.8	1.8%
	8	24.2 ± 1.5	11.1 ± 2.2	11.1 ± 2.2	108.2 ± 21.6	3.2 ± 0.6	1.6%
	9	25.1 ± 1.5	10.1 ± 2.0	10.1 ± 2.0	92.6 ± 18.4	2.8 ± 0.6	6.2%
	10	31.6 ± 1.7	8.0 ± 1.3	8.0 ± 1.3	61.1 ± 10.2	1.9 ± 0.3	14.7%
	11	24.8 ± 1.7	8.7 ± 2.1	8.7 ± 2.1	60.8 ± 14.6	2.0 ± 0.5	2.4%
	12	28.5 ± 1.9	6.7 ± 1.4	6.7 ± 1.4	51.3 ± 11.0	1.6 ± 0.4	8.5%
	13	16.7 ± 2.6	11.5 ± 5.4	11.5 ± 5.4	171.0 ± 80.1	4.4 ± 2.1	2.3%
	14	27.5 ± 1.3	10.8 ± 1.8	10.8 ± 1.8	96.5 ± 15.7	2.9 ± 0.5	7.1%
Wing	1	33.6 ± 1.3	66.8 ± 7.0	66.8 ± 7.0	34.3 ± 3.6	2.7 ± 0.3	8.4%
	2	34.5 ± 1.3	83.9 ± 8.7	83.9 ± 8.7	38.0 ± 3.9	3.1 ± 0.3	12.0%
	3	42.0 ± 2.6	7.6 ± 1.2	7.6 ± 1.2	26.1 ± 4.1	1.1 ± 0.2	17.3%
	4	29.8 ± 1.1	19.5 ± 2.2	19.5 ± 2.2	90.1 ± 10.3	3.4 ± 0.4	3.2%
	5	29.2 ± 1.7	8.5 ± 1.7	8.5 ± 1.7	112.4 ± 22.2	3.0 ± 0.6	0.1%
N88	1	44.8 ± 3.2	9.7 ± 1.7	9.7 ± 1.7	53.5 ± 9.5	1.9 ± 0.3	18.0%

Note. The columns give (1) Source ID, (2) dust temperature, (3) total dust mass, (4) total gas mass, (5) ${{\rm{H}}}_{2}$ gas density, (6) ${{\rm{H}}}_{2}$ column density, and (7) free–free contribution ratio to 1.1 mm total flux. Total gas mass includes the atomic and molecular phases of hydrogen, and the other gas components such as He. ${{\rm{H}}}_{2}$ gas density and column density are estimated assuming that all hydrogen component consists of ${{\rm{H}}}_{2}$ molecules.

A machine-readable version of the table is available.

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To understand the heating mechanism of the cold dust, we investigated the relations between the dust temperature and the star-formation activity of the 1.1 mm objects. The Spitzer 24 μm data is widely used as a tracer of embedded star-formation activity (e.g., Calzetti et al. 2007). Thus, it is useful to determine the significance of the compact and massive star-forming activity that accompanies the strong UV radiation field to the dust temperatures of the 1.1 mm objects. Some part of the 24 μm flux is contributed by stochastically heated PAH emission, which accompanies the diffuse cold dust component, and we removed the PAH contribution using an empirical relation, ${L}_{24\mu {\rm{m}}}-0.14{L}_{8\mu {\rm{m}}}$, obtained by Draine & Li (2007). The ratio of the 24 μm luminosity, connected to massive star-formation activity, to the dust mass corresponds to the intensity of the radiation field that contributes to the dust heating. It is, therefore, expected that the ratio shows good correlation with the dust temperature. Figure 7 shows the relation between the dust temperature and the ratio of the corrected 24 μm luminosity to the dust mass. These parameters are well fitted to an exponential function and have a correlation coefficient of r² = 0.881. This strongly supports the idea that the origin of the variety of dust temperatures is the interstellar radiation field from young and massive stars formed inside each 1.1 mm object.

