PGC 0029664 1188757: Photometry of two interacting galaxies and their debris
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(2) 1952. LIRA & GALAZ. Vol. 140. Figure 1. R image obtained at the Walter Baade telescope (left). Details for PGC0029664 (right). Table 1 Basic Parameters of Galaxies Name (1). R.A.(J2000) (2). Decl.(J2000) (3). Morph. Type (4). Size (5). DL (6). PGC 0029664 PGC 1188757. 10:11:05.2 10:11:08.9. +01:13:27 +01:14:19. SBbca Sc/db. 30.1 × 13.3 14.1 × 4.3. 138.7 139.7. Notes. (1) Name from the Principal Galaxies Catalog. (2) Right ascension J2000. (3) Declination J2000. (4) Morphological type from the Paturel et al. (1989)a and this paper.b (5) Major and minor diameters in kpc. (6) Computed luminosity distance in Mpc using the redshifts of z = 0.033073 for PGC 0029664 and z = 0.033300 for PGC 1188757. Table 2 Solution for the Photometric Calibrations ZB a. CB b. kB c. ZV a. CV b. kV c. ZR a. CR b. kR c. ZI a. CI b. kI c. −26.81 −0.04 0.26 −26.74 −0.06 0.10 −26.87 0.01 0.07 −26.28 −0.004 0.03 (0.06) (0.005) (0.06) (0.04) (0.003) (0.04) (0.03) (0.004) (0.003) (0.12) (0.001) (0.002) Notes. a Photometric zero point in the respective filter with the rms error in parentheses. b Color term coefficient in the respective filter with the rms error in parentheses. c Extinction coefficient in the respective filter with the rms error in parentheses.. 2.3. Photometric Calibrations Photometric calibrations were performed using standard stars from Landolt (1992), computed using the IRAF/PHOTCAL package. We observed five standard stars in each band, providing zero points and color terms for the following transformation equations: B = mB − ZB − CB (B − V ) − kB XB. (1). V = mV − ZV − CV (B − V ) − kV XV. (2). R = mR − ZR − CR (V − R) − kR XR. (3). I = mI − ZI − CI (V − I ) − kI XI ,. (4). where B, V, R, and I are the calibrated magnitudes in the Johnson–Cousins standard system; mB , mV , mR , and mI are the instrumental magnitudes; ZB , ZV , ZR , and ZI are the photometric zero points; kB , kV , kR , and kI are the extinction coefficients; XB , XV , XR , and XI are the mean air masses for the moment of observation; and CB , CV , CR , and CI are the color terms. The corresponding coefficients are presented in Table 2.. The errors for the aperture photometry are computed using the standard rules of error propagation from the original measured fluxes, and considering also the S/N of the different stars and the detector features (read-out noise and gain values). 2.4. Morphology PGC0029664 is classified as SBbc by Paturel et al. (1989) and as an Irregular “interacting” galaxy by Impey et al. (1996). Impey et al. (1996) have set the gray-scaling parameters to values that emphasize the low surface brightness features, but precluding some details in the galaxy nucleus or other highdensity regions. This results in significant differences in the morphological classification respect other authors. The nearly edge-on companion galaxy PGC1188757 has not been classified morphologically by other authors. From our images, we classify this last galaxy as an Scd. PGC0029664 presents a bulge from which two spiral arms proliferate. From an eyeball estimation, the bulge has a size of ∼6 kpc. An inner disk (or bulge disk) is also apparent, where spiral arms are very well defined. Outside the external disk, the spiral arms loose their form and dissipate. The high brightness of the spiral arms, especially in the blue band, suggests young.
