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1. INTRODUCTION

Despite early discoveries of OB stars and molecular gas in the outer Milky Way (MW; e.g., Fich and Blitz (1984); Brand and Wouterloot (1988)), not much attention had been paid to molecular gas in galaxy outskirts primarily because there was a notion that virtually no star formation occurs there. This notion was altered entirely by the Galaxy Evolution Explorer (GALEX), which revealed that ultraviolet emission often extends far beyond the edges of optical disks (namely, extended ultraviolet disks, or XUV disks; Thilker et al (2005); Gil de Paz et al (2007b)). The UV emission suggests the presence of massive stars, at least B stars, and hence that there was recent star formation within the lifetime of B stars (∼ 100 Myr). These young stars must have been born nearby, perhaps requiring unnoticed molecular gas and clouds somewhere in the extended galaxy outskirts. Average gas densities there are extremely low compared to typical star-forming regions within the MW. Understanding the conditions of parental molecular gas in such an extreme condition is vital to expand our knowledge of the physics of star formation. We need to understand the internal properties of molecular clouds, including the atomic-to-molecular gas phase transition, the distribution of molecular clouds, and the external environment in galaxy outskirts.

A blind search for molecular gas has been difficult for the large outskirts of nearby galaxies due to the limited capability of existing facilities. The Atacama Large Millimeter/submillimeter Array (ALMA) improved the sensitivity remarkably, but even ALMA would need to invest hours to days to carry out a large areal search for molecular gas over extended disks. This review summarizes the current knowledge on molecular gas and star formation in the outskirts, but this research field is still in a phase of discovery. The space to explore is large, and more systematic understanding will become possible with future observations.

Studies of molecular gas in the outskirts will also reveal the yet unknown physical properties of the interstellar medium (ISM) in the outskirts. Most observational tools were developed and calibrated in the inner parts of galactic disks and may not be applicable as they are to the outskirts. Many studies are subject to systematic biases, especially when molecular gas in the outskirts is compared with inner disks. For example, the rotational transition of carbon monoxide (CO) is often used to measure the mass of molecular gas in normal galaxies; however, its presence and excitation conditions depend on the metal abundance, stellar radiation field, internal volume and column densities, and kinetic temperature, all of which may change in the outskirts.

In this review, we start from a summary of how the ISM evolves in the inner parts of the MW and nearby galaxies with an emphasis on molecular gas (Sect. 2). We then discuss the observational methods, including the equations needed to plan for a future observational search of molecular gas with a radio telescope (Sect. 3). We explain the potential effects of applying these equations under the extreme conditions in galaxy outskirts, which may cause systematic biases when the ISM is compared between galaxies' inner parts and outskirts (Sect. 3.4). Although not many observations have been carried out in galaxy outskirts, we summarize the current state of molecular gas observations in spiral (Sect. 4) and elliptical galaxies (Sect. 5) and in galaxy groups and clusters (Sect. 6). We finish the review with possible future directions (Sect. 7). The term “outskirts” is abstract and has been used differently in different contexts. In this review we use this term for the area beyond the optical radius of galaxy, e.g., beyond r25, which is the radius where the B-band surface brightness of a galaxy falls to 25 mag arcsec−2. We should, however, note that in some circumstances r25 is not defined well, and we have to rely on a loose definition of “outskirts”.

The measurements of gas properties, such as molecular mass, often depend on some assumptions of the gas properties themselves. However, galaxy outskirts are an extreme environment, and the assumptions based on previous measurements in inner disks may not be appropriate. This problem needs to be resolved iteratively by adjusting the assumptions to match future observations. We therefore spend a number of pages on the methods of basic measurements (Sect. 3), so that the equations and assumptions can be revisited easily in future studies. Readers who already understand the basic methods and assumptions may skip Sect. 3 entirely and move from Sect. 2 to Sect. 4.

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