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While the spectroscopic analysis of H ii regions in the inner disks of spiral galaxies is a well-developed activity in extragalactic astronomical research, starting with the pioneering work by Searle (1971) and Shields (1974), who provided the first evidence for the presence of exponential radial abundance gradients in nearby galaxies such as M33 and M101, the investigation of ionized nebulae located beyond the boundary of the main star-forming disk has begun only recently. Observationally, the main difficulty in measuring chemical abundances in these outlying H ii regions is represented by their intrinsic faintness. In fact, these nebulae are typically ionized by single hot stars, as deduced from their Hα luminosities (Gil de Paz et al. 2005; Goddard, Kennicutt and Ryan-Weber 2010), that are on average about two orders of magnitude fainter than those of the giant H ii regions that are routinely observed in the inner disks (Bresolin et al. 2009b; Goddard et al. 2011). This section reviews the investigations of the chemical abundances in the outskirts of nearby spiral galaxies, focusing mostly on oxygen in H ii regions. Studies of metals in old stars and nebular nitrogen are also briefly discussed.

3.1. Early Work

Spectra of a handful of outlying H ii regions in the disks of the late-type spirals NGC 628, NGC 1058 and NGC 6946, known for their extended H i distributions, were first obtained by Ferguson, Gallagher and Wyse (1998). These authors found that the oxygen abundance gradients measured in the inner disks appear to continue to large galactocentric distances, beyond the isophotal radii 2 R25, and reaching low metallicities, corresponding to 10−15 percent of the Solar value. Unfortunately, the small sample size concealed the possibility, demonstrated by later work, that the radial metallicity trends could actually be different between inner and outer disks. van Zee et al. (1998) also presented spectra of outlying H ii regions in a sample of 13 spirals, but the number of objects observed near R25 and beyond was very small.

The oxygen abundance in the outer disks of two iconic representatives of the class of extended UV (XUV) disk galaxies discovered by the GALEX satellite, M83 (Thilker et al. 2005) and NGC 4625 (Gil de Paz et al. 2005), was investigated by means of multi-object spectroscopy with the Magellan and Palomar 200-inch telescopes by Gil de Paz et al. (2007). These authors also found a relatively low metal content, around 10−20 percent of the Solar value, utilizing a combination of photoionization models and the R23 strong-line abundance diagnostic. This study, however, was also limited by the small number of H ii regions observed at large galactocentric distances, especially for the galaxy NGC 4625. A feature of the M83 radial abundance gradient that was suggested by Gil de Paz et al. (2007), i.e., a sudden drop in metallicity at a galactocentric distance of 10 kpc, but whose presence was dubious due to the uncertain O/H ratios derived from R23, was also detected later in the larger sample of outlying H ii regions studied by Bresolin et al. (2009b). Gil de Paz et al. (2007) remarked that such a sharp decrease, if present, could be the signature of a transition to an outer disk where the star formation efficiency is significantly lower compared to the inner disk.

3.2. M83: a Case Study

The first investigation to obtain robust chemical abundances — via a variety of nebular metallicity diagnostics — in the outer disk of a single galaxy was carried out by Bresolin et al. (2009b), who obtained spectra of ionized nebulae in the outer disk of M83 (NGC 5236) with the ESO Very Large Telescope. Of these H ii regions, 32 lie at galactocentric distances larger than the isophotal radius, extending out to 22.3 kpc (2.64 R25) from the galaxy centre.

The principal chemical abundance properties of the outer disks of spiral galaxies, confirmed by subsequent investigations of other targets (as discussed in the next pages), are all showcased in this prototypical XUV disk galaxy (see Fig. 1):

Figure 1

Figure 1. The radial oxygen abundance gradient in M83, determined from two different diagnostics (R23 in the top panel, N2O2, as calibrated empirically by Bresolin (2007), in the bottom panel), from H ii regions located in the inner disk (⋄ symbols: Bresolin and Kennicutt 2002; Bresolin et al. 2005) and in the outer disk (∘ symbols: Bresolin et al. 2009b). The symbol represents [O iii] λ4363-based O/H values. Linear regressions to the radial abundance gradient are shown as separate dot-dashed lines for the inner and outer portions of the galactic disks. The two dotted lines represent the range of gradient slopes measured by Ho et al. (2015) from a sample of 49 galaxies: dlog(O/H) / dR = −0.39 ± 0.18 dex R25−1. Adapted from the data published by Bresolin et al. (2009b).

