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Michael W. Regan

Bio: Michael W. Regan is an academic researcher from Space Telescope Science Institute. The author has contributed to research in topics: Galaxy & Star formation. The author has an hindex of 44, co-authored 86 publications receiving 7794 citations. Previous affiliations of Michael W. Regan include National Radio Astronomy Observatory & Association of Universities for Research in Astronomy.


Papers
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Journal ArticleDOI
TL;DR: The SIRTF Nearby Galaxy Survey (SINGS) as mentioned in this paper is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies and its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components across a broad range of galaxy properties and star formation environments.
Abstract: The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multiwavelength library of ancillary and complementary observations, including radio continuum, H i, CO, submillimeter, BVRIJHK ,H a ,P aa, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

1,284 citations

Journal ArticleDOI
TL;DR: In this article, the Schmidt power law was used to study the relationship between the star formation rate (SFR), surface density, and gas surface density in the spiral galaxy M51a (NGC 5194), using multiwavelength data obtained as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS).
Abstract: We have studied the relationship between the star formation rate (SFR), surface density, and gas surface density in the spiral galaxy M51a (NGC 5194), using multiwavelength data obtained as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS). We introduce a new SFR index based on a linear combination of Hα emission-line and 24 μm continuum luminosities, which provides reliable extinction-corrected ionizing fluxes and SFR densities over a wide range of dust attenuations. The combination of these extinction-corrected SFR densities with aperture synthesis H I and CO maps has allowed us to probe the form of the spatially resolved star formation law on scales of 0.5-2 kpc. We find that the resolved SFR versus gas surface density relation is well represented by a Schmidt power law, which is similar in form and dispersion to the disk-averaged Schmidt law. We observe a comparably strong correlation of the SFR surface density with the molecular gas surface density, but no significant correlation with the surface density of atomic gas. The best-fitting slope of the Schmidt law varies from N = 1.37 to 1.56, with zero point and slope that change systematically with the spatial sampling scale. We tentatively attribute these variations to the effects of areal sampling and averaging of a nonlinear intrinsic star formation law. Our data can also be fitted by an alternative parameterization of the SFR surface density in terms of the ratio of gas surface density to local dynamical time, but with a considerable dispersion.

624 citations

Journal ArticleDOI
TL;DR: The Spitzer Survey of Stellar Structure in Galaxies (S^4G) as mentioned in this paper is a collection of 2331 galaxies using the Infrared Array Camera (IRAC) at 3.6 and 4.5 μm.
Abstract: 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°,m_(Bcorr) 1') survey of 2331 galaxies using the Infrared Array Camera (IRAC) at 3.6 and 4.5 μm. Each galaxy is observed for 240 s and mapped to ≥ 1:5 × D_(25). The final mosaicked images have a typical 1 σ rms noise level of 0.0072 and 0:0093 MJy sr^-1 at 3.6 and 4.5 μm, respectively. Our azimuthally averaged surface brightness profile typically traces isophotes at μ_(3.6μm (AB)(1σ) ~ 27 mag arcsec^(-2), equivalent to a stellar mass surface density of ~1 M_⊙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 large, unbiased, and extremely deep sample of all Hubble types from dwarfs to spirals to ellipticals will allow for detailed structural studies, not only as a function of stellar mass, but also as a function of the local environment. The data from this survey will serve as a vital testbed for cosmological simulations predicting the stellar mass properties of present-day galaxies. This article introduces the survey and describes the sample selection, the significance of the 3.6 and 4.5 μm bands for this study, and the data collection and survey strategies. We describe the S^4G data analysis pipeline and present measurements for a first set of galaxies, observed in both the cryogenic and warm mission phases of Spitzer. For every galaxy we tabulate the galaxy diameter, position angle, axial ratio, inclination at μ_(3.6μm)(AB) = 25:5, and 26:5 mag arcsec^(-2) (equivalent to ≈μ_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, using the galaxy-fitting code GALFIT, we deconstruct each galaxy 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 and will enable a variety of scientific investigations, some of which are highlighted in this introductory S^4G survey paper.