(3) No. 6, 2010. PHOTOMETRY OF TWO INTERACTING GALAXIES. 1953. 41 40. 39. 38. 37 36. 19. 35. 11. 20. 34. 10. 33. 12 21. 14. 5. 13. 32. 4. 31 3. 30. 15. 16. 22 17. 1. 6. 29. 2. 27. 7. 18. 28. 8 9. 25. 24. 23. 26. 46 47. 48. 45. 44. 43. 42. Figure 2. Sizes and spatial positions of the colors measurements. Top figure is PGC0029664 with the tail, and bottom figure is PGC1188757 with the bridge.. stellar populations. A long and unreported bridge of low surface brightness material (∼22.5–23.8 B mag arcsec−2 ), sized about 12 kpc and detectable with large telescopes or long exposures, goes from one of the spirals arms of PGC0029664 to PGC1188757. At the beginning of this bridge there is evidence of more compact material which, although not a dominant feature in PGC0029664, attract our attention as a possible starforming region. PGC0029664 has two secondary and weaker spiral arms which have no continuity along the disk, blending into the dispersed material. The distribution of apparent starforming regions varies along the spiral arms and bridge, and can be appreciated at the end of the spiral arms. It is worth noting that in the bridge there are prominent bright regions, which are very likely star-forming regions.. 3. LOCAL STELLAR POPULATIONS In order to provide some basic data on the stellar populations embedded in the different regions observed in the studied interacting system, we obtained photometric measures at several positions, including the tail and bridge. Colors were measured using circular apertures, shown in Figure 2, while colors and surface brightnesses in the B band are listed in Table 3. At this point, we can compare our colors with those from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7). Given the complex morphology of the galaxies under study, the only object that can be compared straightforwardly because its simple shape is the galaxy PGC1188757, which has ugriz Sloan photometry as a single object (available.
(4) 1954. LIRA & GALAZ. Vol. 140. Table 3 Colors for the Different Locations of the Interacting System PGC0029664-1188757, Correlated with Positions Shown in Figure 2 V −R (4). V−I (5). ID (1). μB (2). 0.29. 0.07. 0.43. 10. . .. 22.77. 24.12. 0.22. 0.11. 0.55. 11. . .. 22.13. 44. . .. 23.65. 0.20. 0.12. 0.44. 12. . .. 29. . . 30. . . 31. . . 32. . . 33. . . 34. . . 35. . . 36. . . 37. . . 38. . . 39. . .. 23.50 23.98 23.73 23.62 24.22 23.00 24.20 24.16 23.94 23.91 24.51. 0.64 0.60 0.45 0.53 0.55 0.39 0.62 0.64 0.59 0.53 0.62. 0.47 0.30 0.27 0.07 0.35 0.23 0.35 0.37 0.30 0.29 0.25. 1.26 0.61 0.62 0.21 0.74 0.61 0.96 0.95 0.85 0.78 0.73. 40. . . 41. . .. 24.28 25.34. 0.47 0.66. 0.12 0.10. 0.54 0.72. ID (1). μB (2). 42. . .. 24.23. 43. . .. B −V (3). V −R (4). V−I (5). 23.10. 0.47 0.32(†) 0.38 0.27(†) 0.56. 0.37 0.21(†) 0.35 0.14(†) 0.37. 0.84 0.47(†) 0.58 0.51(†) 0.89. 13. . .. 23.08. 0.53. 0.38. 0.85. 14. . . 15. . . 16. . . 17. . . 18. . . 19. . . 20. . . 21. . . 22. . . 23. . . 24. . .. 23.10 22.92 23.24 23.39 23.70 23.63 23.21 23.73 23.66 23.01 22.57. 25. . . 26. . .. 22.83 23.00. 0.54 0.50 0.55 0.55 0.56 0.49 0.37 0.54 0.46 0.66 0.54 0.33(†) 0.53 0.54. 0.38 0.39 0.38 0.36 0.22 0.27 0.25 0.24 0.24 0.46 0.41 0.17(†) 0.37 0.35. 0.77 0.86 0.79 0.75 0.34 0.75 0.60 0.46 0.33 0.97 0.81 0.58(†) 0.71 0.64. 27. . .. 23.18. 0.56. 0.32. 0.61. 28. . .. 22.74. 0.42. 0.31. 0.56. 0.36 0.56 0.51. 