Free Access
Issue
A&A
Volume 635, March 2020
Article Number A197
Number of page(s) 6
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201937384
Published online 03 April 2020

© ESO 2020

1. Introduction

There is plenty of observational work in the literature reporting a moderate statistical increase (by factor of a few) of the star formation rate (SFR) in interacting galaxies (e.g. Larson & Tinsley 1978; Bergvall et al. 2003; Smith et al. 2007; Woods & Geller 2007; Li et al. 2008; Robaina et al. 2009; Knapen & James 2009; Ellison et al. 2013; Barrera-Ballesteros et al. 2015; Brassington et al. 2015; Knapen et al. 2015). It is known that the most extreme starbursts, such as ultraluminous infrared galaxies (ULIRGs), are almost always interacting or merging (e.g. Joseph & Wright 1985); it is likely that this interaction stimulates the high SFR in such rare objects. The SFR enhancement has been found to be larger for smaller nuclear separation in galaxy pairs (Pan et al. 2019). However, Pearson et al. (2019) conclude that the SFR of merging galaxies is not significantly different from the SFR of non-merging galaxies based on the use of convolutional neural networks applied to over 200 000 galaxies. The astrophysics behind the SFR enhancement in interacting galaxies remains a matter of intense investigation.

An important ingredient for probing the link between SFR and interactions is the atomic and molecular gas content. Early work by Combes et al. (1994) reported an enhancement of the CO(1–0) luminosity (and SFR) in tidally perturbed objects, indicating that the mass of molecular gas is higher in interacting systems. Using a sample of 107 visually classified post-merger galaxies, Ellison et al. (2018) find that merged galaxies exhibit an atomic gas fraction enhancement compared with the control sample (extended GALEX Arecibo SDSS Survey; Catinella et al. 2018) of the same stellar mass: they conclude that quenching is not a result of post-merger gas exhaustion (see also Pan et al. 2018). Lisenfeld et al. (2019) find no enhancement of the total H I+H2 gas mass fraction in major-merger pairs, relative to non-interacting comparison samples. Larson et al. (2016) report a dependence of the molecular gas mass fraction on the merger classification stage. They postulate that interactions sweep the available atomic hydrogen from the galaxy outskirts into the central regions, where it is converted into H2 and, eventually, into newly formed stars. In this process, the encounter geometry is an important factor for the gas inflow, as shown in the simulations of Blumenthal & Barnes (2018). Analysis of interactions in the SIMBA cosmological simulation by Rodríguez Montero et al. (2019) reveals that major mergers (≤4:1) induce SFR enhancements owing to an increase of the H2 content at low masses, but when M* ≥ 1010.5M such an enhancement is attributed to a higher star formation (SF) efficiency associated with denser gas.

This work aims at understanding in more detail why galaxies exhibit extreme SFRs, whether they are interacting or not. Expanding the work by Knapen et al. (2015) and Knapen & Cisternas (2015), we use their sample of ∼1500 nearby galaxies drawn from the Spitzer Survey of Stellar Structure in Galaxies (S4G; Sheth et al. 2010). Without an anchoring of the analysis at cold gas masses, the desired comparison with theoretical predictions and simulations would remain incomplete. In this work we address this impediment using archival 21 cm H I integrated data. An additional goal is to shed light on the physical properties of the starburst galaxies with an emphasis on the central stellar mass concentration and their gas content.

2. Sample and data

Our parent sample is the S4G, which is a magnitude- and diameter-limited survey that comprises 2352 galaxies with distances ≲40 Mpc observed in the 3.6 μm and 4.5 μm bands with the Infrared Array Camera (IRAC; Fazio et al. 2004) installed on board the Spitzer Space Telescope (Werner et al. 2004).

We use the catalogue of interacting and merging galaxies in S4G of Knapen et al. (2014). Our sample comprises 1341 galaxies. Of these, 16 are currently merging (class A), 39 appear highly distorted as a result of interaction with a companion (class B), and 84 present minor ongoing interaction (class C). The remaining 1202 galaxies constitute our control sample (CS) of non-interacting galaxies. We have excluded 138 galaxies from the Knapen et al. (2014) catalogue sample that belong to class “0”, namely systems that have a close companion, but show no signs of interaction; this companion is within a radius of five times the diameter of the sample galaxy, has a recession velocity within ±200 km s−1, and is not more than 3 mag fainter.

