Herschel Spectroscopy of the Taffy Galaxies (UGC 12914/12915 = VV 254): Enhanced [C ii] Emission in the Collisionally Formed Bridge

Spectroscopic observations of the Taffy were made using the Photodetector Array Camera and Spectrometer (PACS; Poglitsch et al. 2010) integral field unit (IFU) on board the Herschel Space Observatory (Pilbratt et al. 2010).¹⁹ The spectra were obtained on 2012 January 30 as part of an open time program (program ID: OT1_pappleto_1; PI: Appleton; obsids = 1342238415/6). The PACS IFU uses a 5 × 5 grid of 94 × 94 spatial pixels to obtain a 47'' × 47'' field of view. Observations of the [C ii]157.74 μm and [O i]63.18 μm lines were made using a 3 × 3 raster map (stepsize = 38 arcsec and repeated 4 and 5 times respectively) in chop-nod mode sampling a region of 1.83 × 1.83 arcmin², which included both galaxies and the bridge. The angular resolution in the [C ii] and [O i] lines was (FWHM) 94 and 38, respectively, sampled onto 94 × 94 pixels. Total integration times per pointing were 23.5 and 50.0 minutes, with a total execution time for the full map of 3.9 and 8.2 hr for the [C ii] and [O i] observations, respectively.²⁰

The data were reduced using the Herschel Interactive Processing Environment (HIPE) version 15.0, and individual spectral-cube pointings were combined and regridded onto a sky projection with a pixel scale of 3''. The spectral resolution in the [C ii] and [O i] lines is 235 km s⁻¹ and 85 km s⁻¹ respectively. The spectra, observed in "range mode" to ensure coverage of the broad velocity field, span wavelengths from 159.4–160.8 μm (2570 km s⁻¹) and 64.0–64.4 μm (1965 km s⁻¹), respectively, for the the [C ii] and [O i] lines, centered on a redshift of z = 0.0145.

Spectra were extracted from the final data cubes from regions covering both galaxies and the bridge, as shown in Figure 1. The four bridge regions were chosen to provide nearly complete coverage of the bridge, and to provide information about how the gas properties vary across this part of the system. Their positions are listed in Table 1. In addition, extraction regions were selected to sample X-H ii, the extended emission from the northwest part of the northern galaxy (UGC 12915), and the nucleus of the southern galaxy (UGC 12914). The nuclear region of the northern galaxy would be contaminated by extended emission from the disk, so no attempt was made to extract a nuclear spectrum from this galaxy.

Table 1.  PACS Spectral Extraction Regions

Region	R.A.a	Decl.a	Size
	(J2000.0)	(J2000.0)
UGC 12914 nuc	0 01 3842	+23 29 0053	183	163
UGC 12915 NWb	0 01 3943	+23 30 0928	191	200
b1	0 01 3842	+23 29 4873	280	168
b2	0 01 3933	+23 29 3183	194	170
b3	0 01 4070	+23 29 3216	183	163
b4	0 01 4147	+23 28 5834	224	168
X-H ii	0 01 4066	+23 29 1514	336	168

Notes.

^aCoordinates of center of the extraction box. ^bNW region was also rotated by 466.

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The Taffy galaxies were observed with the SPIRE Fourier Transform Spectrometer (Griffin et al. 2010) on OD 1125 (program ID: OT1_pappleto_1; PI: Appleton; obsids = 1342246980) with a total integration time of 3.7 hr on-source. The FTS has two detector arrays, the Spectrometer Long Wave (SLW) and the Spectrometer Short Wave (SSW), which covered the long and short wavelength ranges, with a small overlap in wavelength. The complete spectrum covers the frequency range 455–1600 GHz (∼194–671μm). The FWHM of the SPIRE beam is 35 arcsec at 809.3 GHz, the rest frequency of the [C i] ${}^{3}{{\rm{P}}}_{2}\to {}^{3}{{\rm{P}}}_{1}$ line. The observations were made with a single pointing in "sparse mode" (no mapping) with a resolving power ranging between 372 < R < 1288 over the full range of the spectra. The central beam was pointed at the bridge center at α(J2000) = 00^h01^m403, δ(J2000) = 23°29'220. Although other beams, apart from the central one, intersect with parts of the two galaxies, the emphasis in the paper is on the bridge, and so the other spectra will not be described in the current paper.

