Near-Infrared Polarimetric Study of N159/N160 Star-Forming Complex in the Large Magellanic Cloud

TL;DR

This study uses near-infrared polarimetry to map magnetic fields in the N159/N160 star-forming complex in the Large Magellanic Cloud, revealing magnetic influence on star formation and interstellar medium structure.

Contribution

It provides the first detailed magnetic field map of N159/N160 using near-infrared polarimetry, linking magnetic structure to star formation processes.

Findings

Magnetic field vectors align with local interstellar structures.

Polarization ratios follow a power-law dependence on wavelength.

Magnetic fields support sequential star formation scenario.

Abstract

We present near-infrared polarimetric results for N159/N160 star-forming complex, in the Large Magellanic Cloud (LMC) with SIRPOL, the polarimeter of the Infrared Survey Facility (IRSF). We separated foreground sources using their visual extinction derived from near-infrared photometric data. The 45 young stellar candidates and 2 high excitation blobs were matched with our sources, and 12 of them showed the high polarization that was not originated from the interstellar dust. We made a polarimetric catalog of 252, 277, and 89 sources at $J$, $H$, and $K_s$ bands, respectively. Based on the ratios of polarization degree among these bands, we verify that the origin of these polarized sources is the dichroic extinction from the aligned interstellar dust by magnetic field and that the ratios follow a power-law dependence of $P_{\lambda}$ $\sim$ $\lambda^{-0.9}$. The linear polarization vectors projected on the H$\alpha$ image of the complex turned out to follow local magnetic field structure. The vector map overlaid on dust and gas emissions shows the close correlation between magnetic field structure and surrounding interstellar medium. We suggest that the derived magnetic field structure supports the sequential formation scenario of the complex.

Tables

Table 1. Table 1: Photometric and polarimetric catalog of YSOs and HPSs the N159/N160 field