0.83 1.34 1.19. 0.60 0.32∗ 0.46 0.18∗. 1.21 0.59∗ 0.95 0.33∗. Bridge. B −V (3) Spiral arms. Tail. Bulge 2. . .. 21.80. 0.67. 0.50. 1.04. 3. . .. 21.58. 0.47. 0.49. 0.83. 4. . . 5. . . 6. . .. 22.72 22.57 21.58. 0.67 0.53 0.67. 0.47 0.43 0.53. 1.03 0.92 1.07. 7. . .. 22.02. 0.67. 0.52. 1.07. 8. . .. 22.52. 0.55. 0.43. 9. . .. 22.57. 0.51. 0.41. Disk 45. . . 46. . . 47. . .. 23.73 24.40 24.00. 0.46 0.40 0.54. 0.94. 1. . .. 21.04. 0.84. 48. . .. 22.38. 0.87 0.38∗ 0.65 0.16∗. Nuclei. Notes. (1) Identification number as in Figure 2. (2) Surface brightness in the B band measured for the position indicated by ID. (3): B − V index measured for the point ID. (4): V − R index measured for the point ID. (5): V − I index measured for the point ID. (*) Corrected from internal extinction, see Section 3.1. (†) Color after the removal of the light contribution of the underlying redder stellar population embedded in the arm. See the text for details.. from the Data Release 7, DR7) with photometric errors of about 2%, except the u band with an error of 50%, which is not used here. Using the values from the SDSS database for PGC1188757, and converting the SDSS magnitudes to the Johnson–Cousins system using the transformation equations of Jordi et al. (2006), we obtain B − V = 0.66, V − R = 0.35, and V − I = 0.88. These values yield absolute color differences between our observed colors and those from the SDSS for PGC1188757, of Δ(B − V ) = 0.01, Δ(V − R) = 0.11, and Δ(V − I ) = 0.10. All these differences are of the order of our photometric errors, implying that our galaxy photometry is reliable and is worth to be compared with synthetic colors. The coordinates of circular apertures were located interactively on each region by identifying by eye possible places of stellar formation. Once the flux is measured to be 1σ above the sky level, the transformation to magnitude is applied, and corrected from Galactic extinction using the maps of Schlegel et al. (1998). 3.1. Extinction As in other extragalactic studies, the knowledge of the amount of internal extinction is crucial when basic properties of stars. populating galaxies are going to be determined from observed colors. Unfortunately, in grand design spirals like PGC0029664, it is impossible to associate a uniform value of internal extinction for the entire galaxy: it can varies significantly from one region to another within the galaxy. In an attempt to give a first approximation of the amount of extinction present in this system, we use the SDSS spectral database to extract some basic information from the emission lines useful to estimate the internal reddening, at least for PGC0029664 for which SDSS spectroscopic data is available. For this galaxy, the spectrum was taken for the nucleus. By measuring the flux ratios between Hα and Hβ (Osterbrock 1989), and assuming that they have a nebular nature, we estimate that the reddening in the very central region of PGC0029664 is significant and equivalent to E(B − V ) ∼ 0.49 mag. Using a standard extinction law for PGC0029664 (Rv = 3.1), we derive an extinction value of Av = 1.52, and using Table 6 from Appendix C of Schlegel et al. (1998) for the Landolt BV RI filters, we obtain E(V − R) = 0.28 and E(V − I ) = 0.62 mags. These figures imply that the nucleus of PGC0029664 is quite extinguished by dust. This agrees with the fact that many such.
