2MASS Galaxy C o l o r s : Hercules Cluster

T. Jarrett, IPAC
T. Chester, IPAC
J. Huchra, CfA
S. Schneider, UMASS
(980331)

to be presented at the June 1998 AAS in San Diego


The Hercules cluster is a nearby (z = 0.03689) gravitationally bound cluster of galaxies. The 2MASS survey observed a large fraction of the cluster multiple times. These data allow a comprehesive test of the 2MASS galaxy photometry and detection completeness and reliability. A detailed description and analysis of these data was given in GALWORKS Performance on the Spiral-Rich Cluster Hercules.

More recently we have compared the observed 2MASS galaxy colors with field galaxies at a variety of redshifts. See the document Analysis of the 2MASS Galaxy Color-Color Plot For Hercules . The main conclusions from this work are reproduced here:

In this document we present updated color-color plots for the Hercules galaxies and we include redshift data to demonstrate that 2MASS can -- in principle -- be used to identify new clusters of galaxies using only their near-infrared colors. This work will be presented at the June 1998 AAS in San Diego.

One 2MASS scan (6 degree X 8.5 arcmin) passing through the Hercules cluster was observed 5 times in order to address photometric repeatibility and C & R issues. The data were observed under good seeing and photometric conditions in May of 1997.

The cluster is rich with spiral type galaxies (it looks quite different from the Coma cluster, for example), which can be seen in the gif image below showing a small piece of one 2MASS scan passing through the core of Hercules. The image shows a 3-color realization of the data (blue = J band, green = H band, and red = Ks band). The scan fraction is broken into three pieces, each side by side (with slight overlap). The width of one scan is about 8.5 arcmin.

For a bigger (width-wise) picture of the cluster, we similarly constructed a 3-color image of the cluster using 2MASS protocamera data (circa 1995), which comprises the entire cluster. The gif below shows the cluster in a 35X35 arcmin image. Note that there is a slight registration differential between the three bands (the reason is that the protocamera was a single-channel instrument; each band was taken at different times).

2MASS Galaxy Photometry: What/How we do it, and Sensitivity Limits

Galaxy photometry is anything but simple. Given the wide range in complexity & extent a galaxy may have in its sky-projected profile, there is no 'ideal' aperture suited toward galaxy photometry. Moreover, historically the apparent brightness of a galaxy has been measured using a number of different methods, leading to a colorful mixture of isophotal fluxes, fixed aperture fluxes, complex metric fluxes (e.g., "petrosian", "kron"), pseudo "total" mags (e.g., via exponential extrapolation) and total mags (using a large aperture), each having circular and elliptical variations. In keeping with this proud tradition, the 2MASS project will perform a plethora of apeture photometry operations.

2MASS will generate integrated mags for galaxies using two-dimensional circular/elliptical apertures including: a series of fixed circular apertures, isophotal apertures (set at the 20 Ks mag per sq. arcsec isophote, and at 21 J mag per sq. arcsec isophote), a "Kron" aperture (2.5 * first moment radius), and an extrapolation aperture (using an exponential fit to the outer radial profile). This translates to a few hundred measurements per source. But, rather than bog down with a mountain of mags, we focus upon two primary varieties: Ks-band fiducial isophotal (elliptical) and fixed circular, radius = 7 arc seconds. The latter is too small for nearby bright galaxies but is sufficient for most of the galaxies that will be observed by 2MASS (Ks > 13) and is well suited to the geometry of galaxies near the sensitivity limit (that is to say, most small galaxies look round in the near-infrared, due to the high surface brightness spheroidal bulge that dominates the 2-micron light from galaxies).

The level-1 specifications for 2MASS galaxy photometry can be viewed in SURVEY LEVEL 1 REQUIREMENTS . In a nutshell, we would like to achieve at least 10% photometry for isophotal H-band mags as faint as 13.8. For J and Ks, this translates to ~13.2 and ~14.3, respectively.