Position			Magnitude							Polarization Properties																HPS	AKARI colorccMid-infrared colors that calculated from $AKARI$ IRC point-source catalog of the LMC (Kato et al., 2012).
${\alpha }_{\circ \mathrm{J2000}.0}$	${\delta }_{\circ \mathrm{J2000}.0}$		$J$		$H$		$K_s$			$P_J$		$P_H$		$P_{K_s}$			${\theta }_J$		${\theta }_H$		${\theta }_{K_s}$		Av	References	TypeaaEvolutionary stages for YSO candidates are labeled as ‘ya’, ‘yb’, and ‘yc’, indicating their stages from I to III, which refer to Chen et al. (2010) and Carlson et al. (2012). The label ‘ysc’, ‘HEB’, and ‘YSO’ indicate young stellar cluster (Chen et al., 2010), HEB objects and massive YSO from Martín-Hernández et al. (2008), respectively. The label ‘CC5’ indicates YSO candidates with water ice absorption, classified by Kato et al. (2012).	No.bbFor the sources that matched with the YSOs candidates from the literature are listed.	S7-S11	N3-S7
			(mag)		(mag)		(mag)			(%)		(%)		(%)			($°$)		($°$)		($°$)		(mag)				(mag)	(mag)
(1)	(2)		(3)	(4)	(5)	(6)	(7)	(8)		(9)	(10)	(11)	(12)	(13)	(14)		(15)	(16)	(17)	(18)	(19)	(20)	(21)	(22)	(23)	(24)	(25)	(26)
5 39 29.02	-69 47 18.69		16.763	0.047	15.791	0.066	14.576	0.058		3.87	3.86	12.57	3.41	18.49	3.73		14.25	20.20	62.04	7.48	48.67	5.65	7.01	1	ya	1	$\cdots$	$\cdots$
5 40 0.48	-69 47 13.02		14.502	0.059	13.583	0.043	12.395	0.028		12.93	3.28	3.06	2.68	$\cdots$	$\cdots$		159.60	7.04	26.79	18.84	$\cdots$	$\cdots$	6.80	1,3	yc,CC5	2	1.40	3.68
5 40 3.40	-69 47 10.02		15.461	0.007	14.849	0.006	14.707	0.013		1.72	0.51	$\cdots$	$\cdots$	$\cdots$	$\cdots$		163.49	8.09	$\cdots$	$\cdots$	$\cdots$	$\cdots$	0.24	1	yc	$\cdots$	$\cdots$	$\cdots$
5 39 33.62	-69 47 1.14		16.726	0.028	16.124	0.026	15.505	0.019		3.13	1.93	$\cdots$	$\cdots$	2.40	2.17		143.59	15.05	$\cdots$	$\cdots$	37.1	19.17	3.23	1,2	yb,ybc	$\cdots$	$\cdots$	$\cdots$
5 39 40.60	-69 46 30.43		16.861	0.055	16.278	0.063	15.546	0.060		8.59	3.63	7.88	3.79	$\cdots$	$\cdots$		27.48	11.14	145.28	12.39	$\cdots$	$\cdots$	3.86	1	yb	$\cdots$	$\cdots$	$\cdots$
5 39 41.68	-69 46 11.31		16.412	0.036	14.416	0.056	12.193	0.045		$\cdots$	$\cdots$	13.03	3.48	9.61	2.58		$\cdots$	$\cdots$	5.98	7.38	7.14	7.43	13.64	1,2	yab,ya	3	$\cdots$	$\cdots$
5 39 37.39	-69 46 8.81		14.573	0.067	13.769	0.083	12.907	0.083		12.92	3.51	5.36	4.80	$\cdots$	$\cdots$		73.11	7.49	93.53	19.08	$\cdots$	$\cdots$	4.82	1	yab	4	$\cdots$	$\cdots$
5 39 35.89	-69 46 3.32		15.451	0.043	14.408	0.058	13.405	0.051		3.02	2.21	$\cdots$	$\cdots$	$\cdots$	$\cdots$		60.37	16.86	$\cdots$	$\cdots$	$\cdots$	$\cdots$	5.78	1	ya	$\cdots$	$\cdots$	$\cdots$
5 39 43.56	-69 45 39.68		18.559	0.056	17.214	0.042	15.942	0.031		$\cdots$	$\cdots$	10.70	2.86	5.70	3.15		$\cdots$	$\cdots$	126.55	7.38	107.14	13.82	7.45	1,2	yc,ya	5	$\cdots$	$\cdots$
5 39 59.23	-69 45 26.02		16.326	0.025	13.882	0.013	11.771	0.004		7.18	1.51	4.56	0.68	3.18	0.29		123.14	5.87	125.68	4.20	133.07	2.59	12.53	1,3	yb,CC5	$\cdots$	1.08	3.20
5 39 52.56	-69 45 16.84		13.883	0.062	13.803	0.020	13.738	0.015		$\cdots$	$\cdots$	2.32	1.26	1.72	1.01		$\cdots$	$\cdots$	98.28	13.62	109.01	14.41	-0.24	1	yc	$\cdots$	$\cdots$	$\cdots$
5 40 9.33	-69 44 53.59		17.918	0.045	16.830	0.052	15.931	0.067		11.67	3.60	7.60	2.87	$\cdots$	$\cdots$		13.00	8.42	77.47	10.11	$\cdots$	$\cdots$	5.19	1	yab	6	$\cdots$	$\cdots$
5 39 45.03	-69 44 50.32		14.693	0.039	14.309	0.043	13.942	0.054		3.32	2.22	$\cdots$	$\cdots$	$\cdots$	$\cdots$		128.82	15.88	$\cdots$	$\cdots$	$\cdots$	$\cdots$	1.60	1	yc	$\cdots$	$\cdots$	$\cdots$
5 39 44.35	-69 44 34.88		16.603	0.026	15.770	0.020	15.218	0.036		$\cdots$	$\cdots$	1.03	1.02	$\cdots$	$\cdots$		$\cdots$	$\cdots$	133.88	20.09	$\cdots$	$\cdots$	2.84	1,2	yab,ya	$\cdots$	$\cdots$	$\cdots$
5 40 3.46	-69 43 55.38		17.993	0.058	16.730	0.078	15.832	0.096		$\cdots$	$\cdots$	5.64	4.12	$\cdots$	$\cdots$		$\cdots$	$\cdots$	50.66	16.85	$\cdots$	$\cdots$	5.60	1	ysc	$\cdots$	$\cdots$	$\cdots$
5 39 29.75	-69 43 33.11		18.660	0.052	17.019	0.028	15.871	0.027		$\cdots$	$\cdots$	7.93	2.83	9.00	3.11		$\cdots$	$\cdots$	26.76	9.60	24.00	9.34	6.64	1	yb	$\cdots$	$\cdots$	$\cdots$
5 39 58.23	-69 47 18.80		16.216	0.029	15.394	0.042	15.246	0.041		$\cdots$	$\cdots$	$\cdots$	$\cdots$	5.92	2.70		$\cdots$	$\cdots$	$\cdots$	$\cdots$	77.83	11.86	0.43	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 30.82	-69 46 48.06		17.169	0.021	16.444	0.016	16.204	0.030		4.32	1.55	5.38	1.21	$\cdots$	$\cdots$		149.11	9.67	143.25	6.28	$\cdots$	$\cdots$	0.87	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 3.77	-69 45 22.15		16.770	0.011	15.802	0.015	15.508	0.022		$\cdots$	$\cdots$	0.92	0.84	5.89	2.38		$\cdots$	$\cdots$	95.04	19.32	158.45	10.73	1.21	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 40.69	-69 45 15.91		19.334	0.090	18.145	0.040	17.358	0.074		$\cdots$	$\cdots$	4.06	3.58	$\cdots$	$\cdots$		$\cdots$	$\cdots$	117.82	18.93	$\cdots$	$\cdots$	4.50	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 40.18	-69 44 54.75		16.499	0.021	16.179	0.037	16.132	0.051		3.15	1.30	$\cdots$	$\cdots$	6.83	3.42		89.36	10.93	$\cdots$	$\cdots$	109.46	12.8	-0.36	2	yb	$\cdots$	$\cdots$	$\cdots$
