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4. MOLECULAR GAS OBSERVATIONS IN THE OUTSKIRTS OF DISK GALAXIES

A primary motivation for molecular gas observations in the outskirts of disk galaxies has been to study molecular clouds and star formation in an extreme environment with lower average density and metallicity. Many researchers highlight that these studies may teach us about the early Universe, where these conditions were more prevalent.

4.1. The Milky Way

The MW is the disk galaxy with the most molecular gas detections in the outskirts, with pioneering studies of the outer disk molecular gas and star formation properties beginning in the 1980s (e.g., Fich and Blitz (1984); Brand and Wouterloot (1988)). The MW can serve as a model for the types of studies that can be done in nearby galaxies with larger and more sensitive facilities. We will use “outer” MW to refer to galactocentric radii between the solar circle (RGal > R = 8.5 kpc) and the edge of the optical disk, which is estimated to be at RGal ∼ 13−19 kpc (Ruffle et al (2007); Sale et al (2010) and references therein). We will use “outskirts” to refer to galactocentric radii beyond the edge of the optical disk.

Only about 2% of the molecular mass of the MW is at RGal > 14.5 kpc (Nakagawa et al (2005) estimated the molecular mass at RGal > 14.5 kpc to be 2 × 107 M while Heyer and Dame (2015) estimated the total molecular mass of the Galaxy to be (1 ± 0.3) × 109 M). N. Izumi (personal communication) collected the known molecular clouds with RGal > 13.5 kpc in the second and third quadrants (Fig. 4). The molecular cloud with the largest known galactocentric radius is probably Digel Cloud 1 with a kinematic galactocentric radius of RGal = 22 kpc, dynamical mass of ∼ 6 × 104 M, and radius of 36 pc (Digel Cloud 2 has a larger kinematic distance of RGal = 24 kpc, but the photometric distance is RGal = 15−19 kpc based on optical spectroscopy of an associated B star; Digel et al (1994); Yasui et al (2006); Yasui et al (2008); Izumi et al (2014)). Digel Cloud 1 is beyond the edge of the optical disk but well within the Hi disk, which extends to RGal ∼ 30 kpc (Digel et al (1994); Ruffle et al (2007) and references therein).

Figure 4

Figure 4. Figure from N. Izumi (personal communication) showing the known molecular clouds at RGal > 13.5 kpc in the second and third quadrants overlaid on an artist's conception of the MW (R. Hurt: NASA/JPL-Caltech/SSC). The colours correspond to the following surveys: orange: Brunt et al (2003), magenta: Sun et al (2015), red: Digel et al (1994), cyan: Brand and Wouterloot (1994), blue: May et al (1997), green: Nakagawa et al (2005), yellow: Vázquez et al (2008). The points represent molecular clouds and the fan-shaped regions represent the survey area. The distances were derived assuming R = 8.5 kpc and a solar orbital speed of V = 220 km s−1

Extremely tenuous H2 gas is mixed with the Hi gas in the Galactic halo with a fraction of H2 over Hi of only 10−4 ∼ −5 (Lehner (2002)). Such tenuous H2 is observed via UV absorption, e.g., toward the Magellanic stream (Lehner (2002)) and high velocity clouds (HVCs; Bluhm et al (2001)). This component is important for understanding the complex physics of the ISM, but is not a major molecular component in galaxy outskirts. We therefore do not discuss this component further in this review.

4.1.1. Properties of Molecular Clouds in the Outer Milky Way

In this Section we highlight studies that have compared the mass, size, and mass surface density of molecular clouds in the outer MW to clouds in the inner MW. Molecular clouds are the site of star formation, and hence, comparisons of their properties between the inner and outer MW is important. In general, molecular clouds in the outer MW have lower mass and mass surface density than clouds in the inner disk. We also describe how molecular clouds have been used to trace spiral arms into the outskirts and to study relatively high-mass star formation.

Heyer and Dame (2015) combined published data on the CO surface brightness out to RGal ∼ 20 kpc. The clouds in the outer MW and outskirts are ∼ 7 times fainter than clouds in the inner MW (and even fainter relative to the Galactic centre). Assuming a constant XCO, this corresponds to a factor of ∼ 7 decrease in the mass surface density of molecular clouds. Heyer and Dame (2015) argued that there is a real decrease in the mass surface density of the molecular clouds, perhaps caused by the lower mid-plane pressure or stronger local FUV radiation field in the outer Galaxy. However, there is also evidence that the outer MW requires a larger XCO to convert the CO surface brightness into the mass surface density (see Sect. 3.4). Therefore the mass surface density likely decreases by somewhat less than a factor of ∼ 7.

