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.

Star Formation in the Milky Way and Nearby Galaxies

We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key program that will map the inner Galactic plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2° × 2° tiles approximately centered at l = 30° and l = 59°. The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores' in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around AV ~ 1 is exceeded for the regions in the l = 59° field; a AV value between 5 and 10 is found for the l = 30° field, likely due to the relatively higher distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm-2. Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.

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Clouds, filaments, and protostars: The Herschel Hi-GAL Milky Way

The Spitzer-COSMOS survey (S-COSMOS) is a Legacy program (Cycles 2+3) designed to carry out a uniform deep survey of the full 2 sq deg COSMOS field in all seven Spitzer bands (3.6, 4.5, 5.6, 8.0, 24.0, 70.0, 160.0 u). This paper describes the survey parameters, mapping strategy, data reduction procedures, achieved sensitivities to date, and the complete data set for future reference. We show that the observed infrared backgrounds in the S-COSMOS field are within 10% of the predicted background levels. The fluctuations in the background at 24u have been measured and do not show any significant contribution from cirrus, as expected. In addition, we report on the number of asteroid detections in the low galactic latitude COSMOS field. We use the Cycle 2 S-COSMOS data to determine preliminary number counts, and compare our results with those from previous Spitzer Legacy surveys (e.g. SWIRE, GOODS). The results from this "first analysis" confirm that the S-COSMOS survey will have sufficient sensitivity with IRAC to detect ~ L* disks and spheroids out to z ~ 3, and with MIPS to detect ultraluminous starbursts and AGN out to z ~3 at 24u and out to z ~1.5-2 at 70u and 160u.

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The Spitzer Legacy Survey of the HST-ACS 2 sq. deg. COSMOS Field: survey strategy and first analysis

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S-COSMOS: The Spitzer Legacy Survey of the Hubble Space Telescope ACS 2 deg^2 COSMOS Field I: Survey Strategy and First Analysis

Context. This study deals with infrared bubbles, the H ii regions they enclose, and triggered massive-star formation on their borders. Aims: We attempt to determine the nature of the bubbles observed by Spitzer in the Galactic plane, mainly to establish if possible their association with massive stars. We take advantage of the very simple morphology of these objects to search for star formation triggered by H ii regions, and to estimate the importance of this mode of star formation. Methods: We consider a sample of 102 bubbles detected by Spitzer-GLIMPSE, and catalogued by Churchwell et al. (2006; hereafter CH06). We use mid-infrared and radio-continuum public data (respectively the Spitzer-GLIMPSE and -MIPSGAL surveys and the MAGPIS and VGPS surveys) to discuss their nature. We use the ATLASGAL survey at 870 μm to search for dense neutral material collected on their borders. The 870 μm data traces the distribution of cold dust, thus of the dense neutral material where stars may form. Results: We find that 86% of the bubbles contain ionized gas detected by means of its radio-continuum emission at 20-cm. Thus, most of the bubbles observed at 8.0 μm enclose H ii regions ionized by O-B2 stars. This finding differs from the earlier CH06 results (~25% of the bubbles enclosing H ii regions). Ninety-eight percent of the bubbles exhibit 24 μm emission in their central regions. The ionized regions at the center of the 8.0 μm bubbles seem to be devoid of PAHs but contain hot dust. PAH emission at 8.0 μm is observed in the direction of the photodissociation regions surrounding the ionized gas. Among the 65 regions for which the angular resolution of the observations is high enough to resolve the spatial distribution of cold dust at 870 μm, we find that 40% are surrounded by cold dust, and that another 28% contain interacting condensations. The former are good candidates for the collect and collapse process, as they display an accumulation of dense material at their borders. The latter are good candidates for the compression of pre-existing condensations by the ionized gas. Thirteen bubbles exhibit associated ultracompact H ii regions in the direction of dust condensations adjacent to their ionization fronts. Another five show methanol masers in similar condensations. Conclusions: Our results suggest that more than a quarter of the bubbles may have triggered the formation of massive objects. Therefore, star formation triggered by H ii regions may be an important process, especially for massive-star formation. Appendices are only available in electronic form at http://www.aanda.org

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A gallery of bubbles. The nature of the bubbles observed by Spitzer and what ATLASGAL tells us about the surrounding neutral material

