2 Observations and data reduction

Figure: From left to right: (a) narrow-band filter image of PN Abell 48 in H$\alpha$ obtained from the SuperCOSMOS Sky H$\alpha$ Survey (SHS; Parker et al., 2005). The rectangles correspond the $25 \times 38$-arcsec${}^{2}$ IFU: 1 (blue) and 2 (red) taken in 2010 April and 2012 August, respectively. Image dimension is $60 \times 60$ arcsec${}^{2}$. (b) Extinction $c({\rm H}\beta )$ map of Abell48 calculated from the flux ratio H$\alpha$/H$\beta$ from fields. Black contour lines show the distribution of the narrow-band emission of H$\alpha$ in arbitrary unit obtained from the SHS. North is up and east is towards the left-hand side.

(a) (b)

Integral field spectra listed in Table 1 were obtained in 2010 and 2012 with the 2.3-m ANU telescope using the Wide Field Spectrograph (WiFeS; Dopita et al., 2010; Dopita et al., 2007). The observations were done with a spectral resolution of $R\sim 7000$ in the 441.5-707.0 nm range in 2010 and $R\sim 3000$ in the 329.5-932.6 nm range in 2012. The WiFeS has a field-of-view of $25\hbox{$^{\prime\prime}$}\times 38\hbox{$^{\prime\prime}$}$ and each spatial resolution element of $1$$\mbox{$.\!\!^{\prime\prime}$}$$0\times0$$\mbox{$.\!\!^{\prime\prime}$}$$5$ (or $1\hbox{$^{\prime\prime}$}\times 1\hbox{$^{\prime\prime}$}$). The spectral resolution of $R\,(=\lambda/\Delta\lambda)\sim3000$ and $R\sim 7000$ corresponds to a full width at half-maximum (FWHM) of $\sim100$ and 45 kms${}^{-1}$, respectively. We used the classical data accumulation mode, so a suitable sky window has been selected from the science data for the sky subtraction purpose.

Table 2: Observed and dereddened relative line fluxes of the PN Abell 48, on a scale where H$\beta =100$. Uncertain and very uncertain values are followed by `:' and `::', respectively. The symbol `*' denotes blended emission lines.

$\lambda_{\rm lab}$(Å)	ID	Mult	$F(\lambda)$	$I(\lambda)$	Err(%)
$3726.03$	[O II]	F1	$20.72$:	$128.96$:	$25.7$
$3728.82$	[O II]	F1	*	*	*
$3868.75$	[Ne III]	F1	$7.52$	$38.96$	$9.4$
$4340.47$	HI5-2	H5	$21.97$	$54.28$:	$17.4$
$4471.50$	He I	V14	$3.76$:	$7.42$:	$12.0$
$4861.33$	HI4-2	H4	$100.00$	$100.00$	$6.2$
$4958.91$	[O III]	F1	$117.78$	$99.28$	$5.3$
$5006.84$	[O III]	F1	$411.98$	$319.35$	$5.2$
$5754.60$	[N II]	F3	$1.73$::	$0.43$::	$40.8$
$5875.66$	He I	V11	$87.70$	$18.97$	$5.3$
$6312.10$	[S III]	F3	$4.47$::	$0.60$::	$46.9$
$6461.95$	C II	V17.04	$3.36$:	$0.38$:	$26.2$
$6548.10$	[N II]	F1	$252.25$	$26.09$	$5.2$
$6562.77$	HI3-2	H3	$2806.94$	$286.00$	$5.1$
$6583.50$	[N II]	F1	$874.83$	$87.28$	$5.3$
$6678.16$	He I	V46	$55.90$	$5.07$	$5.3$
$6716.44$	[S II]	F2	$85.16$	$7.44$	$5.1$
$6730.82$	[S II]	F2	$92.67$	$7.99$	$5.5$
$7135.80$	[Ar III]	F1	$183.86$	$10.88$	$5.2$
$7236.42$	C II	V3	$29.96$:	$1.63$:	$20.7$
$7281.35$	He I	V45	$11.08$::	$0.58$::	$41.3$
$7751.43$	[Ar III]	F1	$111.83$::	$4.00$::	$34.5$
$9068.60$	[S III]	F1	$1236.22$	$19.08$	$5.3$
$c({\rm H}\beta )$				$3.10 \pm 0.04$
H$\beta$/10$^{-13}$ $\frac{\rm erg}{{\rm cm}^2{\rm s}}$		$1.076 \pm 0.067$		$1354.6 \pm 154.2$