5 39 53.15	-69 44 16.84		17.779	0.032	17.029	0.029	16.746	0.050		7.03	2.59	3.59	2.59	$\cdots$	$\cdots$		90.63	9.88	147.58	16.75	$\cdots$	$\cdots$	1.20	2	yb	$\cdots$	$\cdots$	$\cdots$
5 40 0.98	-69 44 7.32		16.495	0.015	15.962	0.028	15.926	0.043		$\cdots$	$\cdots$	$\cdots$	$\cdots$	4.27	3.03		$\cdots$	$\cdots$	$\cdots$	$\cdots$	161.77	16.54	-0.38	2	yb	$\cdots$	$\cdots$	$\cdots$
5 39 21.16	-69 44 8.25		16.431	0.015	15.526	0.018	14.949	0.015		1.59	0.95	1.08	0.97	1.87	1.65		147.03	14.65	138.25	19.06	87.87	18.93	2.97	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 3.85	-69 44 8.54		16.599	0.008	15.986	0.011	15.890	0.021		$\cdots$	$\cdots$	1.59	0.84	9.24	2.61		$\cdots$	$\cdots$	129.36	13.35	170.73	7.76	-0.04	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 11.01	-69 43 53.08		17.215	0.015	16.607	0.015	16.568	0.040		2.86	1.32	1.66	1.28	14.99	4.33		51.84	11.98	173.32	17.46	105.16	7.93	-0.37	2	ya	7	$\cdots$	$\cdots$
5 40 15.40	-69 43 1.28		17.103	0.013	16.313	0.015	16.082	0.033		$\cdots$	$\cdots$	3.72	1.08	2.85	2.69		$\cdots$	$\cdots$	4.49	7.99	41.81	19.63	0.83	2	ya	$\cdots$	$\cdots$	$\cdots$
5 40 19.37	-69 43 0.12		17.639	0.034	17.000	0.028	16.919	0.057		10.18	2.86	$\cdots$	$\cdots$	9.44	5.74		107.91	7.75	$\cdots$	$\cdots$	37.36	14.86	-0.06	2	ybc	8	$\cdots$	$\cdots$
5 39 15.01	-69 42 41.27		18.181	0.036	17.479	0.032	17.198	0.061		9.54	3.81	4.59	2.82	$\cdots$	$\cdots$		51.32	10.60	124.97	14.96	$\cdots$	$\cdots$	1.19	2	yab	$\cdots$	$\cdots$	$\cdots$
5 40 15.32	-69 42 23.43		17.192	0.036	16.383	0.053	16.187	0.056		$\cdots$	$\cdots$	$\cdots$	$\cdots$	$\cdots$	$\cdots$		$\cdots$	$\cdots$	$\cdots$	$\cdots$	$\cdots$	$\cdots$	0.80	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 58.75	-69 41 10.03		17.894	0.030	17.234	0.026	17.053	0.048		6.85	3.21	3.07	2.19	$\cdots$	$\cdots$		56.29	12.14	28.76	16.63	$\cdots$	$\cdots$	0.54	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 13.78	-69 40 27.42		18.758	0.054	17.823	0.029	17.551	0.082		$\cdots$	$\cdots$	9.96	3.69	$\cdots$	$\cdots$		$\cdots$	$\cdots$	117.57	9.94	$\cdots$	$\cdots$	1.27	2	ya	$\cdots$	$\cdots$	$\cdots$
5 40 26.68	-69 39 36.28		17.108	0.023	16.472	0.012	16.313	0.053		2.89	2.64	5.98	1.64	$\cdots$	$\cdots$		70.31	19.30	15.67	7.55	$\cdots$	$\cdots$	0.39	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 38.67	-69 39 3.53		17.646	0.057	15.902	0.032	14.392	0.034		27.67	6.71	8.61	2.29	8.98	2.62		99.32	6.74	45.25	7.34	36.49	8.01	8.94	2	ya	9	$\cdots$	$\cdots$
5 40 47.19	-69 37 5.67		17.111	0.022	16.248	0.008	16.166	0.059		3.05	1.76	1.96	1.44	$\cdots$	$\cdots$		32.53	14.31	97.95	16.98	$\cdots$	$\cdots$	-0.05	2	ya	$\cdots$	$\cdots$	$\cdots$
5 39 8.48	-69 44 14.26		16.004	0.011	15.288	0.006	15.120	0.013		1.58	0.87	1.26	0.43	3.42	1.40		32.11	13.81	176.29	9.19	7.03	10.88	0.41	3	CC5	$\cdots$	0.78	4.52
5 39 37.58	-69 45 40.16		14.308	0.007	12.975	0.007	12.346	0.003		2.11	0.33	1.41	0.34	0.41	0.23		179.34	4.40	163.88	6.81	176.72	14.29	3.26	3	CC5	$\cdots$	0.58	5.04
5 40 1.07	-69 43 23.96		16.759	0.013	16.111	0.015	15.990	0.026		2.78	1.11	4.29	1.06	$\cdots$	$\cdots$		52.67	10.56	87.22	6.85	$\cdots$	$\cdots$	0.13	3	CC5	$\cdots$	1.46	4.87
5 40 2.78	-69 41 18.75		17.397	0.012	16.659	0.016	16.570	0.036		6.13	1.61	4.72	1.21	10.26	4.41		2.54	7.26	96.26	7.13	126.54	11.28	-0.06	3	CC5	$\cdots$	1.78	4.05
5 39 31.19	-69 36 37.14		15.983	0.012	14.638	0.007	14.152	0.019		4.07	1.09	1.56	0.67	3.33	1.45		129.88	7.40	110.7	11.31	14.27	11.39	2.38	3	CC5	$\cdots$	1.02	3.39
5 39 49.48	-69 38 3.27		15.352	0.009	15.199	0.005	15.160	0.024		1.80	0.63	2.06	0.55	$\cdots$	$\cdots$		155.72	9.49	39.01	7.35	$\cdots$	$\cdots$	-0.39	3	CC5	$\cdots$	1.70	4.45
5 39 52.72	-69 36 34.69		16.876	0.015	15.784	0.003	15.491	0.028		9.76	1.65	3.98	0.79	8.78	3.55		166.06	4.76	59.33	5.53	135.37	10.73	1.19	3	CC5	$\cdots$	0.78	4.01
5 39 59.46	-69 37 29.89		16.900	0.027	15.178	0.021	13.274	0.011		4.66	2.35	3.16	1.43	$\cdots$	$\cdots$		173.23	12.90	79.40	11.79	$\cdots$	$\cdots$	11.28	3	CC5	$\cdots$	1.69	3.81
5 40 11.57	-69 33 15.60		13.936	0.023	13.354	0.015	13.095	0.015		$\cdots$	$\cdots$	$\cdots$	$\cdots$	2.16	1.25		$\cdots$	$\cdots$	$\cdots$	$\cdots$	105.00	14.30	0.98	3	CC5	$\cdots$	0.77	3.38
5 39 43.23	-69 38 54.01		13.722	0.082	13.011	0.083	12.374	0.068		25.49	4.81	15.01	6.14	9.35	5.38		11.18	5.30	29.57	10.84	8.27	14.27	3.42	4	HEB	10	$\cdots$	$\cdots$
5 39 45.90	-69 38 39.32		13.640	0.087	13.017	0.068	11.156	0.078		$\cdots$	$\cdots$	$\cdots$	$\cdots$	52.79	6.99		$\cdots$	$\cdots$	$\cdots$	$\cdots$	173.87	3.76	10.51	4	HEB	11	$\cdots$	$\cdots$
5 39 44.12	-69 38 33.47		15.213	0.061	14.886	0.045	14.199	0.047		8.81	4.93	24.38	4.82	5.18	2.85		142.24	13.98	43.44	5.55	16.10	13.80	3.52	4	YSO	12	$\cdots$	$\cdots$