SPIRE data reduction was performed using HIPE 15. The observed lines were extracted using a custom routine, after subtracting the appropriate background, which was observed on that date and at the same resolution. This routine implemented a Sinc function to match the line profiles.

Broadband multi-wavelength coverage of the Taffy system was used to construct SEDs for several extraction regions (see the later discussion). These data were extracted from various archives, and included UV coverage from Galaxy Evolution Explorer (Martin et al. 2005), visible wavelengths from the Sloan Digital Sky Survey DR 7 (SDSS²¹ ), mid-IR from Spitzer IRAC and MIPS (Fazio et al. 2004; Rieke et al. 2004; Werner et al. 2004, from NASA's Infrared Science Archive—IRSA²² ), and far-IR PACS photometry from the Herschel Science Archive.²³

To estimate the physical properties of the galaxies, we have used MAGPHYS (da Cunha et al. 2008, 2010) to fit the observed SEDs with sets of model templates. This code accounts for the global energy balance between the optical and the infrared and uses a Bayesian approach that draws from a large library of random models encompassing all plausible parameter combinations to derive the probability distribution function of various galaxy parameters, such as the stellar masses, star formation rates (SFRs), and both the total- and far-infrared luminosity for each of the regions. These values are listed in Table 2.

To provide background on our observations, we present CO (Gao et al. 2003) and H i (Condon et al. 1993) contours of the Taffy system superimposed on an optical image of the galaxies in Figures 1(a) and (b). The CO interferometer observations, which have a similar spatial resolution to the PACS [C ii] observations, show a clear bridge extending from UGC 12915 to UGC 12914. The H i observations, with much poorer spatial resolution, show a large concentration of H i peaking between the galaxies. In addition to the difference in spatial resolution, the main centroid of the H i emission may be offset toward the northwest compared with the CO emission.

Figure 1(c) presents integrated emission contours of [O i]63 μm derived from the PACS observations. The inner disks of both UGC 12915 and UGC 12914 are strongly detected in [O i]. A ridge of emission in UGC 12915 follows the dark dust lanes in the disk, and extends toward the northwest, where the emission becomes more extended. In UGC 12914, the emission is concentrated toward the nucleus, and in two bright regions further out in the disk. Clumpy emission from [O i] is present between the galaxies, including the X-H ii region, and a prominent clump between the galaxies. Fainter emission is present in the bridge, but this is not well represented in the integrated map.

Figures 1(d) and (e) show the distribution of [C ii] emission superimposed on both the optical image and the 70 μm PACS dust continuum image obtained from the Herschel Science Archive. The [C ii] distribution is similar to that of the [O i], though there are some differences. For example, the [C ii] emission appears to be strong relative to the [O i] in the northwest part of UGC 12915. Furthermore, the nucleus of UGC 12914 seems more prominent in the [O i] emission, as compared with [C ii]. The [C ii] is also well-correlated with the faint dust emission, including faint emission in the bridge. It is notable that the southern boundary of UGC 12914 is very sharply defined, similar to the CO and radio continuum distributions (the latter is not shown here, but see Condon et al. 1993). The emission does not extend into the outer southern ring of UGC 12914, indicating that the ISM of this galaxy has been severely stripped from the disk by the collision with UGC 12915.

Based on the gas distribution, several spectral extraction boxes were defined, as shown in Figure 1(f). The large polygons are intended to capture all of the emission from the galaxies. The bridge is broken up into four regions b1–b4 to allow us to look for variations in the gas properties of the bridge. We also extracted a spectrum from the unusual extragalactic X-H ii region, and the nucleus of UGC 12914. The UGC 12915 nucleus could not be isolated in either the [C ii] or 70 μm emission and was not extracted. Instead, we extracted the extended emission in the northwest part of UGC 12915. This is a region where the underlying far-IR continuum is weak, but the [C ii] emission is quite pronounced.