Fig. 1 also suggests the presence of a ∼ 0.2 dex oxygen abundance discontinuity beyond the isophotal radius. This feature is not confirmed by all abundance diagnostics considered (see also Pilyugin, Grebel and Mattsson 2012), nor is it detected in other extended disk galaxies, except NGC 4625 (Goddard et al. 2011) but it also appears in the data presented by Gil de Paz et al. (2007). It resembles the break occurring for the α-elements measured for Cepheids in the Milky Way at a galactocentric distance of approximately 9 kpc (Lépine et al. 2014).

The observations in M83 demonstrate that spectroscopy of H ii regions located in the extended, gas-rich disks of spiral galaxies allows us to probe the present-day chemical abundances of external galactic disks out to nearly three isophotal radii, equivalent — in the case of M83 — to more than 20 kpc. The interesting, and somewhat surprising, result is found that the radial metallicity distribution becomes virtually flat in the extended outskirts, with a value of at least 1/3 of Solar. The extended disk appears to be chemically over-abundant for its very large gas mass fraction.

3.3. Other Systems

In this Section the results obtained from other investigations of single targets or small samples of galaxies, essentially confirming and expanding the general picture outlined in Sect. 3.2 for M83, are reviewed. In addition to H ii regions as primary probes of the present-day metallicity, information from the older stellar content is included. Some galaxies display a flattening of the gas metallicity already inside the main disk (R < R25), perhaps as a result of gas flows induced by the gravitational potential of a stellar bar (Martin and Roy 1995; Zahid and Bresolin 2011; Marino et al. 2012). Here I will focus on outer disk systems exclusively.

H I-selected galaxies The oxygen abundances of outlying H ii regions in a sample of 13 H i-selected galaxies were measured using the R23 method by Werk et al. (2011). The sample is dominated by interacting systems and galaxies displaying a disturbed, extended neutral gas morphology. In most cases a flat radial abundance distribution was found across most of the disk of these systems, although the number of H ii regions observed per galaxy is sometimes too small to infer variations between the inner and outer disks. Thus the flattening observed can be of a different nature in this kind of galaxies (see the discussion below) compared with relatively isolated galaxies such as M83, where the flattening is observed to occur only in the outer disk.

Werk et al. (2011) showed that the oxygen abundances in the outskirts of the galaxies included in their sample are considerably higher than expected, given their large gas content compared with the total barionic (stars + gas) mass. The same result was described by Bresolin et al. (2009b) and Werk et al. (2010) for the extended disks of M83 and the blue compact dwarf galaxy NGC 2915, respectively. While for typical star-forming galaxies the effective oxygen yield, defined by

Equation 3


where µ is the gas mass fraction and ZO is the metallicity equivalent to the O mass fraction 3, lies below the theoretical oxygen yield for a stellar population, yO ≃ 0.007 (e.g., Kobayashi et al. 2006), in the case of the outer disks the opposite holds, with yO,eff > 0.02 (Werk et al. 2011; López-Sánchez et al. 2015). This in essence implies that the oxygen abundances measured in the gas located in the outskirts of these galaxies, including the XUV disk systems, characterized by extended H i envelopes, exceed the values predicted by the closed-box galactic chemical evolution model. How this level of chemical enrichment can be attained, given the low values measured for the star formation rate, will be addressed in Sect. 5.