(5) No. 6, 2010. PHOTOMETRY OF TWO INTERACTING GALAXIES. 1955. Table 4 General Comparison of Colors in the Different Regions in Each Galaxy PGC0029664 (1) B − V V − R V − I . PGC1188757. Tail. Bridge. Nucleus (2). Bulge (3). Spiral arms (4). Nucleus (5). Disk (6). (7). (8). 0.87(0.05) 0.38∗ (0.05) 0.60(0.05) 0.32∗ (0.05) 1.21(0.05) 0.59∗ (0.05). 0.59(0.08). 0.51(0.07). 0.47(0.06). 0.56(0.05). 0.24(0.04). 0.47(0.04). 0.34(0.06). 0.48(0.08). 0.26(0.12). 0.10(0.02). 0.98(0.09). 0.69(0.18). 0.65(0.05) 0.16∗ (0.05) 0.46(0.05) 0.18∗ (0.05) 0.95(0.05) 0.33∗ (0.05). 1.12(0.21). 0.74(0.05). 0.47(0.05). Notes. (1) Color average index. (2) Nucleus of PGC0029964 (point 1 in Figure 2). (3) Bulge of PGC0029964 (points 2–9 in Figure 2). (4) Spiral arms of PGC0029964 (points 10–28 in Figure 2). (5) Nucleus of PGC1188757 (point 48 in Figure 2). (6) Disk of PG1188757 (points 45–47 in Figure 2). (7) Tail (points 29–41 in Figure 2). (8) Interaction bridge (points 42–44 in Figure 2). The error associated with each measure is indicated in parenthesis. (*) Values with asterisks indicate colors corrected from internal extinction, as explained in Section 3.1.. disturbed systems are found to be nuclear starburst, like this case, which also fits with this picture when the near-IR photometry is considered (Galaz et al. 2002). Thus, the nuclear colors should be corrected from this extinction, and in consequence the age-metallicity analysis which follows from colors within this region should be considered with caution. In Tables 3 and 4, we show observed and extinction-corrected colors for the nucleus of PGC0029664. A similar analysis is done for PGC1188757, using the same extinction values as obtained for PGC0029664, because for this galaxy we do not have spectral data available (see Table 4). In the more external regions, away from the nucleus, we cannot establish an accurate value for the extinction. However, as in other studies, we can assume that the extinction is smaller than the one observed in the nucleus, and so colors could better represent differences in ages and metallicity. Moreover, in the debris and well-dispersed tidal features of PGC0029664, extinction could be quite lower than in the arms, in accordance to lower metallicities, reaching typically in these kind of interactions about 1/5 of solar values. 3.2. Colors For estimating the instrumental error of colors we assume a Poisson noise and obtain values smaller than 0.02 mag in B − V and V − R, and smaller than 0.08 mag in V − I. These errors increase exponentially as a function of the B surface brightness (see Figure 3(a) and Table 3), while the color standard deviation is large (see Figure 3(b)). The color dispersion is similar in all bands. The nucleus of PGC0029664 (point 1) is 0.23 mag redder than the bulge (within the inner ∼6 kpc from the center), suggesting a conspicuous difference in their stellar populations. However, this conclusion should be taken with caution, since the nucleus of PGC0029664 is also heavily extinguished, with a color excess E(B−V ) = 0.49 (see Section 3.1). In fact, the large color dispersion in the bulge could be understood also by dust concentrations, a normal expectation in interacting, and barred systems. Usually, the dust surrounding the galactic disk looses energy due to gravitational interactions with other galaxies and concentrates around the nucleus of the host galaxy (Barnes & Hernquist 1996). The larger spiral arm (points 23–29 in Figure 4) is 0.26 mag bluer toward the end, with similar colors to those observed in the secondary spiral arms (points 10–18 and 19–22). This suggests that these regions present similar stellar populations. A more. precise color estimate for the stellar formation regions observed in the spiral arms requires a subtraction of the