Using the repeated observations of the Hercules cluster (5 in total), we can accurately characterize the quality of the photometry that 2MASS is to generate -- the repeatibility of the Hercules photometry gives a direct measure of the 'internal' accuracy of the 2MASS photometry. The plots below show the repeatibility of Hercules galaxies using both isophotal and fixed circular radius = 7" apertures. We have performed a detailed analysis of the photometric error tree and further explored the uncertainties inherent to our isophotal photometry. See Analysis of Photometric Noises for 2MASS and Error Analysis for Circular Isophotal Magnitudes and Error Tree for 2MASS Galaxy Photometry .

Photometric Repeatibility: Isophotal Aperture
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note: small magenta points demark "expected" or measured mag uncertainty (dominated by sky noise) 		Photometric Repeatibility: Circular radius = 7"
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It is evident that we match or exceed the level-1 requirements for galaxy photometry based upon our internal repeatibility. This assumes that our external errors (zero point calibration, color terms, etc) are small in comparison to the internal errors. It is also clear that using a small circular radius is the way to go for most analysis work involving integrated flux (unless large galaxies are the main focus).

Field Galaxy and Hercules Cluster (J-H) vs. (H-Ks) Colors

The data come from a series of 6° scans over the Hercules cluster of galaxies, with coverage between declinations of 14-20°. The Hercules cluster is located near 17.75°. Hence it is expected that most of the galaxies found by 2MASS will be physically in the Hercules cluster.

The J-H vs. H-Ks color plot for Hercules galaxies with high SNR (delta mag < 0.20) shows a noticeable concentration of points near a mean color of J-H = 0.70 and H-Ks = 0.32 mag (see upper two panels of left plot below). This concentration demarks the location of the Hercules cluster in the JHKs color space. To demonstrate this, we have applied K-corrections to the colors assuming an average redshift of 0.038 and K-corrections corresponding to SAB type galaxies with no evolution (from models Bruzual A., G., & Charlot, S. 1993, ApJ, 405, 538, and McLeod, B. A., & Rieke, M. J. 1995, ApJ, 454, 611; tables provided by Roc Cutri) and K-corrections corresponding to ellipticals with passive evolution (using models of Bruzual & Charlot; tables courtesy of Peter Eisenhardt and Adam Stanford). The lower panels (see left figure below) show the Hercules galaxy colors with spiral K-corrections applied. The K-correction curves are shown in light grey, each point represents 0.1 in redshift.

It is reassuring that the K-corrected value of the concentration color point is almost precisely what would be expected from an equal mixture of spiral and elliptical galaxies. The conclusion is that the concentration of points in the 2MASS color-color plot is indeed due to the Hercules cluster.

Many galaxies have colors that are shifted to significantly larger values of H-Ks and a somewhat larger range of J-H. These effects are consistent with color changes caused by observing galaxies at a range of redshifts from z = 0.0 to beyond z = 0.1. It is clear from the error bars that many galaxies must lie beyond the z=0.11 points.

We can compare the colors of cluster galaxies with those found in the field. The plot below (right hand side) shows relatively nearby elliptical and spiral galaxies, as well as more distant galaxies, including Seyfert type galaxies. 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 compares favorably with the 2MASS fixed circular radius = 7 arcsec aperture. Frogel et al (1985) give the reddening and K-corrected colors of 19 Sc galaxies observed with a 6.6" aperture. Finally, John Huchra has supplied us with data taken by Aaronson, Huchra and Mould of tbs galaxies of many different morphological types. Their JHK observations have much larger apertures which are similar in size to the 2mass effective isophotal apertures. No reddening or K-corrections have been applied to these data, but most of these galaxies are at small enough redshifts that this is a small effect. The K-correction tracks are shown for SAB type galaxies and for E/S0 types.

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. 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 most of those galaxies with large H-K colors are Seyfert galaxies.