The total SFRs used in this work are taken from Querejeta et al. (2015), who calculated these values from the global IRAS photometry at 60 μm and 100 μm, following Larsen & Richtler (2000). Total stellar masses (M*) are taken from Muñoz-Mateos et al. (2015) and are used to calculate specific SFRs (sSFR=SFR/M*). The same SFR values were applied by Knapen et al. (2015), who used M* estimates from contaminant-free mass maps (Querejeta et al. 2015) derived from 3.6 and 4.5 μm imaging. In this work we opt to consistently use the 3.6 μm passband for stellar mass inferences, including central concentration. Besides taking into account H I information, a further improvement with respect to the work by Knapen et al. (2015) is our use of the more refined morphological classifications by Buta et al. (2015), instead of those from HyperLEDA (Paturel et al. 2003). Atomic gas masses (in M) are estimated as (e.g. Giovanelli et al. 1988; Erwin 2018; Díaz-García et al. 2019a)

(1)

where m21c is the corrected 21 cm line flux in magnitude from HyperLEDA1 (available for 98% of the galaxies in our sample) and D is the distance to the galaxy (in Megaparsec) adopted by Muñoz-Mateos et al. (2015). From this, gas depletion times (in yr) are calculated as (e.g. Knapen & James 2009)

(2)

where the factor 2.3 is applied to correct for helium and molecular gas content (Meurer et al. 2006), and the factor 0.6 accounts for the fraction of formed stars which is recycled to the interstellar medium (James et al. 2008).

We also use the inner gradient of the stellar component of the rotation curve (dRv*(0), from Díaz-García et al. 2016a) as a proxy of the central stellar mass concentration (e.g. Erroz-Ferrer et al. 2016; Díaz-García et al. 2016b, 2019b). Specifically, dRv*(0) is obtained from a polynomial fit to the inner part of disc+bulge component of the circular velocity, following Lelli et al. (2013), and taking the linear term as an estimate of the inner slope (80 of the 139 interacting and 768 of the 1202 non-interacting galaxies in our sample have reliable measurements of dRv*(0)).

For every galaxy in our sample, we calculate the enhancement (E) in the global SFR, specific SFR, H I gas mass, gas fraction, and gas depletion time by dividing these parameters by the median values for a control sample: E(SFR) = SFR/⟨SFR⟩cs, E(sSFR) = sSFR/⟨sSFR⟩cs, E(MHI) = MHI/⟨MHIcs, E(MHI/M*) = [MHI/M*]/⟨MHI/M*cs, and E(τ) = τ/⟨τcs. A separate control sample is constructed for each individual galaxy, which encompasses all the non-interacting galaxies with similar values of T-type (±1) and total stellar mass (±0.2 dex), following the criteria of Knapen et al. (2015). Whenever the control sample of a certain galaxy has less than 5 objects (47 cases), this galaxy is not included in our statistics. The median number of galaxies in the control samples is 46. In 17 cases (16 from CS, 2 of class A) with uncertain T-type, we selected the control sample based on M* exclusively.

Starbursts are defined as those galaxies having E(SFR) > 4. Different SFR enhancement cut-offs (also on the sSFR) were tested by Knapen & Cisternas (2015, their Fig. 1), showing consistency in their result that the fraction of interacting galaxies is enhanced in starbursts. We checked and confirmed that the presented observational trends are qualitatively the same when starbursts are defined as E(SFR) > 4 − 5 or E(sSFR) > 4 − 5, fully in line with what Knapen & Cisternas (2015) reported.

3. Results

Depletion times and gas fractions decrease with increasing total stellar mass (Fig. 1). When studied in the Hubble sequence, τ and MHI/M* are larger for larger T among the spirals (Fig. 2). Curiously enough, τ seems to be larger than average among lenticulars, although the sampling of S0s is not optimal because of the bias of the S4G towards late-type, gas-rich systems.

thumbnail Fig. 1.

Gas depletion time (upper panel) and gas fraction (middle panel) as a function of total stellar mass. Non-interacting and interacting galaxies, according to Knapen et al. (2014), are shown in grey and black, respectively. Non-interacting and interacting starbursts are shown in blue and red, respectively (see legend). The lines correspond to the linear fits to the galaxy points. In the lower panel we represent a histogram of galaxies vs. M* (bins of 0.25 dex).

For a given T or M* bin, the average τ and MHI/M* are not significantly different for interacting and non-interacting galaxies (black and grey symbols and lines, respectively), as seen in both Figs. 1 and 2. To confirm this result, we performed a two-sample Kolmogorov–Smirnov (K–S) test (IDL implementation kstwo.pro written by W. Landsman, following Press et al. 1986), finding that the cumulative distribution function of τ and MHI/M* is similar for interacting and non-interacting galaxies, as indicated by the large (> 0.01) p-values (0.27 and 0.46, respectively).

thumbnail Fig. 2.

