5.7 Comparison with Single Aperture Photometry

The 2MASS project was fundamentally designed to image and extract the positions and fluxes of stars and small extended objects. Larger objects are more difficult to quantify due to the scale of background variations¨sky removal ultimately limits the ability to measure the extended flux of a large object. The Large Galaxy Atlas represents an attempt to bridge the gap between small galaxies, well characterized by the 2MASS data reduction pipeline, and the largest galaxies known. We have made every effort to carefully remove the backgrounds and measure the basic properties of the largest galaxies. However, due to the unique nature of the data and final products, comparisons that can be made at large scales with independent measurements are few and far between.

We are able to make modest comparisons with single-aperture measurements carried out at considerably smaller scales. These comparisons validate the basic background removal and photometric methods of the ATLAS, but do not address the large-scale (>tens of arcminutes) measurements. The advent of large format NIR array camera will make this comparison possible in the future.

The first set of comparisons are made with single-aperture (circular) JHK photometry as compiled in de Vaucouleurs & Longo (1988), coming from several sources including seminal work of de Vaucouleurs, Frogel, Aaronson and collaborators. We then focus on the H-band measurements of Aaronson, Mould and Huchra (1980). Note: we do not correct for reddening or inclination effects, and we do not correct for the different photometric systems--both of which should play at only the few percent level.

Table 3 lists the JHK comparison between single-aperture measurements (compiled in de Vaucouleurs & Longo 1988) and the equivalent measurement from the 2MASS Large Galaxy Atlas image. Columns (1) and (2) give the common name and classification; (3) circular aperture in arcmin; (4) J-band photometry from 2MASS; (5) difference between 2MASS and single aperture J measurements; (6) H-band photometry from 2MASS; (7) difference between 2MASS and single aperture H measurements; (8) Ks-band photometry from 2MASS; and (9) difference between 2MASS and single aperture K measurements. The agreement between 2MASS and aperture measurements is generally quite good, with differences less than a few percent for the 2MASS bands. There are three significant outliers: M33, M110 and NGC 2403, whose aperture measurements are some 30% fainter compared to 2MASS. These galaxies are all very large and late-type (we claim even M110, see Section 5.5), with significant low surface brightness emission extended far beyond the relatively tiny apertures. It is quite possible that the single-aperture measurements have inadvertently over-subtracted the "sky" by subtracting the galaxy from itself (via the beam-switch or "chop" measurement). There may be other effects related to stellar contamination and differing aperture coordinate positions (see below). To be fair, the 2MASS mosaics may also by systematically too bright due to excess airglow emission.

Aaronson, Mould and Huchra (1980; hereafter AMH) in a series of studies measured the H-band properties of M31, M33 and the M81/M101 groups. Their data consisted of single-channel photometric measurements with different sized circular apertures. Here we match their largest circular aperture (which is still small compared to the isophotal size of the galaxies) to make a direct comparison between flux measurements (note that there is some redundancy with the H measurements in Table 3). We do not correct for any instrumental differences between the 2MASS and CIT systems (see Frogel et al 1978), which are probably less than a few percent different (Carpenter 2001). For additional comparison purposes, we include the B-band characteristic diameter and circular aperture measure from AMH and the 2MASS total H-band mag. One should keep in mind that we are comparing single-channel photometry with imaging photometry: AMH do not remove foreground stars, while 2MASS does; and hence, we would expect AMH to be systematically brighter than 2MASS, assuming the backgrounds are properly removed, although this difference should vanish for the brightest galaxies (and other effects, such as redundant background subtraction may in the end dominate; see above). The results are given in Table 4. Columns (1-2) are the name and morphology, (3) circular aperture in arcmin, (4) AMH integrated H-band mag, (5) 2MASS integrated H-mag, (6) difference between AMH and 2MASS, (7) from the RC2, the final reduced B-band 25 mag arcsec-2 diameter, (8) the diameter corresponding to log(D-0.5/D1) = -0.5, and (9-10) the 2MASS "total" diameter and integrated H-band mag. AMH quote a nominal photometric error of 3%, although it would not be surprising if the uncertainties are in fact higher due to the persistent H-band airglow fluctuations.

The results show good agreement for the largest galaxies, M31 (5%), M33 (20%), M81 (1%) and NGC 253 (2%). Taken at face value and combined with the results of Table 3, we conclude that the 2MASS mosaics and subsequent measurements of the largest (and most difficult to construct) galaxies are valid and the methods robust. An important caveat is that we are not addressing the integrated flux at the lowest surface brightness levels, but instead the inner regions of these large galaxies (e.g., the M31 AMH aperture is considerably smaller than the 2MASS isophotal or total elliptical apertures).

The fainter galaxies, however, show much larger variations (ranging from 4 to 50%). These galaxies are not only faint in integrated flux but also much fainter in surface brightness¨all are extreme late-type galaxies. Inspection of the images reveals the complexity of measuring late and Magellanic-type galaxies: they are nearly invisible in the 2MASS H-band images, and furthermore, have poorly defined nuclei (or none at all, see NGC 2366). Hence, the comparison between AMH and 2MASS must factor in both formal S/N differences as well as coordinate position mis-alignments. Moreover as noted above, the 2MASS measurements include removal of foreground stars, while AMH aperture photometry does not (consistent with AMH being, for the most part, systematically brighter than 2MASS). Star removal is relatively important for these faint low surface brightness galaxies.


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