The mass function of molecular clouds in the outer MW (9.5 kpc ≲ RGal ≲ 13.5 kpc in this study) has a steeper power law index than that in the inner MW, such that the outer disk hosts more of its molecular mass in lower-mass clouds (Rosolowsky (2005), based on the 330 deg2 Heyer et al (1998) catalogue and analysis in Heyer et al (2001) and Brunt et al (2003)), although this conclusion may at some level be a result of variable angular resolution (Heyer and Dame (2015)). The mass function of the outer MW shows no clear evidence for a truncation at the high-mass end, but under some assumptions Rosolowsky (2005) estimated that the maximum molecular cloud mass is ∼ 2−3 × 105 M. In contrast, Rosolowsky (2005) concluded that the inner MW shows a clear truncation with maximum molecular cloud mass of ∼ 3 × 106 M. Because of the small number of known clouds, the apparent lack of massive clouds in the outer MW might be due to a sampling effect. This possibility should be addressed in future studies, as a truncation, if it exists, would be an important clue to understanding cloud physics in the outskirts.

Heyer et al (2001) concluded that the size distribution of molecular clouds in the outer MW is similar to the distribution in the inner MW from Solomon et al (1987), but note that surveys with fewer clouds and different galactocentric distance ranges reached different conclusions. May et al (1997) concluded that outer MW clouds have smaller sizes than the inner MW while Brand and Wouterloot (1995) concluded that the outer MW clouds have larger sizes than inner MW clouds at the same mass. While there are conflicting results in the literature, it seems natural to conclude that an outer MW cloud must have a larger radius than an inner MW cloud at the same mass because it appears that the mass surface density of clouds is lower in the outer MW (see above and Heyer and Dame (2015)).

Molecular gas observations in the outskirts of the MW have been used to identify spiral arms. Dame and Thaddeus (2011) discovered a spiral arm in the first quadrant at RGal ∼ 15 kpc, based on Hi and CO data. Their new arm is consistent with being an extension of the Scutum-Centaurus arm. Sun et al (2015) also used Hi and CO data to discover an arm in the second quadrant at RGal = 15 − 19 kpc. This arm could be a further continuation of the Scutum-Centaurus arm and the Dame and Thaddeus (2011) arm. These kinds of studies are important not only to map the spiral structure of the MW, but also to help understand the observation that star formation in the outskirts of other galaxies often follows spiral arms.

Another important goal of molecular gas studies in the outskirts of the MW has been to understand the connection with star formation: Milky Way under low density and metallicity conditions. For example, Brand and Wouterloot (2007) studied an IRAS-selected molecular cloud with a mass of 4.5 − 6.6 × 103 M at RGal ∼ 20.2 kpc. They discovered an embedded cluster of 60 stars and the lack of radio continuum emission limits the most massive star to be later than B0.5. In addition, Kobayashi et al (2008) studied Digel Cloud 2, which is really two clouds each with a mass of ∼ 5 × 103 M. They discovered embedded clusters in each of the clouds. One cluster likely contains a Herbig Ae/Be star and there are also several Herbig Ae/Be star candidates, a B0-B1 star, and an Hii region nearby. Therefore, high-mass star formation has occurred near this low-mass molecular cloud. We encourage more study on the relationship between cloud mass and the most massive star present, as extragalactic studies can trace O and B stars relatively easily, but have difficulty detecting the parent molecular clouds (see Sect. 4.2.1).

In the outskirts of the MW and other galaxies, it is important to ask what triggers molecular cloud and star formation. In Digel Cloud 2, star formation may have been triggered by the expanding Hi shell of a nearby supernova remnant (Kobayashi and Tokunaga (2000); Yasui et al (2006); Kobayashi et al (2008)) while Izumi et al (2014) hypothesized that the star formation in Digel Cloud 1 may have been triggered by interaction with a nearby HVC.

4.2. Extragalactic Disk Galaxies

We can study molecular gas in more varied environments by moving from the MW to extragalactic disk galaxies. In this Section, we use “outskirts” to refer to galactocentric radii greater than the optical radius (RGal > r25).