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We also found that the dust temperature of the Wing region is significantly higher than the other regions. The average dust temperatures and the standard deviations are 27.8 ± 2.8, 28.5 ± 5.4, and 33.8 ± 5.1 K in the SW, NE, and Wing regions, respectively. The differences between the Wing and other regions are significant, with the p-values of 0.59 (SW and NE), 0.01 (SW and Wing), and 0.08 (NE and Wing) according to the Mann–Whitney U test (e.g., Wall & Jenkins 2003). The corresponding objects of the Wing region (N81 and N83/84) are known as very active H ii regions, accompanying O-type stars (Testor & Lortet 1987; Heydari-Malayeri et al. 1999). In the Wing region, the number density of low-mass stars is smaller than that in the SW and NE regions, where the stellar bar is located. Therefore, it is likely that the contribution to the dust temperature comes mainly from the strong radiation field caused by local young massive stars. This situation is different from that in some nearby galaxies that show radial gradients of the dust temperature, mainly caused by low-mass stellar populations (e.g., Engelbracht et al. 2010; Komugi et al. 2011; Galametz et al. 2012; Groves et al. 2012; Tabatabaei et al. 2014). From a different perspective, Gordon et al. (2014) revealed variations of the dust emissivity spectral index, β, in different regions of the SMC. This may also explain the discrepancy of the dust temperature between the Wing and NE/SW regions when assuming the higher β value of 1.5 in the Wing region (29.0 ± 4.0 K).

6.2.1. Comparisons with the Other Wavelength Data

We focus on the nature of the detected 1.1 mm objects by investigating the correlations with gas and star-formation tracers. We compared the 1.1 mm data with the survey data of atomic hydrogen (H i), CO, and continuum emission at 24 μm. The H i data is the combination of the Australia Telescope Compact Array and the 64 m Parkes telescope (Stanimirovic et al. 1999). The CO (J = 1–0) survey data was taken by NANTEN (Mizuno et al. 2001). The 24 μm data was provided by the SAGE-SMC program of the Spitzer Space Telescope (Gordon et al. 2011). The spatial resolutions of these data are 98'', 26, and 6'' for H i, CO, and 24 μm, respectively.

Figure 8 shows the comparison between H i, CO, 1.1 mm, and 24 μm in the N66, N75–78, N81, N83/84, N88, and SW regions. We describe the comparison for each region below.

• 1.  
  N66: the most active star-forming region in the SMC. The NANTEN CO emission is seen in the northern part of the 1.1 mm object. Rubio et al. (2000) reported CO (J = 2–1) detection in the northern and central parts, with the central part being weaker than the northern part. In contrast, the southern part shows a high H i column density. We note that the H i profile shows two velocity components at 120 and $160\,\mathrm{km}\,{{\rm{s}}}^{-1}$, and CO emission is associated with the $160\,\mathrm{km}\,{{\rm{s}}}^{-1}$ component. The 24 μm emission is located in the central region of the 1.1 mm object.
• 2.  
  N75–78: there are some compact 1.1 mm objects. The peaks of the 1.1 mm objects do not always exactly co-located with peaks of the H i column density, but they exist in the high H i column density region. The 1.1 mm emission also seems to be a more compact structure than the H i emission.
• 3.  
  N81, N83/84, and N88: these are known as the active star-forming sites associated with the H ii regions. The peaks of the 1.1 mm objects in these regions are in very good agreement with the 24 μm emission.
• 4.  
  SW: this region is located on the main part of the SMC bar, and shows bright CO emission, elevated star-formation activity, and a high H i column density. The CO emission appears to be associated with the 1.1 mm objects. Mopra observation shows that the NANTEN CO clouds are resolved into smaller CO objects (Muller et al. 2013). The 24 μm emission is located on the peak of the 1.1 mm objects. The large 1.1 mm objects, such as SW-1 and 2, are in agreement with the high column H i region, but the 1.1 mm structures appear to be sharper than the H i column density. The other 1.1 mm objects also have smaller structures than the H i distribution. We note that H i spectra show complex velocity structures distributed around 120 and $160\,\mathrm{km}\,{{\rm{s}}}^{-1}$, and CO emission is mainly associated with the $120\,\mathrm{km}\,{{\rm{s}}}^{-1}$ component.

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The three main findings from the comparisons between the 1.1 mm objects and the other tracers in the representative regions in the SMC are summarized as follows.