underlying stellar populations. This is the case of the regions marked 10, 11, and 24, as seen in the images. In Table 3, the colors with a (†) symbol represent the colors of these stellar formation regions corrected from the underlying (redder) color of the arm where these regions are sited. To obtain these colors, we have computed the magnitudes of the closest arm portions near these stellar formation regions and subtracted them from the magnitudes we directly measure for these spots. For all the three stellar formation regions, colors are bluer than the rest of the arm regions. Interestingly, all these three regions have similar colors when compared with the bridge regions (see Table 3), suggesting that the bridge is partially composed by genuine stellar formation regions. The bridge exhibits blue regions with colors similar to those obtained for other interacting spirals by, for example, Schombert et al. (1990), suggesting that very young stars are observed. It is worth mentioning that the nucleus of PGC1188757 is bluer than the disk (and even more without taking into account extinction), except near the bridge where colors are similar to those of the spiral arms and the tail (V − I and V − R). By comparing the observed colors with those from spectrophotometric models of galaxy evolution, it is possible to constrain the age and the metallicity of the dominant stellar populations for each region of the system. In all the discussion, the reader must consider that an internal extinction could be present. In fact, as explained in Section 3.1, the nucleus of PGC0029664 is significantly extinguished, as probably is PGC1188757, and therefore much of the conclusions involving age and metallicity of the central regions must be taken with a cautionary flag. 4. THE INTERACTING SYSTEM Synthesis models such as those developed by Bruzual & Charlot (BC1993, BC1996, BC2003) are useful tools in the understanding of galaxy formation and evolution. These models use a wealth of information relating stellar evolution theories and spectral libraries. They include a set of parameters, such as the initial mass function (IMF, ξ (M)), the stellar formation rate (SFR, ψ(t)), and outcomes of semi-empirical theories of chemical enrichment (Z(t)). These models provide, among many other quantities, the integrated colors of stellar populations as a function of stellar age and evolving metallicity, colors which will be compared with our observations..
(6) 1956. LIRA & GALAZ. Vol. 140. (a). (b). 0.08. 0.8. 0.06. 0.6. 0.04. 0.4. 0.02. 0.2. 0. 20. 22. 24. 0. 26. 20. 22. 24. 26. Figure 3. Estimated errors for colors. Poisson noise (a) and the standard deviations of the mean colors (b) are shown as a function B surface brightness. 1. 0.8. (a). 0.8. (b). 0.6. 0.6 0.4 0.4 0.2. 0.2 0. 0. 10. 20. 30. 1.5. 40. 50. 0. 24. 0.5. 22. 0. 10. 20. 30. 40. 10. 20. 30. 26. (c). 1. 0. 0. 50. 20. 40. 50. (d). 0. 10. 20. 30. 40. 50. Figure 4. B − V , V − R, and V − I colors, and B surface brightness. The number on x-axis corresponds to the location in Figure 2.. Here, we use the BC2003 model to provide B − V , V − R, and V − I as a function of age. We consider a simple instantaneous burst of stellar formation (SSP) with an exponential decline of stellar formation as function of time. The IMF used is that of Salpeter (1955), with lower and upper mass limit of 0.1–100 M , respectively, for a metallicity range of. 0.005–2.5 Z . The timescale indicates different amplitudes of the stellar formation which decreases exponentially from the stellar formation burst (t0 ) to present, with τ > 0 for SFRs ∝ e−t/τ , and τ = 0 for SFRs ∝ δ(t − t0 ). We use an exponential timescale similar to that of the stellar formation processes for disks, i.e., of the order of 1 Gyr. Given the evidence for the age.