Interacting systems The majority of the galaxies studied by Werk et al. (2011) are located in interacting systems. Nebular oxygen abundances have been measured in the main star-forming disks and along tidal features of interacting and merging systems by various authors (e.g. Rupke, Kewley and Chien 2010; Rich et al. 2012; Torres-Flores et al. 2014), finding significantly flatter radial distributions compared to non-interacting systems. These studies are supported by numerical simulations (Rupke, Kewley and Barnes 2010; Torrey et al. 2012), showing that gas flows induced by galaxy interactions redistribute the gas in such a way that the original abundance gradients — present in the galactic disks before the merging process — flatten progressively with merger stage.

This redistribution and radial mixing of metals can take place over very large distances, in extreme cases reaching several tens of kpc. For example, Olave-Rojas et al. (2015) measured the chemical abundances of H ii regions located along the main tidal tail of NGC 6845A, part of a compact, interacting group of galaxies, out to almost 70 kpc from the centre (approximately 4 R25). The radial oxygen distribution displays a remarkably shallow gradient.

An interesting interacting system is represented by NGC 1512, which is experiencing an encounter with the companion galaxy NGC 1510. The system, an XUV disk galaxy, is embedded in a very extended H i envelope, with a radius of 55 kpc (Koribalski and López-Sánchez 2009), in which low-level star formation is taking place, as shown by the far-UV and Hα emission originating from low-luminosity stellar complexes and associated H ii regions. The flat and relatively high O/H abundance values, 12 + log(O/H) ≃ 8.3, out to a galactocentric distance of ∼30 kpc, have been studied by Bresolin, Kennicutt and Ryan-Weber (2012) and, more recently, by López-Sánchez et al. (2015). The latter authors point out the effect of the interaction on the outlying northern H i spiral arm, where the O/H values have a much larger dispersion than in the opposite side of the galaxy, which remains relatively undisturbed by the ongoing interaction.

Old stars The data discussed so far refer only to the present-day metallicities, as derived from H ii region emission. It is also possible to infer the chemical composition of the outer disks of nearby spirals from stellar photometry of older populations, in particular of red giant branch (RGB) stars. The method requires the photometry of individual stars, and as such has successfully been applied only to nearby systems, out to approximately 3 Mpc. Care must be taken in interpreting these photometric metallicities when discussing disk radial gradients, because of the potential contamination from halo stars.

The metallicity can be derived from a comparison of the observed stellar colours, such as VI, with theoretical stellar tracks, exploiting the fact that these broad-band colours are more sensitive to metallicity than age. In this way, Worthey et al. (2005) obtained a flat metallicity gradient in the outer disk of M31, between 20 and 50 kpc from the galaxy centre (1 to 2.5 R25), with a mean value of [Z/H] ≃ −0.5. The published H ii region abundances (e.g., Zurita and Bresolin 2012; Sanders et al. 2012) do not extend beyond 25 kpc, and thus whether a similar behaviour is encountered for the younger stellar populations cannot be verified. A flat gradient, with an approximately Solar O/H value, extending out to ∼100 kpc, has been reported for planetary nebulae by Balick et al. (2013) and Corradi et al. (2015). These authors attribute this finding to a star formation burst following interactions and merger processes, perhaps related to an encounter with M33, that occurred approximately 3 Gyrs ago.

Vlajić, Bland-Hawthorn and Freeman (2009, 2011) measured the metallicity of RGB stars from deep g′ and i′ Gemini photometry in the outer disks of the two Sculptor Group spirals NGC 300 and NGC 7793, out to 15 kpc (2.3 R25) and 11.5 kpc (2.4 R25), respectively. Both galaxies, like M31, display purely exponential surface brightness profiles out to these large galactocentric distances, indicating that the halo contribution is probably negligible. The possible connection between gas-phase metallicity and surface brightness profiles will be briefly discussed in Sect. 4.1.