• 1.  
  The spatial correlation between the 1.1 mm emission and the H i column density is not necessarily good, and the structure of the H i column density has a smoother distribution than that of the 1.1 mm emission.
• 2.  
  The CO emission shows good spatial correlation with the 1.1 mm emission, and the 1.1 mm objects appear to trace the spatially resolved structure of the NANTEN CO objects. CO molecules exist on the inner side of the self-shielding surface of ${{\rm{H}}}_{2}$ clouds (Tielens & Hollenbach 1985), suggesting that a part of the 1.1 mm object consists of ${{\rm{H}}}_{2}$ gas.
• 3.  
  The 24 μm emission is associated with most 1.1 mm peaks. Since the 24 μm emission is a tracer of the star-formation activity, the 1.1 mm emission around the 24 μm sources corresponds to the envelope of high-density gasses that forms massive stars.

The 1.1 mm objects can be classified into two types in terms of star-formation activity: the first classification encompasses the objects that are associated with H ii regions, Spitzer and Herschel YSO candidates (Bolatto et al. 2007; Seale et al. 2014), or bright 24 μm sources (>10 MJy sr⁻¹), and the second accounts for the objects that are not associated with these star-formation signatures. Among the 44 objects, 34 objects are associated with signs of star formation, and the other 10 objects are not. The 1.1 mm objects can also be classified into two types in terms of whether or not they are associated with the CO emission. The existence of CO molecules in the 1.1 mm objects is judged by the detection of the CO (J = 1–0) line using the data obtained by the Mopra 22 m (beam size of 33'' FWHM and typical virial mass of ≳6 × 10³ M_⊙, Muller et al. 2003, 2010, 2013) and NANTEN 4 m (26 and ≳6 × 10⁴ M_⊙, Mizuno et al. 2001) telescopes. The Mopra data is roughly comparable to the detection mass limit of the 1.1 mm objects (≳7 × 10³ M_⊙). Among the 44 objects, 23 objects are associated with the CO emission, and 21 objects are not. Table 5 shows the results of the classification by the existence of star-formation activity and CO detection in the 1.1 mm objects.

Table 5.  Association with the Signs of Star-formation Activity and CO Emission

ID		Star formation	CO emission
SW	1	N27a/DEM S40b/S3MC 72–76,78,79c/24 μm	NANTEN/MOPRA
	2	N20-23,25,26/DEM S35-38/S3MC 57,58,61–64,67/24 μm	NANTEN/MOPRA
	3	N13AB/DEM S16/S3MC 16,19/24 μm	NANTEN/MOPRA
	4	N12A/DEM S23/S3MC 16,19/24 μm	NANTEN/MOPRA
	5	N30A/DEM S45/S3MC 37,38/24 μm	NANTEN/MOPRA
	6	N11/24 μm	⋯
	7	S3MC 23/24 μm	NANTEN/MOPRA
	8	N33/S3MC 95,97/24 μm	⋯
	9	S3MC 43/24 μm	MOPRA
	10	N51/DEM S72/S3MC 126,127/24 μm	⋯
	11	S3MC 17,18,20/24 μm	NANTEN/MOPRA
	12	⋯	⋯
	13	⋯	⋯
	14	24 μm	NANTEN/MOPRA
	15	S3MC 146,147/24 μm	⋯
	16	DEM S39	⋯
	17	⋯	⋯
	18	24 μm	⋯
	19	⋯	⋯
	20	⋯	NANTEN/MOPRA
	21	S3MC 111/24 μm	⋯
	22	S3MC 22	⋯
	23	S3MC 70/24 μm	⋯
	24	⋯	⋯
NE	1	N66ABC/DEM S103/S3MC 188–192,194–198,200,203,204/24 μm	NANTEN/MOPRA
	2	N78ABD/DEM S27/S3MC 243,245,246,248/24 μm	MOPRA
	3	N77AB/DEM S117ab/S3MC 222,223/24 μm	NANTEN/MOPRA
	4	N76A/S3MC 237/24 μm	⋯
	5	N62/DEM S93/S3MC 171,173,175/24 μm	⋯
	6	S3MC 225–227,229,231/24 μm	NANTEN/MOPRA
	7	S3MC 256/24 μm	⋯
	8	DEM S116/S3MC 218	MOPRA
	9	⋯	⋯
	10	⋯	⋯
	11	24 μm	MOPRA
	12	S3MC 251/24 μm	MOPRA
	13	⋯	⋯
	14	⋯	NANTEN/MOPRA
Wing	1	N84ABD/DEM S151,152/S3MC 278,280/24 μm	NANTEN/MOPRA
	2	N83ABC/DEM S147,148/S3MC 269,271,272/24 μm	NANTEN/MOPRA
	3	N81/DEM S138/S3MC 264/24 μm	⋯
	4	N84C/DEM S149/24 μm	NANTEN/MOPRA
	5	DEM S150/S3MC 276/24 μm	NANTEN/MOPRA
N88	1	N88/DEM S161/24 μm	MOPRA