Figure 2 shows velocity contour maps for the [C ii] line. As previously noted by many authors, the two galaxies appear to be counter-rotating. The emission in UGC 12914 is from v ∼ −400 to 500 km s⁻¹, with negative velocities at the northern end of the galaxy and positive toward the south.²⁴ The emission at the southern end of the galaxy extends along the visible spiral arm farther than is apparent from the integrated contours of Figure 1, with extended emission from v ∼ 200 to 400 km s⁻¹. On the other hand, UGC 12915 is detected from v ∼ −300 to 750 km s⁻¹ with negative velocities toward the south and positive toward the north: the opposite of UGC 12914. The emission peak is confined to the same compact area as the 70 μm emission at all velocities. The extended emission in the northwest part of the galaxy is evident from v ∼ 150 to 450 km s⁻¹. This is a wider velocity range than the emission along the southern arm of UGC 12914.

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Emission in the bridge is detected from −300 to 350 km s⁻¹, starting just south of the northern arm of UGC 12914 and moving across the bridge toward the center of UGC 12915 at increasing velocities. The X-H ii region near UGC 12915 appears at velocities from v ∼ −100 to 300 km s⁻¹. While much of the emission from X-H ii appears at the same velocities as the diffuse bridge emission, it is also strong from 200 to 300 km s⁻¹, where the diffuse bridge emission has largely faded.

[O i] velocity contours are presented in Figure 3. UGC 12915 is detected from v = −250 to 600 km s⁻¹ and, as in [C ii], shows extended emission to the northwest. Clumpy extended emission appears to extend along the northwest disk of UGC 12915, and extend into the bridge from ∼0 to 200 km s⁻¹. Starting at ∼200 km s⁻¹, and continuing through ∼350 km s⁻¹, the emission extends well past the northern arm. The X-H ii region is better defined in the [O i] maps than in [C ii]. The narrow northwestern arm of UGC 12914 extends out of the disk and into the bridge at velocities of −300 to −200 km s⁻¹.

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Figure 4 shows the profiles of the [C ii] (black lines) and [O i] (green lines) lines for each of the extraction regions. The [C ii] is strong (peak flux densities of 50 and 25 Jy integrated over the two galaxies; labeled N and S for UGC 12915 and UGC 12914 respectively), and weaker in the bridge and the X-H ii region (in the peak flux range of a few jansky; labeled b1 – b4 and X-H ii). The bridge and X-H ii extractions show very broad line profiles that suggest multiple components. Similar, though weaker and noisier, broad profiles are also seen in the [O i] extractions (green lines) shown in Figure 4. The [O i]63 μm observations have higher resolving power (85 km s⁻¹) and are better able to resolve the complex velocity structure of the bridge. For example, the X-H ii region spectrum in [O i] shows a narrower high-velocity feature and a broader low-velocity feature, consistent with the lower resolution [C ii] profile. The [O i] emission is weakly detected at positions b2 and b3, is marginally detected in b1, and is not detected in b4. As in the case of the [C ii] profile, the [O i] emission at bridge positions b1 and b2 shows multiple components, whereas b3 is better represented by a single component.

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The line fluxes and velocity components for the [C ii] and [O i] lines are given in Tables 3 and 4, respectively, with line ratios provided in Table 5. Line fluxes were measured using the ISO Spectral Analysis Package (Sturm et al. 1998). Each line was measured using either a single or double line fit to these spectra. The two components usually provided a better match to [C ii] profiles. There were, however, cases where the data were fit with a single component, such as b2 and b3, where the [O i] is weak. In b1 and b4, the [O i] emission is sufficiently weak that we are only able to provide 3σ upper limits.

Table 5.  Line Ratios

Region	[C ii]/[O i]	[C ii]/COa	[C ii]/LFIRb
			Percent
UGC 12915	2.75 ± 0.44	669 ± 67	0.86 ± 0.09
UGC 12914	2.58 ± 0.39	465 ± 47	0.94 ± 0.10
UGC 12914 nuc	2.23 ± 0.32	403 ± 40	0.74 ± 0.07
UGC 12915 NW	2.71 ± 0.40	1286 ± 129	4.35 ± 0.44
b1	≥5.22	909 ± 91	2.73 ± 0.36
b2	1.39 ± 0.24	348 ± 35	1.80 ± 0.24
b3	2.75 ± 0.66	218 ± 22	3.31 ± 0.35
b4	≥0.59	191 ± 19	1.78 ± 0.20
X-H ii	1.90 ± 0.27	236 ± 24	1.72 ± 0.18

Notes.