Table 2. Table 2: Criteria for the best quality polarization in the N159/N160 field

	N159				N160
	$J$	$H$	$K_s$		$J$	$H$	$K_s$
$P/{\sigma }_P$		$>$3				$>$3
$P$		10				10
Mag	$\le$17.0	$\le$16.7	$\le$14.5		$\le$16.5	$\le$16.2	$\le$14.0
Total number	159	157	56		93	120	33

Table 3. Table 3: Photometric and polarimetric catalog of the sources in the N159/N160 field of the LMC a a Only a portion of the catalog is listed in Table  3 .

	Position			Magnitude							Polarization Properties														NICER
Field	${\alpha }_{\circ \mathrm{J2000}.0}$	${\delta }_{\circ \mathrm{J2000}.0}$		$J$		$H$		$K_s$			$P_J$		$P_H$		$P_{K_s}$			${\theta }_J$		${\theta }_H$		${\theta }_{K_s}$			Av
				(mag)		(mag)		(mag)			(%)		(%)		(%)			($°$)		($°$)		($°$)			(mag)
(1)	(2)	(3)		(4)	(5)	(6)	(7)	(8)	(9)		(10)	(11)	(12)	(13)	(14)	(15)		(16)	(17)	(18)	(19)	(20)	(21)		(22)	(23)
N159	5 39 39.31	-69 47 21.83		15.363	0.006	14.195	0.010	13.828	0.004		3.97	0.38	2.01	0.61	2.94	0.57		47.44	2.71	61.31	8.28	50.31	5.45		1.63	0.90
N159	5 40 13.13	-69 47 16.40		17.320	0.013	16.545	0.022	16.305	0.100		$\cdots$	$\cdots$	4.92	1.49	$\cdots$	$\cdots$		$\cdots$	$\cdots$	83.32	8.30	$\cdots$	$\cdots$		1.05	1.09
N159	5 39 43.87	-69 47 13.21		16.756	0.013	15.539	0.013	15.046	0.016		8.02	1.16	6.00	0.80	2.54	1.55		55.28	4.11	52.22	3.80	70.53	14.92		2.44	0.90
N159	5 39 27.05	-69 47 11.64		16.133	0.009	14.809	0.013	14.302	0.009		4.03	0.66	3.77	0.71	2.52	0.68		28.63	4.62	48.45	5.33	31.62	7.44		2.52	0.90
N159	5 39 45.63	-69 47 08.93		17.051	0.009	16.138	0.012	15.851	0.023		5.89	1.18	2.77	0.88	3.63	2.43		45.11	5.64	58.10	8.64	88.84	15.94		1.16	0.91
N159	5 39 24.00	-69 47 01.85		16.601	0.011	15.954	0.014	15.759	0.022		4.82	0.90	1.73	0.91	7.64	2.95		54.51	5.27	132.06	13.39	98.95	10.3		0.59	0.91
N159	5 40 01.63	-69 46 55.35		13.383	0.004	12.517	0.008	12.289	0.002		1.51	0.26	$\cdots$	$\cdots$	0.27	0.27		173.08	4.79	$\cdots$	$\cdots$	18.53	20.07		0.77	0.90
N159	5 39 25.47	-69 46 59.48		16.882	0.015	16.060	0.009	15.807	0.021		3.60	0.92	2.20	0.64	3.83	2.16		64.98	7.11	42.07	8.02	4.73	14.04		0.94	0.91
N159	5 39 30.82	-69 46 48.06		17.169	0.021	16.444	0.016	16.204	0.030		4.32	1.55	5.38	1.21	$\cdots$	$\cdots$		149.11	9.67	143.25	6.28	$\cdots$	$\cdots$		0.87	0.92
N159	5 39 50.21	-69 46 43.60		16.487	0.013	15.801	0.010	15.611	0.020		3.75	0.90	$\cdots$	$\cdots$	6.39	2.17		24.23	6.66	$\cdots$	$\cdots$	103.61	9.22		0.55	0.91

Full text

infrared: ISM — infrared: stars — ISM: individual objects (N159, N160) — ISM: magnetic fields — ISM: structure — Magellanic Clouds

1 Introduction

It is important to understand the formation and evolution of stars in molecular clouds, because they are the most fundamental building blocks of the universe. Magnetic fields have been considered as one of the main forces influencing the star formation process, thus there have been many studies on them both theoretically and observationally (Shu, Adams, & Lizano, 1987; Heiles & Crutcher, 2005; McKee & Ostriker, 2007; Crutcher, 2012; Dobbs et al., 2014). Li et al. (2014) reviewed the observations of magnetic fields by their scales, and discussed the relation between magnetic fields and the formation process of molecular clouds and stars. Since magnetic fields are generally associated with the interstellar medium, revealing their structure of magnetic fields is essential to understand the morphology of star-forming regions and their surrounding interstellar medium.

Polarimetry is the general method for examining magnetic fields. Near-infrared polarimetric measurements in star-forming regions provide us with the magnetic field structure by observing the dichroic extinction of background stars, whose starlights are polarized when they pass through the magnetically aligned dust grains within molecular clouds. Various polarimetric studies of magnetic fields have been performed in Galactic star-forming regions (Kandori et al., 2007; Tamura et al., 2007; Kusakabe et al., 2008; Saito et al., 2009; Kwon et al., 2011; Santos et al., 2014; Kusune et al., 2015).

The Large Magellanic Cloud (LMC) is one of the nearest galaxies, and therefore it has been one of the best observed targets in studies of the star-forming regions and giant molecular clouds (GMCs) within a galaxy. The northeastern part of the LMC includes the root of the molecular stream extending southward from the 30 Doradus complex and has been the focus of numerous studies of the star-forming regions (i.e. the 30 Doradus, N158, and N159/N160 complexes) and molecular clouds (Cohen et al., 1988; Kutner et al., 1997; Fukui et al., 1999; Ott et al., 2008; Pineda et al., 2009). Nakajima et al. (2007) first studied the central part of 30 Doradus using near-infrared polarimetric observations, and discovered its magnetic field distribution, which showed a U-shaped structure in the northern area and aligned about 70$\arcdeg$ and 45$\arcdeg$ in the western and southeastern areas, respectively. Kim et al. (2011) observed a wider region of 20$\arcmin$ $\times$ 20$\arcmin$ region around 30 Doradus, and they used the proper motion data to separate the Galactic foreground sources. They confirmed that most polarized sources showed their polarization angles of 75$\arcdeg$. Kim et al. (2016) extended the survey area to 39$\arcmin$ $\times$ 69$\arcmin$, and found patterns following the molecular stream. However, the N159/N160 complex still shows the veiled structure of the magnetic fields. Since this region has active star formation, H II regions, and interesting molecular cloud structures (see Section 2 for details), we expect that the magnetic fields might be influenced by those environments to make construct complex structure.