For both NGC 300 and NGC 7793 the stellar metallicity flattens out to an approximately constant value, or even slightly increases with radius in the outer disk, in contrast with the exponential decline inferred from H ii regions and young stars in the inner disk. The measured metallicity is quite low, [Fe/H] ≃ −1 for the outer disk of NGC 300, and [Fe/H] ≃ −1.5 (or even lower, depending on the age of the stars) for NGC 7793, but could be compatible with the present-day metallicity of the inner disk if the chemical enrichment due to stellar evolution between the time probed by the RGB stars (8−12 Gyr ago) and the present epoch is taken into account. These results are made somewhat uncertain by the age-metallicity degeneracy, the assumption of a single age for the RGB stars and the potential effects of stellar migration.

Other XUV disks The chemical abundances of outer H ii regions in a few XUV disk galaxies (as defined in the catalogue by Thilker et al. (2007)), in addition to M83 and NGC 1512, have been presented by different authors. For convenience, Table 1 summarizes these studies. These investigations differ somewhat in spectroscopic depth, abundance diagnostics adopted and radial coverage, but they tend to provide a unified picture regarding the abundance gradients, in particular the presence of a break occurring approximately at the isophotal radius, as a dividing point between the inner disk, characterized by an exponential nebular abundance gradient, and the outer disk, with a shallower or flat abundance gradient.

Table 1. Nebular abundance studies in XUV disks

Galaxy References Largest radius
(R25 units)

NGC 628 Rosales-Ortega et al. 2011 a 1.7
NGC 1512 Bresolin et al. 2012 2.2
López-Sánchez et al. 2015 2.8
NGC 3621 Bresolin et al. 2012 2.0
NGC 4625 Goddard et al. 2011 2.8
NGC 5236 (M83) Bresolin et al. 2009 2.6

a Data for objects lying beyond R25 extracted from Ferguson, Gallagher and Wyse (1998).

Not included in Table 1 is NGC 3031 (M81), for which a flat outer gradient has been suggested, but this result relies on a very small sample of outlying H ii regions (Patterson et al. 2012; Stanghellini et al. 2014). A more recent work by Arellano-Córdova et al. (2016) does not find evidence for a flat gradient out to a galactocentric distance of 33 kpc (2.3 R25). This seems to be consistent with the shallow overall abundance gradient, both in dex kpc−1 and normalized to the isophotal radius, which is possibly the consequence of galaxy interactions. It is also worth pointing out that the abundance break observed in NGC 3621 by Bresolin, Kennicutt and Ryan-Weber (2012) has been confirmed by the independent spectroscopic analysis of five blue supergiant stars, straddling the isophotal radius, by Kudritzki et al. (2014). The stellar metallicities are intermediate between the nebular metallicities determined from the N2 and R23 diagnostics. Finally, it is important to notice that the sample presented above includes fairly isolated systems (NGC 3621, M83, NGC 628), ruling out the possibility that abundance breaks and significant metal mixing develop only as a consequence of recent galaxy interactions.

The Milky Way Evidence for a flattening of the abundance gradient in the outer disk of the Milky Way comes from observations of various metal tracers, which also sample populations with different ages: Cepheid variables (Korotin et al. 2014), open clusters (Magrini et al. 2009; Yong, Carney and Friel 2012), and H ii regions (Vílchez and Esteban 1996; Esteban et al. 2013). This break appears at a galactocentric distance around 12 kpc, extending outwards to 19–21 kpc, as shown from either Cepheids (Genovali et al. 2015) or open clusters (Carraro et al. 2004). While the flattening in the Cepheids chemical abundances is still somewhat controversial (e.g., Lemasle et al. 2013), the open clusters show a clear bimodal radial gradient in metallicity (Yong, Carney and Friel 2012), the outer gradient being quite shallow, with a characteristic outer disk metallicity [Fe/H] ≃ −0.3 ± 0.1. Further studies of the behaviour of the radial distribution of the stellar metallicity (and chemical element patterns) in the outer disk of the Galaxy will be important to constrain models of the chemical evolution of the Milky Way, and the effects of the corotation resonance and stellar radial migration (Mishurov, Lépine and Acharova 2002; Lépine et al. 2011; Korotin et al. 2014).

