Free Access
Issue
A&A
Volume 553, May 2013
Article Number L7
Number of page(s) 5
Section Letters
DOI https://doi.org/10.1051/0004-6361/201321460
Published online 14 May 2013

© ESO, 2013

1. Introduction

The advent of large area surveys has enormously advanced our knowledge of galaxy formation and evolution in recent decades. In particular, the surveys consisting of imaging and single fiber spectroscopy (e.g. 2dFGRS, Folkes et al. 1999; SDSS, York et al. 2000; GAMA, Driver et al. 2011) have provided invaluable information on redshift as well as other galactic properties such as star formation rates (SFR) or metallicities, via the intensities of the most conspicuous emission lines. However, aperture effects are always present in such studies because of the limited (and variable with redshift) coverage of the individual galaxies. Consequently, a fraction of the total flux at all wavelengths is lost and, to date, no recipe to correct for this effect has proved satisfactory, although several studies of aperture effects have been carried out (e.g. Brinchmann et al. 2004; Ellis et al. 2005; Kewley et al. 2005; Salim et al. 2007; Kronberger et al. 2008; Gerssen et al. 2012; Zahid et al. 2013; Hopkins et al. 2013).

In practice, there are two main problems: one is the size of the projection on the sky of the aperture relative to the physical dimensions of the galaxy. The second one, which mainly affects single-fiber (or very small aperture) spectrographs (e.g. 2′′ for 2dFGRS and GAMA; 3′′ for SDSS), is the precise position of the aperture relative to the galaxy center. This aperture effect has implications for spectral stacking studies, since the fraction of a galaxy covered by a fixed aperture varies with redshift. Therefore using a realistic aperture correction is a sine qua non condition to carry out the analysis of aperture spectra of galaxies at different epochs, to derive their fundamental properties, and to discuss their evolution.

Gerssen et al. (2012) have recently carried out a study of a reduced sample of SDSS star-forming galaxies using the VIMOS integral field spectrograph (Le Fèvre et al. 2003) to map (with full spatial coverage) some properties related to star formation diagnostics. These authors found a high dispersion when comparing their results with color-based SDSS extrapolations. This suggests that full spatial coverage is needed to produce proper corrections for emission line intensities, and large samples of galaxies covering all morphological types are required.

The Calar Alto Legacy Integral Field Area Survey (CALIFA; Sánchez et al. 2012a) is observing a statistically well-defined sample of 600 galaxies in the local Universe with the Potsdam Multi Aperture Spectrograph in the PPAK mode (Roth et al. 2005). Galaxies have been selected from the SDSS-DR7 catalog (Abazajian et al. 2009) to fulfill the following conditions: (1) they fall in the redshift range 0.005 < z < 0.03; and (2) their isophotal diameters in the r′ band (isoA_r in SDSS-DR7) are in the range 45′′ < isoA_r < 80′′. Combining these two criteria results in a sample of galaxies that fall completely within the PPAK field-of-view and covers all relevant emission lines with a single spectral setup. The CALIFA galaxies completely sample the color-magnitude diagram down to Mr =  −19 mag (see Sánchez et al. 2012a). In summary, the data provided by CALIFA are well-suited to testing the aperture effects discussed above, thanks to complete spectral and spatial coverage of the sample galaxies, and providing aperture correction factors as a function of aperture size for quantities that vary smoothly with the latter. In our analysis, we adopt the following cosmological parameters: H0 = 72 km s-1 Mpc-1, ΩM = 0.27 and Ωλ = 0.73.

