Photometric calibration

Photometric calibration is necessary to transfer the observed flux to a standard system of the observed galaxies. Usually the zeropoint is determined with the formula

$$m_{fit}\ =\ m_{obs}\ + a\cdot X\ +\ b\cdot (J-K)\ +\ \textrm{zeropoint}\ ,$$ (1)

where a$\cdot X$ is the extinction term with $X$ the airmass and $b\cdot (J-K)$ the colour term. Normally standard stars are observed to obtain the zeropoint, but because no standard stars had been observed in the CA run we had to put everything into one zeropoint.
The photometric calibration was done by doing aperture photometry of all known stars on the frame of the galaxy in the 2MASS catalogue⁷ of point sources. For matching the fields we used the Aladin Sky Atlas⁸. We applied three different methods to determine the zeropoints. The 2MASS catalogue provides a list of stars and their magnitudes in the JHK bands which can be selected for the region around our galaxies on the 2MASS images. The 2MASS images are already provided with a zeropoint, which in addition allows us to use their stars to determine the zeropoint of our images independently. The provided stars were matched with stars from our own image and the magnitude read. For the selected stars aperture photometry was done to determine the flux of the stars in our own images. Galaxies (still in the point source catalogue), saturated and crowded regions, or stars too close to the galaxy or the edge of the frame, were not taken into account. By comparing the 2MASS magnitudes and the fluxes the mean zeropoint for each image was calculated. Another test was done by doing also aperture photometry on the stars in the 2MASS image and determining the stellar magnitude manually.

TABLE 4

PHOTOMETRY CA SET

Galaxy	Filter	Zeropoint$^{a}_{CM}$	Zeropoint$^{b}_{MM}$	Zeropoint$^{c}_{B}$	Night
		[mag]	[mag]	[mag]
(1)	(2)	(3)	(4)	(5)	(6)
IC 3322A	J	25.21 $\pm$ 0.09	25.18 $\pm$ 0.07	25.33	2
IC 3322A	K'	23.56 $\pm$ 0.05	23.57 $\pm$ 0.08	23.60	2
NGC 2424	J	25.39 $\pm$ 0.08	25.39 $\pm$ 0.14	25.48	3
NGC 2591	K'	23.43 $\pm$ 0.12	23.50 $\pm$ 0.19	23.48	5
NGC 5290	J	25.09 $\pm$ 0.06	25.09 $\pm$ 0.11	25.20	1
NGC 5290	K'	23.49 $\pm$ 0.09	23.50 $\pm$ 0.11	23.49	1
NGC 5348	J	24.99 $\pm$ 0.06	25.29 $\pm$ 0.10	25.23	5
NGC 5981	J	25.24 $\pm$ 0.13	25.31 $\pm$ 0.08	25.43	3
Notes: (1) Galaxy. (2) Filter. (3) Zeropoint by aperture photometry on Standard Stars. (4) Zeropoint by aperture photometry on 2MASS stars with $C$atalogue $M$agnitudes. (4) Zeropoint by aperture photometry on 2MASS stars with $M$agnitudes measured $M$anually the 2MASS images. (5) Zeropoint by aperture photometry on the galaxy bulges in both images. (6) Observation night.

As we were uncertain of the apertures 2MASS used, aperture photometry was done on the bulge for comparison. Four circular apertures were made around the center of the galaxy with the largest circle including the whole bulge. Within each circle the flux was calculated and compared to similar circles on the galaxy in the 2MASS image, resulting in a value for the zeropoint. Making larger circles included too much background that influenced the zeropoint value. The aperture photometry on the bulge was done because this had also been done in the 2MASS catalogue, but comparison with those values were not directly possible because they used ellipse shaped areas for their photometry and the exact shape of the ellipse was not given. Thus we used the bulge photometry to have another indication of the zeropoint value when comparing them to the other methods. The results of the different methods are shown in Table 4.
The zeropoint values we found with the 2MASS magnitudes were similar zeropoints we derived by manually determining the magnitudes. However, the stellar magnitudes from 2MASS are determined in a way we could not fully recover (see Skrutskie et al. 2006), while the 2MASS images available on their webserver have been changed in pixel resolution. This makes our manual determination of the magnitudes more uncertain and inaccurate and creating in most cases larger standard deviations, even when we removed all stars that had a magnitude error larger than 0.15.
The UKIRT set did have standard stars observed (taken from Persson et al. 1998). We had too few standard stars to disentangle an extinction and colour term for the zeropoints. The zeropoints showed no dependency airmass of the standard star, so we decided to combine all factors into the zeropoint. We took the average of each set of images. For each night the mean zeropoint for the J- and K'-band were calculated to provide a zeropoint for all respective galaxies (see Table 5).