Notes.

^aThe Henize Catalog of SMC Emission Nebulae (Henize 1956). ^bMagellanic Cloud Emission-line Survey DEM Catalog: Small Magellanic Cloud (Davies et al. 1976). ^cS³MC Young Stellar Object Catalog (Bolatto et al. 2007, Table 4).

A machine-readable version of the table is available.

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6.2.2. Physical Property Comparisons with the Previous GMC Observations

The physical properties estimated by the SED fits also revealed the nature of the 1.1 mm objects. The masses and radii of the 1.1 mm objects are in the range of 4 × 10³–3 × 10⁵ M_⊙ and up to 40 pc, respectively. In addition, the hydrogen molecule densities of the 1.1 mm objects, assuming spherical symmetry, are a few × 10–100 H₂ cm⁻³. The statistical characteristics of the mass, size, and density of the 1.1 mm objects are roughly consistent with the GMC properties obtained by CO observations toward our galaxy (e.g., Blitz 1993) and the LMC (Fukui et al. 2008), supporting the idea that the detected 1.1 mm objects correspond to GMCs.

The mass spectrum of the 1.1 mm objects is shown in Figure 9. The cumulative number count of the 1.1 mm objects, N, was fitted by a power law with dN/dM ∝ M^α, where the power index α =  −1.76 ± 0.13, with gas masses higher than 8 × 10³ M_⊙. Recently, van Kempen et al. (submitted to A&A) revealed that the power index α = −2.0 ± 0.2, using the high-resolution CO (J = 2–1) data in the SMC, and the result was also consistent with the power index of the 1.1 mm objects. The power index is also consistent with that of the Large Magellanic Cloud (α = −1.75 ± 0.06, Fukui et al. 2008). On the other hand, the index is deeper than that of our galaxy (α ≈ −1.5, e.g., Casoli et al. 1984; Solomon & Rivolo 1989; Digel et al. 1996; Dobashi et al. 1996), meaning that it could be responsible for the differences in the environment of formation or dissipation of the GMCs in the SMC.

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6.2.3. The Nature of the 1.1 mm Objects

6.2.4. Evolution of the 1.1 mm Objects

It seems reasonable to suppose that the detected 1.1 mm objects trace gas clouds massive and dense enough to form massive stars. Here we examine the existence of the difference in the physical properties of classified objects.

Figure 10 shows the histograms of the physical properties (dust temperature, gas mass, and radius) of the 1.1 mm objects, classified according to the existence of star-formation activity. Table 6 shows the median values¹⁹ of the physical parameters in some classification. We note that the dust temperatures, gas masses, and radii of the star-forming objects are higher or larger than those of starless objects. There are significant differences in the median values between the starless and star-forming clouds, with p-values of 0.01, 0.20, and 0.18 for the dust temperature, dust mass, and radius, respectively, determined using the Mann–Whitney U test. Figure 11 also shows that the starless clouds concentrate in regions of low dust temperature and small gas mass.

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Table 6.  Numbers and Median Values of Physical Parameters of the 1.1 mm Objects

	Numbers	Tdust	Mgas	Radius
		(K)	(×103 M⊙)	(pc)
All	43	28.5	13.9	8.8
Starless	10	25.1	11.2	7.6
Star-forming	33	29.2	17.0	10.1

Note. N88-1 is excluded from the statistics.

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As a result, the dust temperature, gas mass, and radius size differences between starless and star-forming clouds show the evolution stage of the GMCs. The same tendencies for the gas mass and size are shown by previous GMC studies based on CO line observations toward the LMC and M33 (Kawamura et al. 2009; Miura et al. 2012).