(7) No. 6, 2010. PHOTOMETRY OF TWO INTERACTING GALAXIES. of the universe by WMAP (13.7 Gyr), and assuming that first stars were formed about 200 Myr after the big bang (Bennett et al. 2003), it is reasonable to assume in the model that the stellar formation in the universe began 12 Gyr ago. This last figure puts a natural upper limit for the ages used by our stellar models. 4.1. Comparing Observations with the Spectrophotometric Model Now we describe the procedure to compare the colors provided by the synthesis model with those provided by observations. As we will show, this procedure considers the intrinsic age-metallicity degeneracy. A point in a space of N dimensions, can be defined by a set of N photometric values (e.g., colors), each associated with a specific region in a galaxy, like a stellar formation region, a region in the spiral arms, etc. Thus, as a galaxy evolves with time t, the synthesis model traces a line in such a space of N dimensions. The age t and metallicity (Z) are parameters which will be fitted in order to obtain the smaller differences between model and observed parameters, in this case colors. The combinations of age, metallicity, IMF, and SFR from the models which best describe the data, will be those that generate synthetic colors which best fit our observed colors, with the constraints given by the age of the first stars. We define the best fit when the distance between points in our N dimension space reaches a minimum value. Note that the distance estimator or similarity among the measured magnitudes has a significant impact on the determination of the physical properties. We use a maximum likelihood estimator, allowing to weight for the differences between the observed and predicted colors (with the corresponding errors). The estimator to be maximized is then defined as 3 1 (cn − Cn )2 2 χobs = , (5) N n=1 ΔCn2 where Cn are the measured colors B − V , V − I, and V − R; cn are the synthetic colors and ΔCn2 are the errors associated with these last ones. With the above expression, it is possible to obtain the solution (t, Z) which best approximates to the observed 2 value. This value is settled as the minimum value of the χobs 2 2 formulation, such that χobs < χN;α , where N is the number of available colors (N = 3). Therefore, N is the degree of freedom of the χ 2 distribution and α is the level of significance. The colors predicted by the composite stellar populations are presented in Table 4. For each region (spiral arms, tail, bridge, and nucleus) we consider the average color, with the standard deviation defined as the associated color errors. We note that the average color of the spiral arms does change (∼0.08 mag) if one excludes from the average computation the colors of the star-forming regions (spots 10, 11, and 24). We compare observations with 12 different synthesis models: 6 considering an instantaneous burst of stellar formation and 6 considering an e-folding SFR decline with a timescale of 1 Gyr. The models that best reproduce the observed properties according to the SFR type are presented in Table 5. Again, the results for the nucleus must be taken with caution. Recall that an extinction of E(B − V ) = 0.49 mag is present, which when included in the analysis, would turn the nucleus less metallic, younger, and even bluer than shown in Table 5. Our results show that at low metallicities, models agree with observations only when they have a large fraction of old stars. On the other hand, when metallicity is large, models are consistent. 1957. Table 5 Parameters of the Synthesis Models which Best Describes the Observed Colors ψ(t) ∝ δ(t − t0 ) Age. (109. yr). ψ(t) ∝ e−t/τ Age (109 yr) (4). Z (Z ) (5). (1). (2). Z (Z ) (3). PGC0029664 Nucleus Bulge Spiral arms. 12 >12 0.9†. 0.4 0.005 0.4†. 5† 3† 3.75. 2.5† 2.5† 0.02. PGC1188757 Nucleus Disk. 1.28 4. 1 0.005. 4.25† 2.4†. 0.4† 2.5†. Tail Bridge. 1.14† 0.18†. 0.4† 0.2†. 4.75 0.9. 0.02 0.02. Notes. (1) Region of the galaxy system. (2) Age in units of Gyr, considering an instantaneous burst of stellar formation. (3) Metallicity in Z considering an instantaneous burst of stellar formation. (4) Age in units of Gyr, considering an exponential decline with a timescale of 1 Gyr. (5) Metallicity in Z , considering an exponential decline with a timescale of 1 Gyr. † Between the two best 2 . The ages are approximated synthesis models, this has the minimum value of χobs 2 . to the minimum value of χobs. with observed colors only when a large fraction of young stars is present. Note that we consider only models where a minimum 2 χobs is obtained (the best model† in Table 5) in the space of solutions and parameters, with the restriction that age must be smaller than the age of the first stars (i.e., t < 12 Gyr). Some solutions do have metallicities where ages are outside this limit, and thus were not considered as plausible solutions. With this method, we reduce the impact of the existing age-metallicity degeneracy. There is one combination of parameters