J-H vs. H-Ks 2MASS Colors for Hercules Galaxies
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upper left panel: colors computed using elliptical isophotal (20 mag/arcsec^2) apertures; mean colors are represented by filled circles and 1-sigma errorbars; individual galaxy colors are denoted by red dots; K-corrections for spirals are represented by the grey line and triangle points -- each point is 0.1 in redshift -- and ellipticals by the grey line and square points; the main sequence (dwarf and giant branches) are shown in green.
upper right panel: colors computed used fixed circular radius=7" apertures
lower panels: colors with K-corrections applied 		J-H vs. H-K Colors for field E, Spiral & distant Seyfert Galaxies
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elliptical galaxies denoted by triangles; spiral galaxies by circles; Huchra fields galaxies by blue squares.


Since the Hercules cluster is a historically well studied region of the sky, redshifts exist for a large fraction of the brighter sources. Both optical and 21-cm redshifts have been obtained for many galaxies in Hercules (cf. Dickey, J. M. 1997, AJ 113; Dressler et al. AJ 95, 284, 1988; Tarenghi et al 1979, ApJ234, 793; Humason Mayall and Sandage 1956 , AJ61, 97).

About 50% of our Hercules galaxies have published redshifts. The plot below (left side) again shows the JHKs colors of these sources, but now we have color coded the points according to redshift. If the source has a redshift, z, less than 0.03, then we mark it with a blue point; for 0.03 < z < 0.05 points marked in green, 0.05 < z < 0.1 points marked in orange and z > 0.1 marked with red. Galaxies physically associated with the Hercules cluster have redshifts clustering around z = 0.038 (green points).

It can be seen that their is a strong correlation with color (in particular, H-Ks) and redshift. The cluster (green points) and galaxies located behind the cluster (orange and red points) follow roughly the K-correction curve. This can be equally demonstrated with a plot of color vs. redshift (see below, right plot). Note that the K-correction tracks for spirals and ellipticals appear to bracket the data, which is consistent with a cluster mixed with these types of galaxies (with Hercules siding toward spiral dominated).

J-H vs. H-Ks 2MASS Galaxies w/ Redshifts
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upper panel: mean colors and 1-sigma error bars
lower panel: all Hercules galaxies, including repeat observations 		J-H and H-Ks Colors vs. Redshift
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These results show that the K-correction plus the restricted range of colors for all galaxy morphological types allows statistical redshifts to be obtained from the 2MASS color-color plot without any additional data.

Moreover, this allows 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 a previously unknown cluster (see plot below). Conversely, a sky plot of sources selected with large values of H-Ks color should not show the location of a nearby cluster (z < 0.1). The cluster nicely separates from the field galaxies when using a restricted color that encompasses the tight concentration of galaxies around J-H = 0.70 and H-Ks = 0.30 (see right panel below; large white points demark restricted galaxies with redshifts). A cluster to the south of the Hercules cluster, called Abell 2152, is part of the supercluster, located just beyond the Hercules cluster, which also separes nicely in color-color space.

Skyplot of Hercules Region Galaxies
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left panel: small white points == All 2MASS galaxies; color-coded large points are galaxies with redshifts
right panel: small white points = all 2MASS galaxies with redshifts; large white points == 2MASS galaxies with 0.60 < J-H < 0.80, 0.18 < H-K < 0.38
small yellow points == all galaxies from Huchra catalog of Hercules;
blue circle denotes the Hercules cluster of galaxies

Summary

The 2MASS project will produce a complete (>90%) and reliable (>98%) catalog of galaxies down to Ks = 13.5 and J = 15 mag. The 10% photometric limits for galaxy photometry is about 15.1 mag at J, 14.2 at H and 13.5 at Ks, using a small circular aperture. The J-H and H-K colors will have sufficient accuracy to differentiate galaxies (at z > 0.02) from stars, and moreover, the change in galaxy color with redshift can be used to obtain statistical redshifts of 2MASS galaxies from the 2MASS galaxy color-color diagram. A sky plot of sources selected to be near the concentration of points in the JHK color-color plane should show a heavy concentration to the location of a previously unknown cluster.