Conclusions
Introduction
Infrared Galaxy Colors Vs. Morphological Type
Change of Infrared Galaxy Colors Vs. Redshift
The 2MASS Galaxy Color-Color Plot For Hercules
Further Work
Footnotes
Acknowledgements
References
The main conclusions are as follows:
G. Neugebauer noticed that the 2MASS Galaxy J-H / H-K color-color plot showed a noticeable elongation that could be caused by an increased scatter in H band fluxes, presumably caused by airglow.
The color-color plot GXN looked at included low SNR sources and, as with most data processed so far, had not had final calibration tweaks applied. The 2MASS Galaxy J-H / H-K color-color plot1, with only higher-SNR sources and final calibration tweaks, looks much better, but is distinct from the point source color-color plot.
Spurred by the investigation into the effects of noise on the color-color plot, we have analyzed the 2MASS Galaxy J-H / H-K color-color plot and found that it is a more powerful tool than any of us had expected.
Surprisingly, there is no paper in the literature that we could find (or that John Huchra knows of) that presents the location of galaxies of different morphological types in the J-H, H-K color-color plot. We found exactly two references in a quick search.
Impey et al 1986 give the reddening and K-corrected colors of 47 optically-selected bright elliptical galaxies observed with a small (5.8 - 7.5") beam, which represents values of log A/D(0) of -2 to -1 for these galaxies. (log A/D(0) is the log of the ratio of the beam aperture to the effective diameter of a galaxy. To be comparable to the 2MASS measurement at an isophote of 20 mag per square arcsecond, the number should close to 0.)
Frogel et al 1985 give the reddening and K-corrected colors of 19 Sc galaxies observed with a 6.6" aperture which also represents values of log A/D(0) of -2 to -1 for these galaxies.
To supplement those data, John Huchra has supplied us with data taken by Aaronson, Huchra and Mould of tbs galaxies of many different morphological types. (J. Elias reports that some of these data are from Aaronson's PhD thesis, which was never entirely published.) Their JHK observations have much larger apertures which are similar in size to the 2mass effective apertures. About 90 of those galaxies have been used so far to check two important effects:
Fig. 1 shows the color-color diagram for the galaxies found in the literature. The spirals form the extended diagonal line at lower right, and the ellipticals group to the upper left. Note the quite tight concentration compared to the observed 2MASS plot.
Fig. 2 shows the color-color diagram for the literature galaxies plus the Huchra galaxy subsample. Most of the Huchra sample falls in the same area defined by the two published samples of elliptical and Sc galaxies. The Huchra data also defines one new region of the color-color plot, which extends the spirals to the upper right. A check of NED shows that almost all of those galaxies with large H-K colors are Seyfert galaxies.
As pointed out by J. Elias, one needs to be cautious in interpreting shifts in colors at the level of 0.05 mag or so, because the J filters are slightly different for Aaronson's data set and because 2MASS uses Ks and not K. Fortunately, any shift in J is not apparent in that plot. Also, the conclusions below are derived from shifts of several tenths of mag and hence are little affected by shifts of 0.05 mag or less.
We conclude that galaxies have a well-defined, restricted location of J-H / H-K colors.
The very definition of redshift is the percentage change of observed wavelength versus rest-frame wavelength for emission from a source, and hence it is not surprising that the color of a galaxy will change with redshift since one is effectively observing a different wavelength region. This change of colors due to redshift is called the K-correction, and has nothing at all to do with the K band. The 2MASS galaxies are expected to have fairly small redshifts up to z ~ 0.2 and hence we can use a simple linear change of the colors with redshift.
For elliptical galaxies, we use the standard change of the colors of elliptical galaxies with redshift. For spiral galaxies, we use the linear trend from z=0.0 to z=0.2 given for the K-correction for an Sab galaxy. The latter plot was supplied by Roc Cutri using data from Brian McLeod. Those data are for a model for the spectral energy distribution of an Sab-type galaxy, redshifted with no luminosity evolution. The highest redshift point on that plot is z=3.9.
The two K-corrections are nearly identical for the H-K color, but have opposite signs for the weak J-H color dependence on redshift.
The data used for this analysis came from a series of 6° scans over the Hercules cluster of galaxies. The scans covered declinations of 14-20° and the Hercules cluster is located near 17.75°. Hence it is expected that many, but not all, of the galaxies found by 2MASS will be physically in the Hercules cluster.
The 2MASS Galaxy Color-Color Plot for Hercules galaxies with high SNR shows a noticeable concentration of points near a mean color of J-H = 0.70 and H-K = 0.32 mag, which is seen best in the right hand plot. Due to the large number of 2MASS points in that plot, we have replaced them with a schematic plot, which shows the reference galaxies mentioned above plus the following:
The K-corrected value of the concentration color point was obtained using a redshift of 0.034 and the Sab value for the K-correction. Applying the elliptical galaxy K-correction would give nearly the same point.
We draw the following conclusions from the above:
These conclusions allow us to predict that a sky plot of sources selected to be near the concentration of points in the color-color plot should show a heavy concentration to the location of the Hercules cluster. Also, a sky plot of sources selected with large values of H-K color should not show the location of the Hercules cluster.
These two predictions were made before any of us had looked at a sky plot of these sources. Because the 2MASS scans cover only 0.14° in R.A. and 6° of declination, the only sensible sky plot to make is a histogram of sources by declination.
The two samples were defined as follows:
The histogram of sources by declination seems to bear out the two predictions. The "color cluster" sample has the largest number of sources exactly in the bin containing the declination of the Hercules cluster at 17.75°. Although there are many fewer sources in the "high H-K" sample, it does not seem to have a strong peak at the declination of the Hercules cluster. However, it is difficult to actually establish this difference at a high level of significance since dupes are present in that plot. The actual number of galaxies in each bin is lower by a factor of up to 5 in most but not all bins.
We have compared the dupe-free numbers in the 16-17° bin. Only 20% of the "color cluster" sample is found in that bin, leading to the prediction that 2.6 of the 13 independent galaxies in the "high H-K" sample should be found in that bin, whereas 6 are present. This result by itself is a 2-sigma result, but there are many distributions caused by chance that would have caused us to make a similar calculation.
We plan on doing a simulation which will clarify the expected range of redshift and percentage of spirals and ellipticals by making different cuts in the 2MASS color-color diagram.
1 This plot is for Hercules data and includes only sources with color errors less than 0.15 mag. The upper discrepant point is caused by a neighboring source which ate most of the H flux and some of the K flux. The discrepant point at the lower right may have a faint red star contributing some flux at K, or may just be the effect of noise on a galaxy with already red colors.
We thank J. Huchra for urging us to investigate the K-corrected galaxy colors versus morphological type and for supplying data used to analyze it. We thank J. Huchra and S. Schneider for other comments on this memo.
Aaronson, Huchra and Mould, private communication from J. Huchra.
Frogel 1985 Ap.J. 298:528.
Impey et al 1986 Ap.J. 309:572.
McLeod, private communication from R. Cutri.
