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Ages of Galaxy Bulges and Disks from Optical and Near-Infrared Colors

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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server Ages of galaxy bulges and disks from optical and near-infrared colors R. F. Peletier and M. Balcells Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, Netherlands ABSTRACT We compare optical and near-infrared colors of disks and bulges in a diameter-limited sample o of inclined, bright, nearby, early-typ e spirals. Color pro les along wedge ap ertures at 15 from the ma jor axis and on the minor axis on the side of the galaxy opp osite to the dust lane are used to assign nominal colors for the inner disks (at 2 scale length) and for the bulges ( 0.5 r ), resp ectively.We estimate that the e ects of dust reddening and the cross-talk b etween ef f the colors of the two comp onents is negligible. We nd that color di erences (bulge { disk) are very small: (U R)= 0:126 0:165, (R K )=0:078 0:165. Disks tend to b e bluer by an amount three times smaller than that rep orted by Bothun & Gregg B84 (1990) for S0's. Color variations from galaxy to galaxy are much larger than color di erences b etween disk and bulge in each galaxy. Probably, the underlying old p opulation of disks and bulges is much more similar than the p opulation paradigm would lead us to b elieve. Implied age di erences, assuming identical metallicities, are less than 30%. Subject headings: Galaxies:Bulges; Galaxies:Disks; Galaxies:Ages; Galaxies:Populations 1. Intro duction How di erent are the colors of bulges and disks of spiral galaxies? According to the Population paradigm, disks of spirals, containing Population I stars, are bluer and younger than bulges, the latter made up of Population I I stars. Also, the widespread b elief that bulges are metal rich (eg. Rich R88 1988) supp orts the notion that bulges are redder than disks. This notion needs to b e reexamined in the lightofthe following: (1) We nd that bulges of early-typ e spirals are not metal rich; their mean metallicities are rarely ab ove solar (Balcells & Peletier BP94 1994, hereafter BP94). Also the metallicity of our Bulge nowadays astro-ph/9602088 18 Feb 1996 is estimated to b e just ab ove solar (McWilliam & Rich MR94 1994). (2) The inner parts of disks are often very dusty(Valentijn V90 1990, Peletier et al. PVMFKB95 1995). Therefore, integrated colors (and colors derived from ellipse tting, see section 2.) trace reddening as much as p opulation characteristics. (3) Colors have b ecome useful age diagnostic to ols with the realization byFrogel F85 (1985) that the combination of optical and near-infrared (NIR) color indices allows to partially disentangle age and metallicity. (4) Integrated colors include contributions from bright, lo calized, blue star forming regions. The colors of the underlying disk contain information on the age of the older comp onents of the disk p opulation. Excluding regions of ongoing star formation and regions with signi cant reddening from the color determinations was not p ossible with ap erture photometry, but can now b e done using two-dimensional digital photometry and mo dern data pro cessing techniques. The recentavailability of large-format two-dimensional near-infrared detectors allows the determination of NIR color pro les without the limitations of studies based on NIR ap erture photometry data. In this {2{ pap er we analyze UBRIJK surface photometry of a sample of early-to-intermediate typ e, inclined disk galaxies. This work is part of a larger study of optical and NIR colors of disks and bulges (Peletier & Balcells, in preparation). Here we highlight that the colors of bulges and disks are very similar. Indeed, contrary to what would b e exp ected if bulges are indeed redder than disks, bulges do not app ear as distinct morphological comp onents in color index maps derived from optical and NIR images. We nd long dust lanes to b e the main morphological comp onent of the color maps. A few galaxies show additional features suchasnuclear sp ots of redder color with a size smaller than the bulge, and progressive bluing in the outer parts of the galaxy. Color variations from galaxy to galaxy are much larger than color variations within each galaxy. To put this result in a more quantitative fo oting, wework with color pro les of disk and bulge determined in a way which minimizes the e ects of dust reddening. Our results, from accurate near- infrared array measurements, show that color di erences b etween bulges and disks are very small. Age determinations derived from optical and NIR color-color diagrams with the use of simple p opulation mo dels indicate that the bulk of the (inner) disk and bulge stars are essentially co eval. At most, disk stars are 2-3 Gyr younger than bulge stars. We disagree with the conclusion of Bothun & Gregg BG90 (1990) that disks are bluer than bulges by(BH)0:4. 