ABSTRACT
We present an analysis of the full bidimensional optical spectral cube of the nearby spiral galaxy NGC 5668, observed with the Pmas fiber PAcK Integral Field Unit (IFU) at the Calar Alto observatory 3.5 m telescope. We make use of broadband imaging to provide further constraints on the evolutionary history of the galaxy. This data set will allow us to improve our understanding of the mechanisms that drive the evolution of disks. We investigated the properties of 62 H ii regions and concentric rings in NGC 5668 and derived maps in ionized-gas attenuation and chemical (oxygen) abundances. We find that while inward of r ∼36'' ∼ 4.4 kpc ∼ 0.36 (D25/2) the derived O/H ratio follows the radial gradient typical of spiral galaxies, the abundance gradient beyond r ∼ 36'' flattens out. The analysis of the multi-wavelength surface brightness profiles of NGC 5668 is performed by fitting these profiles with those predicted by chemo-spectrophotometric evolutionary models of galaxy disks. From this, we infer a spin and circular velocity of λ = 0.053 and vc = 167 km s−1, respectively. The metallicity gradient and rotation curve predicted by this best-fitting galaxy model nicely match the values derived from the IFU observations, especially within r ∼36''. The same is true for the colors despite some small offsets and a reddening in the bluest colors beyond that radius. On the other hand, deviations of some of these properties in the outer disk indicate that a secondary mechanism, possibly gas transfer induced by the presence of a young bar, must have played a role in shaping the recent chemical and star formation histories of NGC 5668.
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Footnotes
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Based on observations collected at the German-Spanish Astronomical Center, Calar Alto, jointly operated by the Max-Planck-Institut für Astronomie Heidelberg and the Instituto de Astrofísica de Andalucía (CSIC).
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Source: NASA/IPAC Extragalactic Database (http://nedwww.ipac.caltech.edu/)
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They adopted a distance of 30 Mpc, consistent with a cosmology with H0 = 50 km s−1 Mpc−1.
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IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.
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Spitzer Proposal ID 69: A Mid-IR Hubble Atlas of Galaxies, PI: G. Fazio.
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Hereafter we use the terms "H ii regions" and "H ii complexes" indistinctly, even although individual H ii regions are not spatially resolved at the physical resolution achieved in our IFS data.
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The [O ii] λλ3726, 3729 doublet is spectrally unresolved in our data, therefore, and hereafter we will refer to it as [O ii] λ3727.
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The limit in the continuum intensity between the two types of spectra was set to 1 × 10−16 erg s−1 Å−1 . For those spectra where the continuum was fainter (in either Hβ or Hα) than this number, no attempt to carry out a full spectral fitting to the continuum was attempted.
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Note that we detect these lines only in 23 H ii regions, so we do not use Hδ and Hγ to derive physical properties.
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The errors are calculated from the residual fit models of the underlying stellar population; we are assuming that these residuals are primarily due to photon noise and sky subtraction. We note that we have pointings with a slightly different spatial resolution and several combinations of exposure time and number of individual exposures; for this reason the continuum level emission and the associated noise was derived for each pointing separately. The sky subtraction to the spectra of NGC 5668 was done, see the data reduction steps in Sánchez (2006), by median average the 331 sky spectra. The resulting spectrum was then subtracted from the cube of NGC 5668. The analysis of these sky-subtracted spectra showed that the error in the continuum level emission increases linearly with the number of fibers (but not to the square root). This indicates that for apertures including a large number of fibers the total error is dominated by the sky-subtraction error.
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We used the value of 12 + log (O/H)☉ = 8.93 as in Kewley & Dopita (2002).
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Note that the scatter of the data points around the model is not entirely due to noise, but also to real features of the galaxy that a smooth model cannot reproduce. This precludes applying the classical statistical formula to translate the χ2 distribution into confidence intervals. The criterion of using χ2 ⩽ 2χ2min is just orientative, based on the results shown in Figure 9. Note also that we are not using the χ2 distributions in a strict statistical way, since many of the required mathematical conditions (Gaussianity of errors, etc.) are not met (see Muñoz-Mateos et al. 2011 for a more extensive discussion).
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One could decide to fit only the color profiles instead. However, that would lead to solutions that might not fit the overall shape of the surface brightness profiles and that would be very sensitive to uncertainties in the model predictions regarding specific colors (associated with limitations in the yields, stellar libraries at non-solar abundances, and uncertainties in the luminosity at near-infrared and UV wavelengths; see Maraston et al. 2006 and Muñoz-Mateos et al. 2011, respectively).
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The lack of strong spiral arms in NGC 5668 is evident even after the examination of the IRAC near-infrared images of the object (see Scarano 2010 and references therein).