This research studied the detailed structure of the magnetic field in the N159/N160 complex. We carried out deep near-infrared polarimetric observations over the two fields of the complex. In Section 2, we describe the N159/N160 complex and review previous studies of this region. Observations and data reduction are described in Section 3. The method to separate foreground sources is also presented in Section 3. The polarimetric results and analysis are presented in Section 4. In Section 5, we discuss the magnetic field structures in the N159/N160 complex with gas and dust distributions, and suggest a star formation scenario in the complex.

2 The N159/N160 Complex

The N159/N160 complex is located in a molecular ridge, about 600 pc south from the 30 Doradus complex. Ever since Henize (1956) cataloged H II regions from the survey of emission nebulae in the LMC, this complex has been the target of various observations (Jones et al., 1986; Johansson et al., 1998; Pak et al., 1998; Bolatto et al., 2000; Jones et al., 2005; Nakajima et al., 2005; Mizuno et al., 2010; Carlson et al., 2012; Galametz et al., 2013). The N159/N160 complex is composed of the N160 and N159 H II regions, and they are spatially separated from each other in the northern and the southern parts of the complex. The H$\alpha$ observations by the Wide Field Imager (WFI) of the ESO 2.2 m telescope (program ID 076.C-0888111Data are available and described at http://archive.eso.org/cms/eso-data/data-packages/30-doradus/30-doradus-reqpage.html.) clearly show the detailed structure of the two regions.

N160 includes H II regions and young stellar clusters. Bright H II regions in N160 are designated using the notation of Henize (1956). The main H II region, with an elongated shape, is composed of N160 A and N160 D (hereafter N160 A+D) at the southwestern part of N160. The southern part of N160 has two bright condensations of H II regions, N160 B and N160 C (hereafter N160 B+C). Heydari-Malayeri & Testor (1986) observed the H II regions in N160 and reported the physical properties of high-excitation blobs (HEBs) and their surrounding nebulae. HEBs are a rare class of the compact H II regions in the Magellanic Clouds. Martín-Hernández et al. (2008) observed HEBs in N160 A with high spatial resolution. A notable feature of the N160 region is a shell structure opened toward the northeast. The parent clouds of the shell are almost dissipated and young clusters are detected inside the shell (Nakajima et al., 2005). Young stellar objects (YSOs) have been observed around the H II regions in N160 (Henning et al., 1998; Nakajima et al., 2005; van Loon et al., 2010; Carlson et al., 2012). Sequential star formation throughout this region was suggested, based on the various evolutionary stages of massive stars in the OB clusters (Nakajima et al., 2005; Fariña et al., 2009).

N159 has bright H II regions and two GMCs, N159E and N159W, located at the eastern and southwestern parts of the region, respectively. In addition, N159 has embedded interesting objects such as an X-ray binary, a supernova remnant, compact H II regions, and radio sources (Hunt & Whiteoak, 1994; Cowley et al., 1995; Chu et al., 1997; Heydari-Malayeri et al., 1999; Testor et al., 2006, 2007; Seok et al., 2013). Jones et al. (2005) located these objects in mid-infrared images observed with the $Spitzer$ $Space$ $Telescope$. Studies of dust emissions unveiled that N159 is younger than N160 and shows active star formation (Galametz et al., 2013). CO observations detected the sites of star formation through cloud-cloud collisions, and revealed the link between high concentrations of molecular gas and star formation (Mizuno et al., 2010; Fukui et al., 2015). Embedded massive YSOs, maser sources, and ultracompact H II regions are distributed around the molecular clouds (Nakajima et al., 2005; Chen et al., 2010).

3 Observations and Data Reduction

3.1 Observations

Simultaneous $JHK_{s}$ polarimetric observations of the N159/N160 complex were performed on 2007 February 3 and 5. We used the near-infrared camera SIRIUS (Nagayama et al., 2003) and the polarimeter SIRPOL (Kandori et al., 2006) of the Infrared Survey Facility (IRSF) 1.4 m telescope at the South African Astronomical Observatory in Sutherland, South Africa. The camera has a field of view of 7$\farcm$7 $\times$ 7$\farcm$7 and a pixel scale of 0$\farcs$45 pixel*-1*. One set of observations for a target field consisted of 20 s exposures at 10 dithered positions for four wave-plate angles (0$\arcdeg$, 45$\arcdeg$, 22$\fdg$5, and 67$\fdg$5) in the $J$, $H$, and $K_{s}$ bands, and the whole sequence is repeated 10 and 9 times for the N159 and N160 fields centered at ($\alpha$, $\delta$)2000 $=$ (5h39m37s.1, -69$\arcdeg$43$\arcmin$45$\farcs$1) and (5h40m05s.6, -69$\arcdeg$36$\arcmin$25$\farcs$8), respectively.

3.2 Data Reduction

We used the DAOPHOT package (Stetson, 1987) of the Image Reduction & Analysis Facility222IRAF is distributed by the US National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. (IRAF) for detection and photometry of the sources. On each wave-plate frame at $J$, $H$, and $K_{s}$, the fluxes of the detected sources were estimated by fitting Penny2-type point-spread function (PSF) to them. The pixel coordinates of the sources were converted to celestial coordinates, based on the 2MASS All Sky Point Source Catalog (Skrutskie et al., 2006). The instrumental magnitudes were converted to 2MASS magnitude following Kim et al. (2016).

The Stokes parameters $I$, $Q$, and $U$ of the sources are calculated using the following equations: $I$ $=$ ($I_{0}$+$I_{22.5}$+$I_{45}$+$I_{67.5}$)/2, $Q$ $=$ $I_{0}-I_{45}$, and $U$ = $I_{22.5}-I_{67.5}$, where $I_{0}$, $I_{22.5}$, $I_{45}$, and $I_{67.5}$ are the fluxes on each wave-plate angle frame (0$\arcdeg$, 45$\arcdeg$, 22$\fdg$5, and 67$\fdg$5). We calculated the flux errors of the sources and derived their polarization uncertainties as described in Kim et al. (2016). The debiased degree of polarization, $P$, and the polarization angle, $\theta$, were calculated as described in Kim et al. (2011).