2. Aperture corrections

The basic tools for this process are the CALIFA reduced cubes from the DR1 sample (Husemann et al. 2013). Our starting point is a larger sample containing 258 galaxies including those of DR1 and some others not yet released to the community. The survey sample comprises 24.4% E-S0 and 75.6% Sa-Sm galaxies. We removed the galaxies suspected of being Active Galactic Nuclei (AGN) according to several criteria based on optical spectra and radio data following the criteria of Best et al. (2012), Kewley et al. (2001, 2006), and Cid Fernandes et al. (2011). We also removed galaxies with morphological types earlier than S0a since we want to focus only on spiral galaxies. To keep only galaxies completely observed within the PPAK field-of-view, we rejected all galaxies for which 2.5R50 ≥ 36′′, where R50 is the radius containing 50% of the Petrosian flux in the r′ band (petroR50_r in SDSS-DR7). After testing several options such us half-light radii from circular and elliptical growth curve analysis, we decided to use petroR50_r as our scale because it results in the largest sample of galaxies fulfilling our requirements, and has the least-dispersed profiles for the growth curves to be discussed later. This left us with a sample of 104 galaxies with morphological types from Sa to Sm completely covered by the PPAK field-of-view. The median values (1σ confidence interval) of the stellar mass, SFR (from dust-corrected Hα luminosities), EW(Hα), and f(Hα)/f(Hβ) distributions of the sample are log M/M = 10.25 ([9.65,10.85]), log SFR/(M yr-1) = 0.45 ([−0.06,0.79]), EW(Hα)/Å = 16.91 ([6.39,28.56]), and f(Hα)/f(Hβ) = 4.75 ([4.00,5.89]). We produced stacked spectra for each galaxy by adding the emission from the spaxels within circular apertures centered at the galaxy’s nucleus and with radii increasing from 1.5′′ by steps of 1.5′′ up to a maximum radius completely covering the hexagonal PPAK aperture. Then we subtracted the stellar continuum to each of our stacked spectra by using the FIT3D code (Sánchez et al. 2011), which yielded a set of spectra of the ionized gas emission. Finally, the fluxes of the emission lines within each aperture were extracted from two independent Gaussian fits with the IDL-based routine MPFITEXPR (Markwardt 2009), one for Hβ and another one for the triplet Hα + [Nii]λ6548, 6583 Å. A more detailed description of this point will be included in a forthcoming paper.

thumbnail Fig. 1

Growth curve of f(Hα) normalized to f(Hα) at 2.5R50, x(α,2p5), as a function of the radius of the aperture. Bold red (blue) lines correspond to the median values for galaxies with b/a > 0.4 (b/a < 0.4). Dashed red and blue lines correspond to 1σ deviations from the median curve. Numbers indicate the size of each subsample. The vertical dashed line corresponds to 0.3R50, which on average corresponds to the FWHM of the CALIFA psf for our sample.

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thumbnail Fig. 2

Top: growth curve of f(Hα)/f(Hβ) normalized to f(Hα)/f(Hβ) at 2.5R50, x(αβ,2p5), as a function of the radius of the aperture. Lines and colors as in Fig. 1. For one of the galaxies the Hβ emission line was not detected in all apertures and was not included. Bottom: same as top panel restricted to galaxies with EW(Hα) ≤ 16.91 Å.

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thumbnail Fig. 3

Top: growth curve of EW(Hα) normalized to EW(Hα) at 2.5R50, x(EWα,2p5), as a function of the radius. Bottom: same as top panel restricted to galaxies with EW(Hα) ≤ 8 Å at 2.5R50.

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3. Results

We illustrate our results with the aperture corrections for three observables: f(Hα), the f(Hα)/f(Hβ) ratio, and the EW(Hα), shown in Figs. 1 to 3. The figures show the median profiles of the growth curves corresponding to these three quantities normalized to the value measured within an external isophote containing most of the optical light of the galaxy. We normalized the profiles to the value measured at 2.5R50 since this is the typical value covered by the PPAK field-of-view for the CALIFA sample. This aperture, 2.5R50, encloses on average ≃90% of the total Hα flux of spiral galaxies1.

Figure 1 shows the median f(Hα) growth curve normalized to f(Hα) at 2.5R50, x(α,2p5). We split our sample according to the axis ratio to take into account the effect that the inclination of the galaxy may have on the values measured through circular apertures. For this, we use the isophotal axis in the r′ band isoA_r and isoB_r from SDSS-DR7. This choice gives average values of ⟨isoB_r/isoA_r⟩ = 0.27 and 0.53, respectively, for each subsample. We verified that other choices like the semi-minor to semi-major axis from elliptical growth curve and light momentum analysis yield basically the same average inclination distributions as our selection. As can be seen, the average curves for both ranges of inclination are quite similar within the 1σ uncertainties. The low thickness of the ionized gas disks, as estimated from the low-velocity dispersion of the Hii regions (≈10 km s-1, Relaño et al. 2005), suggests that no significant differences in the Hα growth curves of face-on and edge-on galaxies should be expected. However, a circular aperture covers different parts of the disk for face-on and edge-on galaxies. Thus, the similarity found for both curves might partly arise because the average inclination values of our subsamples are far from the extreme face-on and edge-on cases and close enough to each other, although we verified that even the curves restricted to galaxies with the most extreme values in our sample are not significantly different from each other. A physically motivated study of the Hα spatial distribution for face-on and edge-on galaxies is beyond the scope of this work and will be part of ongoing work devoted to the SFR in CALIFA galaxies. Similarly, aperture corrections as a function of Hubble type require a larger sample and will be part of future papers on this topic. We list in Table 1 the median growth curve and the 1σ uncertainties of f(Hα) as a function of the aperture radius. Because galaxies with low and high inclination show very similar results, these values correspond to our complete working sample.

