2 Observations

  
2.1 Moderate spectral resolution K-band spectroscopy

Spectroscopy in the K-band was performed with ISAAC at VLT-UT1 on April 17 2000. The spectrograph was configured to have a 0 $.\!\!^{\prime\prime}$3 slit and the medium resolution grating was tilted such that the central wavelength was 2.31 $\mu$m with a total wavelength coverage from 2.25 to 2.37 $\mu$m. This was sufficient to include the ¹²CO (2-0) and (3-1) absorption bands and at a resolution ( $\lambda /\Delta \lambda$) of about 9000. Observations of late-type supergiant stars were taken so that they could be used as templates for the determination of the velocity dispersions. Various templates were observed, however, due to incorrect classifications in the literature, only one of them actually was an M-type supergiant. More template stars were observed by Linda Tacconi during an observing run on August 15 2000 using the same instrumental set-up. Observations were performed by nodding along the slit and dithering the source position from one exposure to the next. Atmospheric calibrator stars were observed several times during the night.

The target clusters were selected using the results from NTT-SOFI imaging observations performed in May 1999 (Mengel, Ph.D. Thesis, der Ludwig-Maximilians-Universität München, 2001; Mengel et al. 2001, in preparation). They included narrow band imaging in the CO(2-0) band-head and revealed those clusters which are at an age where the near-infrared emission is dominated by red supergiants. The clusters with the highest equivalent width in the CO band-heads, highest luminosity, and high extinction were selected as the ISAAC targets (for the location of all observed clusters see Fig. 1). Two target clusters were positioned in the first slit position. These were two clusters with a separation of $\sim$5 $^{\prime\prime}$ located in the region where the galaxy discs apparently overlap ("the overlap region''). One of them has an optical counterpart, [W99]15, the other one is detectable only in the I-band, therefore denoted as a "Very Red Object (VRO)'' by Whitmore & Schweizer 1995 (with the designation: [WS95]355). Their detectability in at least one band from the HST imaging program of Whitmore et al. (1999) was a necessary additional constraint, since the compactness of each cluster means that their radii must be measured from HST data (see Sect. 3.4). The second slit position included only one bright cluster ([W99]2), which has a relatively low extinction and was also observed with UVES. Due to the extreme narrowness of the slit, object drifts made several re-acquisitions necessary, especially when the object was transiting. The ISAAC observations are summarized in Table 2.

Figure 1: ISAAC Ks-band image of NGC 4038/4039 (taken during a run in April 2001) with labeling of the clusters that were observed using ISAAC and UVES spectroscopy.

 

 

Table 1: Summary of ISAAC spectroscopy. The first slit position included two clusters, the second one cluster. The supergiants were observed to provide template spectra for the determination of velocity dispersions. The slit width was 0 $.\!\!^{\prime\prime}$3, resulting in a resolution R = $\lambda /\Delta \lambda$ of 9000. Seeing variations are expected to have no effect on the spectral resolution given the narrow slit width. The total integration time was split up into several integrations of duration $T_{\rm int}$. The K-band magnitudes were determined from an ISAAC Ks-band image (aperture diameter 1 $.\!\!^{\prime\prime}$74). The seeing value refers to an average for the monitored V-band seeing during the integration. The stars were observed during two observing runs (2000 and 2001), and those marked with LT were kindly observed by Linda Tacconi during another run, because the 2000 run had resulted in only one useful supergiant spectrum.

Position	Object	mV	mK	mK0	$T_{\rm int}$	Total ON time	Seeing	Comments
		[mag]	[mag]	[mag]	[s]	[s]
Slit 1	[W99]15	19.4	15.9	15.8	600	15600	0 $.\!\!^{\prime\prime}$7	Difficulties keeping both
	[WS95]355	$\approx$22.1	15.7	15.4	600	15600		targets in slit during 2 integrations
Slit 2	[W99]2	17.0	14.0	14.0	600	2400	0 $.\!\!^{\prime\prime}$9
and the following stars: K5Ib (HD 183589), K5Ib (HD 53177, LT), M0I (CD-44 11324, LT),
M0-1Iab (HD 42475), M1I (BD+0 4030), M1Iab (HD 98817), M1Ib (HD 163755), M2Iab (SAO 15241),
M3Ia-Iab (HD 103052), M3.5I (BD+13 1212), M5Iab (HD 142154)