TABLE 5

STANDARD STAR PHOTOMETRY UKIRT SET

Star	Filter	Airmass	Zeropoint$^{a}$	Night
P9105	J	1.11	26.08 $\pm$ 0.01	1
P9105	J	1.03	26.06 $\pm$ 0.01	1
P9122	J	1.20	26.02 $\pm$ 0.02	1
P9138	J	1.12	26.06 $\pm$ 0.01	1
P9148	J	1.05	26.09 $\pm$ 0.02	1
mean	J		26.06 $\pm$ 0.03	1
P9105	K'	1.10	25.58 $\pm$ 0.02	1
P9105	K'	1.03	25.55 $\pm$ 0.01	1
P9122	K'	1.20	25.52 $\pm$ 0.01	1
P9138	K'	1.11	25.53 $\pm$ 0.02	1
P9148	K'	1.05	25.59 $\pm$ 0.02	1
mean	K'		25.55 $\pm$ 0.03	1
P9105	J	1.29	26.14 $\pm$ 0.02	2
P9105	J	1.27	26.11 $\pm$ 0.02	2
P9105	J	1.06	26.08 $\pm$ 0.02	2
P9122	J	1.36	26.02 $\pm$ 0.03	2
P9122	J	1.18	26.04 $\pm$ 0.02	2
P9138	J	1.20	26.07 $\pm$ 0.02	2
mean	J		26.08 $\pm$ 0.04	2
P9105	K'	1.26	25.64 $\pm$ 0.07	2
P9105	K'	1.06	25.61 $\pm$ 0.06	2
P9122	K'	1.35	25.57 $\pm$ 0.05	2
P9122	K'	1.18	25.58 $\pm$ 0.05	2
P9138	K'	1.21	25.60 $\pm$ 0.06	2
mean	K'		25.60 $\pm$ 0.03	2
Notes: $^{a}$ Zeropoints in magnitudes.

To test the reliability of the photometric calibration of the CA set via 2MASS in comparison with the standard stars, we compared the 2MASS calibration also for the UKIRT set (see Table 6). As the image frames of the UKIRT set are small and don't encompass the whole galaxy, a comparison with the 2MASS catalogue was done only where possible. Larger deviations are probably caused by a lack of sufficient stars we could use. Aperture photometry was done on the bulge, resulting in comparable values with the standard star zeropoints, except for UGC 3186, which is probably caused by it being very faint and small.
To obtain more external comparisons for our photometry, we searched in the literature. Unfortunately we did not find any aperture photometry in the bands we used for any of our galaxies and only found one contour map. Comparing contours provides only a crude comparison but is useful to see if the zeropoints deviate by less than 0.2 magnitude. The used contour map was for IC 2531 by Kuchinski et al. (1996). As an other internal comparison we compared the contour maps of NGC 2424, a galaxy that is observed in both sets in the same band. For both maps the comparison shows that the error is smaller than 0.2, showing that our zeropoints are sufficiently accurate.

TABLE 6

PHOTOMETRY UKIRT SET

Galaxy	Filter	Zeropoint$_{SS}$	Zeropoint$_{CM}$	Zeropoint$_{MM}$	Zeropoint$_{B}$	Night
		[mag]	[mag]	[mag]	[mag]
(1)	(2)	(3)	(4)	(5)	(6)	(7)
IC 2531	J	26.06 $\pm$ 0.03	25.97 $\pm$ 0.09	26.15 $\pm$ 0.15	26.04	1
IC 2531	K'	25.55 $\pm$ 0.03	25.49 $\pm$ 0.26	25.54 $\pm$ 0.24	25.54	1
NGC 0973	J	26.06 $\pm$ 0.03	26.07 $\pm$ 0.13	26.24 $\pm$ 0.07	26.08	1
NGC 0973	K'	25.60 $\pm$ 0.03	25.53 $\pm$ 0.06	25.80 $\pm$ 0.24	25.59	2
NGC 1886	J	26.06 $\pm$ 0.03			26.08	1
NGC 1886	K'	25.55 $\pm$ 0.03			25.61	1
NGC 2424	J	26.08 $\pm$ 0.04			26.13	2
NGC 2424	K'	25.60 $\pm$ 0.03			25.62	2
UGC 3186	J	26.08 $\pm$ 0.04			27.84	2
UGC 3186	K'	25.60 $\pm$ 0.03			25.36	2
UGC 4277	J	26.06 $\pm$ 0.03	26.01 $\pm$ 0.15	26.13 $\pm$ 0.05	26.06	1
UGC 4277	K'	25.60 $\pm$ 0.03	25.38 $\pm$ 0.18	25.96 $\pm$ 0.40	25.51	2
Notes: (1) Galaxy. (2) Filter. (3) Zeropoint by aperture photometry on Standard Stars. (4) Zeropoint by aperture photometry on 2MASS stars with $C$atalogue $M$agnitudes. (5) Zeropoint by aperture photometry on 2MASS stars with $M$agnitudes measuered $M$anually on the 2MASS images. (6) Zeropoint by aperture photometry on the galaxy bulges in both images. (7) Observation night.