As in Fig. 1 but as a function of the revised Hubble stage. The running median (error bars obtained via bootstrap re-samplings) is shown for interacting (black) and non-interacting (grey) galaxies separately. In the upper and central panels we added small random offsets (≲0.3) to the T values in the x-axis (integers) to avoid point overlapping.

The distribution of M* for non-interacting galaxies peaks at 1010 − 10.25M (lower panel of Fig. 1), while for interacting systems it peaks at slightly larger masses M* ≈ 1010.5 − 10.75M. Interacting galaxies spread evenly across the Hubble sequence (lower panel of Fig. 2), but only 5 S0s in our sample are interacting. K–S tests indicate that the two arrays of M* and T values for interacting and CS galaxies are drawn from different distributions (p-values of 3.8 × 10−5 and 3.7 × 10−3, respectively).

Figure 3 shows the enhancements on the SFR, H I gas mass and gas fraction (normalised by M*), and gas depletion timescale for multiple interaction classes. Median factors (and uncertainties obtained via bootstrap re-sampling) are indicated at the top of the panels. The median enhancement in SFR (E(SFR)) increases with increasing degree of interaction, where class A presents a median factor of 1.9 ± 0.5 higher SFR than the median for the CS galaxies (in agreement with Knapen et al. 2015). Likewise, highly interacting systems are characterised by a median E(τ) of 0.4 ± 0.2. Regarding atomic gas masses, class B has median E(MHI) and E(MHI/M*) of 1.9 ± 0.3 and 1.7 ± 0.3, respectively, but classes A and C do not show significant differences as compared to the CS. Extreme enhancements can be found in galaxies with different degrees of interaction or in complete isolation. Finally, enhancements on MHI, MHI/M*, and τ are compared to that of the SFR in Fig. 4. The quantities E(MHI) and E(MHI/M*) hardly show any correlation with E(SFR), as indicated by Spearman’s correlation coefficients (significances) ρ = 0.19 (p = 3.6 × 10−12) and ρ = 0.18 (p = 6 × 10−11), respectively, while a more robust correlation between E(SFR) and E(τ) is found (ρ = −0.45, p = 0, naturally expected from the definition of τ).

thumbnail Fig. 3.

Interaction class (A = merging, B = highly distorted due to interaction, C = minor interaction, and N = control sample) vs. SFR, gas mass, gas fraction, and depletion time, all normalised to CS median values (from left to right). We show the running median and confidence limit based on bootstrap re-sampling (values are shown at the top).

thumbnail Fig. 4.

Enhancement of H I gas masses (left), gas fractions (centre), and gas depletion times (right) as a function of enhancement on SFRs.

There are 45 starburst galaxies in our sample, i.e. fulfilling E(SFR) > 4. These are typically massive systems with M* ≳ 1010 M (Fig. 1) and Hubble stages T ≲ 5 (Fig. 2) (80% of the cases). For a given M* or T bin, starbursts present lower gas depletion timescales than the median. However, their gas fractions are not larger than the median. This means that the lower τ is only determined by the enhanced SFR. There are three massive systems in our sample (NGC 2894, IC 2461, and UGC 07522) that present high depletion timescales even though they are starbursts. All three of these massive systems are S0 and S00 galaxies.

Starbursts are characterised by higher central concentrations of stars (Fig. 5): for a given M* bin, galaxies with E(SFR) > 4 show higher values of dRv*(0). Interacting and non-interacting galaxies are not significantly different in dRv*(0) for a certain M*. However, as interacting galaxies are in general slightly more massive than their CS counterparts, their global dRv*(0) distribution peaks for slightly higher values, and thus the significance level of the K–S statistic between the subsamples of interacting and CS galaxies is fairly low (p-value of 0.0036).

thumbnail Fig. 5.

Inner slope of the stellar component of the rotation curve (direct measurement of the central stellar mass concentration) vs. total stellar mass for interacting (black) and non-interacting (grey) galaxies, while highlighting the starbursts (blue and red). The lines correspond to the linear fit to the data clouds.

4. Discussion

Using refined morphological types (Buta et al. 2015) for the control sample definition, we confirm the result by Knapen et al. (2015) that the enhancement in SFR for highly interacting galaxies is a factor of ∼2 higher than the average for CS galaxies. Our results are consistent with the expectations from the simulations by Di Matteo et al. (2007, 2008), who showed a similar enhancement of (circumnuclear) SF due to galaxy collisions, which drive the formation of non-axisymmetries that trigger inward gas flows (Barnes & Hernquist 1991; Mihos & Hernquist 1996).

