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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Title/Abstract Page: DENIS Observations of Multibeam
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Contents Page: Volume 16, Number 1

Cross-identification of HI-detected galaxies on DENIS images

Figure 2 displays the distribution of MB galaxies in the ZOA (large circles). Dots indicate galaxies found in the optical B-band search with a diameter limit of

$D = 0\hbox{$.\mkern-4mu^\prime$}2$ (Woudt et al. 1998). Contours indicate extinction levels determined from the DIRBE maps (Schlegel et al. 1998), which are scaled to independent extinction determinations from Mg₂-indices in this region (Woudt 1998). The thick contour corresponds to

$A_B = 2\hbox{$.\!\!^{\rm m}$}75$ ; this is the completeness limit for galaxies with an extinction-corrected diameter of

$D^{o}\!=1\hbox{$.\mkern-4mu^\prime$}3$ in the deep optical ZOA galaxy catalogue. The other contours indicate $A_B=5^{\rm m}$ and $10^{\rm m}$ , where the Milky Way is opaque in the optical.

**Figure 2:** Galaxy distribution in part of the southern ZOA with galaxies found in an optical search (small dots) and galaxies detected with the shallow MB survey (large circles). The superimposed contours represent absorption levels of
$A_B = 2\hbox{$.\!\!^{\rm m}$}75$ ( *thick line*),

$5\hbox{$.\!\!^{\rm m}$}0$ ,

$10\hbox{$.\!\!^{\rm m}$}0$ (see text for details). MB galaxies for which DENIS data are available are marked with a large dot, those re-identified on DENIS images with a cross, and those without an optical counterpart with a star.
$\begin{figure}\vspace{-2cm} \begin{center} \hfil\psfig{file=mb_map.eps,angle=270,height=13cm}\vspace {-2cm}\hfil \end{center}\end{figure}$

For 24 of the galaxies detected in the shallow MB-ZOA survey (cf. Henning et al., this volume), DENIS survey images (

$12\hbox {$^\prime $}\times 12\hbox {$^\prime $}$ ) covering the full positional uncertainty region (

$\sim\!4\hbox{$^\prime$}\times \sim\!4\hbox{$^\prime$}$ ) were currently available. They are displayed as large dots (including 8 detections for which only partial coverage of the HI positional uncertainty region was available on existing DENIS images). For 16 out of the 24 a clear counterpart could be identified (crosses); of these, 6 are invisible in the B-band due to large foreground extinctions (stars). For 4 out of the 24 MB-ZOA galaxies the cross-identifications are uncertain, and for the remaining 4 galaxies no counterparts could be identified. These galaxies either lie behind a very thick extinction layer (e.g., one galaxy at

$b \simeq 0{^\circ}$ with

$A_B \sim 70^{\rm m}$ ), or they are late-type spirals or irregular galaxies of very low surface brightness and hence below the magnitude limits of the DENIS survey. The HI survey, however, is not affected by the foreground extinction, and spiral galaxies can be found in the NIR at lower Galactic latitudes and higher foreground extinction levels than in the optical.

In Figure 3, DENIS images of five MB galaxies are presented. The I_c -band is shown in the upper panel, the J-band in the middle and the K_s-band in the bottom panel. From left to right the foreground extinction increases. The first galaxy is a nearby ( $v\!=\!1445\,$ km ${\rm s}^{-1}$ ) Lauberts galaxy (ESO223-G12) at

$b=+4\hbox{$.\!\!^\circ$}8$ and

$A_B = 3\hbox{$.\!\!^{\rm m}$}9$ . It is prominent in all three NIR passbands (note the larger image scale for this galaxy, i.e.,

$3\hbox{$.\mkern-4mu^\prime$}3$ instead of

$1\hbox{$.\mkern-4mu^\prime$}7$ ). The second galaxy, also detected with IRAS, at

$(b,A_B) = (-3\hbox{$.\!\!^\circ$}4,4\hbox{$.\!\!^{\rm m}$}4$ ) is slightly more distant ( $v\!=\!2789\,$ km ${\rm s}^{-1}$ ). This galaxy has also been identified in B and is quite distinct in the NIR. The third and fourth galaxies at

$(b,A_B) \simeq (3\hbox{$.\!\!^\circ$}4, 4\hbox{$.\!\!^{\rm m}$}8)$ and

$(2\hbox{$.\!\!^\circ$}3, 8\hbox{$.\!\!^{\rm m}$}4)$ , respectively, have no optical counterparts. Here, the extinction effects are quite obvious: while both have a similar appearance in the J-band, the third is very faint in K_s and the fourth very faint in I_c. The fifth example has been detected at the highest extinction so far:

$(b,A_B,v) \simeq (+1\hbox{$.\!\!^\circ$}2,11\hbox{$.\!\!^{\rm m}$}8, 3963\,$ km ${\rm s}^{-1}$ ). It is fully obscured in the I_c-band and barely visible in the J-band.

**Figure 3:** DENIS survey images (before bad pixel filtering) of five galaxies detected in HI with the MB survey at
$\vert b\vert \le 5{^\circ }$ ; the I_c band image is at the top, J in the middle, and K_s at the bottom.
$\begin{figure} \vspace{-2cm} \begin{center} \hfil\psfig{file=mb5_f.ps,height=12cm}\hfil \end{center}\end{figure}$

Cross-identifications of galaxies detected in the MB survey are not always unambiguous. The positional accuracy for galaxies detected in the blind HI search is of the order of

$\sim\!4\hbox{$^\prime$}$ . Sometimes more than one possible counterpart lies within this area as illustrated in Figure 4. The position of this MB galaxy is indicated by a white rectangle on the DENIS I_c -band image; the foreground extinction is A_B=4.0. Four galaxies can be seen in its vicinity: an edge-on spiral with a small companion (right and above the rectangle), an early-type spiral (left) and a late-type spiral with low surface brightness (left and above the early-type spiral). Although the last seems the most likely candidate given the HI parameters ( $v\!=\!2903\,$ km ${\rm s}^{-1}$ , $w\!=\!177\,$ km ${\rm s}^{-1}$ , $I\!=\!13.1\,$ Jykm ${\rm s}^{-1}$ ) and the other galaxies may be further away or have less HI mass and hence not be detectable with the shallow HI survey, the cross-identifications are not always clearcut. For such cases, follow-up observations, either with HI mapping or optical and NIR spectroscopy, are required.