Trisha Mizusawa

RECONSTRUCTING THE STELLAR MASS DISTRIBUTIONS OF GALAXIES USING S4G IRAC 3.6 AND 4.5 μm IMAGES. I. CORRECTING FOR CONTAMINATION BY POLYCYCLIC AROMATIC HYDROCARBONS, HOT DUST, AND INTERMEDIATE-AGE STARS

With the aim of constructing accurate two-dimensional maps of the stellar mass distribution in nearby galaxies from Spitzer Survey of Stellar Structure in Galaxies 3.6 and 4.5 μm images, we report on the separation of the light from old stars from the emission contributed by contaminants. Results for a small sample of six disk galaxies (NGC 1566, NGC 2976, NGC 3031, NGC 3184, NGC 4321, and NGC 5194) with a range of morphological properties, dust content, and star formation histories are presented to demonstrate our approach. To isolate the old stellar light from contaminant emission (e.g., hot dust and the 3.3 μm polycyclic aromatic hydrocarbon (PAH) feature) in the IRAC 3.6 and 4.5 μm bands we use an independent component analysis (ICA) technique designed to separate statistically independent source distributions, maximizing the distinction in the [3.6]-[4.5] colors of the sources. The technique also removes emission from evolved red objects with a low mass-to-light ratio, such as asymptotic giant branch (AGB) and red supergiant (RSG) stars, revealing maps of the underlying old distribution of light with [3.6]-[4.5] colors consistent with the colors of K and M giants. The contaminants are studied by comparison with the non-stellar emission imaged at 8 μm, which is dominated by the broad PAH feature. Using the measured 3.6 μm/8 μm ratio to select individual contaminants, we find that hot dust and PAHs together contribute between ~5% and 15% to the integrated light at 3.6 μm, while light from regions dominated by intermediate-age (AGB and RSG) stars accounts for only 1%-5%. Locally, however, the contribution from either contaminant can reach much higher levels; dust contributes on average 22% to the emission in star-forming regions throughout the sample, while intermediate-age stars contribute upward of 50% in localized knots. The removal of these contaminants with ICA leaves maps of the old stellar disk that retain a high degree of structural information and are ideally suited for tracing stellar mass, as will be the focus in a companion paper.

Reconstructing the Stellar Mass Distributions of Galaxies Using S4G IRAC 3.6 and 4.5 μm Images. II.: The Conversion from Light to Mass

We present a new approach for estimating the 3.6 μm stellar mass-to-light (M/L) ratio Υ_3.6 in terms of the [3.6]-[4.5] colors of old stellar populations. Our approach avoids several of the largest sources of uncertainty in existing techniques using population synthesis models. By focusing on mid-IR wavelengths, we gain a virtually dust extinction-free tracer of the old stars, avoiding the need to adopt a dust model to correctly interpret optical or optical/near-IR colors normally leveraged to assign the mass-to-light ratio Upsilon. By calibrating a new relation between near-IR and mid-IR colors of giant stars observed in GLIMPSE we also avoid the discrepancies in model predictions for the [3.6]-[4.5] colors of old stellar populations due to uncertainties in the molecular line opacities assumed in template spectra. We find that the [3.6]-[4.5] color, which is driven primarily by metallicity, provides a tight constraint on Upsilon3.6, which varies intrinsically less than at optical wavelengths. The uncertainty on Υ3.6 of ~0.07 dex due to unconstrained age variations marks a significant improvement on existing techniques for estimating the stellar M/L with shorter wavelength data. A single Υ3.6 = 0.6 (assuming a Chabrier initial mass function (IMF)), independent of [3.6]-[4.5] color, is also feasible because it can be applied simultaneously to old, metal-rich and young, metal-poor populations, and still with comparable (or better) accuracy (~0.1 dex) than alternatives. We expect our Υ3.6 to be optimal for mapping the stellar mass distributions in S4G galaxies, for which we have developed an independent component analysis technique to first isolate the old stellar light at 3.6 μm from nonstellar emission (e.g., hot dust and the 3.3 polycyclic aromatic hydrocarbon feature). Our estimate can also be used to determine the fractional contribution of nonstellar emission to global (rest-frame) 3.6 μm fluxes, e.g., in WISE imaging, and establishes a reliable basis for exploring variations in the stellar IMF.