^aThe PACS [C ii] and BIMA CO beams have comparable sizes of 94 × 94 and 99 × 97, respectively. The uncertainties in the CO fluxes were assumed to be 10%. ^b42–122 μm.

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We note that in the bridge position b3, the observed line ratio of [C ii]/[O i] = 2.75 ± 0.66, which is similar to that seen in several of in the intergalactic shocked regions in the SQ system (Appleton et al. 2013), and with similar [C ii] surface intensity. By analogy with that work, we estimate similar derived properties to that diffuse gas (T ∼ 200 K; n ∼ 10³ cm⁻³) in the Taffy bridge (see Figures 6 and 7 of that paper).

The emission in [C ii] from the NW-disk extraction region in UGC 12915 is quite strong and broad, given its weak far-IR emission (see Figure 1(f)), and is much weaker in [O i]. It is possible that this emission is bridge material superimposed on the northern disk, as it shares similar properties to other material in the bridge. X-ray emission, which we suggested was shock-heated gas left over from the initial collision between the Taffy galaxies (Appleton et al. 2015), also occupies this region of the northwest disk of UGC 12915 and the bridge.

Both the [C ii] and [O i] profiles of the nucleus of UGC 12914 (labeled S-nuc in Figure 4) show broad lines (FWHM 354 and 288 km s⁻¹, respectively, for [C ii] and [O i] after deconvolving with the instrument profile). The Chandra X-ray spectral index measurements of Appleton et al. (2015) showed possible evidence for a weak low-luminosity AGN in this galaxy.

To compare the PACS spectra of the extracted regions with the Condon et al. (1993) and CO data from BIMA CO (1–0) (Gao et al. 2003), we obtained calibrated data cubes of both sets of observations and performed extractions over similar areas to those of Figure 1. Figure 5 shows the emission profiles of [C ii] with H i and CO (1–0), and Figure 6 compares the [O i] and CO (1–0) line profiles. The CO (1–0) spatial resolution (98 × 98) is similar to that of the [C ii], but the H i data has much poorer resolution (18'' × 18''), and so this must be taken into account. All of the Herschel spectral extraction boxes are large enough to include at least one beam of the H i VLA data. Nevertheless, these latter profiles are likely to be more affected by resolution effects than the other spectra. In some of our comparisons, we have convolved the CO and [C ii] data to the same resolution as the H i data.

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There are several regions in the Taffy bridge where the [C ii], H i, and CO (1–0) profiles differ, even after taking into account the differing resolutions. The biggest differences are between the CO and the H i in regions b2, b3, and in X-H ii. The H i is double-peaked, with a low- and high-velocity component that is similar to the [C ii] profiles, whereas the CO is single-peaked with the high-velocity component being dominant. This is particularly striking in the X-H ii spectrum, where the CO (1–0) profile shows almost no emission in the low-velocity component, whereas the H i and [C ii] profiles exhibit both components. In region X-H ii, the [O i] emission is more strongly peaked on the high-velocity component, corresponding with the CO, but also shows weaker emission in the low-velocity component of the double profile. In other regions of the bridge, the situation is less clear-cut. For example, in b2 and b3, the CO contains a bright high-velocity component, but a weaker low-velocity component, both of which are represented in [C ii] and weakly in [O i].

The fact that the [C ii] and H i share similar double-peaked line-shapes in parts of the bridge, but that the CO (1–0)-emitting gas primarily dominates the higher-velocity peak, shows that the two kinematic components contain a different mix of H i and CO-bright H₂ (we cannot, of course, rule out CO-faint H₂; Wolfire et al. 2010). Both velocity components also contain an unknown (but potentially large) fraction of mid-IR-emitting warm molecular hydrogen (the Spitzer IRS observations did not have sufficient resolution to separate the high- and low-velocity components; Peterson et al. 2012). It is therefore very likely that both neutral and molecular hydrogen contribute to the collisional excitation of the [C ii]157 μm line in the presence of a small degree of UV ionization. We will return to this point in Section 4.

