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5. MOLECULAR GAS OBSERVATIONS IN THE OUTSKIRTS OF EARLY-TYPE GALAXIES

Early-type galaxies were historically viewed as “red and dead,” with little gas to form new stars. However, more recent surveys have found reservoirs of cold gas both at galaxy centres and in the outskirts. Molecular gas in the centres of early-type galaxies can have an internal and/or external origin while the molecular gas in the outskirts often originated in a gas-rich companion that has interacted or merged with the early-type. As in all of the environments we have explored, stimuli can also trigger new molecule formation in the outskirts of early-types.

We start with a review of Hi in the inner and outer regions of early-type galaxies to put the molecular gas observations in context. The ATLAS3D survey detected Hi in 32% of 166 early-type galaxies in a volume-limited sample, down to a 3σ upper limit of MHI = 5 × 106 − 5 × 107 M. Atomic gas in the outskirts of early-type galaxies is even relatively common, as 14% of the ATLAS3D sample have Hi that extends out to more than 3.5 times the optical effective radius (Serra et al (2012)).

Most surveys of molecular gas in early-type galaxies have focussed on the inner regions. 22% of 260 early-type galaxies in the ATLAS3D sample were detected in CO, down to a 3σ upper limit of MH2 ∼ 107 − 108 M (Young et al (2011); see also Sage and Wrobel (1989); Knapp and Rupen (1996); Welch and Sage (2003); Combes et al (2007); Welch et al (2010)). Within the areas searched, the molecular gas is generally confined to the central few kpc and is distributed in disks, bars plus rings, spiral arms, or with a disrupted morphology (Young (2002); Welch and Sage (2003); Young et al (2008); Davis et al (2013); Alatalo et al (2013)).

One important motivation for studies of molecular gas in early-type galaxies has been to determine whether the gas is of internal or external origin. Some of the molecular gas has likely either been present since the galaxies transitioned to being early-type or has accumulated from stellar mass loss (Faber and Gallagher (1976); Young (2002); Young et al (2008); Mathews and Brighenti (2003); Ciotti et al (2010)). In contrast, some molecular gas has likely been accreted more recently through minor mergers and/or cold accretion. This external origin is most clearly exhibited by galaxies that display a misalignment between the kinematic axes of the molecular/ionized gas and the stars (Young et al (2008); Crocker et al (2008); Davis et al (2011); Alatalo et al (2013)). In particular, Alatalo et al (2013) concluded that 15 galaxies out of a sample of 40 show a kinematic misalignment of at least 30 degrees, which is consistent with gas accretion via minor mergers.

The majority of accreting gas is perhaps in the atomic form, but the outskirts of early-type galaxies also offer the opportunity to study recently accreted molecular gas, which has mainly been detected in polar rings of elliptical and S0 galaxies (see Fig. 7 for an example). These polar rings are present in about 0.5% of nearby S0 galaxies (Whitmore et al (1990)). CO has been detected in polar rings at galactocentric radii of 12 kpc in NGC 660 (Combes et al (1992)) and 2 kpc in NGC 2685 (Schinnerer and Scoville (2002); see also Watson et al (1994); Galletta et al (1997); Combes et al (2013)). Published values for the mass of molecular hydrogen in the polar rings range from 8−11 × 106 M in NGC 2685 (Schinnerer and Scoville (2002)) to 109 M in NGC 660 (Combes et al (1992)), although the handful of polar rings with CO detections are likely biased towards high MH2.

Figure 7

Figure 7. Figure 2 from Watson et al (1994) showing the Caltech Submillimeter Observatory CO(2−1) spectra (left) at three pointings, which are indicated by circles in the B-band image of the polar-ring galaxy NGC 4650A (Whitmore et al (1987)) on the right. Watson et al (1994) estimated the mass of molecular hydrogen in the polar ring of NGC 4650A to be MH2 = 8−16 × 108 M. © AAS. Reproduced with permission

Polar rings are likely caused by tidal accretion from, or a merger with, a gas-rich companion and are stable on timescales of a few Gyr as a result of self gravity (Bournaud and Combes (2003)). The molecular gas observations generally support this hypothesis because the molecular gas masses are consistent with those of a dwarf or spiral galaxy (Watson et al (1994); Galletta et al (1997); Schinnerer and Scoville (2002)).

Mergers between an early-type galaxy and a gas-rich companion can manifest in non-polar ring systems as well. Buta et al (1995) studied the spheroid-dominated spiral galaxy NGC 7217 and concluded that most of the molecular mass is in an outer star-forming ring at RGal ∼ 0.6 r25 that could have an H2 mass that is equal to or greater than the Hi mass. More recent work by Sil'chenko et al (2011) indicates that minor mergers may be responsible for the outer ring structures.

Molecular gas has also been detected in shells at a galactocentric radius of 15 kpc (1.16 r25) in the elliptical galaxy Centaurus A (Charmandaris et al (2000)). Charmandaris et al (2000) calculated the mass of molecular hydrogen in the CenA shells to be MH2 = 4.3 × 107 M. Like polar rings, shells are likely caused by galaxy interactions and Charmandaris et al (2000) concluded that CenA interacted with a massive spiral galaxy rather than a low-mass dwarf galaxy because of the large total gas mass and large ratio of molecular to atomic gas in CenA. Additional molecular cloud formation may have been triggered by the interaction between the shells and the CenA radio jet (see also Salomé et al (2016)).

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