We study the characteristics of molecular gas in the central regions of spiral galaxies on the basis of our CO(J = 1-0) imaging survey of 20 nearby spiral galaxies using the NRO and OVRO millimeter arrays. Condensations of molecular gas at galactic centers with size scales 1 kpc and CO-derived masses Mgas(R < 500 pc) ~ 108-109 M☉ are found to be prevalent in the gas-rich ~ L* galaxies. Moreover, the degree of gas concentration to the central kiloparsec is found to be higher in barred systems than in unbarred galaxies. This is the first statistical evidence for the higher central concentration of molecular gas in barred galaxies, and it strongly supports the theory of bar-driven gas transport. It is most likely that more than half of molecular gas within the central kiloparsec of a barred galaxy was transported there from outside by the bar. The supply of gas has exceeded the consumption of gas by star formation in the central kiloparsec, resulting in the excess gas in the centers of barred systems. The mean rate of gas inflow is statistically estimated to be larger than 0.1-1 M☉ yr-1. There is no clear correlation between gas mass in the central kiloparsec and the type of nuclear spectrum (H II, LINER, or Seyfert), suggesting that the amount of gas at this scale does not determine the nature of the nuclear activity. There is, however, a clear correlation for galaxies with larger gas-to-dynamical mass ratios to have H II nuclear spectra, while galaxies with smaller ratios show spectra indicating active galactic nuclei (AGNs). This trend may well be related to the gravitational stability of the nuclear gas disk, which is generally lower for larger gas mass fractions. It is therefore possible that all galaxies have active nuclei, but that dwarf AGNs are overwhelmed by the surrounding star formation when the nuclear molecular gas disk is massive and unstable. The theoretical prediction of bar dissolution by condensation of gas to galactic centers is observationally tested by comparing gas concentration in barred and unbarred galaxies. If a bar is to be destroyed so abruptly that the gas condensation at the nucleus does not have enough time to be consumed, then there would be currently unbarred but previously barred galaxies with high gas concentrations. The lack of such galaxies in our sample, together with the current rates of gas consumption at the galactic centers, suggests that the timescale for bar dissolution is larger than 108-1010 yr or a bar in a L* galaxy is not destroyed by a condensation of 108-109 M☉ gas in the central kiloparsec.

Bar-driven Transport of Molecular Gas to Galactic Centers and Its Consequences

The mid-infrared is an optimal window to trace stellar mass in nearby galaxies and the 3.6μm 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 (S^4G). This survey consists of images in two infrared bands (3.6 and 4.5μm), 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 3.6μm 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 3.6μm 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(±0.057) x [3.6]-[4.5])-0.336(± 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.

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

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.

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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

Context. A variety of formation scenarios have been proposed to explain the diversity of properties observed in bulges. Studying their intrinsic shape can help to constrain the dominant mechanisms at the epochs of their assembly. Aims. The structural parameters of a magnitude-limited sample of 148 unbarred S0–Sb galaxies were derived in order to study the correlations between bulges and disks, as well as the probability distribution function of the intrinsic equatorial ellipticity of bulges. Methods. We present a new fitting algorithm (GASP2D) to perform two-dimensional photometric decomposition of the galaxy surface-brightness distribution. This was assumed to be the sum of the contribution of a bulge and disk component characterized by elliptical and concentric isophotes with constant (but possibly different) ellipticity and position angles. Bulge and disk parameters of the sample galaxies were derived from the J-band images, which were available in the Two Micron All Sky Survey. The probability distribution function of the equatorial ellipticity of the bulges was derived from the distribution of the observed ellipticities of bulges and misalignments between bulges and disks. Results. Strong correlations between the bulge and disk parameters were found. About 80% of bulges in unbarred lenticular and early

/pdf/structural-properties-of-disk-galaxies-i-the-intrinsic-44cy71mjoa.pdf

Structural properties of disk galaxies - I. The intrinsic equatorial ellipticity of bulges

We have analyzed the radial distribution of old stars in a sample of 218 nearby face-on disks, using deep 3.6um images from the Spitzer Survey of Stellar Structure in Galaxies (S4G). 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 case of barred galaxies, the ratio between the break and the bar radii (Rbr/Rbar) depends strongly on the total stellar mass of the galaxy. For galaxies more massive than 10^10 Msun, the distribution is bimodal, peaking at Rbr/Rbar~2 and ~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 Msun, breaks are found up to ~10 Rbar, 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-axysymmetric features can be responsible not only for those breaks at 2 Rbar, but also for many of those found at larger radii.

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K. Y. Lo

Papers

Evidence for nonaxisymmetric nuclear bulges in spiral galaxies