Figure 7 shows the spectrum of the bridge split into several pieces to zoom in on the relevant detected lines. Several lines are detected, including the neutral carbon lines [C i] ${}^{3}{{\rm{P}}}_{2}\to {}^{3}{{\rm{P}}}_{1}$ (rest frequency 809.3 GHz = 370.4 μm), [C i] ${}^{3}{{\rm{P}}}_{1}\to {}^{3}{{\rm{P}}}_{0}$ (rest frequency 492.2 GHz = 609.1 μm; hereafter [C i] (2–1) and [C i] (1–0) respectively). The ¹²CO (6–5) and ¹²CO (5–4) are weakly detected at a lower level of significance. The ¹²CO (4–3) line appears to be stronger; however, it is very close to the band edge so its existence must be treated with caution. We also measured a 3σ upper limit for the [N ii]205 μm line. The line measurements are presented in Table 6.

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Table 6.  SPIRE FTS Line Fluxes from Taffy Bridge^a

Line	νobs	FWHMb	Vhelio (unc)	Line Flux (unc)	Detector
	(GHz)	(arcsec)	(km s−1)	(×10−17 W m−2)
12CO 4−3	453.93	43	4699 ± 28	[1.33 ± 0.09]c	SLWC3
12CO 5−4	567.80	35	4473 ± 44	0.68 ± 0.08	SLWC3
12CO 6−5	681.09	32	4571 ± 76	0.33 ± 0.09	SLWC3
[C i] 1−0	484.74	38	4591 ± 29	1.20 ± 0.09	SLWC3
[C i] 2−1	797.50	35	4452 ± 29	0.80 ± 0.09	SLWC3
[N ii]205 μm	1440.78	18	⋯	<0.08d	SSWD4

Notes.

^aObservations centered on α(J2000) = 00^h01^m403, δ(J2000) = 23^d29^m220. ^bThe dimensions of the SLW FWHM FTS beam is not proportional to ν⁻¹ and is somewhat noncircular because of the design of the feed-horn (see Figure 5.18 of the SPIRE Observer's Manual; http://herschel.esac.esa.int/Docs/SPIRE/html/spire_om.html). ^cUncertain detection because the line is close to the SLW lower band edge. ^d3σ upper limit.

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The detection of two [C i] lines is perhaps not surprising given that they likely trace diffuse molecular gas with similar density to that of the CO (1–0) emitting gas (Papadopoulos & Greve 2004). The critical densities of the [C i] (2–1) and (1–0) are 500 and 1000 cm⁻³, respectively, which is very similar to that of CO (1–0) (440 cm⁻³). Interestingly, the ratio of the [C i] (2–1) and (1–0) lines, R_{[C i]} = 0.7 ± 0.1, is an outlier when compared with samples of normal and luminous infrared galaxies (LIRGs), where typical galaxies show values in the range 0.1 < R_{[C i]} < 0.5, and is similar to that of NGC 6240 (Jiao et al. 2017). This latter system's value of R_{[C i]} = 0.81 ± 0.09 is unusual, even compared with most (U)LIRGs, and is believed to be primarily shock-heated. We can quantify this if the [C i] lines are optically thin. In that case the excitation temperature of the gas can be estimated from Walter et al. (2011) to be T_ex = 38.8 × [ln(2.11/R_{[C i]})]⁻¹ K, or 35 K. This relatively high excitation temperature for [C i], and presumably CO (1–0) emitting molecular gas, is another indication of the unusual nature of the Taffy bridge, again suggesting shock heating as a viable mechanism for heating the gas.

Finally, extracting the [C ii]157.7 μm line flux from the same regions as the SPIRE SSW and SLW beams (5.4 and 22.0 × 10⁻¹⁷ W m⁻² respectively), yields [C ii]/[C i] = 11 (summing the fluxes from the [C i] (2–1) and (1–0) transitions) and [C ii]/[N ii] > 54. The value of [C ii]/[C i] is relatively low compared with values of ∼35 for active star-forming galaxies (derived from results shown in Lu et al. 2015, 2017). This is consistent with the low measured [C ii]/CO(1–0) ratios (see Section 3.4.1), since the [C i] and the CO (1–0) sample similar phases.

The nondetection of the [N ii]205 μm line, and a measured lower limit to the ratio of [C ii]/[N ii] > 54 (where the [C ii] is extracted from the larger SLW beam), implies a small ionized gas fraction in the bridge. For pure ionized gas, the ratio [C ii]/[N ii] ∼ 4 and is almost independent of density. In normal galaxies, [C ii] emission arises primarily from the neutral gas phase, with typical values of [C ii]/[N ii] lying between 11 and 22 (Croxall et al. 2017, based on KINGFISH galaxy sample). The lower limit of this ratio for the Taffy bridge implies that most of the [C ii] originates in a neutral or molecular phase.

