KIR: First Light

René Doyon

Observatoire du Mont Mégantic et Département de physique, Université de Montréal,
C.P. 6128, Succ. Centre-ville, Montréal, QC, Canada H3C 3J7
Electronic-mail: doyon@astro.umontreal.ca

Abstract:

This paper presents a subset of first light images obtained with KIR, the new 1024$\times$1024 high-spatial resolution near-infrared camera used with the adaptive optics bonnette of the CFHT.

Introduction

  KIR is a high-spatial resolution near-infared (1-2.5 $\mu$m) camera used at the f/20 focus of the Adaptive Optics Bonnette (AOB) of the CFHT. The camera houses a 1024$\times$1024 HgCdTe array with cold transfer optics yielding an image scale of 0.035. The instrument was built jointly by the Université de Montréal, the Observatoire Midi Pyrénées and the CFHT corporation. Performance and full description of KIR are given in Doyon et al.(1998).

Observations

Following a successful engineering run in September 1997, KIR was scheduled for first light on December 7th 1997 on a program (2 nights) devoted to starburst galaxies. Observations at J, H and K were obtained on the ultraluminous IRAS galaxy NGC1614, the ``canonical'' starburst NGC7714 and the blue compact dwarf He 2-10. The wave front sensor of the AOB was centered on the nucleus except for He 2-10 where a 14.5 mag star located $\sim$20'' away from the galaxy was used for closing the loop. The seeing conditions were excellent (0.6-0.7''); the spatial resolution obtained was typically 0.2-0.3''. In the following we describe some of the results obtained on He 2-10.

He 2-10 is the prototype for Wolf-Rayet galaxies. Whether the starburst activity in this object is stochastic (self-induced) in nature or due to a merger event is very uncertain. Optical images show two distinct centers of activity (A: the nucleus and B, $\sim$8 east of A) reminiscent of an on-going merger event but the smooth elliptical envelope surrounding both regions has also been taken as evidence that He 2-10 is a single dwarf elliptical experiencing stochastic star formation.

 

Figure 1:   K-band image of He 2-10 showing faint details in the outer region of the galaxy. East is left and north is up. The faint sources to the east have $m_{\rm K}=18-19$.

Figure 1 shows a K-band image of He 2-10 obtained with KIR. The near-infrared morphology of region B is more reminiscent of a tidal tail than a small companion. There is also a feature north-west of the nucleus which looks very much like a faint tidal tail. The overall K-band morphology is similar (on smaller scale) to the famous ``Antennae''. This, combined with the smooth and faint elliptical envelope detected on the K image suggest that He 2-10 is probably a merger (in a well advanced state) of two gas-rich dwarf ellipticals.

The HST UV image of He 2-10 (Conti & Vacca, 1994) shows a cluster of strong UV knots in the nucleus with luminosities consistent with young (1 to 10 Myrs) globular clusters of masses between 10⁵ and 10⁶ $M_{\sun}$. The authors argued that these globulars could have been formed as a result of a merger event. The K-band morphology of He 2-10 certainly strengthen this scenario.

The unsharp-masked K image (low-frequency component subtracted) of the central 5of He 2-10 is shown in Figure 2, reveiling a clumpy structure similar to the HST image. Thanks to the relatively large field of view of KIR, it was possible to get an accurate registration of both KIR and HST images. The result is shown in Figure 2. While the UV sources are close to K sources, the overall spatial correlation is not very good. This is especially true for two K sources (one to the NW and the other to the SE) that have no UV counterpart nearby. These two sources are extremely red in J-H and H-K suggesting that they are significantly affected by extinction. Furthermore, they are very close to 10 $\mu$m sources (Sauvage et al., 1997). This association suggests that these sources are sites of vigorous star formation activity still embedded in dust and thus undetectable in the UV.

 

Figure 2:   Unsharp-masked K-band image of the central region of He 2-10. Crosses correspond to loci of HST UV sources. Note the relatively poor spatial correlation between the UV and K sources.

Yet extinction cannot explain everything. The JHK colors in the vicinity of the UV cluster suggest that this region is suffering from relatively low and uniform extinction. A more likely explanation for the poor spatial correlation between the UV and K sources in this region is that the latter are sampling relatively evolved (>10 Myrs) and short-lived starburst regions. These K sources were probably very strong in the UV a few Myrs ago before the OB stars turned into red supergiants. Conversely, the current UV cluster will probably disappear in the UV in a few Myrs when the OB stars become red supergiants.

It is interesting to estimate the luminosities of the K sources in the vicinity of the UV cluster. For instance, the brightest source has an absolute K magnitude of -14.8 (distance of 9 Mpc assumed for He 2-10). From comparison with starburst models we can infer that such a cluster would have a mass between 10⁵ and 10⁶ $M_{\sun}$ depending on the age. This is the same masss range inferred for the UV sources.

Both UV and infrared observations suggest that there are relatively young and massive star clusters in the central region of He 2-10. It is attractive to argue that these clusters are proto-globular clusters formed as a result of a merger event. Are there older globular clusters in He 2-10? There are certainly candidates. The K image in Figure 1 shows three sources east of the nucleus with $M_{\rm K}$=-11.7 corresponding, at the distance of He 2-10, to the peak of the galactic globular clusters luminosity function.