We use the population synthesis code STARLIGHT to t spectral synthesis models to all our spectra and we compute some characteristic properties of the underlying stellar component as the luminosity-weighted and mass-weighted stellar age and metallicity (see, e.g. Asari et al. 2007). We also use the model ts to correct our spectra for underlying stellar absorption.
Our galaxy sample spans a considerable range in its emission-line properties,
3.5. Spectral synthesis model tting 49
Figure 3.10: Integrated 1D observed spectra for each galaxy or part of a galaxy obtained at INT2.5m and WHT4.2m.
50Chapter 3. Spatially resolved spectroscopy of star-forming galaxies in the Hercules cluster
Figure 3.10: Continued.
3.5. Spectral synthesis model tting 51
Figure 3.10: Continued.
52Chapter 3. Spatially resolved spectroscopy of star-forming galaxies in the Hercules cluster
Figure 3.10: Continued.
3.5. Spectral synthesis model tting 53
Figure 3.10: Continued.
54Chapter 3. Spatially resolved spectroscopy of star-forming galaxies in the Hercules cluster from systems with very strong Balmer emission lines to systems where the Hβ is just detectable above a high level of stellar continuum. Especially in the latter cases, an accurate correction for underlying stellar absorption is crucial. The usual practice of adopting an EWabs ∼ 2Å for stellar Balmer lines, though generally justiable, bears the risk of introducing signicant systematic errors in the analysis of systems with faint emission lines. This is because the exact value forEWabs depends on the stellar population mixture i.e. the SFH in a galaxy.
For example,González Delgado et al.(1999) infered for a single stellar population (SSP) a monotonous increase of the EWabs's for the Balmer Hβ through Hδ lines from≤3 Å at an age≤3 Myr to>10 Å at ages∼0.5 Gyr, followed by a decrease for older ages. Star-forming galaxies, consisting of dierent SSPs of young to interme- diate age, in addition to a (signicantly) older underlying host galaxy, likely follow a considerably more complex time evolution in their luminosity-weighted EWabs. This issue has been explored in much detail by e.g. Guseva et al. (2001) who have considered a variety of SFHs and mass proportions between the young and older stellar components in SF dwarf galaxies. Obviously, for systems with low emission-lineEWs, improper correction for underlying stellar absorption may prop- agate into signicant errors in e.g., the intrinsic extinction or spectroscopic classi- cation using BPT (Baldwin et al. 1981) diagrams. For example, for an intermediate luminosity-weighted age of 0.2−0.6 Gyr, where BalmerEWabs are the largest, the underestimation of Balmer line uxes due to incomplete correction for underlying stellar absorption may shift a star forming galaxy on the log([Oiii]λ5007/Hβ) vs.
log([Nii]λ6583/Hα) BPT diagram into the zone of active galactic nuclei (AGNs).
The determination of the EWabs may be carried out by simultaneous tting of two Gaussians to each emission and absorption Balmer line prole. Another adequate, and arguably more ecient, technique for correcting for underlying stellar absorption is the tting and subsequent subtraction of the observed spectrum of a synthetic stellar spectrum in order to isolate the net nebular line emission uxes. To this end, we used the population synthesis code STARLIGHT (Cid Fernandes et al.
2004, 2005a,b; Garcia-Rissmann et al. 2005). This code synthesizes the observed stellar continuum as due to the superposition of SSPs of dierent age and metallicity.
The SSP library used is based on stellar models by Bruzual & Charlot(2003) for a Salpeter IMF for three metallicities (Z = 0.004,0.008,0.02)4 and 59 ages (between 0.25 Myr and 13 Gyr). Output of the model is the list of those SSPs with a mass contributionMi>0(%) and their luminosity contributionLi(%) to the wavelength interval to which the input spectra have been normalized (51005300 Å in this work).
Prior to modeling the observed spectra were de-redshifted and resampled to 1 Å pixel−1. Additionally, emission lines and residuals from the night sky subtraction were identied and interactively agged using a code developed by us. Throughout, we preferred not to correct the input spectra for intrinsic extinction or to strongly constrain its allowed range in the STARLIGHT models for a twofold reason: rst,
4The solar metallicity is assumed Z¯ = 0.0134, which is translated to an oxygen abundance 12+log(O/H)¯=8.69 (Asplund et al. 2009).
