Detection of Low Surface Brightness Galaxies with 2MASS

T. Jarrett, IPAC
(970420)


Introduction

It has become fashionable in recent years to promote low surface brightness galaxies as the harbingers of the so called "missing" dark (re: baryonic) matter in the universe. Their mass to light ratio is large, primarily because they are so faint, but the relative significance of their overall mass to a galaxy cluster (where they predominently live) is not clear. The true nature of these objects is still not well known, including their stellar population(s), metallicity, star formation efficiency & evolution, and as noted, their overall mass. A number of large-area coverage studies have been carried out to to address these issues (to site just a few: Sandage etla . 1985; Schombert et al. 1992; Impey et al. 1996; Bothun et al. 1993; Bernstein et al. 1995; Ulmer et al. 1996; Sprayberry et al. 1996; O'Neil et al. 1997; Impey & Bothun 1997).

The definition of a low surface brightness galaxy (LSB) is somewhat vague, but the basic characteristic is that their central surface brightness is fainter than 22 or 23 mag per sq. arcsec at B (cf. Bothun et al. 1991). Their apparent size can have a wide range in value, from the smallest dwarf ellipticals with scale lengths of ~1 arcsec, to tens of arcseconds for the biggest galaxies LSBs in the local group.

Adopting the central surface brightness criteria of 22 mag per sq. arcsec at B as the definition of an LSB galaxy, we can estimate the surface brighteness in the near-infrared using a color of (B-K) = 3 or 4 (B-I ranges from 2 to 3 for LSBs, O'Neil 1997, private comm). Thus, our definition of an LSB galaxy is central surface brigtness fainter than:

The 2MASS survey will cover the entire sky, north and south, at the near-infrared bands of J, H and K, with a limiting J magnitude of about 15.0 for galaxies. 2MASS sacrifices depth for areal coverage, but still should be sensitive to the bright end of the LSB spectrum (see below). The typical sky brightness in a 2MASS image is:

Based on the flux limits of the survey and the typical background noise, 2MASS should be sensitive to LSBs with central surface brightness up to 20 mag per sq. arcsec, or equivalent to a B surface brightness of 23 to 24 mag per sq. arcsec. This corresponds to "bright" LSB galaxies, easily covered in deep photographic and CCD optical limited-coverage surveys now underway. Nevertheless, 2MASS will see a lot of LSB galaxies due to the coverage. This memo provides some preliminary information on what we should expect to see in 2MASS with LSB galaxies. Here I describe the results of an intensive look at six-degree scans (each one sq. degree in area) through the core of COMA and the SA 57 region near the galactic pole.

This work is to be presented at the joint 2MASS - DENIS Euroconference in Paris during June of 1997.

The COMA and SA57 data single channel data sets were converted to "3-channel" sets in order to be compatible with the GALWORKS pipeline software. A brief description of this procedure is given in the following line.

  • Construction of 3-Channel Data Set

    • For a description of the GALWORKS algorithm of detection of low central surface brightness galaxies, see the memo

    Further information regarding the COMA data set and galaxy detection can be found in the documents:

    Central Surface Brightness

    Three classes of objects are considered here: stars, high surface brightness galaxies (HSB) and low surface brightness galaxies (LSB). The latter is defined to be any galaxy that satisfies at least one of the following criteria:

    Outstanding Issues

    Do the LSB galaxies represent a distinct population? The plots above suggest that LSBs are simply a faint extension of the HSB or normal galaxy surface brightness curve. There are a number of LSBs that are situated well above the HSB population, but they tend to be very faint (integrated flux) subject to significant distortion from background noise. For the brighter LSB galaxies, the central surface brightness is not signicantly different than HSB galaxies with comparable integrated flux (see the J band in particular). For these two scans (comprising 2 sq. degrees) there are only 10 or 20 LSB galaxies, so any conclusions made from this data is very tentative at best. The 2MASS survey will provide more than enough information to address this issue in a significant manner.

    LSB Images

    The following set of images correspond to the brightest of LSB galaxy members of the COMA and Sa 57 regions of study (one coadd each = 1 sq. degree each). Note however, that the typical integrated flux is still fainter than the limit at which the 2MASS survey will be reliable and complete.

    Each image is about ~230 Kb.

    COMA


    SA57 : scan 039


    Surface Brightness Profiles

    The following plots show the mean surface brightness profiles for the representative sample of LSB galaxies in this study. An exponential was fit to the mean surface brightness vs. radius for each band. For the brighter galaxies in which the elliptical parameters were well determined, the radial profile was constructed from elliptical annuli. For most of the remaining LSB galaxies, the annuli were circular.

    The exponential was fit over a radial range of 2 to 10 arc seconds -- comprising most (if not) all of the area that is not lost in the background noise for a typical LSB galaxy. The inner 2 arc seconds was avoided to minimize the effect of the PSF. Only points with >2*sigma where used in the fit, where sigma represents the uncertainty in the mean surface brightness measurement (poisson statistics).

    The derived scale length (see below) ranged from a low of ~1 arcseconds (typical of the fainter LSBs -- indicative of our lack of sensititivy), corresponding to about 500 pc (assuming H0 = 75 km/s/Mpc and DM = 34.9 for COMA) to 10 arcseconds for bright LSBs with a very flat profile, corresponding to ~4.6 kpc for COMA galaxies.

    The radial surface brightness, u(r), can be expressed in terms of the scale length as follows:

    Each plot is about ~7 Kb.

    COMA


    SA 57


    It can be seen from the radial profiles that a simple exponential fits the data very well in most cases. (There are a few instances in which we did not attempt to fit the K band profile due to the faintness of the object -- the error bars in the radial profile belie this condition.) The results suggest that we are either measuring the "disks" of the LSB galaxies, as opposed to a "spherical" or "bulge" component which we would expect to have a profile more similar to a r**1/4 law, or it simply means that LSBs are well fit by exponentials, regardless of whether they are dwarf ellipticals or dwarf spirals. This result is consistent with that seen in the optical for COMA LSB galaxies (see Bernstein et al. 1995). Again, the conclusions made here are preliminary, subject to further investigation with a larger areal coverage.

    References