3.3 Separation of Foreground Sources

The magnetically aligned dust grains generate the polarized light with E-vector direction parallel to the magnetic field, but only from the stars behind the field. In addition, the light of the foreground stars in front of the ISM field is not affected by the magnetic field of our interest. Therefore, the polarization of the foreground stars contaminates our results that reveal the magnetic fields of the N159/N160 complex. We separated the foreground stars out from the detected sources using their visual extinction, $A_{\mathrm{V}}$, derived with the Near-Infrared Color Excess Revisited (NICER) technique (Lombardi & Alves, 2001). The NICER technique is the revised version of that of Lada et al. (1994), and it can measure the interstellar extinction from multi-band observations of molecular clouds. We estimated visual extinctions of stars in two target fields and a control field with their near-infrared colors using the NICER algorithm333Using the C. Beaumont Interactive Data Language (IDL) implementation. The control field (yellow box in Figure 1(a)) is a nearby extinction-free region centered at ($\alpha$, $\delta$)2000 $=$ (5h36m08s.3, -69$\arcdeg$26$\arcmin$00$\farcs$2). The $JHK_{s}$ photometric data of the control field are from Kim et al. (2016).

Figure 1(b) shows the distribution of visual extinctions in the control and target fields. The histogram for the control field peaks at 0.01 mag and with a standard deviation of 0.47 mag. Based on these values, we regarded the sources with $A_{\mathrm{V}}$ $\leq$ 0.48 mag as the foreground stars in front of our target field. A total of 1062 and 640 sources in the N159 and N160 fields, respectively, remained as the background stars for examining the polarimetric properties.

4 Results

4.1 Highly Polarized Sources

Highly polarized sources (HPSs) are mostly due to the intrinsic polarization by circumstellar dust around the stars themselves, not due to the dichroic absorption by foreground dust (Kandori et al., 2007). These sources should be separated from from our polarization sources to trace the magnetic fields that make interstellar dust align.

YSOs are one of the plausible sources of the intrinsic polarization, due to the dust grains in their circumstellar disk and envelope. The N159/N160 complex contains quite a number of YSOs. We matched our sources with the YSO candidates from Chen et al. (2010) and Carlson et al. (2012). In addition, we used the catalog of the AKARI Infrared Camera (IRC) survey in the LMC (Ita et al., 2008; Kato et al., 2012) and classified new YSO candidates in this complex, using the color-color diagram (Figure 2). Most sources in the diagram follow the sequence of dusty C-rich stars denoted group CC3 in Kato et al. (2012), but the sources redder than CC3 were found to be above [N3]$-$[S7] $\sim$ 3.0 mag. Shimonishi et al. (2008) mentioned that these redder sources are the YSO candidates with the water ice absorption at 3 $\mu$m. We considered the nine redder sources to be new YSO candidates in the N159/N160 complex. Two HEBs and one YSO from Martín-Hernández et al. (2008) were also detected in our observations.

A total of 45 YSO candidates and 2 HEBs in previous works were found in our catalog. Twelve of them show polarization degrees larger than 10 %, and polarization signal-to-noise ratios (hereafter $P/\sigma_{P}$) greater than 3. In Table Near-Infrared Polarimetric Study of the N159/N160 Star-Forming Complex in the Large Magellanic Cloud, we compiled near-infrared photometric, polarimetric data, and information from other literature for the all 45 matched YSO candidates.

Figure 3 a) shows the polarization vectors of the 12 highly polarized sources and locations of the matched YSO candidates on the H$\alpha$ image from the WFI/ESO archive. We excluded the highly polarized sources listed in Table Near-Infrared Polarimetric Study of the N159/N160 Star-Forming Complex in the Large Magellanic Cloud in the analysis. The Other 35 YSO candidates did not satisfy the polarization quality criterion of $P/\sigma_{P}$ $>$ 3 or showed the polarization pattern caused by the dichroic extinction due to interstellar dust. A fraction of the intrinsic component might exist in their polarization, but the dominant fraction is probably the dichroic effect due to the aligned interstellar dust, which is strong enough to make the intrinsic component negligible.

4.2 Estimation of the Polarization Accuracy

In order to study the magnetic field in the N159/N160 complex, we set the selection criterion of $P/\sigma_{P}$ $>$ 3. Most sources with this criterion showed their polarization degrees smaller than 10 % (Kandori et al., 2007; Nakajima et al., 2007; Santos et al., 2014).

In general, fainter sources have larger polarization uncertainties than brighter ones. Figure 4 shows the trend between polarization uncertainty and the magnitude of our sources. We set the photometric limits at $J$, $H$, and $K_{s}$ to be 17.0, 16.7, and 14.5 mag for the N159 field and 16.5, 16.2, and 14.0 mag for the N160 field, respectively, so that stars brighter than these limits have a polarization uncertainty smaller than 1 %. We assume that the polarization of sources brighter than these magnitude limits are measured with sufficiently a high accuracy. The criteria for selecting final catalog sources are given in Table 2. Table 3 lists the photometric and polarimetric data of the selected sources in the N159 and N160 fields. The $A_{\mathrm{V}}$ values obtained using the NICER algorithm are also listed.

In order to confirm the interstellar polarization of the catalog sources, we examined the efficiency of the interstellar polarization for the 252, 277, and 89 sources in the N159/N160 complex. In general, interstellar polarization gives the upper limit of the polarization degree, depending on the near-infrared color (Jones, 1989). Figure 5 shows that most of the sources have polarization degrees lower than the upper limit of the interstellar polarization at each band. On the contrary, most of the foreground sources exceed the upper limits. Hence, we conclude that the polarization of the sources is mainly caused by interstellar dust aligned due to the magnetic field in the N159/N160 complex.

4.3 Wavelength Dependence of Polarization

Serkowski, Mathewson, & Ford (1975) empirically modeled the wavelength dependence of polarization for the Milky Way. The empirical relationship can be represented by a power law of $P_{\lambda}$ $\propto$ $\lambda^{-\beta}$, with $\beta$ of 1.6$-$2.0 at the near-infrared wavelengths from 1.25 to 2.2 $\mu$m (Martin & Whittet, 1990; Nagata, 1990; Martin et al., 1992). In the case of the LMC, however, the relation has a shallower slope than that of the Milky Way, with $\beta$ $=$ 0.9 (Nakajima et al., 2007; Kim et al., 2016).