3.4. Results from Galaxy Surveys

More recent results about the chemical abundances of the outer disks of spiral galaxies in the nearby Universe have been published as part of relatively large spectroscopic surveys, largely dedicated to the measurement of emission-line abundances of the interstellar medium in the parent galaxies. These surveys, based on 4 m-class telescope observations, do not reach emission line levels as faint as those probed by some of the single galaxy work illustrated earlier. Therefore, weak lines such as [O iii] λ4363 remain undetected in the low-luminosity H ii regions located in the galactic outskirts. In addition, these surveys have provided metallicity information out to ∼1–1.5 R25, i.e., to considerably smaller galactocentric distances than possible with 8 m-class facilities (see Table 1). On the other hand, the large number of galaxies (hundreds) provides essential statistical information about the properties of the abundance gradients, that are necessary to establish, for instance, how common radial metal distribution breaks are within the general population of spiral galaxies. Furthermore, such surveys also enable the investigation of possible correlations between abundance gradients and galactic attributes, such as mass, star formation rate, and structural properties (e.g., the presence or absence of bars).

Integral Field Spectroscopy Sánchez-Menguiano et al. (2016) presented oxygen abundance measurements obtained by the Calar Alto Legacy Integral Field Area (CALIFA) project (Sánchez et al. 2012) in 122 face-on spiral galaxies. Adopting the O3N2 nebular diagnostic, they confirmed earlier results, obtained from the same survey (Sánchez et al. 2014), that a flattening of the gas-phase oxygen abundance taking place around a galactocentric distance corresponding to twice their effective radii 4 (Re, measured in the r band) is a common occurrence in spiral disks. About 82% of the sample with reliable abundance data in the outer disks show this effect, with no apparent dependence on galactic mass, luminosity, and morphological type. The oxygen abundance in the inner disks, on the other hand, follows a gradient having a characteristic slope of approximately −0.07 dex Re−1, except for the very central parts (Sánchez et al. 2014). This common behaviour is illustrated in Fig. 2, which displays data extracted from Sánchez-Menguiano et al. 2016, their Fig. 9.

In order to make these results more easily comparable with those presented earlier, where the radial normalization is done relative to the isophotal radius, we need to define a relation between Re and R25, which depends on the central surface brightness value (µ0) for the adopted exponential brightness profile. Taking µ0 = 21.65 mag arcsec−2 from Freeman (1970), and using µ(R) = µ0 + 1.086 R / Rd, one obtains R25 = 1.84 Re. Thus, the flattening in the abundance gradient observed for the CALIFA sample of galaxies to occur at R ∼ 2 Re or, equivalently, around RR25, is consistent with what is reported in Sect. 3.2 and 3.3. The O3N2-based oxygen abundances measured in the outer disks, out to ∼ 1.5 R25, are also roughly consistent with those presented earlier, in particular they represent a significant fraction of the Solar value, e.g., approximately 0.5 (O/H) for the intermediate mass bin shown in Fig. 2.

Figure 2

Figure 2. Mean radial oxygen abundance profiles measured for a sample of face-on spirals from the CALIFA survey. The data are plotted in bins of 0.25 Re, for three different galaxy mass ranges, as indicated in the plot. The upper scale, drawn in units of the isophotal radius, assumes a central surface brightness µ0 = 21.65 mag arcsec−2. Adapted from Sánchez-Menguiano et al. (2016), Fig. 9.

Long-slit spectra For completeness, it is worth mentioning some additional surveys that obtained spectroscopic observations of the ionized gas in regions close to the edges of spiral galaxies, even though the radial coverage is not as extended as in the cases discussed so far, and is generally limited to regions inside the isophotal radius. Moran et al. (2012) obtained long-slit spectra along the major axis of 174 star-forming galaxies from the GALEX Arecibo Sloan Digital Sky Survey (Catinella et al. 2010) with stellar mass M > 1010 M, and determined O3N2-based gas-phase oxygen abundances in spatial bins for 151 galaxies displaying emission lines. However, their data extend to galactocentric distances of about 1.5 R90 5, or approximately 0.9 R25 according to the transformation between the two normalization radii estimated by these authors. Thus, these chemical abundances still refer to the main star-forming disk, and should not be compared directly with the outer disk abundance properties presented in the previous sections. Interestingly, however, for about 10% of their galaxies Moran et al. (2012) measured a significant drop in O/H around R = R90, whose magnitude correlates with the total H i mass fraction. These authors suggest that the downturn in oxygen abundance results from the accretion of relatively metal-poor gas in the outer regions of these galaxies.