2. The data o Our sample consists of inclined galaxies (i 50 ), of galaxy typ e S0 to Sb c. The sample comprises the 30 galaxies of Table 1 of BP94, except for the two that were outside the declination range of UKIRT. The optical colors of the bulges have b een analyzed in that pap er. In short, the optical data consist of U,B,R and I surface photometry, obtained with a 400590 EEV CCD chip at the PF camera of the 2.5m INT telescop e at La Palma. The pixelsize was 0.549", and the e ective seeing on the images lies b etween 1.2" and 1.6". The images were taken under photometric conditions, and at elding was accurate to ab out 0.2%. The colors were corrected for Galactic extinction (BP94). We obtained near-infrared images for this sample at the United Kingdom Infrared Telescop e (UKIRT) using IRCAM3 (Puxley & Aspin PA94 1994), an infrared camera equipp ed with a 256 256 InSb array. The camera gives a pixel size of 0.291 arcsec and a eld of 75 arcsec. Cosmetically, the arrayisvery clean, with less than 1% bad pixels. For every ob ject we to ok images in 10 p ositions, of several readouts each, making up a total integration time of 100 s p er p osition. The ob ject was moved around on the chip on 6 of these exp osures, while 4 consisted of blank sky, ab out 10 arcmin away from the galaxy. The frames were at elded using median sky at elds, and mosaics were made aligning the individual frames. The e ective seeing on the nal frames was b etween 0.8" and 1.0". The frames here were also taken under photometric conditions, with errors for individual stars of 0.03 mag. Comparison with ap erture photometry shows that the maximum zero p oint errors are 0.1 mag in J and K . All galaxies were observed in K . In addition, 20 galaxies were also observed in the J -band. In Fig. ?? we show 'real-color' URK-images for this sample. To make the images we rst calibrated the frames in each band photometrically, and then aligned the three images using the p ositions of stars. Color pro les for bulges and disks were obtained by measuring surface brightness pro les on each o band along wedge-shap ed ap ertures. For the bulge pro les, wedges were 22.5 wide, centered on the disk semi-minor axes as determined by elliptical ts to the galaxy's image. In galaxies with prominent dust lanes, only the semi-minor axis away from the dust lane was used for subsequent analysis. Otherwise, we to ok the mean of the two pro les. Details on the metho d are given in Balcells & Peletier BP94 (1994). The o o disk surface brightness pro les were measured along 10 -wide wedge ap ertures centered 15 away from the {3{ disk ma jor axis, as determined in the K -band image. Wechose these ap ertures so as to avoid measuring over the prominent dust lanes near the ma jor axis of inclined galaxies. Wethus measure the lightofthe disk on the far side of the galaxy which is not extincted by the dust on the near side of the disk. The color o pro les could b e slightly a ected byanyvertical disk color gradients for inclinations ab ove80 . Wechose the wedge-ap erture metho d of determining color pro les over the more common metho d of integrating azimuthally along b est- t ellipses (Terndrup et al. T94 1994; Peletier et al. PVMF94 1994; de Jong dJ95 1995). For galaxies at high inclinations, such as those in our sample, the two metho ds givevery di erent results. The di erence is higher for galaxies with redder colors, a sign that the dust is resp onsible for the observed di erences. The e ects of dust in the wedge color pro les can b e estimated from the amplitude of the disk color gradients in various bands. For our sample, dust e ects app ear to b e small for galaxy typ es up to Sab and for some Sb's. 3. Colors and color di erences Colors in bulges and disks b ecome bluer radially outward (de Jong, dJ95 1995; BP94 BP94). Gradients are small enough however that we assign representativevalues for the color of each comp onent. For bulges, we take the color at 0.5R or at 5 arcsec, whichever is larger.
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