Infra-red imaging of perpendicular nested bars in spiral galaxies with the Infra-red Camera at the Carlos Sanchez Telescope S.N. Kemp (skemp@ll.iac.es) Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain S.J. Chan (sjchan@homemail.com) Institute of Astronomy, Madingley Road, Cambridge, UK J.E. Beckman (jeb@ll.iac.es) and P.L. Hammersley (plh@ll.iac.es) Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain 1998/02/11 Abstract. Images are presented of some of the galaxies in the sample of the BARS International Time Project in the near-infrared JHK s bands, obtained with the Infra-Red Camera at the 1.5-m Carlos Sanchez Telescope, Teide Observatory, between 1996-98. We discuss the scientific importance of imaging galaxies in the near-infrared bands and recent and future developments concerning the Infra-red Camera. 1. Introduction and Scientific Background In the context of the International Time Project BARS, data have been obtained relating to the structure and evolution of bars in galaxies. These data include broad band UBVRI optical imaging of the disks, bulges and bars of a group of nearby spiral galaxies, and spectral data for the bars. One of the principal aims of the project is to use the methods of stellar population synthesis to investigate the evolution of the bars. During the past 10 years a series of models of increasing sophistication have been developed to investigate the evolution of stellar populations in galaxies. The advances are based on two principles: taking into account the complexity, without using rough approximations (but always comparing with simpler analytic models to see qualitatively if the numerical models have not lost the basic physical principles), and using restricted, well-defined zones of a galaxy to avoid the confusion that results from combining populations that have developed under different conditions. The range of the models, which continue improving constantly, can be seen in two articles (Vazdekis et al. 1996, 1997), which deal with applications to ellipticals and lenticulars. Such objects have simpler histories of star formation than spirals, but we found that it was possible with them to infer a consistent history, given photometric data and spectroscopy of sufficient spatial resolution. In these models, and in general, the photometry gives a global mark, to avoid large errors in specc 1999 Kluwer Academic Publishers. Printed in the Netherlands. kluwnewirpos.tex; 30/03/1999; 21:34; p.1
2 S.N. Kemp et al. Figure 1. J image of NGC 5850 with a total exposure of 27 minutes. Note twisting of isophotes indicating inner feature perpendicular to main bar ifying the balance between stars of different types in a population and the spectroscopy helps to estimate more directly parameters with some ambiguity such as the metallicity and the fractions of stars of different luminosity classes. As an integral part of these investigations of bars, we obtained time at the Carlos Sanchez Telescope, Teide Observatory, Tenerife during the BARS project in 1996-97 and also later in May 1998 to obtain images in J and K s of the galaxies under study. Extending the baseline of the photometry into the infra-red bands allows us to estimate with much more confidence the fraction of dwarfs and late-type giants in a population; basically comparing V?I with I? K distinguishes between more dwarfs or more giants in a region under study (I? K less red than V? I implies a lower fraction of giants). In addition, the images in J and K s will permit us to calibrate and eliminate the effects of interstellar dust in the photometry. This is possible for two reasons. The dust features have a characteristic geometry in the bars, where they are seen as dark lanes in the leading edge of the bars. In general terms the dust is organised in patches that correspond with the neutral gas clouds (atomic and molecular) and with sufficient angular resolution it is recognisable as such. Manipulating the images in UBV RIJK, making maps in I? K and comparing with maps in B?V for example, it is normally possible to find and calibrate the dust clouds (where they are too faint to detect in this manner, their presence will not affect the results of the stellar photometry). kluwnewirpos.tex; 30/03/1999; 21:34; p.2
Infra-red imaging of barred galaxies 3 Figure 2. J band image of NGC 7479 of 13 minutes total exposure. This detection serves two purposes: to fine-tune the stellar photometry, and quantify the amount of dust, in order to include its effect in models of the interstellar medium of the bars. Comparing the stellar population inferred from the colours corrected for dust with the dwarf and giant star spectroscopy (especially the Ca II triplet), there is a good chance of a self-consistent solution for a defined zone of a galaxy. 2. Observations and Results So far (as at 1 December 1998) we have had three separate observational periods with the CAIN (infra-red camera) on the 1.5-m Carlos Sanchez Telescope, Teide Observatory, Tenerife, plus some test observations in late 1996. The first period in January 1997 was affected by instrumental problems. The second and third periods suffered from poor weather. In all, out of 21 nights obtained, we have only had one photometric night and 5 nights of poor quality data (cirrus etc.). We have obtained further time from 22-30 December 1998 and have applied for more time in March 1999 in semester 99A. During our observing time thus far, the detector of CAIN had 256256 pixels of HgCdTe (NICMOS 3) sensitive in the interval 1 2.5 µm. The pixel scale was 0.39 2 arcseconds and hence the detector field was 100100 arcseckluwnewirpos.tex; 30/03/1999; 21:34; p.3