MIPSGAL is a 278 deg^2 survey of the inner Galactic plane using the Multiband Infrared Photometer for Spitzer aboard the Spitzer Space Telescope. The survey field was imaged in two passbands, 24 and 70 μm with resolutions of 6″ and 18″, respectively. The survey was designed to provide a uniform, well-calibrated and well-characterized data set for general inquiry of the inner Galactic plane and as a longer-wavelength complement to the shorter-wavelength Spitzer survey of the Galactic plane: Galactic Plane Infrared Mapping Survey Extraordinaire. The primary science drivers of the current survey are to identify all high-mass (M > 5 M⊙) protostars in the inner Galactic disk and to probe the distribution, energetics, and properties of interstellar dust in the Galactic disk. The observations were planned to minimize data artifacts due to image latents at 24 μm and to provide full coverage at 70 μm. Observations at ecliptic latitudes within 15° of the ecliptic plane were taken at multiple epochs to help reject asteroids. The data for the survey were collected in three epochs, 2005 September–October, 2006 April, and 2006 October with all of the data available to the public. The estimated point-source sensitivities of the survey are 2 and 75 mJy (3 σ) at 24 and 70 μm, respectively. Additional data processing was needed to mitigate image artifacts due to bright sources at 24 μm and detector responsivity variations at 70 μm due to the large dynamic range of the Galactic plane. Enhanced data products including artifact-mitigated mosaics and point-source catalogs are being produced with the 24 μm mosaics already publicly available from the NASA/IPAC Infrared Science Archive. Some preliminary results using the enhanced data products are described.

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MIPSGAL: A Survey of the Inner Galactic Plane at 24 and 70 μm

The Spitzer First Look Survey (FLS) provided an initial characterization of the infrared sky at Spitzer wavelengths and sensitivities. The ecliptic plane component (EPC) of the FLS concentrated on two 0.13 deg2 fields at a solar elongation of 115° and ecliptic latitudes (β) of 0° and +5°. The FLS-EPC explored the small asteroid counts at 8 and 24 μm, with a detection limit down to ~0.08 and 0.8 mJy, respectively, and a completeness limit almost twice as deep as the 8 μm equivalent flux density of the previous deepest mid-IR survey. The FLS-EPC also provided initial characterization of the zodiacal light near the ecliptic plane. Fifteen known and 19 unknown asteroids were identified, and asteroids detected at both wavelengths displayed similar 8 to 24 μm flux ratios of ~0.1. Comparing number counts for the β = 0° and +5° fields indicates a slower-than-anticipated drop-off in contrast to predicted scale heights, possibly due to the presence of higher inclination objects in the small population sampled by Spitzer. The measured zodiacal light background was found to be within 5% of Spitzer model predictions at 24 μm.

The Spitzer First Look Survey—Ecliptic Plane Component: Asteroids and Zodiacal Background

Studying materials released from Jupiter-family comets (JFCs)—as seen in their inner comae, the envelope of gas and dust that forms as the comet approaches the Sun—improves the understanding of their origin and evolutionary history. As part of a coordinated, multiwavelength observing campaign, we observed comet 45P/Honda–Mrkos–Pajdušáková during its close approach to Earth in 2017 February. Narrowband observations were taken using the Bok 90″ telescope at Kitt Peak National Observatory on February 16 and 17 UT, revealing gas and dust structures. We observed different jet directions for different volatile species, implying source region heterogeneity, consistent with other ground-based and in situ observations of other comet nuclei. A repeating feature visible in CN and C2 images on February 16 was also observed on February 17 with an interval of 7.6 ± 0.1 hr, consistent with the rotation period of the comet derived from Arecibo Observatory radar observations. The repeating feature’s projected gas velocity away from the nucleus is 0.8 km s−1, with an outflow velocity of 0.5 km s−1. A bright compact spot adjacent to the nucleus provides a lower limit of the amount of material released in one cycle of ∼9.2 kg, depending on composition—a quantity small enough to be produced by repeated exposure of nucleus ices to sunlight. This repeating CN jet, forming within 400 km of the nucleus, may be typical of inner-coma behavior in JFCs; however, similar features could be obscured by other processes and daughter product species when viewed from distances further than the scale length of CN molecules.

E. L. Ryan

Papers

MIPSGAL: A Survey of the Inner Galactic Plane at 24 and 70 μm

The Spitzer First Look Survey—Ecliptic Plane Component: Asteroids and Zodiacal Background

Repeating Gas Ejection Events from Comet 45P/Honda–Mrkos–Pajdušáková