4.2.1. Molecular Gas Detections

Numerous attempts to detect CO beyond the optical radius in the disks of spiral galaxies have failed, although many of the non-detections are unpublished (Watson et al (2016); Morokuma-Matsui et al (2016); J. Braine, F. Combes, J. Donovan Meyer, and A. Gil de Paz, personal communications). To our knowledge, there are only four isolated spiral galaxies with published CO detections beyond the optical radius (Braine and Herpin (2004); Braine et al (2007); Braine et al (2010); Braine et al (2012); Dessauges-Zavadsky et al (2014)). Table 1 summarizes the number of detected regions and their range of galactocentric radii and molecular gas masses. Extragalactic studies have not yet reached the molecular gas masses that are typical in the outskirts of the MW (2−20 × 103 M for the eleven Digel clouds at RGal = 18−22 kpc; Digel et al (1994); Kobayashi et al (2008); see also Braine et al (2007)).

Table 1. Extragalactic disk galaxies in relative isolation with CO detections beyond the optical radius (Braine and Herpin (2004); Braine et al (2007); Braine et al (2010); Braine et al (2012); Dessauges-Zavadsky et al (2014)). For M33, the molecular gas mass is for one of the detected clouds. For M63, the molecular gas mass is based on a sum of the CO line intensities in twelve pointings, two of which are detections. The NGC 4414, NGC 6946, and M63 masses were computed assuming XCO = 2 × 1020 cm−2 (K km s−1)−1.

Galaxy Detected Galactocentric Molecular Method used for Mass
  Regions Radius Gas Mass  
  (#) (r25) (105 M)  

NGC 4414 4 1.1 − 1.5 10−20 Within 21” IRAM 30 m beam
NGC 6946 4 1.0 − 1.4 1.7−3.3 Within 21” IRAM 30 m beam
M33 6 1.0 − 1.1 0.43 Virial mass using resolved PdBI data
M63 2 1.36 7.1 Sum of 12 IRAM 30 m pointings

It would be useful to be able to predict where CO will be detected in the outskirts of disk galaxies, both as a test of our understanding of the physics of CO formation and destruction in extreme conditions (see Sect. 3.4) and to help us efficiently collect more detections. Most of the published CO studies selected high Hi column density regions or regions near young stars traced by Hα, FUV, or FIR emission. None of these selection methods is completely reliable. Braine et al (2010) concluded that CO is often associated with large Hi and FIR structures, but it is not necessarily located at Hi, FIR, or Hα peaks. Many factors might affect the association between Hi, CO and star formation tracers. For example, the star forming regions may drift away from their birthplaces over the 10−100 Myr timescales traced by Hα, FUV, and FIR emission. In addition, feedback from massive stars might destroy molecular clouds more easily in the low-density outskirt environment. Finally, higher-resolution Hi maps may show better correlation with CO emission. Sensitive, large-scale (> kpc) maps of the outskirts of disk galaxies may allow for a more impartial study of the conditions that maximize the CO detection rate.

4.2.2. Star Formation in Extragalactic Disk Galaxies

It is generally accepted that stars form from molecular gas (e.g., Fukui and Kawamura (2010)) and that an important stage before star formation is the conversion of Hi to H2 (e.g., Leroy et al (2008)). A main tool to study the connection between gas and star formation is the Kennicutt-Schmidt law (Schmidt (1959); Kennicutt (1998)), which is an empirical relationship between the star formation rate (SFR) surface density (ΣSFR) and the gas surface density. Within the optical disk of spiral galaxies, there is an approximately linear correlation between ΣSFR and the molecular hydrogen surface density (ΣH2) but no correlation between ΣSFR and the atomic hydrogen surface density (ΣHI; e.g., Bigiel et al (2008); Schruba et al (2011)).

The majority of the published work connecting the SFR and gas density in the outskirts of disk galaxies has focused on the atomic gas because molecular gas is difficult to detect (Sect. 4.2.1) and because the ISM is dominantly atomic in the outskirts, at least on ≳ kpc scales. Bigiel et al (2010) concluded that there is a correlation between the FUV-based ΣSFR and ΣHI in the outskirts of 17 disk galaxies and 5 dwarf galaxies. They measured a longer depletion time in the outskirts, such that it will take on average 1011 years to deplete the Hi gas reservoir in the outskirts versus 109 years to deplete the H2 gas reservoir within the optical disk. Roychowdhury et al (2015) reached a similar conclusion using Hi-dominated regions in disks and dwarfs, including some regions in the outskirts, although they concluded that the depletion time is somewhat shorter than in the outskirts of the Bigiel et al (2010) sample (see also Boissier et al (2007); Dong et al (2008); Barnes et al (2012)). The correlation between ΣSFR and ΣHI is surprising because there is no correlation within the optical disk. Bigiel et al (2010) suggested that high Hi column density is important for determining where stars will form in the outskirts.