  
2.2 High-resolution optical spectroscopy

High-resolution optical echelle spectroscopy was performed using UVES at VLT-UT2 on the night of 18 April 2000. The instrument was configured with a slit width of 1 $^{\prime\prime}$, which resulted in a resolution of $R \approx$ 38000 (depends on wavelength). A dichroic was placed in the light path allowing the use of both the red and blue arms of the spectrograph. However, in this paper we will discuss only the results obtained with data from the red arm. The central wavelength of the red arm was shifted to 8400 Å since part of the CaT (at NGC 4038/4039 redshift of $z \sim$ 0.005 located roughly at: 8540 Å, 8585 Å and 8705 Å) would have fallen right in the small gap between the two CCDs that cover the lower and the upper part of the spectra in the red arm. The CCDs were read out with a binning of 2 $\times$ 2, 50 kHz and high gain. For the cluster selection we applied the same criteria as for the clusters observed with ISAAC, however, for the UVES observations, we selected only those with relatively low optical extinction and bright I-band magnitudes. To boost the efficiency of the observing, we additionally required the clusters to have a nearby cluster within the slit of UVES (which is only 10 $^{\prime\prime}$ due to the echelle design). The clusters observed were [W99]1 and [W99]43 in one slit position, [W99]10 and [W99]16 in a second slit position. Unfortunately, [W99]43 and [W99]10 turned out to be too faint to extract a spectrum of sufficient S/N for further analysis. A third slit position covered [W99]2, common to ISAAC and UVES. Obtaining two independent observations of [W99]2 provides an independent estimate of the velocity dispersion to test the uncertainties of our measurements and to gauge whether there might be some systematic differences in velocity dispersions determined from the CaT in the optical with UVES versus that obtained from the CO band-head with ISAAC in the near-IR. A range of template supergiants was observed (early K through late M-type supergiant stars), but also several hot main sequence stars (late O through mid B-type main sequence stars). Massive main sequence stars are expected to dominate the blue spectrum and to contribute significantly to the flux in the I-band (Bruzual & Charlot 1993). Due to the high dispersion and faint sky background, integration times could be very long and were only limited by the desire to keep the number of cosmic ray hits down to a reasonable limit. The UVES observations are summarized in Table 2.

 

 

Table 2: Summary of UVES spectroscopy of science targets (star clusters and template stars). The spectral resolution was $R \approx$ 38000, and differences in the seeing for different objects had no measurable effect on the shape of the absorption features in the stellar spectra.

Position	Object	mV	mK	mK0	$T_{\rm int}$	$T_{\rm tot}$	Seeing	Comments
		[mag]	[mag]	[mag]	[s]	[s]
Slit 1	[W99]1	17.6	14.7	14.7	4800	9600	0 $.\!\!^{\prime\prime}$7	required pausing of integration
	[WS95]43	20.0			4800	9600		due to A.O. software crash
Slit 2	[W99]10	19.0	15.6	15.6	4800+1800	11400	0 $.\!\!^{\prime\prime}$85	required pausing of integration
	[W99]16	19.0	15.5	15.5	4800+1800	11400		due to A.O. software crash
Slit 3	[W99]2	17.0	14.0	14.0	4600	4600	1 $.\!\!^{\prime\prime}$2
And the following stars: M3Iab (HD 303250), M5I (HD 142154), M1Ib (HR 6693), MIa-Ib (HR 4064)
K7IIa (HD 181475), K5Ib (HR 7412), K5II (HR 7873), K3Ib-IICN (HR 6959), K3II (HR 6842)
K2.5IIb (HR 7604), K2II (HR 6498), K2Ib (BM Sco), K0.5IIb (HR 6392), K0Ia (HR 6392)
B5V (HR 5914), B5V (HR 5839), B2V (HR 6028), B2V (HR 6946) with integration times
between 1 s and 20 s, and some atmospheric and flux calibrators.