THICK DISKS OF EDGE-ON GALAXIES SEEN THROUGH THE SPITZER SURVEY OF STELLAR STRUCTURE IN GALAXIES (S4G): LAIR OF MISSING BARYONS?

Most, if not all, disk galaxies have a thin (classical) disk and a thick disk. In most models thick disks are thought to be a necessary consequence of the disk formation and/or evolution of the galaxy. We present the results of a study of the thick disk properties in a sample of carefully selected edge-on galaxies with types ranging from T = 3 to nT = 8. We fitted one-dimensional luminosity profiles with physically motivated functions—the solutions of two nstellar and one gaseous isothermal coupled disks in equilibrium—which are likely to yield more accurate results nthan other functions used in previous studies. The images used for the fits come from the Spitzer Survey of Stellar nStructure in Galaxies (S^4G). We found that thick disks are on average more massive than previously reported, nmostly due to the selected fitting function. Typically, the thin and thick disks have similar masses. We also found nthat thick disks do not flare significantly within the observed range in galactocentric radii and that the ratio of nthick-to-thin disk scale heights is higher for galaxies of earlier types. Our results tend to favor an in situ origin for most of the stars in the thick disk. In addition, the thick disk may contain a significant amount of stars coming nfrom satellites accreted after the initial buildup of the galaxy and an extra fraction of stars coming from the secular heating of the thin disk by its own overdensities. Assigning thick disk light to the thin disk component may lead to an underestimate of the overall stellar mass in galaxies because of different mass-to-light ratios in the two disk components. On the basis of our new results, we estimate that disk stellar masses are between 10% and 50% higher than previously thought and we suggest that thick disks are a reservoir of “local missing baryons.”

The Spitzer Survey of Stellar Structure in Galaxies (S4G): Precise Stellar Mass Distributions from Automated Dust Correction at 3.6 μm

The mid-infrared is an optimal window to trace stellar mass in nearby galaxies and the 3.6 m 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 μm and obtain stellar mass maps for more than 1600 galaxies available from the Spitzer Survey of Stellar Structure in Galaxies (SG). This survey consists of images in two infrared bands (3.6 and 4.5 m m ), and we use the Independent Component Analysis (ICA) method presented in Meidt et al. to separate the dominant light from old stars and the dust emission that can significantly contribute to the observed 3.6 m 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 m 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: M L log( ) = 0.339( 0.057) uf0b1 ́ ([3.6] [4.5]) 0.336( 0.002) uf0b1 . 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.

A CLASSICAL MORPHOLOGICAL ANALYSIS OF GALAXIES IN THE SPITZER SURVEY OF STELLAR STRUCTURE IN GALAXIES (S4G)

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.

THE IMPACT OF BARS ON DISK BREAKS AS PROBED BY S 4 G IMAGING.

We have analyzed the radial distribution of old stars in a sample of 218 nearby face-on disks, using deep 3.6 mu m images from the Spitzer Survey of Stellar Structure in Galaxies. In particular, we have studied the structural properties of those disks with a broken or down-bending profile. We find that, on average, disks with a genuine single-exponential profile have a scale length and a central surface brightness which are intermediate to those of the inner and outer components of a down-bending disk with the same total stellar mass. In the particular case of barred galaxies, the ratio between the break and the bar radii (R-br/R-bar) depends strongly on the total stellar mass of the galaxy. For galaxies more massive than 10(10) M-circle dot, the distribution is bimodal, peaking at R-br/R-bar similar to 2 and similar to 3.5. The first peak, which is the most populated one, is linked to the outer Lindblad resonance of the bar, whereas the second one is consistent with a dynamical coupling between the bar and the spiral pattern. For galaxies below 10(10) M-circle dot, breaks are found up to similar to 10 R-bar, but we show that they could still be caused by resonances given the rising nature of rotation curves in these low-mass disks. While not ruling out star formation thresholds, our results imply that radial stellar migration induced by non-axisymmetric features can be responsible not only for those breaks at similar to 2 R-bar, but also for many of those found at larger radii.