The study of the connection between molecular gas and star formation in the outskirts has been limited by the few molecular gas detections. Figures 5 and 6 show the relationship between ΣSFR and ΣH2 for the molecular gas detections from Table 1 plus a number of deep CO upper limits. In both panels the SFR was computed based on FUV and 24 µm data to account for the star formation that is unobscured and obscured by dust.

Figure 5

Figure 5. Figure 7 from Dessauges-Zavadsky et al (2014) showing the molecular-hydrogen Kennicutt-Schmidt relation for the star forming regions in the UV-complex at r = 1.36 r25 in M63 (red points) compared to regions within the optical disk (blue points). The blue line shows the fit for the optical disk. The black lines represent constant star formation efficiency, assuming a timescale of 108 years. Credit: Dessauges-Zavadsky et al (2014), reproduced with permission © ESO

Dessauges-Zavadsky et al (2014) studied a UV-bright region at r = 1.36 r25 in the XUV disk of M63 (Fig. 5). They detected CO in two out of twelve pointings and concluded that the molecular gas has a low star formation efficiency (or, equivalently, the molecular gas has a long depletion time) compared to regions within the optical disk. They suggested that the low star formation efficiency may be caused by a warp or by high turbulence. Watson et al (2016) measured a deep CO upper limit in a region at r = 3.4 r25 in the XUV disk of NGC 4625 and compiled published CO measurements and upper limits for 15 regions in the XUV disk or outskirts of NGC 4414, NGC 6946, and M33 from Braine and Herpin (2004) and Braine et al (2007); Braine et al (2010) (see Table 1 and Fig. 6). They concluded that star-forming regions in the outskirts are in general consistent with the same ΣSFR - ΣH2 relationship that exists in the optical disk. However, some points are offset to high star formation efficiency (short depletion time), which may be because the authors selected Hα- or FUV-bright regions that could have already exhausted some of the molecular gas supply (as in Schruba et al (2010); Kruijssen and Longmore (2014)).

Figure 6

Figure 6. The molecular hydrogen Kennicutt-Schmidt relation for the remaining star forming regions that are beyond the optical radius in isolated extragalactic disk galaxies and have published CO detections or deep upper limits. The solid line shows the fit for the optical disk of normal spiral galaxies at ∼ kpc resolution, with the 1σ scatter shown by the dotted lines (Leroy et al (2013)). This figure was originally presented in Fig. 4 in Watson et al (2016)

We should ask what stimulates the formation of molecular gas and stars in the outskirts of disk galaxies. Thilker et al (2007) suggested that interactions may trigger the extended star formation in XUV disks while Holwerda et al (2012) suggested that cold accretion may be more important. Bush et al (2008); Bush et al (2010) carried out hydrodynamic simulations and concluded that spiral density waves can raise the density in an extended gas disk to induce star formation (see also Sect. 4.1.1. of Debattista et al., this volume).

The state-of-the-art data from SINGS (Kennicutt et al (2003)), the GALEX Nearby Galaxy Survey (Gil de Paz et al (2007a)), THINGS (Walter et al (2008)), and HERACLES (Leroy et al (2009)) brought new insight into the Kennicutt-Schmidt law within the optical disk of spirals. Deeper CO surveys over wider areas in the outskirts could bring a similar increase in our understanding of star formation at the onset of the Hi-to-H2 transition. In such wide-area studies, one should keep in mind that the “standard” physical condition of gas in inner disks could change in the outskirts, which could affect the measurements (Sect. 3.4).

4.2.3. Theory

This Chapter focuses on observations, but here we briefly highlight theoretical works that are related to molecular gas in the outskirts. The majority of the relevant theoretical studies have concentrated on the origin of gas in the outskirts (e.g., Dekel and Birnboim (2006); Sancisi et al (2008); Sánchez Almeida et al (2014); Mitra et al (2015)) and star formation in the outskirts (Bush et al (2008); Bush et al (2010); Ostriker et al (2010); Krumholz (2013); Sánchez Almeida et al (2014); see also Roškar et al (2010); Khoperskov and Bertin (2015)). Krumholz (2013) is particularly relevant because he extended earlier work to develop an analytic model for the atomic and molecular ISM and star formation in outer disks. Krumholz assumed that hydrostatic equilibrium sets the density of cold neutral gas in the outskirts and was able to match the Bigiel et al (2010) observations that show a correlation between ΣSFR and ΣHI (see also Sect. 7 of Elmegreen and Hunter, this volume).

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