3.4.1. CO (1–0) versus [C ii] Line Fluxes in the Taffy

The SFRs for the two galaxy extraction regions estimated using MAGPHYS SED fits are 2.77 ± 0.30 and 1.13 ± 0.01 M_☉ yr⁻¹ for UGC 12915 and 12914, respectively. These values are in good agreement with those of Appleton et al. (2015) who estimated the SFR using several different methods.

The SFR can also be estimated using the [C ii] luminosities. Using the calibration of Herrera-Camus et al. (2015) with no IR color correction, we find SFRs of 5.2 M_☉ yr⁻¹ for UGC 12915 and 2.6 M_☉ yr⁻¹ for UGC 12914. For the two galaxies, the [C ii]-based estimate is higher than the SED-based estimate by a factor of ∼2. However, the IR color temperatures of both galaxies are on the lower end of the distribution for normal galaxies, and this would act to reduce the overall SFR derived from this correlation, bringing them into closer agreement with the SED method.

More interesting are the SFRs in the bridge and X-H ii. Based on the SED fit, we estimate a total bridge SFR of 0.13 M_☉ yr⁻¹, excluding region X-H ii. This is in approximate agreement with estimates using the 24 μm emission from the same regions (Appleton et al. 2015). On the other hand, if the [C ii] line luminosity is used to calculate the SFR, we estimate a much higher value of 0.64 M_☉ yr⁻¹. Although there is a large spread in the SFR/[C ii] calibration, this excess implies that the [C ii] emission is boosted by processes other than normal star formation processes.

To explore this further, we attempt to quantify this excess in Figure 8, where we plot the SFR surface density Σ_SFR, determined from the Spitzer MIPS 24 μm image of the system, with the [C ii] surface brightness Σ_{[C ii]}. The figure is taken from Herrera-Camus et al. (2015), and includes a density distribution of points taken from a sample of 3486 regions in 46 normal galaxies. The two Taffy galaxies are marked as black circles in the diagram. The other extracted regions are marked in green. Except for the UGC 12914 nucleus, and region b4, most of the points, including the galaxies as a whole, lie on the extreme edge of the distribution for regions in normal galaxies. This makes the point that the [C ii] emission appears generally boosted, compared with that expected for normal star formation. Alternatively, Figure 8 could be interpreted as showing lower SFR surface densities for a given [C ii] surface density, compared with normal galaxies. However, as we shall see, it is more likely that the [C ii] is boosted. We note that the region in the extreme northwest part of UGC 12915 (labeled NW disk in the figure) also shows unusually high [C ii] emission. In this respect, it shows similarities with the bridge. Indeed, kinematically, it may be a projection of part of the northern bridge emission onto the galaxy (see Figures 2 and 3).

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We can also compare the Taffy results with commonly measured properties of galaxies, such as the far-IR luminosity and color temperature. Figure 9 shows L_{[C ii]}/L_FIR plotted against L_FIR for the Taffy extraction regions. For comparison, we plot normal star-forming galaxies from Malhotra et al. (2001) and regions from the group-wide shock in SQ (Appleton et al. 2013). We also show nuclei and extended emission from LIRGs in the GOALS sample (Díaz-Santos et al. 2013, 2014). The full-galaxy extractions and the UGC 12914 nucleus fall at the upper end of L_{[C ii]}/L_FIR values for star-forming galaxies and LIRGs, so the emission from these regions is likely to be PDR-related. All of the bridge regions, the X-H ii region, and the extended emission from the northwest region of UGC 12915 fall well above the distribution of normal galaxies, again supporting an enhancement in the [C ii] bridge emission.