The positions observed on the PN are shown in Fig. 1(a). The centre of the IFU was placed in two different positions in 2010 and 2012. The exposure time of 20min yields a signal-to-noise ratio of $S/N \gtrsim 10$ for the [O III] emission line. Multiple spectroscopic standard stars were observed for the flux calibration purposes, notably Feige110 and EG274. As usual, series of bias, flat-field frames, arc lamp exposures, and wire frames were acquired for data reduction, flat-fielding, wavelength calibration and spatial calibration.

Data reductions were carried out using the IRAF pipeline WIFES (version 2.0; 2011 Nov 21).² The reduction involves three main tasks: WFTABLE, WFCAL and WFREDUCE. The IRAF task WFTABLE converts the raw data files with the single-extension Flexible Image Transport System (FITS) file format to the Multi-Extension FITS file format, edits FITS file key headers, and makes file lists for reduction purposes. The IRAF task WFCAL extracts calibration solutions, namely the master bias, the master flat-field frame (from QI lamp exposures), the wavelength calibration (from Ne-Ar or Cu-Ar arc exposures and reference arc) and the spatial calibration (from wire frames). The IRAF task WFREDUCE applies the calibration solutions to science data, subtracts sky spectra, corrects for differential atmospheric refraction, and applies the flux calibration using observations of spectrophotometric standard stars.

Figure: Maps of the PN Abell 48 in H$\alpha$ $\lambda$6563 Å (top) and $[$N II$]$ $\lambda$6584 Å (bottom) from the IFU ( ${\rm PA}=0^{\circ }$) taken in 2010 April. From left to right: spatial distribution maps of flux intensity, continuum, LSR velocity and velocity dispersion. Flux unit is in $10^{-15}$ ergs${}^{-1}$cm${}^{-2}$spaxel${}^{-1}$, continuum in $10^{-15}$ ergs${}^{-1}$cm${}^{-2}$Å${}^{-1}$spaxel${}^{-1}$, and velocities in kms${}^{-1}$. North is up and east is towards the left-hand side. The white contour lines show the distribution of the narrow-band emission of H$\alpha$ in arbitrary unit obtained from the SHS.

A complete list of observed emission lines and their flux intensities are given in Table 2 on a scale where H$\beta$ = 100. All fluxes were corrected for reddening using $I(\lambda)_{\rm corr}=F(\lambda)_{\rm obs}10^{c({\rm H}\beta)[1+f(\lambda)]}.$ The logarithmic $c({\rm H}\beta )$ value of the interstellar extinction for the case B recombination ( $T_{\rm e}=10\,000$K and $N_{\rm e}=1000$cm$^{-3}$; Storey & Hummer, 1995) has been obtained from the H$\alpha$ and H$\beta$ Balmer fluxes. We used the Galactic extinction law $f(\lambda)$ of Howarth (1983) for $R_V = A(V)/E(B-V)=3.1$, and normalized such that $f({\rm H}\beta)=0$. We obtained an extinction of $c({\rm H}\beta)=3.1$ for the total fluxes (see Table 2). Our derived nebular extinction is in excellent agreement with the value derived by Todt et al. (2013) from the stellar spectral energy (SED). The same method was applied to create $c({\rm H}\beta )$ maps using the flux ratio H$\alpha$/H$\beta$, as shown in Fig. 1(b). Assuming that the foreground interstellar extinction is uniformly distributed over the nebula, an inhomogeneous extinction map may be related to some internal dust contributions. As seen, the extinction map of Abell48 depicts that the shell is brighter than other regions, and it may contain the asymptotic giant branch (AGB) dust remnants.