557 citations

Journal ArticleDOI
TL;DR: In this article, the authors compared the spectral properties of the SFR indicators using the monochromatic infrared dust emission centered at 70 and 160 μm and derived a reference SFR indicator for use in their analysis.
Abstract: Spitzer data at 24, 70, and 160 μm and ground-based Hα images are analyzed for a sample of 189 nearby star-forming and starburst galaxies to investigate whether reliable star formation rate (SFR) indicators can be defined using the monochromatic infrared dust emission centered at 70 and 160 μm. We compare recently published recipes for SFR measures using combinations of the 24 μm and observed Hα luminosities with those using 24 μm luminosity alone. From these comparisons, we derive a reference SFR indicator for use in our analysis. Linear correlations between SFR and the 70 μm and 160 μm luminosity are found for L(70) ≳ 1.4 × 10^(42) erg s^(–1) and L(160) ≳ 2 × 10^(42) erg s^(–1), corresponding to SFR ≳ 0.1-0.3 M_☉ yr^(–1), and calibrations of SFRs based on L(70) and L(160) are proposed. Below those two luminosity limits, the relation between SFR and 70 μm (160 μm) luminosity is nonlinear and SFR calibrations become problematic. A more important limitation is the dispersion of the data around the mean trend, which increases for increasing wavelength. The scatter of the 70 μm (160 μm) data around the mean is about 25% (factor ~2) larger than the scatter of the 24 μm data. We interpret this increasing dispersion as an effect of the increasing contribution to the infrared emission of dust heated by stellar populations not associated with the current star formation. Thus, the 70 (160) μm luminosity can be reliably used to trace SFRs in large galaxy samples, but will be of limited utility for individual objects, with the exception of infrared-dominated galaxies. The nonlinear relation between SFR and the 70 and 160 μm emission at faint galaxy luminosities suggests a variety of mechanisms affecting the infrared emission for decreasing luminosity, such as increasing transparency of the interstellar medium, decreasing effective dust temperature, and decreasing filling factor of star-forming regions across the galaxy. In all cases, the calibrations hold for galaxies with oxygen abundance higher than roughly 12 +log(O/H) ~ 8.1. At lower metallicity, the infrared luminosity no longer reliably traces the SFR because galaxies are less dusty and more transparent.

421 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present an analysis of the molecular gas distributions in the 29 barred and 15 unbarred spirals in the BIMA CO (J = 1-0) Survey of Nearby Galaxies (SONG).
Abstract: We present an analysis of the molecular gas distributions in the 29 barred and 15 unbarred spirals in the BIMA CO (J = 1-0) Survey of Nearby Galaxies (SONG). For galaxies that are bright in CO, we confirm the conclusion by Sakamoto et al. that barred spirals have higher molecular gas concentrations in the central kiloparsec. The SONG sample also includes 27 galaxies below the CO brightness limit used by Sakamoto et al. Even in these less CO-bright galaxies we show that high central gas concentrations are more common in barred galaxies, consistent with radial inflow driven by the bar. However, there is a significant population of early-type (Sa-Sbc) barred spirals (6 of 19) that have no molecular gas detected in the nuclear region and have very little out to the bar corotation radius. This suggests that in barred galaxies with gas-deficient nuclear regions, the bar has already driven most of the gas within the bar corotation radius to the nuclear region, where it has been consumed by star formation. The median mass of nuclear molecular gas is over 4 times higher in early-type bars than in late-type (Sc-Sdm) bars. Since previous work has shown that the gas consumption rate is an order of magnitude higher in early-type bars, this implies that the early types have significantly higher bar-driven inflows. The lower accretion rates in late-type bars can probably be attributed to the known differences in bar structure between early and late types. Despite the evidence for bar-driven inflows in both early and late Hubble-type spirals, the data indicate that it is highly unlikely for a late-type galaxy to evolve into an early type via bar-induced gas inflow. Nonetheless, secular evolutionary processes are undoubtedly present, and pseudobulges are inevitable; evidence for pseudobulges is likely to be clearest in early-type galaxies because of their high gas inflow rates and higher star formation activity.

323 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, the authors review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies.
Abstract: We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star-formation rates are discussed, and updated prescriptions for calculating star-formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds.

2,525 citations

Journal ArticleDOI
TL;DR: In this paper, an overall theoretical framework and the observations that motivate it are outlined, outlining the key dynamical processes involved in star formation, including turbulence, magnetic fields, and self-gravity.
Abstract: We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation—turbulence, magnetic fields, and self-gravity— are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.

