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Register a new userUnveiling the Structure of Barred Galaxies at 3.6 $mu {rm m}$ with the Spitzer Survey of Stellar Structure in Galaxies (S$^4$G): I. Disk Breaks
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We have performed two-dimensional multicomponent decomposition of 144 local barred spiral galaxies using 3.6 $mu {rm m}$ images from the Spitzer Survey of Stellar Structure in Galaxies. Our model fit includes up to four components (bulge, disk, bar, and a point source) and, most importantly, takes into account disk breaks. We find that ignoring the disk break and using a single disk scale length in the model fit for Type II (down-bending) disk galaxies can lead to differences of 40% in the disk scale length, 10% in bulge-to-total luminosity ratio (B/T), and 25% in bar-to-total luminosity ratios. We find that for galaxies with B/T $geq$ 0.1, the break radius to bar radius, $r_{rm br}/R_{rm bar}$, varies between 1 and 3, but as a function of B/T the ratio remains roughly constant. This suggests that in bulge-dominated galaxies the disk break is likely related to the outer Lindblad Resonance (OLR) of the bar, and thus moves outwards as the bar grows. For galaxies with small bulges, B/T $<$ 0.1, $r_{rm br}/R_{rm bar}$ spans a wide range from 1 to 6. This suggests that the mechanism that produces the break in these galaxies may be different from that in galaxies with more massive bulges. Consistent with previous studies, we conclude that disk breaks in galaxies with small bulges may originate from bar resonances that may be also coupled with the spiral arms, or be related to star formation thresholds.
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The mid-infrared is an optimal window to trace stellar mass in nearby galaxies and the 3.6$mu m$ IRAC band has been exploited to this effect, but such mass estimates can be biased by dust emission. We present our pipeline to reveal the old stellar flux at 3.6$mu m$ and obtain stellar mass maps for more than 1600 galaxies available from the Spitzer Survey of Stellar Structure in Galaxies (S$^{4}$G). This survey consists of images in two infrared bands (3.6 and 4.5$mu m$), and we use the Independent Component Analysis (ICA) method presented in Meidt et al. (2012) to separate the dominant light from old stars and the dust emission that can significantly contribute to the observed 3.6$mu m$ flux. We exclude from our ICA analysis galaxies with low signal-to-noise ratio (S/N < 10) and those with original [3.6]-[4.5] colors compatible with an old stellar population, indicative of little dust emission (mostly early Hubble types, which can directly provide good mass maps). For the remaining 1251 galaxies to which ICA was successfully applied, we find that as much as 10-30% of the total light at 3.6$mu m$ typically originates from dust, and locally it can reach even higher values. This contamination fraction shows a correlation with specific star formation rates, confirming that the dust emission that we detect is related to star formation. Additionally, we have used our large sample of mass estimates to calibrate a relationship of effective mass-to-light ratio ($M/L$) as a function of observed [3.6]-[4.5] color: $log(M/L)=-0.339 (pm 0.057) times ([3.6]-[4.5]) -0.336 (pm 0.002)$. Our final pipeline products have been made public through IRSA, providing the astronomical community with an unprecedentedly large set of stellar mass maps ready to use for scientific applications.
The Spitzer Survey of Stellar Structure in Galaxies (S$^4$G, Sheth et. al. 2010) is a deep 3.6 and 4.5 $mu$m imaging survey of 2352 nearby ($< 40$ Mpc) galaxies. We describe the S$^4$G data analysis pipeline 4, which is dedicated to 2-dimensional structural surface brightness decompositions of 3.6 $mu$m images, using GALFIT3.0 citep{peng2010}. Besides automatic 1-component Sersic fits, and 2-component Sersic bulge + exponential disk fits, we present human supervised multi-component decompositions, which include, when judged appropriate, a central point source, bulge, disk, and bar components. Comparison of the fitted parameters indicates that multi-component models are needed to obtain reliable estimates for the bulge Sersic index and bulge-to-total light ratio ($B/T$), confirming earlier results citep{laurikainen2007, gadotti2008, weinzirl2009}. In this first paper, we describe the preparations of input data done for decompositions, give examples of our decomposition strategy, and describe the data products released via IRSA and via our web page ({bf tt www.oulu.fi/astronomy/S4G_PIPELINE4/MAIN}). These products include all the input data and decomposition files in electronic form, making it easy to extend the decompositions to suit specific science purposes. We also provide our IDL-based visualization tools (GALFIDL) developed for displaying/running GALFIT-decompositions, as well as our mask editing procedure (MASK_EDIT) used in data preparation. In the second paper we will present a detailed analysis of the bulge, disk, and bar parameter derived from multi-component decompositions.