In this work we take a step forward by adding the cold gas masses to the analysis for the different interacting classes. Studying all subclasses of interacting galaxies separately is undoubtedly insightful, yet uncertainties associated with the beam size for the far-infrared (IRAS) and 21 cm data (data from different telescopes are gathered in LEDA) may affect the measurements, and thus care must be taken to tone down conclusions based on interacting class. In certain cases (class A) strongly interacting galaxies may be so close that they could have been observed together, resulting in artificially increased values of MHI, SFR, or τ. We argue that such effects do not strongly affect our statistics when all interacting classes are grouped.

Knapen & James (2009) showed that H I total and relative mass are not strongly dependent on the absence or presence of a close companion, but do depend strongly on T-type. This is confirmed in this work with a four times larger sample (Fig. 2). Numerical models by Di Matteo et al. (2007) showed that the gas content in the merging event is not the main parameter governing the SF efficiency. Observational work by Violino et al. (2018) finds that gas fractions and depletion times in galaxy pairs are consistent with those of non-mergers whose SFRs are similarly raised. In addition, recent work by Ellison et al. (2018) shows that there is no correlation between the H I gas fraction enhancement of a galaxy and its SFR enhancement (see their Fig. 7). We assessed this in our sample and showed that, indeed, E(SFR) is barely correlated with E(MHI) and E(MHI/M*) (Fig. 4).

Interestingly, we find that in highly distorted galaxies (class B) the H I mass is significantly enhanced (almost a factor of 2 higher than the CS, on average). This is not the case in class A, where the merger-driven enhanced SFR (plausibly prolonged for several billion years) might have started to quench the galaxy, given the low depletion timescales. On the other hand, the cold gas could have been heated or removed via feedback from active galactic nuclei (e.g. Harrison et al. 2018; Ellison et al. 2019, and references therein). Also, Di Matteo et al. (2007) found that strong tidal interactions can remove a large amount of gas from the galaxy discs that is not fully re-acquired in the last merging phases. Finally, we note that the correlation between E(SFR) and E(τ) is not very tight (Fig. 4), which emphasises how controversial the definition of starburst can be (see Knapen & James 2009).

In class A galaxies (mergers), we find the median τ to be a factor of 0.4 ± 0.2 that of the CS. This is consistent with the findings by Saintonge et al. (2012), who showed that among the gas-rich disc-dominated population galaxies those undergoing mergers or presenting disturbed morphologies are characterised by short depletion times, roughly a factor of two shorter when compared to the CS. Saintonge et al. also note that not all mergers or interactions are necessarily associated with episodes of efficient SF, which is also the case in our work; we also find low values of E(τ) among all interacting classes.

Our analysis reveals that starbursts are characterised by a higher stellar mass central concentration when compared to a control sample, as measured from 3.6 μm imaging. This is most likely related to the fact that these galaxies have mainly harboured SF in the circumnuclear regions. This may imply that the SF enhancement is not associated with a single instantaneous central SF burst, but instead with either continuous SF over a period of 108 − 9 yr (e.g. Knapen & James 2009), probably resulting from large-scale gas inflow driven by stellar non-axisymmetries, or a series of bursts over a significant timescale (e.g. Allard et al. 2006). In addition, gas depletion times are longer in bulge-dominated galaxies according to Saintonge et al. (2012). However, we checked that the M*-τ relation is not segregated as a function of dRv*(0) (the bulge mass is larger for larger dRv*(0); Díaz-García 2016). From SDSS-IV MaNGA data, Pan et al. (2019) suggest that interaction-triggered SF is not necessarily limited to the circumnuclear regions, but the enhancement is centrally peaked. Recent work by Ellison et al. (2020) using MaNGA and ALMA data for 12 galaxies shows that an elevated SF efficiency is the fundamental driver for central starbursts, which have contributions from both interactions and secular mechanisms. In our work, high central stellar mass concentrations are detected for most starbursts regardless of whether they are interacting or not.

5. Summary and conclusions

We used a sample of 1341 S4G galaxies from the catalogue on interacting and merging galaxies produced by Knapen et al. (2014) to analyse the effect of galaxy-galaxy interactions on SFRs, atomic gas masses (MHI) and fractions (MHI/M*), and depletions times (τ), using archival 21 cm H I data from HyperLEDA and IRAS far-infrared fluxes (Querejeta et al. 2015). We also studied the global physical properties of starburst galaxies, such as total stellar masses (M*, from Muñoz-Mateos et al. 2015) and central concentration (traced from the inner slope of the stellar component of the rotation curves, dRv*(0), using 3.6 μm imaging, from Díaz-García et al. 2016a). Starbursts are defined as galaxies that have a factor > 4 enhanced SFR relative to a control sample, which comprises non-interacting systems with ±0.2 dex in M* and ±1 in T (types from Buta et al. 2015). The main results of this paper are the following:

  • We confirm that the distribution of gas fraction MHI/M* and depletion time τ is similar in interacting and control sample galaxies, but depends on M* and T (e.g. Knapen & James 2009).