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A similar conclusion is reached by plotting L_{[C ii]}/L_FIR against the dust temperature as measured by F_{60 μm}/F_{100 μm}, as shown in Figure 10. Again, the full-galaxy extractions and UGC 12914 nucleus have L_{[C ii]}/L_FIR ratios consistent with normal star-forming galaxies. The bridge regions, X-H ii, and northwest region of UGC 12915 are all well above the values of normal galaxies in the Malhotra et al. (2001) sample. The low F_{60 μm}/F_{100 μm} temperatures in the bridge show that the dust is cool there, compared with warm gas temperatures. This is typical of shock-heated regions where the gas is heated more efficiently than the dust grains—a result that is contrary to that of a star-forming PDR, where much of the heating from young stars goes into warming the grains.

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Since the Taffy bridge contains large amounts of H i, it is possible that H i could contribute to the collisional excitation of [C ii] (Madden et al. 1997; Nikola et al. 1998). Indeed, as we have shown in Section 3.3, H i alone dominates the low-velocity component of the bridge, with little detected CO emission at those velocities. Although we cannot rule out "dark" molecules at those velocities (Wolfire et al. 2010), it is worth exploring whether H i can provide part of the explanation for the [C ii] emission. The surface intensity of [C ii], I_C+ is given by the equation

$$\begin{eqnarray*}&&{I}_{C+}=h\nu A/4\pi [2{{\rm{e}}}^{-91/T}/(1+2{{\rm{e}}}^{-91/T}+{n}_{\mathrm{Hcr}}/{n}_{{\rm{H}}})]{\chi }_{C+}{{\rm{N}}}_{{\rm{H}}}{{\rm{\Phi }}}_{b},\end{eqnarray*}$$

where h is Planck's constant, A is the Einstein coefficient for the transition (we assume 2.29 × 10⁻⁶ s⁻¹), T is the temperature, and n_H is the density of H i. The critical density of C ii for collisions with H i, n_Hcr, is a slowly decreasing function of temperature (Goldsmith et al. 2012), and is ∼3000 cm⁻³ at T = 100 K. χ_C+ is the fractional abundance of carbon. If the carbon is evenly split between the neutral and ionized forms, and assuming solar abundance, we adopt χ_C+ ∼ 1.5 × 10⁻⁴. N_H is the column density of neutral hydrogen, and Φ_b is the beam filling factor, which we assume is unity. The assumption of solar abundance in the bridge is based on the belief that most of the gas in the bridge originated in the galaxies before the collision.

Clearly the density and temperature of the H i are not constrained by the observations, and so we use the equation to provide an order of magnitude estimate of the importance of collisions with H i. If we take the peak value of N_H = 1.13 × 10²¹ cm⁻² (Condon et al. 1993), T = 100 K, n_H = 100 cm⁻³ (assuming a cold neutral medium), and with the assumptions given above, we find I_C+ = 0.98 ×10⁻⁵ erg s⁻¹ cm⁻² sr⁻¹. This is close to the observed peak [C ii] surface density in the bridge at position b3 of 1.3 ×10⁻⁵ erg s⁻¹ cm⁻² sr⁻¹. Dropping the H i temperature to 50 K (given that the bridge is likely in adiabatic expansion as the galaxies pull away) would decrease I_C+ by a factor of 3. Alternatively, increasing the H i density by a factor of two would more than double the value of I_C+. Given the large uncertainties in the many assumed quantities, including the filling factor, we can conclude that reasonable properties for the H i could potentially give rise to a significant fraction of the observed [C ii] emission in the bridge.

However, we know that H i is not the only likely collisional partner with ionized carbon atoms. We already know from the previous Spitzer observations that the warm molecular gas is strongly radiating, so these molecules, which are likely to be shock-heated (Peterson et al. 2012), must also play a role in exciting the [C ii] transition.²⁵ Unlike the case of the H i, we have better constraints on the properties of the warm H₂ emission. For example, we know the temperature and column density of the bulk of the gas from fitting to the H₂ excitation diagram for the pure rotational lines. Adopting a temperature for the warm H₂ of 160 K, a density of 1000 cm⁻³ (by analogy with the similar gas in SQ), a molecular column density of 3.2 × 10²⁰ cm⁻², and a critical density of 5900 cm⁻³, the expected intensity of [C ii] from this collisionally excited warm H₂ would be 3.1 × 10⁻⁵ erg s⁻¹ cm⁻² sr⁻¹. This is clearly more than enough to explain the observed [C ii] intensity. We conclude that, within the considerable uncertainties, both H i and warm H₂ are likely drivers of the enhanced [C ii] emission seen in the bridge.