2 Observations

A field of $\sim$ $2'\times1'$ centered on the main body of NGC1569 was observed for 20 hours with MERLIN (6 antennae) on Jan. 31 and Feb. 1, 1999. The observing frequency was 4.994GHz ( $\lambda = 6$cm) with a bandwidth of 15MHz in both circular polarizations. The data were taken in spectral-line mode (32$\times$1MHz channels). The QSOs 0552+398 (6.4Jy) and 0402+682 (0.18Jy) were used as flux density and phase calibrators.

NGC1569 was also observed with MERLIN (7 antennae, including the Lovell telescope) at 1.412GHz ( $\lambda = 21$cm) for 36 hours in Apr. (9, 10, 11th) and May (3rd) 1999, with a bandwidth of 15MHz in both polarizations. The passband and relative gain of the antennae were determined from observations of QSO 0552+398 (1.75Jy); the phases were determined from observations of QSO 0402+682 (0.15Jy).

Images were produced with the AIPS tasks IMAGR and deconvolved with CLEAN (Clark 1980). Table 1 summarizes the details of these observations. At 1.4GHz, the rms-noise ( $\sigma_{\rm n}$) in source-free fields is consistent with the expected thermal noise level; for unknown reasons, at 5GHz the rms-noise is some 40% too high as compared to other observations (see Tarchi et al. 2000). At 1.4GHz and 5GHz, respectively, sources weaker than $\sim$0.07 mJy/beam (=3 $\sigma_{\rm n}$) and $\sim$0.21 mJy/beam (=3 $\sigma_{\rm n}$) are therefore not detected.

 

 

Table 1: MERLIN observations of NGC1569; values for natural weighting.

Frequency	Beam	Beam at	rms-noise ($\sigma$$_{\rm n}$)
	HPBW	NGC1569a	[$\mu$Jy/beam]
1.4GHz	$0.21''\times0.19''$	$\rm 2.2\,pc\times2.0$pc	$\sim$25
5GHz	$0.05''\times0.05''$	$\rm0.5\,pc\times0.5$pc	$\sim$70
a For a distance of 2.2Mpc.

Since the Wardle telescope was not available for observations, the shortest baseline at 1.4GHz was 7.5km (Darnhall-Lovell telescopes) so that the array was not sensitive to extended structures larger than $\theta^{\rm 1.4\,GHz}_{\rm max} = 5.8''$ (equivalent to 70pc). At 5GHz the shortest baseline was 8.7km (Darnhall-Mark2 telescopes) so that, similarly, $\theta^{\rm 5\,GHz}_{\rm max}$ = 1.4'' (equivalent to 17pc). However, these fields are large enough for a search for RSNe, which appear point-like, and SNRs.

We compare the MERLIN observations with 20cm and 6cm VLA observations made on Sep. 24, 1982 (B-array: 20cm and 6cm) and Nov. 22, 1983 (A-array: 20cm) by van der Hulst et al. (2001; abbreviated vdHGI). The radio images obtained with the VLA were made with 1.18'' resolution (Gaussian beam) at both frequencies. The spectral indices ($\alpha$) given below are derived from the VLA measurements.

At the distance of 2.2Mpc, 1'' is equivalent to 11pc linear scale. The astrometric accuracy of the MERLIN observation is $\Delta$$\alpha$ = $\Delta$$\delta$ $\approx$ 0.01'', of the VLA observation $\Delta$$\alpha$= $\Delta$$\delta$ $\approx$ 0.05 to 0.1''.

We denote the MERLIN-detected sources by M, the VLA-detected soures (vdHGI) by VLA.

 

 

Table 2: 1.4GHz MERLIN-detected sources in NGC1569.

Source	RA(2000)	Dec(2000)	Peak Flux	Integr. Flux	Peak Fluxa	Spectral	Objectc
	[ $^{\rm h~ m~ s}$]	[ $\hbox{$^\circ$ }$ ' '']	[mJy/beam]	[mJy]	[mJy]	Index ($\alpha$)b
M-1	4 30 46.94	64 51 00.6	0.16 (0.02) (6$\sigma$$_{\rm n}$)	0.57 (0.20)	$1.353\pm0.039$	- $0.02\pm0.05$	Waller No.2
M-2	4 30 46.51	64 50 53.4	0.12 (0.02) (5$\sigma$$_{\rm n}$)	0.45 (0.25)	$0.848\pm0.015$	- $0.56\pm0.05$	non-thermal
M-3	4 30 47.02	64 51 06.7	0.10 (0.02) (4$\sigma$$_{\rm n}$)	0.10 (0.04)	$1.107\pm0.071$	- $0.58\pm0.10$	non-thermal
M-4	4 30 49.20	64 51 04.3	0.11 (0.02) (4 $\sigma_{\rm n}$)	0.11 (0.04)	$0.411\pm0.028$	$0.04\pm0.11$	Waller No.5
M-5	4 30 49.50	64 50 59.3	0.09 (0.02) (4 $\sigma_{\rm n}$)	0.09 (0.04)	$0.232\pm0.016$	- $0.02\pm0.13$	thermal
M-6	4 30 54.13	64 50 43.5	0.19 (0.02) (8 $\sigma_{\rm n}$)	1.54 (03.0)	$1.888\pm0.027$	- $0.56\pm0.03$	non-thermal
a VLA observation (van der Hulst et al. 2001), b $S \propto \nu^{{\alpha}}$, 1.4-5 GHz, c H II region number from Waller (1991);
thermal source.

Figure 2: 1.4GHz MERLIN source M-1 (Table 2). The contour interval is 20$\mu$Jy/beam with the first negative and positive contour at 50 $\mu$Jy/beam (=2$\sigma$$_{\rm n}$). The synthesized beam is shown in the lower right corner. The crosses indicate the center position of the star clusters No. 6, at $\sim$3pc to the West, and No. 7, at $\sim$12pc to the South (Table 4). The tentative sources M-b,c,d (Table 3) are indicated.

Figure 3: 1.4GHz MERLIN source M-2 (Table 2). The contour interval is 20$\mu$Jy/beam with the first negative and positive contour at 50 $\mu$Jy/beam (=2$\sigma$$_{\rm n}$). The synthesized beam is shown in the lower right corner. The cross indicates the center position of the star cluster No. 5, $\sim$15pc to the North-East (Table 4).

Figure 4: 1.4GHz MERLIN source M-3 (Table 2). The contour interval is 20$\mu$Jy/beam with the first negative and positive contour at 50 $\mu$Jy/beam (=2$\sigma$$_{\rm n}$). The synthesized beam is shown in the lower right corner. The cross indicates the center position of the star cluster No. 8, $\sim$2pc to the South-East (Table 4).

Figure 5: 1.4GHz MERLIN source M-4 (Table 2). The contour interval is 20$\mu$Jy/beam with the first negative and positive contour at 50 $\mu$Jy/beam (=2$\sigma$$_{\rm n}$). The synthesized beam is shown in the lower right corner.

Figure 6: 1.4GHz MERLIN source M-5 (Table 2). The contour interval is 20$\mu$Jy/beam with the first negative and positive contour at 50 $\mu$Jy/beam (=2$\sigma$$_{\rm n}$). The synthesized beam is shown in the lower right corner.