2 The sample, the observations, and the colors

The ISO Key Project for Normal Galaxies (PI: G. Helou, KP) was proposed to study the ISM of a broad range of normal galaxies using several instruments aboard ISO. The sample, described in Malhotra et al. (2001), is designed to capture the great diversity among galaxies, in terms of morphology, luminosity, far-infrared-to-blue luminosity ratio FIR/B, and IRAS colors. The project obtained ISO observations of 69 galaxies, including nine relatively nearby and extended objects. The remaining 60 galaxies cover the full range of observed morphologies, luminosities, and star-formation rates seen in normal galaxies, and includes five dwarf irregulars discussed in Hunter et al. (2001). Two of these, NGC 1156 and NGC 1569, are part of our observed sample, and are also the two nearest objects (6.4 and 0.9 Mpc, respectively). The median distance of the KP sample (including these nearby objects, and with H₀=75 km s^-1/Mpc) is 34 Mpc (Dale et al. 2000). This means that with the 14 $^{\prime\prime}$ observing aperture we are sampling a central region roughly 1 kpc in radius. We observed 26 KP galaxies.

We also selected seven quiescent ellipticals from the sample of early-type galaxies observed with ISO by Malhotra et al. (2000), or with published JHK photometry (Frogel et al. 1978). Several of the latter do not have the rich array of ISO observations obtained for the KP galaxies.

2.1 Observations

The observations were acquired at the 1.5 m f/20 Infrared Telescope at Gornergrat (TIRGO), with a single-element InSb detector. The photometer is equipped with standard broadband filters (J 1.2 $\mu$m, H 1.6 $\mu$m, K 2.2 $\mu$m, and $L^\prime$ 3.8 $\mu$m) with diaphragms in the focal plane defining the aperture dimensions. The galaxy coordinates were taken from NED and checked with the DSS; all observations were acquired with a 14 $^{\prime\prime}$ aperture, after maximizing the infrared signal. Sky subtraction was performed with a wobbling secondary at frequencies that ranged from 2.1 to 12.5 Hz, according to the integration time for the individual measurement. The modulation direction was in a EW direction, with an amplitude of roughly 3$^\prime$. Beam switching was used to eliminate linear variations in sky emission.

Photometric calibration was achieved by similarly observing several standard stars nightly from the CIT (Elias et al. 1982) and the ARNICA (Hunt et al. 1998) standard lists. Nightly scatter of the photometric zero point was typically 5% or better in JHK and 8% or better in $L^\prime$.

The final photometry for the 33 galaxies observed is reported, together with their basic data, in Table 1. In what follows, we have transformed K-$L^\prime$ to K-L (this last is centered at 3.5 $\mu$m) using the transformation by Bessell & Brett (1988), and we will use K-L to denote such colors. In the various plots and when testing for correlations, the NIR data have also been corrected for i) Galactic extinction according to Schlegel et al. (1998) and Cardelli et al. (1989), and ii) K dimming using the precepts mentioned in Hunt & Giovanardi (1992).

Figure 1: NIR color-color diagram; the K-L color in the top panel is transformed from K-L' as described in the text. Filled symbols refer to galaxies with K-L > 1.0. The 7 quiescent ellipticals are indicated with an E. Shown as stars are H  II galaxies taken from Glass & Moorwood (1985) and starbursts from Hunt & Giovanardi (1992), and shown as asterisks the BCDs (see text). Mixing curves show how the colors change when various physical processes increasingly contribute to the emission observed (see Hunt & Giovanardi 1992); the end points of the mixing curves indicate equal K-band contributions from stars and the process (hot dust, ionized gas, A stars). The tick marks on the extinction line show unit AV magnitude increments.

   
2.2 JHKL colors

The NIR colors of the observed galaxies are shown in Fig. 1. The filled symbols represent those (10) galaxies with $K-L\ga 1.0$, that is a rising continuum (see below). Also shown in the diagram are H  II galaxies taken from Glass & Moorwood (1985) and starbursts from Hunt & Giovanardi (1992). Three low-metallicity blue compact dwarfs (BCDs) are also shown as asterisks: NGC 5253 (Glass & Moorwood 1985), II Zw 40 (Thuan 1983), but no J, and SBS 0335-052 (Hunt et al. 2001). The BCDs have the most extreme colors since they tend to be quite blue in J-H (because of low metallicity and youth), and red in K-L (because of ionized gas and hot dust).

Mixing curves show how the colors change when various physical processes increasingly contribute to the emission observed (see Hunt & Giovanardi 1992); they illustrate that, on the basis of NIR color, it is possible to distinguish among stellar photospheric emission, "passive'' dust in extinction, and "active'' dust in emission. H-K tends to be red (>0.35) for both dust extinction and dust emission, while K-L is red only because of emission by dust or ionized gas (see the free-free line). Thus red K-L can be used to signal a substantial contribution from hot dust emission in the observing aperture. The 600 and 1000 K mixing curves span the observed colors quite well, and although the VSG emission is not thermalized, the curves show that the NIR colors are consistent with dust at these temperatures.

The K-L color can also provide an estimate of the slope of the spectral energy distribution (SED) between 2 and 4 $\mu$m. Using the zero-magnitude fluxes given in Koornneef (1983), and assuming a power-law dependence $f_\nu \propto \nu^\alpha$, we find that $K-L~=~-0.50~\alpha\ +\ 0.95$. Therefore, $K-L\ga 1.0$ is where the slope $\alpha$ changes sign and becomes negative, signifying a continuum $f_\nu$ rising with increasing wavelength; when K-L < 1.0, $\alpha$ is positive. The mean $\alpha$ averaged over the entire KP sample is > 0, with $f_\nu~\propto~\nu^{+0.65}$ (Helou et al. 2000). If we include the ellipticals, the median K-L of our 33 galaxies is 0.50, which gives $\alpha~=~0.90$, steeper than, but consistent with, Helou et al. (2000). If only the ISO Key Project galaxies are considered, median K-L = 0.68, corresponding to $\alpha~=~0.54$, remarkably close to that reported in Helou et al. The median K-L for the ellipticals only is 0.21, which corresponds to a very steep falling continuum with $\alpha~=~1.5$.

The mixing curves shown in Fig. 1 illustrate what fraction of the observed flux is due to hot dust; they assume a mix of stellar photospheres with intrinsic stellar color (K-L)_* = 0.3, or $(K-L^\prime)_*~=~0.5$, plus dust emission. When the SED is flat at 4  $\mu$m ( $K-L~\approx~1.0$, slope $\alpha~=~0$), 600 K hot dust comprises roughly 5% of the total K-band flux; hotter dust (e.g., 1000 K) would constitute 30%. It is unlikely that flat or rising continua are due to free-free emission from ionized gas, since even with a 50% emission fraction from gas, the continuum is still falling [ (K-L) = 0.7]. This means that K-L, or alternatively the slope of the 4 $\mu$m continuum, is a remarkably sensitive diagnostic of hot dust: small K-band fractions (5-30%) of dust emission cause large variations (0.6 mag) in the K-L color. A 50/50 K-band mix of hot dust and stars would produce a K-L color between 1.2 and 2.7, depending on the dust temperature; NGC 4519, the galaxy with the reddest H-K (=0.82), may contain such a mix.