2,522 citations

Journal ArticleDOI
TL;DR: The Multiband Imaging Photometer for Spitzer (MIPS) as discussed by the authors provides long-wavelength capability for the mission in imaging bands at 24, 70, and 160?m and measurements of spectral energy distributions between 52 and 100?m at a spectral resolution of about 7%.
Abstract: The Multiband Imaging Photometer for Spitzer (MIPS) provides long-wavelength capability for the mission in imaging bands at 24, 70, and 160 ?m and measurements of spectral energy distributions between 52 and 100 ?m at a spectral resolution of about 7%. By using true detector arrays in each band, it provides both critical sampling of the Spitzer point-spread function and relatively large imaging fields of view, allowing for substantial advances in sensitivity, angular resolution, and efficiency of areal coverage compared with previous space far-infrared capabilities. The 24 ?m array has excellent photometric properties, and measurements with rms relative errors of about 1% can be obtained. The two longer-wavelength arrays use detectors with poor photometric stability, but a system of onboard stimulators used for relative calibration, combined with a unique data pipeline, produce good photometry with rms relative errors of less than 10%.

2,370 citations

Journal ArticleDOI
TL;DR: In this paper, the star formation efficiency (SFE) per unit of gas in 23 nearby galaxies and compare it with expectations from proposed star formation laws and thresholds was measured, and the authors interpreted this decline as a strong dependence of giant molecular cloud (GMC) formation on environment.
Abstract: We measure the star formation efficiency (SFE), the star formation rate (SFR) per unit of gas, in 23 nearby galaxies and compare it with expectations from proposed star formation laws and thresholds. We use H I maps from The H I Nearby Galaxy Survey (THINGS) and derive H2 maps of CO measured by HERA CO-Line Extragalactic Survey and Berkeley-Illinois-Maryland Association Survey of Nearby Galaxies. We estimate the SFR by combining Galaxy Evolution Explorer (GALEX) far-ultraviolet maps and the Spitzer Infrared Nearby Galaxies Survey (SINGS) 24 ?m maps, infer stellar surface density profiles from SINGS 3.6 ?m data, and use kinematics from THINGS. We measure the SFE as a function of the free fall and orbital timescales, midplane gas pressure, stability of the gas disk to collapse (including the effects of stars), the ability of perturbations to grow despite shear, and the ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly constant at (5.25 ? 2.5) ? 10?10 yr?1 (equivalent to an H2 depletion time of 1.9 ? 109 yr) as a function of all of these variables at our 800 pc resolution. Where the interstellar medium (ISM) is mostly H I, however, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2r 25-0.25r 25. We interpret this decline as a strong dependence of giant molecular cloud (GMC) formation on environment. The ratio of molecular-to-atomic gas appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H2-dominated to H I-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe?phase balance in the H I, H2 formation and destruction, and stellar feedback?governs the formation of GMCs from H I.

1,888 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a comprehensive analysis of the relationship between star formation rate surface density and gas surface density at sub-kpc resolution in a sample of 18 nearby galaxies and investigate how the star formation law differs between the H2 dominated centers of spiral galaxies, their H I dominated outskirts and the H I rich late-type/dwarf galaxies.
Abstract: We present a comprehensive analysis of the relationship between star formation rate surface density, ΣSFR, and gas surface density, Σgas, at sub-kpc resolution in a sample of 18 nearby galaxies. We use high-resolution H I data from The H I Nearby Galaxy Survey, CO data from HERACLES and the BIMA Survey of Nearby Galaxies, 24 μm data from the Spitzer Space Telescope, and UV data from the Galaxy Evolution Explorer. We target seven spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H2 dominated centers of spiral galaxies, their H I dominated outskirts and the H I rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N = 1.0 ± 0.2 relates ΣSFR and ΣH2 across our sample of spiral galaxies, i.e., that H2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ~2 × 109 years. The range of ΣH2 over which we measure this relation is ~3-50 M ☉ pc–2, significantly lower than in starburst environments. We find the same results when performing a pixel-by-pixel analysis, averaging in radial bins, or when varying the star formation tracer used. We interpret the linear relation and constant depletion time as evidence that stars are forming in giant molecular clouds with approximately uniform properties and that ΣH2 may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas surface density (Σgas) and ΣSFR varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between ΣHI and ΣSFR. As a result, the star formation efficiency (SFE), ΣSFR/Σgas, varies strongly across our sample and within individual galaxies. We show that this variation is systematic and consistent with the SFE being set by local environmental factors: in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. We attribute the similarity to common environments (low density, low metallicity, H I dominated) and argue that shear (which is typically absent in dwarfs) cannot drive the SFE. In addition to a molecular Schmidt law, the other general feature of our sample is a sharp saturation of H I surface densities at ΣHI ≈ 9 M ☉ pc–2 in both the spiral and dwarf galaxies. In the case of the spirals, we observe gas in excess of this limit to be molecular.

1,842 citations