The Calcium Triplet In Elliptical Galaxies

As proposed by Dressler [4], the Ca II triplet lines at 8498A, 8542A, and 8662A can be used as excellent diagnostic tools for measuring the kinematics and metallicity in elliptical galaxies. Such data would provide important observational tests for theoretical scenarios of galaxy formation and evolution. In this context, it would be particularly important to measure the lines' strength as a function of galactocentric radius. However, the Ca II lines are located in a wavelength region which is heavily affected by bright OH night sky lines, rendering the spectroscopy of faint surface brightness regions unreliable, i.e. in particular at large radii, which would be necessary for the determination of a line strength gradient. The situation is illustrated with an example for the dwarf elliptical NGC221 (M32), shown in Figure 2 as a 2MASS image at 1.2 ^m (obtained from NASA/IP AC), and a corresponding surface brightness plot obtained as a horizontal cut through the center of the galaxy. The two crosses indicate positions of the center and of the effective radius, where the surface brightness drops by more than two orders of magnitude.

First of all, 3D spectroscopy provides a significant advantage over classical slit spectroscopy, since binning of spaxels can be used to improve the Signal-to-Noise Ratio (SNR) in low brightness regions quite dramatically ([5], see Figure 3). Secondly, spectroscopy in the faint outskirts of the galaxy will be sensitive to systematic errors occurring during the process of sky background subtraction, in particular at the wavelengths of bright night sky emission lines [6]. This is where the N&S beam switching technique provides an invaluable gain over the conventional method of obtaining object and blank sky frames at different times, as the quasi-simultaneous

N&S exposure of object and sky can be evaluated with practically zero residuals, even at the brightest sky lines (see Figure 4).

Figure 2. 2MASS image and surface brightness profile of NGC221.

Figure 3. IFUs allow the coadding of several spatial elements to improve the SNR. Sky spectra are obtained at a blank field near the object. N&S observations involve telescope offsets form object to sky, and vice versa, while shuffling charge on the CCD as shown in Figure 1.

In a project by J. Cenarro (PI), N. Cardiel, and collaborators, to study calcium triplet gradients in elliptical galaxies, PMAS data for a total of 11 objects have been obtained from 3 observing runs in August 2003, January 2004, and May 2005. Here we present first results for the galaxy NGC221. Figure 5 shows the Ca II line strengths as a function of radius as obtained from single spaxels (from the center to ~ 4") , from coadding 15-17 spaxels (the 5 plots symbols with small error bars between 4" and 10"), and by coadding the entire IFU (256 spectra) at 36" (one effective radius). Note that binning improves the SNR significantly at 4"< r < 10", even when the surface brightness has decreased with regard to the inner region. This is particularly obvious in the outer bin at 36", where a nominal SNR gain of 16* has been achieved. The error bar is of the same order of magnitude as for the central unbinned spectrum. Clearly, opening the slit to a width of 16 arcsec to achieve the same effect would be impractical for conventional spectroscopy. Translating the SNR gain into light collecting power, the equivalent in terms of telescope size would be a 14 m telescope (as opposed to the 3.5 m telescope used for these observations!).

Figure 4. Sky subtraction with N&S spectroscopy, example for NGC221. The subtraction of sky from galaxy+sky is performed arithmetically at the level of corresponding CCD pixels, which have experienced nearly identical optical transmission and atmospheric observing conditions between object and sky subexposures (for technical details [3]). Data reduction and analysis performed with the reduceme package of Cardiel [7]. Note: This plot is shown in color in the reference plot section in the front of these proceedings.

Figure 4. Sky subtraction with N&S spectroscopy, example for NGC221. The subtraction of sky from galaxy+sky is performed arithmetically at the level of corresponding CCD pixels, which have experienced nearly identical optical transmission and atmospheric observing conditions between object and sky subexposures (for technical details [3]). Data reduction and analysis performed with the reduceme package of Cardiel [7]. Note: This plot is shown in color in the reference plot section in the front of these proceedings.

To our knowledge, this the first time that a reliable Ca II gradient with accurate sky subtraction has been measured for an elliptical galaxy. The surprising result is that the abundance gradient is flat, which is in contrast with the known Mg index gradient for this galaxy [7]. Work is in progress to reduce and analyze the full sample of objects, to investigate whether this is a peculiar property of NGC221, or whether other objects show the same phenomenon. We conclude that this is a most striking and unexpected first result, which may have a significant impact on the understanding of stellar populations in elliptical galaxies, and scenarios of galaxy evolution in general.

Figure 5. Ca II line strength as a function of radius for NGC221.

Figure 5. Ca II line strength as a function of radius for NGC221.

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