which better reproduces the observed colors of the nucleus and bulge of PGC0029664. Such a model has an exponential SFR, as we 2 see in Figure 5. We obtain in these regions a maximum χobs ≈ 0.09 (Equation (5) considering the minimum values of the estimator). This corresponds to an average difference between the observations and the model of ∼0.03 mag for a typical color uncertainty of 0.06 mag. On the other hand, the model which best reproduces the observations for the spiral arms data turn to be an instantaneous burst of stellar formation (Figure 5). Here, we 2 obtain a minimum χobs ≈ 0.14, corresponding to an average difference between the observations and the model of ∼0.06 mag, for a typical color uncertainty of 0.10 mag. Note that if we consider colors of the stellar formation spots in the spiral arms corrected from the underlying populations residing in the spiral arms, then the range of age of the spiral arms are only slightly reduced, since these spots do not have a heavy weight into the average computation. Our computations show that they are only ∼5% younger than indicated by the dashed line in the middle panel of Figure 6. This yields an age of the spiral arm of about the same age than the bulge (see Section 4.2). The stellar populations of PGC1188757 are better represented by an exponential SFR (Figure 6). For the disk, we obtain a 2 2 minimum χobs ≈ 0.55, and for the nucleus a χobs ≈ 0.10. These correspond to ∼0.10 and ∼0.04 mag of difference between the observations and the model, considering a typical color error of 0.08 mag. The synthesis model which best reproduces the observed colors of the interaction region (tail and bridge) is an instantaneous burst of stellar formation (Figure 7). In the tail, we obtained a 2 minimum χobs ≈ 0.50 corresponding to an average difference.
(8) 1958. LIRA & GALAZ. 2 obtained with two synthesis models applied to the Figure 5. Minimum χobs nucleus (top panel), bulge (middle panel), and spiral arms (bottom panel) of PGC0029664, according to Table 5, that best reproduces the observed colors. The solid line represents an instantaneous burst of stellar formation. Dashed lines indicate an e-folding decline with a timescale of 1 Gyr.. Figure 6. Identical to Figure 5 but for galaxy PGC1188757. Nucleus (top panel) and disk (bottom panel).. between the observations and the model of ∼0.17 mag for a color uncertainty of ∼0.14 mag. Finally, we obtain a minimum 2 χobs ≈ 7.50 in the bridge. This means a difference between the observations and the model of ∼0.14 mag (considering a color error of 0.03 mag). 4.2. Ages and Metallicities We estimate ages and metallicities of the composite stellar populations using the BC2003 models and the maximum likelihood estimator already described (Equation (5)). There are basically two different groups of estimates, depending on the. Vol. 140. Figure 7. Identical to Figure 5 but for the interaction region. Tail (top panel) and bridge (bottom panel).. function adopted for the SFR. We examine the ages and metallicities of the different components of the system as follows. 1. PGC0029664. When using an instantaneous burst of stellar formation, the models only give low thresholds for the ages of the usually oldest structures (bulge and nuclear regions). The bulge being older than 6 Gyr and the nucleus older than 8 Gyr (see Figure 5). However, for the spiral arms, the instantaneous burst gives a rather well constrained age of 1 Gyr. When using an SFR with an exponential decline, these oldest components appear to have well-constrained ages, the nucleus with an age of ∼5 Gyr, and the bulge slightly younger, with 3 Gyr. The spiral arms, on the other hand, appear slightly older than the bulge, with 4 Gyr. This last result could be a direct consequence of the fact that when computing colors of the stellar formation regions, one should subtract the underlying light of the disk and arm component. However, when this procedure is done, the average age of the arms does not change significantly (<5%), since in terms of color surface density these spots actually occupy a small surface of the total extent of the spiral arms. A more precise computation of the age of the different components of the spiral arms should involve the modeling of the spatial distribution of the different stellar populations along the arms, which should at least consider two stellar underlying populations, e.g., the disk and the younger populations typically observed in an spiral arm. 2. PGC1188757. For this system, the instantaneous burst gives a constrained age for the nucleus (1 Gyr) and a less constrained age for the disk (3–10 Gyr, see Figure 6). The fact that the nucleus appear younger than the disk in this galaxy suggests a possible secular evolution for PGC1188757, not surprising after the observed major interaction. However, the exponentially declining SFR give better constrained ages and gives more conservative age estimates, yielding a nucleus of ∼4 Gyr, older than the disk of 2 Gyr..