Similar long-slit observations have been carried out by Carton et al. (2015) for 50 H i-rich galaxies, part of the Bluedisk survey (Wang et al. 2013), with a radial coverage extending to about 2 R90 in some cases. Also for these targets a steepening of the radial abundance distribution is observed at large radii. However, in this work the oxygen abundance downturn is not found to correlate with the H i properties of the parent galaxies as instead found in the work by Moran et al. (2012).

3.5. Nitrogen Abundances

The investigation of nitrogen abundances in extragalactic H ii regions, and in particular of the N/O abundance ratio, provides important constraints on the chemical evolution of galaxies. This springs from the fact that, while oxygen is the nucleosynthetic product of massive stars (M > 8 M), nitrogen, whose abundance can in general be easily measured in nebular spectra 6, originates mostly in intermediate-mass (M = 1−8 M) stars (Henry and Worthey 1999). A look at the N/O ratio variation as a function of metallicity O/H in extragalactic nebulae reveals a bimodal behaviour. The N/O ratio is approximately constant [log(N/O) ≃ −1.4, but with a large scatter, see Garnett 1990] below 12 + log(O/H) = 8.0, and increases with O/H at larger metallicities. This is interpreted in terms of a primary production of nitrogen, which is what is predominantly being measured at low metallicity, and of a secondary component, proportional to the oxygen abundance, dominating at high O/H (Vila Costas and Edmunds 1993). The N/O ratio measured in outer disk H ii regions conforms to this trend, as shown by Bresolin, Kennicutt and Ryan-Weber (2012). Since the oxygen abundances measured in outer spiral disks are generally below the level at which secondary nitrogen production becomes predominant, the radial trend of the N/O abundance ratio is virtually flat, with log(N/O) ≃ −1.3 to −1.5, in these outer regions (Bresolin et al. 2009b; Berg et al. 2012; López-Sánchez et al. 2015). A similar behaviour has recently been observed by Croxall et al. (2016) in M101 (also an XUV galaxy, Thilker et al. 2007), with the onset of the flattened N/O radial distribution occurring around 0.7 R25. These results stress the fact that primary production of nitrogen dominates in the H ii regions populating the outskirts of spiral galaxies.

In summary, flat abundance gradients and relatively high oxygen abundances appear to be common features of star-forming outer disks.

2 At the isophotal radius R25 the surface brightness measures 25 mag arcsec−2, and is often reported in the B photometric band, as in the Third Reference Catalogue of Bright Galaxies (de Vaucouleurs et al. 1991). Back.

3 The gas mass fraction is µ = Mgas / (Mgas + Mstars). The O mass fraction (‘metallicity’) and the abundance by number are linked by the relation ZO = 11.81 (O/H). The coefficient of proportionality is calculated as 16 X, adopting the hydrogen mass fraction for Solar composition from Asplund et al. (2009). Back.

4 The effective radius Re encloses 50% of the light, integrated by adopting an exponential radial profile of the surface brightness (i.e., not including the contribution from the bulge): I = I0 exp[−(R / Rd)], with I0 the central intensity and Rd the disk scale-length. The effective radius is given by Re = 1.678 Rd (e.g., Graham and Driver 2005). Back.

5 R90 encloses 90% of the galaxy light, including the bulge. Back.

6 The N/O abundance ratio is obtained from the [N ii] λλ6548,6583 / [O ii] λ3727 line ratio, using the commonly adopted ionization correction scheme N+/O+ = N/O. Back.

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