Figure 6 plots our selected sources with $P/\sigma_{P}$ $>$ 5 and $\sigma_{P}$ $\leq$ 1 % to show the wavelength dependence of the polarization degrees. The best-fit slopes of $P_{H}$/$P_{J}$ $=$ 0.71 and $P_{K_{s}}$/$P_{H}$ $=$ 0.76 indicate that the interstellar polarization of the N159/N160 complex also follows the power-law index of $\beta$ $=$ 0.9, rather than the case of the Milky Way.

4.4 Linear Polarization Vector Map

We can assume that the direction of a polarization vector is parallel to the sky-projected component of the magnetic field. In Figure 7, we draw the polarization vectors of our best-quality sources on the H$\alpha$ image to map the magnetic field structure of the N159/N160 complex. The magnetic field structure shows different trends between N159 and N160.

In the N160 region, polarization vectors are roughly aligned with the axis of the open H$\alpha$ shell. However, the vectors in the northwestern region (red dotted ellipse in Figure 7) clearly follow a different pattern, when compared with those inside of the H$\alpha$ shell. The vectors in the south of N160 B+C and N160 A+D show a U-shaped distribution (green dotted curve in Figure 7), which is similar to the H$\alpha$ feature in this region. The magnetic field structure inferred from the distribution of polarization vectors in the N160 region supports the expanding shell in the region.

N159 shows a complex magnetic field structure associated with H$\alpha$ emission. Polarization vectors in N159E are distributed around the lower part, while those in N159W are on the outskirts of the H$\alpha$-emitting cloud. The western side of N159 has no significant H$\alpha$ emission, and the polarization vectors in this region are mostly aligned with the polarization angle of about 30$\arcdeg$.

The peaks in the distribution of the polarization angles of each region in Figure 8 are good indicators of the major directions of the magnetic fields. Those directions are also appeared in the polarization vector map (Figure 7). In the case of N160, one of the two peaks, around 30$\arcdeg$, is related to the magnetic field inside the H$\alpha$ shell, while the other, around 170$\arcdeg$, is related to the field on the northwestern part. Histograms for N159 have a single peak around 10\arcdeg$$-30$\arcdeg$ and this is mostly caused by the magnetic field on the western side of the region. The polarized sources in N159E and N159W have various polarization angles and are responsible for the relatively large dispersion of the histogram compared to that for N160.

5 Discussion

5.1 Magnetic Field Structures with Molecular Cloud Emissions

The magnetic field structures in star-forming regions are constrained by the environment (gas and dust). To reveal the nature of the magnetic field structures in the N159/N160 complex, we have compared our data with the dust and gas emissions. To trace the emission features of dust and ionized gas in the N159/N160 complex, the IRAC 24 $\mu$m data from the Spitzer SAGE (Meixner et al., 2006) and the Herschel 100 $\mu$m data from the HERITAGE project (Meixner et al., 2010) were combined with the vector map on the H$\alpha$ image(Figure 9).

As seen in Figure 9, most of the H$\alpha$ shell structures in N160 encounter the boundary of dust emission at 100 $\mu$m. This trend is more clearly seen at the southern shell structure rather than the northern one. The red dotted ellipse region of the northern shell shows a leak feature of the H$\alpha$ emission extending to the dust cavity, and the polarization vectors in this region are well-aligned in the direction of the cavity. The southern regions of N160 B+C and N160 A+D show the U-shaped feature of the polarization vectors at the edge of strong H$\alpha$ and 100 $\mu$m emissions. In addition, the magnetic field structures inside the shell structure are distributed along the direction from northeast to southwest, which is consistent with the triggered direction of the sequential cluster formation suggested by Nakajima et al. (2005). We also found that some polarization vectors are seen along the shell structure like an expanding shell.

Polarization vectors in the N159 region are mostly found near H II regions showing the dust emissions at 24 $\mu$m and 100 $\mu$m (yellow-composite feature in Figure 9). In order to understand the complex structure of the magnetic fields in N159, we compared our vector map with the CO gas distribution. Mizuno et al. (2010) observed the 12CO($J$ $=$ 4$-$3) and 12CO($J$ $=$ 3$-$2) rotational lines in N159 with the NANTEN2 (Fukui et al., 2008) and ASTE (Ezawa et al., 2004) submillimeter telescopes, respectively. We compared the integrated intensity map of 12CO($J$ $=$ 3$-$2) (Mizuno et al., 2010) with our polarization vector map on the H$\alpha$ image in Figure 10. Strong concentrations of the CO distribution (white X symbols in Figure 10) are located at the regions where H$\alpha$ emission is obscured by the high extinction of the molecular gas. In the case of N159W, the magnetic fields appear around the 24 $\mu$m emission and CO peak. The configuration of the complex magnetic field structure in N159 appears to have been affected by the expansion and evolution of the H II regions in the central N159 and the southern N159W. Jones et al. (2005), Chen et al. (2010), and Bernard et al. (2016) proposed expansions of the H II regions in N159E and N159W. They also reported that the locations of several compact H II regions and young stellar clusters in N159 are likely to be linked with the molecular gas and are mainly distributed on the edge of the ionized H II regions in N159E and N159W.

The western region of N159 does not show any interesting features of dust and gas emissions, while the magnetic fields are in a uniform direction, as shown by the polarization vectors at this region. It indicates that this region was not affected by the star-forming activities in the N159/N160 complex and that a larger scale magnetic field was already formed toward this region before the beginning of the star formation in the nascent molecular cloud in N159 and N160.

In conclusion, the polarization vector map with dust and gas emission features indicates that the magnetic field structures in N159 and N160 were affected by different star-forming activities in each region. These different patterns of the magnetic field support the suggestion that N159 and N160 are in the different evolutionary stages based on the large-scale sequential star formation.