3.5. Spectral synthesis model tting 55 the extinction coecient c(Hβ) computed prior to correction of Balmer emission line uxes for underlying stellar absorption is, especially for galaxies with faint nebular emission, uncertain. An a priori de-reddening of the input spectra using this approximatec(Hβ) value would therefore yield no advantage towards reliability and computational expense. For the same reason, and in order to minimize biases in the STARLIGHT solutions, we have allowed the intrinsic extinction to vary between +4.5 and −0.7 V mag. The allowance for a negative AV is justied by possible uncertainties in ux calibration, the (partial) coverage of the physical spatial area of the galaxy, as well as the incompleteness of the SSP library. As evident from Table 3.2, solutions with AV≤0 are the exception in our analysis. Secondly, xing the intrinsic extinction in the STARLIGHT models to the observed c(Hβ) would imply that ionized gas and stars are subject to equal extinction. No compelling evidence exists for this as yet. On the other hand, for the sake of simplicity and to keep computational time low, we have throughout assumed a single foreground extinction model, although provision is given in STARLIGHT to solve for a dierent extinction in the young (≤10 Myr) and older stellar population.
A typical example of the spectral synthesis for a galaxy with relatively faint nebular emission in our sample is shown in Figure 3.11. The left panel shows the observed, normalized spectrum (orange color) with the best-tting synthetic spec- trum overlaid (blue color). Flagged intervals in each spectrum are marked with shaded vertical bars. The smaller panels to the upper-right and lower-right show the age distribution of the SSPs evaluated by STARLIGHT as a function of, respec- tively, their luminosity and mass fraction in %. Thin vertical lines in the upper right panel indicate the ages of the library SSPs. The shaded area in the lower diagram shows the smoothed Mi distribution and is meant as schematic illustration of the SFH. It may be seen from the left panel that synthetic spectra t well broad stel- lar absorption features, thus permitting an adequate determination and correction for underlying absorption. The corrected emission line uxes, as measured from the observed spectra after subtraction of the synthetic stellar spectra are listed in Table 3.3(see Section 3.6for a detailed discussion).
Although an elaborated study of the SFH of the sample galaxies is beyond the scope of this work, we took advantage of the STARLIGHT models to compute some characteristic properties of our sample galaxies. These include the luminosity- weighted and mass-weighted metallicityZ?,LandZ?,M and stellar ageτ?,Landτ?,M along with their standard deviations (see, e.g.,Asari et al. 2007). These quantities and the intrinsic extinctionAVof the stellar component, are listed in Table 3.2and will be further discussed in Section3.10.
56Chapter 3. Spatially resolved spectroscopy of star-forming galaxies in the Hercules cluster
Figure 3.11: A typical example of spectral synthesis with STARLIGHT for the galaxy KUG1602+175. The left panel shows the observed, normalized spectrum (orange color) with the best-tting synthetic spectrum overlaid (blue color). Flagged intervals in each spectrum are marked with shaded vertical bars. The smaller panels to the upper-right and lower-right show the age distribution of the SSPs evaluated by STARLIGHT as a function of, respectively, their luminosity and mass fraction in %. Thin vertical lines in the upper right panel indicate the ages of the library SSPs. The shaded area in the lower diagram shows the smoothed Mi distribution and is meant as schematic illustration of the SFH.