  • In merging galaxies, the median enhancement in τ and SFR is a factor 0.4 ± 0.2 and 1.9 ± 0.5 that of the average for control sample galaxies, respectively (in line with earlier work by e.g. Saintonge et al. 2012; Knapen et al. 2015).

  • Highly distorted interacting galaxies have a factor of 1.9 ± 0.3 enhanced H I mass. This is not the case in merging systems, where the (interaction-triggered) higher star formation efficiency might have started to quench the GALAXIES.

  • Starbursts are typically early-type massive galaxies (M*  ≳  1010M, T  ≲  5) and can be either interacting or non-interacting.

  • For a given M* bin, starbursts present lower gas depletion time τ than the average, yet they have similar MHI/M*. An enhancement of the SFR does not imply an enhancement of the relative and absolute amount of gas, or vice versa.

  • For a given M* bin, starbursts are characterised by higher central stellar concentrations. This points to these systems having undergone continuous circumnuclear star formation over a period of 108 to 109 yr, nourished by gas inflow that is driven by both interactions and non-axisymmetries.

This work highlights how the study of large comprehensive samples of nearby galaxies with accurately determined physical properties allows us to shed light on the boost of SF that certain galaxies experience and, in particular, the connection of this boost with galaxy-galaxy interactions. Specifically, it provides observational evidence that starbursts harbour long-lasting SF in circumnuclear regions, independent of whether they are interacting or not, and that galaxy mergers produce a moderate enhancement (by a factor of ∼2) of the SF efficiency, in line with theoretical predictions that are based on numerical models.


1

We acknowledge the usage of the database http://leda.univ-lyon1.fr

Acknowledgments

We thank the anonymous referee for comments that improved this paper. We acknowledge financial support from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement No 721463 to the SUNDIAL ITN network, from the State Research Agency (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU) and the European Regional Development Fund (FEDER) under the grant with reference AYA2016-76219-P, and from the IAC project P/300724 which is financed by the Ministry of Science, Innovation and Universities, through the State Budget and by the Canary Islands Department of Economy, Knowledge and Employment, through the Regional Budget of the Autonomous Community. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 893673. JHK acknowledges financial support from the Fundación BBVA under its 2017 programme of assistance to scientific research groups, for the project “Using machine-learning techniques to drag galaxies from the noise in deep imaging”. We thank Ute Lisenfeld for comments on the manuscript, and Miguel Querejeta for fruitful discussions. Facilities: Spitzer (IRAC).

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All Figures

thumbnail Fig. 1.

Gas depletion time (upper panel) and gas fraction (middle panel) as a function of total stellar mass. Non-interacting and interacting galaxies, according to Knapen et al. (2014), are shown in grey and black, respectively. Non-interacting and interacting starbursts are shown in blue and red, respectively (see legend). The lines correspond to the linear fits to the galaxy points. In the lower panel we represent a histogram of galaxies vs. M* (bins of 0.25 dex).

In the text
thumbnail Fig. 2.

As in Fig. 1 but as a function of the revised Hubble stage. The running median (error bars obtained via bootstrap re-samplings) is shown for interacting (black) and non-interacting (grey) galaxies separately. In the upper and central panels we added small random offsets (≲0.3) to the T values in the x-axis (integers) to avoid point overlapping.

In the text
thumbnail Fig. 3.

Interaction class (A = merging, B = highly distorted due to interaction, C = minor interaction, and N = control sample) vs. SFR, gas mass, gas fraction, and depletion time, all normalised to CS median values (from left to right). We show the running median and confidence limit based on bootstrap re-sampling (values are shown at the top).

In the text
thumbnail Fig. 4.

Enhancement of H I gas masses (left), gas fractions (centre), and gas depletion times (right) as a function of enhancement on SFRs.

In the text
thumbnail Fig. 5.

Inner slope of the stellar component of the rotation curve (direct measurement of the central stellar mass concentration) vs. total stellar mass for interacting (black) and non-interacting (grey) galaxies, while highlighting the starbursts (blue and red). The lines correspond to the linear fit to the data clouds.

In the text

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