The Spitzer Survey of Stellar Structure in Galaxies S^4G is an Exploration Science Legacy Program approved for the Spitzer post-cryogenic mission. It is a volume-, magnitude-, and size-limited (d < 40 Mpc, |b| > 30 degrees, m_(Bcorr) < 15.5, D25>1) survey of 2,331 galaxies using IRAC at 3.6 and 4.5 microns. Each galaxy is observed for 240 s and mapped to > 1.5 x D25. The final mosaicked images have a typical 1 sigma rms noise level of 0.0072 and 0.0093 MJy / sr at 3.6 and 4.5 microns, respectively. Our azimuthally-averaged surface brightness profile typically traces isophotes at mu_3.6 (AB) (1 sigma) ~ 27 mag arcsec^-2, equivalent to a stellar mass surface density of ~ 1 Msun pc^-2. S^4G thus provides an unprecedented data set for the study of the distribution of mass and stellar structures in the local Universe. This paper introduces the survey, the data analysis pipeline and measurements for a first set of galaxies, observed in both the cryogenic and warm mission phase of Spitzer. For every galaxy we tabulate the galaxy diameter, position angle, axial ratio, inclination at mu_3.6 (AB) = 25.5 and 26.5 mag arcsec^-2 (equivalent to ~ mu_B (AB) =27.2 and 28.2 mag arcsec^-2, respectively). These measurements will form the initial S^4G catalog of galaxy properties. We also measure the total magnitude and the azimuthally-averaged radial profiles of ellipticity, position angle, surface brightness and color. Finally, we deconstruct each galaxy using GALFIT into its main constituent stellar components: the bulge/spheroid, disk, bar, and nuclear point source, where necessary. Together these data products will provide a comprehensive and definitive catalog of stellar structures, mass and properties of galaxies in the nearby Universe.
The Spitzer Survey of Stellar Structure in Galaxies (S4G) is the largest available database of deep, homogeneous middle-infrared (mid-IR) images of galaxies of all types. The survey, which includes 2352 nearby galaxies, reveals galaxy morphology only minimally affected by interstellar extinction. This paper presents an atlas and classifications of S4G galaxies in the Comprehensive de Vaucouleurs revised Hubble-Sandage (CVRHS) system. The CVRHS system follows the precepts of classical de Vaucouleurs (1959) morphology, modified to include recognition of other features such as inner, outer, and nuclear lenses, nuclear rings, bars, and disks, spheroidal galaxies, X patterns and box/peanut structures, OLR subclass outer rings and pseudorings, bar ansae and barlenses, parallel sequence late-types, thick disks, and embedded disks in 3D early-type systems. We show that our CVRHS classifications are internally consistent, and that nearly half of the S4G sample consists of extreme late-type systems (mostly bulgeless, pure disk galaxies) in the range Scd-Im. The most common family classification for mid-IR types S0/a to Sc is SA while that for types Scd to Sm is SB. The bars in these two type domains are very different in mid-IR structure and morphology. This paper examines the bar, ring, and type classification fractions in the sample, and also includes several montages of images highlighting the various kinds of stellar structures seen in mid-IR galaxy morphology.
(Abridged) The Spitzer Survey for Stellar Structure in Galaxies (S4G) and its more recently approved extension will lead to a set of 3.6 and 4.5 micron images for 2829 galaxies, which can be used to study many different aspects of the structure and evolution of local galaxies. We collected and re-processed optical images in five bands from the Sloan Digital Sky Survey for 1657 galaxies, which are publicly released with the publication of this paper. We observed, in only the g-band, an additional 111 S4G galaxies in the northern hemisphere with the 2.5 m Liverpool Telescope, so that optical imaging is released for 1768 galaxies, or for 62% of the S4G sample. We visually checked all images. We noted interactions and close companions in our optical data set and in the S4G sample, confirming them by determining the galaxies radial velocities and magnitudes in the NASA-IPAC Extragalactic Database. We find that 17% of the S4G galaxies (21% of those brighter than 13.5 mag) have a close companion (within a radius of five times the diameter of the sample galaxy, a recession velocity within 200km/s and not more than 3 mag fainter) and that around 5% of the bright part of the S4G sample show significant morphological evidence of an ongoing interaction. This confirms and further supports previous estimates of these fractions. The over 8000 science images described in this paper, the re-processed Sloan Digital Sky Survey ones, the new Liverpool Telescope images, the set of 29 false-colour pictures, and the catalogue of companion and interacting galaxies, are all publicly released for general use for scientific, illustrative, or public outreach purposes.
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