(9) No. 6, 2010. PHOTOMETRY OF TWO INTERACTING GALAXIES. 3. Bridge and tail. For these observed components, the instantaneous burst gives ages of 1 Gyr for the tail and less than 1 Gyr (180 Myr) for the bridge (see Figure 7). The exponentially declining SFR gives, on the other hand, a less constrained age for the tail, between 3 and 8 Gyr, and a much better constrained age for the bridge, of about 1 Gyr. We emphasize that the color of the spots observed in the bridge are quite similar to those observed in the regions 10, 11, and 24 of the spiral arms, which are very likely spots of recent star formation. This suggests that at least some regions observed in the bridge are composed by young stars. Regarding metallicities, the models give rather unsurprising values, with the bulge and nucleus of PGC0029664 as metal rich components (2.5 Z ), and the spiral arms and tail as metal-poor components (0.4 Z ). The associated error in the metallicity is about 0.9 Z . The above results show that when trying to fit broadband colors with spectrophotometric models of galaxy evolution, the resulting ages of the different galaxy components (and the associated errors) are very sensitive to the form of the SFR. 5. CONCLUSIONS We have presented basic photometric data and a simple spectrophotometric analysis of an interacting system composed by two spiral galaxies (PGC0029664 and PGC1188757). By comparing the observed colors with synthetic colors from Bruzual & Charlot models of galaxy evolution Bruzual & Charlot (2003, BC2003), we are able to constrain age and metallicity of the stellar components which compose the many different conspicuous structures in the system, likely formed by the interaction. PGC0029664 presents two very well defined spiral arms emerging from the inner disk region, becoming diffuse toward the external disk region. A tail of dispersed material is evident from the largest spiral arm, and a bridge of material ∼12 kpc long goes from the tail to PGC1188757. It is possible to affirm that in the interaction bridge very bright regions are observed. These regions have similar colors to some bright spots observed in the spiral arms of PGC0029664, which are regions with very blue and young stars. In fact, the BC2003 models suggest that the bridge is composed by quite young (180 Myr) and metalpoor (0.2 Z ) stellar populations. This suggests that the bridge was formed recently, probably less than 1 Gyr ago. Using BC2003 models, and using the color index obtained for different regions of the system (bulge, disk, tail, and bridge), we constrain age and metallicity of the stellar populations embedded in the different observed structures. The large galaxy (PGC0029664) has stellar populations with probable ages between 0.9 and 5 Gyr, and metallicities between 0.4 Z and 2.5 Z . The youngest and metal-poor stellar populations are located in the spiral arms (0.9 Gyr, 0.4 Z ) and the tail (1.14 Gyr, 0.4 Z ). From the SDSS optical spectrum, we estimate the internal extinction present in the very center of PGC0029664 as E(B − V ) = 0.49 mag, which is consistent with other spirals suffering interactions. This suggests that the nucleus of PGC0029664, and very likely also the nucleus of PGC1188757, are significantly extinguished, so their ages and metallicity are overestimated. Observed colors of the stellar populations of the nucleus of the satellite galaxy (PGC1188757) matches with an age of 4.25 Gyr and a metallicity of 0.4 Z (younger than PGC0029664).. 1959. On the other hand, the stellar populations embedded in the disk are ∼2.4 Gyr old, but metal-rich (∼2.5 Z ). We are grateful to the staff of Las Campanas Observatory, in particular the Magellan telescope engineers and assistants. This research was supported by FONDECYT grant 1085267 and by FONDAP “Center for Astrophysics” grant 15010003. We thank the anonymous referee, who helped to improve the content and form of this paper. This research has used the Sloan Digital Sky Survey (SDSS) database. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. 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