3.5. Spectral synthesis model tting 57
Table3.2:STARLIGHTOutput GalaxyZ?,LZ?,Mτ?,Lτ?,MAV GyrGyrmag PGC05185a0.014±0.0080.012±0.0080.94±0.301.08±0.280.98 PGC05185b0.009±0.0050.007±0.0042.02±1.027.28±3.071.68 PGC05185c0.014±0.0070.009±0.0042.64±1.248.54±2.071.32 IC1173a0.016±0.0060.019±0.0097.73±1.9211.10±0.80−0.21 IC1173e0.018±0.0090.016±0.0072.09±0.725.66±1.860.63 KUG1603+179Aa0.014±0.0070.012±0.0053.69±1.356.60±1.81−0.56 KUG1603+179Ab0.015±0.0060.018±0.0117.27±2.2511.62±0.760.91 KUG1603+179Ac0.012±0.0040.008±0.0054.80±1.9811.07±0.771.09 NGC6050Aa0.011±0.0040.014±0.0058.10±1.9311.36±0.580.47 NGC6050Ab0.013±0.0030.012±0.0055.67±1.8711.58±0.540.41 NGC6050B0.007±0.0020.004±0.0025.82±1.358.54±0.97−0.12 LEDA15435860.006±0.0020.008±0.0031.98±0.916.66±1.710.42 NGC6045a0.015±0.0050.018±0.0104.87±1.9410.82±1.200.57 NGC6045b0.013±0.0040.017±0.0085.42±1.7410.10±1.310.27 NGC6045c0.016±0.0080.017±0.0138.30±2.4512.68±0.282.13 NGC6045d0.013±0.0050.015±0.0084.25±1.7811.14±1.381.33 NGC6045e0.011±0.0070.010±0.0081.06±0.302.71±1.600.93 KUG1602+174A0.007±0.0020.008±0.0034.66±1.348.34±0.870.19 LEDA0847190.008±0.0040.012±0.0044.45±1.418.34±1.530.15 PGC057077a0.013±0.0060.018±0.0174.24±2.8612.03±1.193.23 PGC057077b0.014±0.0090.013±0.0081.75±0.412.10±0.60−0.70 UGC10190a0.007±0.0030.005±0.0023.43±1.468.14±1.910.22 UGC10190b0.007±0.0020.006±0.0025.85±1.2210.04±0.710.79 UGC10190c0.014±0.0050.009±0.0055.45±2.1611.63±0.640.82 LEDA1405680.008±0.0040.006±0.0030.86±0.454.84±2.170.56 D97ce2000.007±0.0030.008±0.0050.73±0.251.09±0.110.33 PGC057064a0.014±0.0050.017±0.0085.06±1.368.13±0.850.74
58Chapter 3. Spatially resolved spectroscopy of star-forming galaxies in the Hercules cluster
Table3.2:STARLIGHTOutput GalaxyZ?,LZ?,Mτ?,Lτ?,MAV GyrGyrmag PGC057064b0.011±0.0040.017±0.0076.56±1.7510.52±0.800.36 LEDA084703int0.009±0.0020.009±0.0021.22±0.262.42±0.500.31 LEDA084703a0.007±0.0050.008±0.0070.71±0.261.00±0.02−0.34 LEDA084703b0.009±0.0030.012±0.0041.97±0.525.93±1.600.50 LEDA084703c0.009±0.0050.008±0.0050.94±0.291.75±0.630.43 KUG1602+1750.013±0.0060.017±0.0073.76±1.549.74±1.550.55 LEDA0847100.015±0.0070.017±0.0082.86±1.249.05±2.051.02 CGCG108-1490.018±0.0080.019±0.0107.68±2.4611.11±1.080.40 KUG1602+174B0.016±0.0070.018±0.0082.91±1.117.76±1.310.48 LEDA0847240.007±0.0030.008±0.0033.15±1.247.57±0.920.37 SDSSJ160556.98+174304.10.008±0.0060.008±0.0070.82±0.491.26±0.210.61 [DKP87]160310.21+175956.70.006±0.0010.006±0.0023.11±1.487.72±0.690.31 SDSSJ160524.27+175329.30.004±0.0040.004±0.0041.06±0.211.06±0.10−0.01 SDSSJ160523.66+174832.30.007±0.0040.004±0.0022.25±0.996.16±2.520.38 IC11820.012±0.0040.014±0.0086.37±1.9711.56±0.881.47 IC1182:[S72]d0.019±0.0110.018±0.0111.92±1.4911.51±0.760.42 SDSSJ160531.84+174826.10.007±0.0030.008±0.0046.11±2.4611.10±1.131.11 SDSSJ160304.20+171126.70.007±0.0020.006±0.0023.88±0.715.78±0.630.48 SDSSJ160520.58+175210.60.008±0.0030.008±0.0035.76±0.657.11±0.460.50 Note:Theluminosity-weightedandmass-weightedstellarmetallicity(e.g.themassfractionofmetalsoverH,wherethesolarvalueis assumedtobeZ¯=0.019)Z?,LandZ?,Mandstellarageτ?,Landτ?,Malongwiththeirthestandarddeviationsandtheintrinsicextinction AVofthestellarcomponentasgivenbyttingSTARLIGHTmodels.