The spitzer survey of stellar structure in galaxies (S^4G): multi-component decomposition strategies and data release

The Spitzer Survey of Stellar Structure in Galaxies (S^4G) is a deep 3.6 and 4.5 μm imaging survey of 2352 nearby (<40 Mpc) galaxies. We describe the S(^4)G data analysis pipeline 4, which is dedicated to two-dimensional structural surface brightness decompositions of 3.6 μm images, using GALFIT3.0. 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. Here, 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 (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. A detailed analysis of the bulge, disk, and bar parameters derived from multi-component decompositions will be published separately.

Breaks in thin and thick disks of edge-on galaxies imaged in the Spitzer Survey of Stellar Structure in Galaxies (S4G)

Breaks in the radial luminosity profiles of galaxies have until now been mostly studied averaged over disks. Here, we study separately breaks in thin and thick disks in 70 edge-on galaxies using imaging from the Spitzer Survey of Stellar Structure in Galaxies. We built luminosity profiles of the thin and thick disks parallel to midplanes and we found that thin disks often truncate (77%). Thick disks truncate less often (31%), but when they do, their break radius is comparable with that in the thin disk. This suggests either two different truncation mechanisms—one of dynamical origin affecting both disks simultaneously and another one only affecting the thin disk—or a single mechanism that creates a truncation in one disk or in both depending on some galaxy property. Thin disks apparently antitruncate in around 40% of galaxies. However, in many cases, these antitruncations are an artifact caused by the superposition of a thin disk and a thick disk, with the latter having a longer scale length. We estimate the real thin disk antitruncation fraction to be less than 15%. We found that the ratio of the thick and thin stellar disk mass is roughly constant (0.2 120 km s^–1, but becomes much larger at smaller velocities. We hypothesize that this is due to a combination of a high efficiency of supernova feedback and a slower dynamical evolution in lower-mass galaxies causing stellar thin disks to be younger and less massive than in higher-mass galaxies.

Grand design and flocculent spirals in the Spitzer survey of stellar structure in galaxies (S4G)

Spiral arm properties of 46 galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S(4)G) were measured at 3.6 mu m, where extinction is small and the old stars dominate. The sample includes flocculent, multiple arm, and grand design types with a wide range of Hubble and bar types. We find that most optically flocculent galaxies are also flocculent in the mid-IR because of star formation uncorrelated with stellar density waves, whereas multiple arm and grand design galaxies have underlying stellar waves. Arm-interarm contrasts increase from flocculent to multiple arm to grand design galaxies and with later Hubble types. Structure can be traced further out in the disk than in previous surveys. Some spirals peak at mid-radius while others continuously rise or fall, depending on Hubble and bar type. We find evidence for regular and symmetric modulations of the arm strength in NGC 4321. Bars tend to be long, high amplitude, and flat-profiled in early-type spirals, with arm contrasts that decrease with radius beyond the end of the bar, and they tend to be short, low amplitude, and exponential-profiled in late Hubble types, with arm contrasts that are constant or increase with radius. Longer bars tend to have larger amplitudes and stronger arms.

Erratum: Morphology and environment of galaxies with disc breaks in the S4G and NIRS0S

We study the surface brightness profiles of disc galaxies in the 3.6 mu m images from the Spitzer Survey of Stellar Structure in Galaxies (S(4)G) and K-s-band images from the Near-Infrared S0-Sa galaxy Survey (NIRS0S). We particularly connect properties of single exponential (type I), downbending double exponential (type II), and upbending double exponential (type III) disc profile types, to structural components of galaxies by using detailed morphological classifications, and size measurements of rings and lenses. We also study how the local environment of the galaxies affects the profile types by calculating parameters describing the environmental density and the tidal interaction strength. We find that in majority of type II profiles the break radius is connected with structural components such as rings, lenses, and spirals. The exponential disc sections of all three profile types, when considered separately, follow the disc scaling relations. However, the outer discs of type II, and the inner discs of type III, are similar in scalelength to the single exponential discs. Although the different profile types have similar mean environmental parameters, the scalelengths of the type III profiles show a positive correlation with the tidal interaction strength.