5.2 Formation Scenario of the N159/N160 Complex and Magnetic Field Structure

The spatial distributions of the Herbig Ae/Be and OB clusters in the N159/N160 complex at the near-infrared bands were revealed by Nakajima et al. (2005). Using the spatial correlations between these young stellar clusters and gaseous components, they suggested a large-scale cluster formation scenario that occurred sequentially from north to south. Possible starting points of the formation are the supergiant shell LMC 2 and a superbubble located northeast and east of the complex, respectively. However, the dominant magnetic field direction in N160 shows the pattern along the northeast to southwest, which is in the same direction to the LMC 2 rather than that to the superbubble. The similar direction between the magnetic field and the sequential formation shows that the LMC 2 is a plausible trigger for this large-scale cluster formation.

Figure 11 (a) shows an illustration of the sequential formation process of the N159/N160 complex and the magnetic field structures at each scenario step. The RGB (Red: H$\alpha$, Green: $V$, and Blue: $B$) composite image of the complex is also displayed to help in understanding each illustrated feature (Figure 11 (b)). LMC 2 (blue-colored amorphous feature), the molecular clouds (orange-colored cloudy feature) of the 30 Doradus complex, and the molecular ridge are shown in Step 1, based on the large-scale H$\alpha$ map of Figure 5(a) in Points et al. (2001). Kim et al. (1999) classified H I shell structures in the LMC by using their shell size, expansion pattern, morphological structure, and associated H$\alpha$ emission. The LMC 2 was classified as an expanding supergiant shell. At Step 1, the interaction between the expansion of LMC 2 and the parent cloud of the complex might influence the initial star formation at N160. As we showed in our previous paper (Kim et al., 2016), the boundary between the western border of LMC 2 and the large-scale pattern of magnetic fields (red dashed line) is well-aligned in the eastern side of the molecular ridge. This may support the process of the expansion of LMC 2 and its triggering of star formation.

At Step 2, the OB star formation begins in the parent cloud of N160. The first-formed cluster, HS 385, is located at the northeastern region of N160, as denoted by a big blue star symbol in Step 3. We suggest that a shock wave driven by stellar radiation and stellar winds from HS 385 sequentially triggered the formation of OB stars through this parent cloud (yellow arrow in Step 3). Actually, most of the OB clusters in the N160 region are detected inside of the shell, as denoted by blue-colored stars in Figure 7 and in this illustration. In addition, their age distribution from Bica et al. (1996) indicated that HS 385 is older than other OB clusters in the N160 region, and the southwestern region of N160 contains the Herbig Ae/Be clusters, with their ages less than 3 Myr (Nakajima et al., 2005).

Step 4 illustrates the expansion of H II regions energized by the OB clusters within the shell, which passed through the south of N160 B+C and N160 A+D, and toward the GMCs of N159. The dominant direction of the magnetic field structure at the interior of the shell (red dotted line in Step 4) is coincided with that of the sequential formation process. The magnetic fields near the southern shell trace its boundary as shown in Figure 9. The U-shaped magnetic field structure in the south of N160 B+C and N160 A+D is bent toward the central region of N159. These features indicate that magnetic fields are associated with the expansion of the H II region in N160. It also implies that the path is propagated sequentially from N160 to the nearby GMC of N159. A similar propagation process is also suggested in the Scorpius$-$Centaurus association by Preibisch & Zinnecker (1999, 2007). They proposed a scenario for the sequentially triggered star formation and dissipation of molecular clouds in three subgroups of Scorpius-Centaurus association, by stellar winds and ionizing radiation from the OB stars therein.

Steps 5 and 6 illustrate the triggered star formation and the related magnetic fields in N159. According to Jones et al. (2005), formation of OB stars at the center of N159 pushed H II regions out to the surrounding molecular cloud, and the subsequent star formation then triggered at the rim of the expansion bubble (dotted circle in Step 6). The distribution of star formation rates from Galametz et al. (2013) supports this triggering process. The magnetic fields in the central part of N159 are distribute on the H II region that is considered to be the initial trigger site for N159. Subsequent triggering processes can also be traced by the magnetic fields, following the boundary of the triggered H II regions (sky-colored clouds in Step 6) located around the rim of the expansion bubble. We suggest that the dynamical star-forming activities and expansion of the H II bubble disturbed and rearranged the nascent magnetic field structure. Uniformly aligned magnetic fields at the southeast and the west of N159, which do not show any star-forming activities and H II regions, support the change in the magnetic field because of these effects.

6 Summary

We conducted near-infrared polarimetry for the stars in the N159/N160 star-forming complex in the LMC. We applied the NICER algorithm to select background stars for the study of the magnetic field in our observation field. The 36 YSO candidates and 2 HEBs were matched, and 12 of them were the sources that are highly polarized, likely by the dust in the circumstellar envelope rather than by the interstellar dust. We excluded them from the final catalog to study only the interstellar polarization in the N159/N160 complex. We newly verified nine YSO candidates using the color-color diagram for the sources matched with the $AKARI$ IRC LMC catalog. The 252, 277, and 89 sources in the N159 and N160 fields with sufficiently good polarization qualities represent dichroic polarization from the interstellar dust in this complex. As other studies (Nakajima et al., 2007; Kim et al., 2016) have suggested, wavelength dependence of polarization turned out to be weaker here than in the case of the Milky Way.

We visualized the structure of the magnetic fields in the N159 and N160 fields, using a polarization vector map projected on the H$\alpha$ image. Polarization vectors showed a complex distribution of the magnetic fields, indicating the interaction between star-forming activities and surrounding interstellar medium. The comparison between the polarization vectors and molecular cloud emissions showed that the magnetic fields are resulted from the different formation histories of N159 and N160. From the distribution of the magnetic field structures, we suggested the plausible scenario that the star formation is sequentially triggered from N160 to N159. We also proposed that ionizing radiation from OB clusters and the expanding H II bubbles in this complex probably affected the nascent magnetic field structure.

This work was supported by the National Research Foundation of Korea (NRF) grant No. 2008-0060544, funded by the Korean government (MSIP). We would like to thank Prof. Yasushi Nakajima for kindly providing comments that improved this paper. This paper uses observations performed at the South African Astronomical Observatory. This publication makes use of data products from the Two Micron All Sky Survey and observations with AKARI. The Two Micron All Sky Survey is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. The AKARI is a JAXA project with the participation of ESA. This research has made use of the NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.