Australia Telescope National Facility

DENIS Observations of Multibeam Galaxies in the Zone of Avoidance
A. Schröder , R.C. Kraan-Korteweg , G.A. Mamon, PASA, 16 (1), in press.

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The DENIS survey

Observations in the NIR can provide important complementary data to other surveys. NIR surveys will
$\bullet$ be sensitive to early-type galaxies -- tracers of massive groups and clusters -- which are missed in IRAS and HI surveys,
$\bullet$ have less confusion with Galactic objects compared to FIR surveys,
$\bullet$ be less affected by absorption than optical surveys.
Furthermore, because recent star formation contributes only little to the flux at this wavelength, the NIR gives a better estimation of the stellar mass content of galaxies and is hence ideally suited for the application of the Tully-Fisher relation.

The DENIS survey has currently imaged 40% of the southern sky in the $I\,<br /> (0.8\,\mu)$ ,

$J\,<br /> (1.25\,\mu)$ and

$K_s (2.15\,\mu)$ passbands with a resolution of

$1\hbox{$^{\prime\prime}$}$ in I_c and

$3\hbox{$^{\prime\prime}$}$ in J and K_s. In a pilot study, we have assessed the performance of the DENIS survey at low Galactic latitudes (Schröder et al. 1997, hereafter Paper I, Kraan-Korteweg et al. 1998, hereafter Paper II). We (a) tested the potential of the DENIS survey to detect galaxies where optical and FIR surveys fail, i.e., at high foreground extinctions and in crowded regions, (b) established that the NIR colours of galaxies lead to values for the foreground extinction, (c) determined preliminary I_c^o, J^o and K_s^o galaxy luminosity functions in the rich cluster A3627, and (d) cross-identified highly obscured galaxies detected in a blind HI search at

$\vert b\vert < 5{^\circ}$ .

Number counts of galaxies are lower in the ZOA due to the high foreground extinction, but the effect depends on wavelength. Interpolating from Cardelli et al. (1989), the extinctions in the NIR passbands are

$A_{I_c}\!=\!0\hbox{$.\!\!^{\rm m}$}45$ ,

$A_J\!=\!0\hbox{$.\!\!^{\rm m}$}21$ , and

$A_{K_s}\!=\!0\hbox{$.\!\!^{\rm m}$}09$ for

$A_B\!=\!1\hbox{$.\!\!^{\rm m}$}0$ , hence the decrease in number counts as a function of extinction is considerably slower in the NIR than in the optical. Figure 1 shows the predicted surface number density of galaxies for DENIS (Mamon et al. 1998) and for B < 19, as a function of Galactic foreground extinction (cf.Paper II).

**Figure 1:** Predicted galaxy counts in B, I_c, J and K_s as a function of absorption in B, for highly complete and reliable DENIS galaxy samples and a
$B_J \leq 19^{\rm m}$ optical sample.
$\begin{figure} \vspace{-2cm} \begin{center} \hfil \psfig{file=gal_cts.eps,height=10cm}\hfil \end{center}\end{figure}$

The NIR becomes notably more efficient at

$A_B \mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$>$}}}2-3^{\rm m}$ . The J-band is the most efficient passband to find galaxies at intermediate extinctions, but at

$A_B \simeq 10^{\rm m}$ K_s becomes superior to J. Even at

$A_B \!>\! 10^{\rm m}$ we can expect to find galaxies in J and K_s. A new air conditioning system for the K_s-band camera installed in April 1997 has decreased its instrumental background by

$0\hbox{$.\!\!^{\rm m}$}7$ , which makes the K_s-band now more comparable to J. These are very rough predictions and do not take into account any dependence on morphological type, surface brightness, orientation and crowding, which may lower the number of actually detectable galaxies (Mamon 1994).

Next Section: Cross-identification of HI-detected galaxies
Title/Abstract Page: DENIS Observations of Multibeam
Previous Section: Introduction
Contents Page: Volume 16, Number 1

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