Table 1

x(α,2p5) and x(αβ,2p5) and their 1σ deviations as a function of the size of the circular aperture normalized to R50.

Table 2

Redshift and absolute magnitude ranges at which the aperture corrections listed in Table 1 are valid for SDSS and SAMI data.

An interesting problem is the effect of the Full Width at Half Maximum (FWHM) of the CALIFA Point Spread Function (PSF, ~3.6′′) on the growth curves of the emission lines. Trujillo et al. (2001) demonstrated that the parameters of a Sérsic profile are affected by the seeing. For exponential disks, which correspond to a n = 1 profile, the central intensity and effective radius are recovered within 20% of their true value, disregarding the ellipticity as far as FWHM/reff ≤ 0.3. This result prevents the use of aperture corrections below this limit since they could be severely affected by seeing. Given that the average value of R50 for our sample is ≈10′′, the range of validity of our aperture correction is valid for radii r/R50 ≳ 0.3. Another limitation is the fact that the observed size of the galaxies decreases as we move to higher redshifts, and this imposes both a lower and an upper limit to the range of magnitudes for which our correction can be applied. Based on average measurements of R50 for spiral galaxies from SDSS, we derived the validity ranges of the aperture corrections at different redshifts. In Table 2, we show these ranges for two representative projects providing spectra with different aperture sizes: SDSS (single-fiber, ?= 3′′) and SAMI (multi-fiber bundle, ?= 15′′, Bryant et al. 2011). In both cases the lower and upper magnitude limits correspond to the magnitudes at which raper/R50 ≥ 0.3 and raper/R50 ≤ 2.5, respectively2, based on their average sizes at the different redshift proved with a sample of SDSS-DR7 galaxies. The applicability of the aperture corrections is meaningful for individual galaxies within the limits listed in Table 2, thus we do not recommend to apply average corrections to large samples of galaxies to correct quantities such as the star formation density from observed Hα luminosities.

Figure 2 (top panel) shows the median f(Hα)/f(Hβ) ratio growth curve normalized to 2.5R50, x(αβ,2p5), again split for low- and high-inclination galaxies. Both curves show a smooth decline from a central value of ≃1.1 to 1.0 at the normalization radius and no significant difference with the inclination of the galaxies. The median growth curve and the 1σ uncertainty for the whole sample are listed in Table 1. The very smooth decline of x(αβ,2p5) supports the earlier result that the dust attenuation shows little dependence on the radial distance (Sánchez et al. 2012b). We examined the effect of non-negligible residual Hβ absorption after subtracting the underlying stellar continuum by restricting the analysis to galaxies with EW(Hα) ≤ 16.9 Å (see lower panel of Fig. 2). Clearly, this growth curve is consistent within the uncertainties with the one corresponding to the whole sample, which suggests that our Hβ fluxes, and thus the f(Hα)/f(Hβ) ratio, are on average free from this effect.

Figure 3 (top panel) shows the median EW(Hα) growth curve, x(EWα,2p5), which as in previous cases is similar for galaxies with low- and high-inclination. This curve grows with radius, ranging from a central value of ≃0.7 to 1 at the normalization radius. The high dispersion of this quantity measured at small apertures (σ > 0.3 at r/R50 ≤ 1.3) is remarkable. The observed dispersion at small radii arises because EW(Hα) is an intensive quantity (in contrast to Hα or Hβ fluxes which are extensive quantities), and thus very low or high values at small radii can be measured because of features such as circumnuclear rings of star formation or nuclear starbursts. This high dispersion at small radii rules out a reliable aperture correction for the EW(Hα). The main implication of the high dispersion at small radii is the possibility that star-forming galaxies observed through a small aperture are misclassified as quiescent if the classification is based only on an extrapolation of the central value of the EW(Hα). To illustrate this, we show x(EWα,2p5) in the bottom panel of Fig. 3 as a function of radius restricted to galaxies with EW(Hα) ≤8 Å. The curve of the low-inclination galaxies is clearly below that of the high-inclination ones, which follows a trend similar to that of the whole sample. As an example, according to this plot a face-on spiral galaxy with (total) EW(Hα) = 6 Å and observed through an aperture covering less than R50 will have a ≃50% chance of showing (observed) EW(Hα) ≤ 3 Å, and thus of being classified as quiescent depending on the adopted limit (previous works separate active and quiescent galaxies at EW(Hα) ≃2−4 Å, see Cid Fernandes et al. 2010). This effect is less dramatic for edge-on galaxies.