4 S.N. Kemp et al. Figure 3. J band image of NGC 7479 of 12 minutes total exposure, taken with the new wide field of CAIN. The increase in depth and field size can be clearly seen the three stars at the extremes of Fig. 2 can be seen near the bar in this image onds. For most of the galaxies in our sample, this is large enough to contain the majority of the bar feature, or at least to indicate the orientation of this feature. Typically individual images were of 20s duration in J, and 5s in K s and short series of these were taken, while similar duration sky (blank field) exposures were taken every 1 2 minutes or so because of rapid variations in infra-red sky brightness. The images were flat-fielded, bad pixels removed, sky-subtracted, processed to reduce electronic noise, aligned and co-added. Images are here presented of NGC 2273, NGC 5850, NGC 7479 and NGC 7743 in J, NGC 5383 in H and NGC 2273 and NGC 7743 in K s (Figures 1-2, 4-8). These are the only images so far obtained in photometric conditions for this project, and when fully calibrated (work in progress) we will be able to align and combine them with similar optical data from the BARS project (e.g. UBV WHT/AUXPORT images of these galaxies) to produce colour maps in e.g. V? K s. In the limited amount of clear weather obtained so far, photometric images in K s of only two galaxies have been obtained for K s a factor of 5 times the exposure time necessary in J was required to produce a good signal-to-noise image, and in the limited time available we have concentrated on obtaining good images in J. We hope to obtain good S/N images in K s of these galaxies on future observing runs, so that we can accomplish the scientific aims referred to above. The images of NGC 2273 (Figs. 5-6) and NGC 7743 (Figs. 7-8) have had a second pass data reduction performed on them in which the objects kluwnewirpos.tex; 30/03/1999; 21:34; p.4
Infra-red imaging of barred galaxies 5 Figure 4. H-band image of NGC 5383 of 36 minutes total exposure. Note that the beginning of a spiral arm can be seen in the upper right part of the field, while the main bar can just be seen running from lower left to upper right (where it is easier to see) in the image. Also some twisting of the isophotes can be seen in the central regions, which may indicate an internal bar are masked out and a better estimate made of the sky level, and the data is then re-processed with the improved sky images. It can be seen that a much cleaner final image is obtained. The orientations of the bar feature can be seen in the images of all of these galaxies, and in some cases structure can be seen within the nuclear region as well. In particular, for three galaxies (NGC 5850 in J Fig. 1; NGC 5383 in H Fig. 4; NGC 2273 in J;K s Figs. 5,6) there appears to be some form of isophote twisting in the central regions which may indicate the presence of an embedded bar feature which in the case of NGC 5850 and NGC 2273 is perpendicular to the main bar. Such near-ir images, which show more faithfully the true stellar and mass distribution unobscured by the effects of dust, are very useful for revealing the presence of such inner features. Such double bars or bars-within-bars features have been observed before, notably by de Vaucouleurs (1974), Buta & Crocker (1993), Wosniak et al. kluwnewirpos.tex; 30/03/1999; 21:34; p.5
6 S.N. Kemp et al. Figure 5. J image of NGC 2273 of 30 minutes total exposure, with 2nd pass reduction performed to clean up the image. Note twisting of isophotes indicating inner feature perpendicular to main bar (1995) and Friedli et al. (1996), while NIR surveys have been carried out by Jungwiert et al. (1997) and Mulchaey et al. (1997). It can be seen that once reliably calibrated, these images will be useful for combining with optical images to produce colour maps, and that, especially considering the improvements discussed below, this telescope/instrument combination is a powerful one for producing good quality infra-red images of barred galaxies. 3. Future Since our last observing run, CAIN has been equipped with new optics, using the same detector. This gives a pixel size of 1 2 arcseconds resulting in a field size of 250 250 arcseconds ( 4:2 4:2 arcminutes). There is now the capability to switch between the wide and narrow (previous) fields. In terms of our project, the wider field offers the possibility of imaging all the regions of the galaxy in which we are interested during the same exposure, while the greater pixel size means that 6 times the amount of light will be falling on each pixel, thus increasing the sensitivity and the depth of the image by a similar amount ( 2 magnitudes). Of course the larger pixel size will mean a loss of spatial resolution, and for our scientific aims (see above) it is important to deal with small defined areas of a galaxy for kluwnewirpos.tex; 30/03/1999; 21:34; p.6
Infra-red imaging of barred galaxies 7 Figure 6. K s image of NGC 2273 of 30 minutes total exposure, with 2nd pass reduction performed to clean up the image. population synthesis to avoid combining populations evolved under different conditions. However the wider field and greater sensitivity will also result in a larger number of better-defined stellar centroids within the image which will increase the accuracy of the image-aligning algorithm, so the effective loss of spatial resolution will not be so large (it can be seen that some of the fields displayed here do not contain any bright stars which can be centroided, and so the individual images have to be aligned using the galaxy centers). As an example, Fig. 3 displays a J image of NGC 7479 obtained during the commisioning phase of the wide-field camera, of similar exposure time to Fig. 2, which is a J image taken with the narrow (old) field. The increase in depth and field size can be clearly seen. References Buta R., Crocker D.A., 1993, AJ, 105, 1344 de Vaucouleurs G., 1974, in Formation of Galaxies, IAU Symp. 58, ed. J.R. Shakeshaft. (Dordrecht: Reideal), 335 Friedli D., Wozniak H., Rieke M., Martinet L., Bratschi P., 1996, A&AS, 118, 461 Jungwiert B., Combes F., Axon D.J., 1997, A&AS, 125, 479 Mulchaey J.S., Regan M.W., Kundu A., 1997, ApJS, 110, 299 Vazdekis A., Casuso E., Peletier R.F., Beckman J.E., 1996, ApJS, 106, 307 Vazdekis A., Peletier R.F., Beckman J.E., Casuso E., 1997, ApJS, 111, 203 Wozniak H., Friedli D., Martinet L, Martin P., Bratschi P., 1995, A&AS, 111, 115 kluwnewirpos.tex; 30/03/1999; 21:34; p.7
8 S.N. Kemp et al. Figure 7. J image of NGC 7743 of 30 minutes total exposure, with 2nd pass reduction performed to clean up the image. Figure 8. K s image of NGC 7743 of 30 minutes total exposure, with 2nd pass reduction performed to clean up the image. kluwnewirpos.tex; 30/03/1999; 21:34; p.8