We conclude that the median f(Hα) growth curve of spiral galaxies is well defined and shows a low dispersion for a range of aperture sizes between 0.3R50 and 2.5R50. The f(Hα)/f(Hβ) ratio growth curve presents a smooth behavior that suggests a low dependence of the dust attenuation with radius. Median aperture corrections for this quantity are also meaningful. The observed high dispersion around the median EW(Hα) growth curve excludes deriving a reliable aperture correction especially at small radii. This high dispersion is a caveat in using the EW(Hα) as the criterion to classify star-forming and quiescent galaxies for which the size of the aperture is ≲R50, because it might result in misclassification of low EW(Hα) galaxies. We finally add that this paper presents only the tip of the iceberg. Aperture effects are widely mentioned but rarely understood. Their influence can seriously affect scientific results through systematic errors (Bland-Hawthorn et al. 2011). Forthcoming papers will be devoted to providing a complete set of diagnostics involving other indicators of SFR and/or abundances, and to applying these results to existing datasets.


1

Estimated by adding the contribution of all the spaxels of the PPAK aperture for each galaxy of the sample to the Hα emission flux. This must be taken as a rough estimate since the largest galaxies in the sample show Hα emission beyond the PPAK aperture.

2

There is another restriction on the upper magnitude limit due to the effect of the PSF on the determination of R50. This upper limit corresponds to the magnitude at which FWHM ≈ R50 and it must be applied when it is more restrictive than the one given in table 2 for each redshift. In the particular case of SDSS images, R50 are not corrected for the psf such that this limit can become relevant at high redshifts. A discussion on the limits of the reliability of R50 at different redshifts can be found in González-Pérez et al. (2011).

Acknowledgments

This study made use of the data provided by the Calar Alto Legacy Integral Field Area (CALIFA) survey (http://califa.caha.es/). The CALIFA collaboration would like to thank the IAA-CSIC and MPIA-MPG as major partners of the observatory, and CAHA itself, for the unique access to telescope time and support in manpower and infrastructures. The CALIFA collaboration also thanks the CAHA staff for the dedication to this project. Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck-Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC). We thank the Viabilidad, Diseño, Acceso y Mejora funding program ICTS-2009-10, for supporting the initial developement of this project. J.I.-P., J.V.M., C.K. and A.M.I. thank the Spanish PNAYA projects Estallidos AYA2010-21887-C04-01. R.A.M. was funded by the Spanish programme of International Campus of Excellence Moncloa (CEI). J.F.-B. acknowledges support from the Ramón y Cajal programme by the Spanish Ministry of Economy and Competitiveness (MINECO). This work has been supported by the Programa Nacional de Astronomía y Astrofísica of MINECO, under grants AYA2010- 21322-C03-01 and AYA2010-21322-C03-02. L.G. and V.S. acknowledge financial support from Fundação para a Ciência e a Tecnologia (FCT) under program Ciência 2008 and the research grant PTDC/CTE-AST/112582/2009. The Dark Cosmology Centre is funded by the Danish National Research Foundation. A.G. acknowledges funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 267251. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

References

All Tables

Table 1

x(α,2p5) and x(αβ,2p5) and their 1σ deviations as a function of the size of the circular aperture normalized to R50.

Table 2

Redshift and absolute magnitude ranges at which the aperture corrections listed in Table 1 are valid for SDSS and SAMI data.

All Figures

thumbnail Fig. 1

Growth curve of f(Hα) normalized to f(Hα) at 2.5R50, x(α,2p5), as a function of the radius of the aperture. Bold red (blue) lines correspond to the median values for galaxies with b/a > 0.4 (b/a < 0.4). Dashed red and blue lines correspond to 1σ deviations from the median curve. Numbers indicate the size of each subsample. The vertical dashed line corresponds to 0.3R50, which on average corresponds to the FWHM of the CALIFA psf for our sample.

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In the text
thumbnail Fig. 2

Top: growth curve of f(Hα)/f(Hβ) normalized to f(Hα)/f(Hβ) at 2.5R50, x(αβ,2p5), as a function of the radius of the aperture. Lines and colors as in Fig. 1. For one of the galaxies the Hβ emission line was not detected in all apertures and was not included. Bottom: same as top panel restricted to galaxies with EW(Hα) ≤ 16.91 Å.

Open with DEXTER
In the text
thumbnail Fig. 3

Top: growth curve of EW(Hα) normalized to EW(Hα) at 2.5R50, x(EWα,2p5), as a function of the radius. Bottom: same as top panel restricted to galaxies with EW(Hα) ≤ 8 Å at 2.5R50.

Open with DEXTER
In the text

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