Tidal disruption events (TDEs), in which stars are gravitationally disrupted as they pass close to the supermassive black holes in the centres of galaxies
, are potentially important probes of strong gravity and accretion physics. Most TDEs have been discovered in large-area monitoring surveys of many thousands of galaxies, and a relatively low rate of one event every 104–105 years per galaxy has been deduced
Ultra-luminous infrared galaxies (ULIRGs: infrared luminosity L IR > 1012 L ⊙ ) 5 represent the peaks of major, gas-rich galaxy mergers in which the merger-induced gas flows concentrate the gas into the nuclear regions, leading to high rates of star formation and accretion onto the central supermassive black holes (SMBHs). The nearby ULIRG F01004-2237 (RA 01 h 02 m 50.007 s, dec. −22 d 21 m 57.22 s (J2000); redshift z = 0.117835) was observed using deep spectroscopic observations in September 2015 as part of a study of 15 ULIRGs to examine the importance of the warm gas outflows driven by active galactic nuclei (AGNs) in such objects 6 . Many of its properties are typical of local ULIRGs, including relatively modest total stellar and SMBH masses (stellar mass M ★ = 1.9 × 1010 solar masses, M ⊙ ; black hole mass M bh = 2.5 × 107 M ⊙ ) 7,8 , and evidence for AGN activity in the form of blue-shifted high ionization emission lines 6 . However, it is unusual in the sense that it is one of the few ULIRGs in which Wolf–Rayet features have been detected at optical wavelengths 9 , indicating the presence of a population of ~3 × 104 Wolf–Rayet stars with ages 3–6 Myr (see Supplementary Information). Also, unlike most ULIRGs for which the central starburst regions are heavily enshrouded in dust, this source has a compact nucleus that is barely resolved in optical and ultraviolet observations with the Hubble Space Telescope (HST) and has been attributed to a population of stars that is both young (<10 Myr) and massive (~3 × 108 M ⊙ ) 10 . Together, these features suggest that we have an unusually clear view of the nuclear star-forming regions in F01004-2237.
Figure 1 compares the optical spectra of F01004-2237 taken in 2015 with earlier spectra taken in 2000. In common with all the other published optical spectra of the source taken between 1995 and 2005 (Supplementary Information), those obtained in 2000 show emission lines characteristic of a composite of regions photoionized by hot stars and AGN, as well as a blend of N iii and He ii emission lines at ~4,660 Å that is characteristic of Wolf–Rayet stars of type WN 9 . However, the 2015 spectra are markedly different: the ~4,660-Å feature is a factor of 5.6 ± 1.1 stronger in flux compared with the 2000 spectra, and the blend has developed broad wings that extend up to ~5,000 km s–1 to the red of the centroid of the narrow He ii λ4686 component (that is, emission at wavelength λ = 4,686 Å); the He i λ5876 line is also notably stronger (by a factor of 3.7 ± 0.2); and new He i emission features have appeared at 3,889, 4,471, 6,678 and 7,065 Å. In contrast, the broad, blueshifted forbidden lines associated with the AGN-induced outflow have not varied significantly between the two epochs (Supplementary Information), whereas the Hβ line has increased by a factor of 1.52 ± 0.12. If the broader component of the blend at 4,660 Å detected in the 2015 spectrum is attributed to He ii λ4686, the ratio of the flux of this broad line to that of the broader Hβ component is He ii λ4686/Hβ = 1.82 ± 0.09; however, the ratio is much larger if we consider only the component of Hβ that has changed between the two epochs: He ii λ4686/Hβ = 5.3 ± 0.85. Similarly, we derive Ηe i λ5876/Hβ = 0.46 ± 0.04 and Ηe i λ5876/Hβ = 1.34 ± 0.23 when comparing the broad Ηe i λ5876 flux to the total and variable broad Hβ component fluxes respectively. In comparison, typical quasars have He ii λ4686/Hβ ≈ 0.02 and Ηe i λ5876/Hβ ≈ 0.009 for their wavelength-integrated broad emission line fluxes 11 .
Alerted by our spectra to the possibility of an unusual transient event in the nucleus of F01004-2237, we examined the Catalina Sky Survey (CSS) 12 database for evidence of variability in its optical continuum over the period 2003–2015. The resulting V-band light curve for F01004-2237 is presented in Fig. 2, where it is compared with those of the other 14 ULIRGs in our spectroscopic sample. Owing to their low spatial resolution, the Catalina measurements include a substantial fraction of the total light of each galaxy, not just that of the nucleus. Whereas the light curves of all the other ULIRGs are flat within ±0.1 magnitudes, that of F01004-2237 showed a significant spike in 2010, when it was 0.45 ± 0.02 magnitudes (a factor of 1.51 ± 0.03) brighter than the average of the four earliest epochs.
A supernova origin for the phenomenon observed in F01004-2237 is ruled out by the fact that the light curve and post-flare spectrum are unlike any known supernova. Although high rates of supernovae are expected in ULIRGs because of their high star formation rates (>100M ⊙ yr−1), and a rate of 4 ± 2 yr−1 has been measured for the closest ULIRG, Arp220, using radio observations 13 , most core-collapse supernovae would not be sufficiently bright to detect in the integrated-light CSS observations of F01004-2237. The peak luminosity of the flare in F01004-2237 (M v < −20.1 mag) approaches that of super-luminous supernovae, which are orders of magnitude less common than typical core-collapse supernovae.
Given the prior evidence for an AGN in F01004-2237, it is also important to consider whether the optical flare and spectral changes observed in F01004-2237 fall within the range of observed AGN activity. High-amplitude (more than a factor of 10) flares are not unprecedented in AGN. However, they are rare in the type of radio-quiet AGN represented by F01004-2237 that lack powerful, synchrotron-emitting jets. Considering the class of ‘changing-look’ AGN in which strong broad emission lines and non-stellar continuum have appeared in optical spectra after a period of apparent quiescence, it is notable that the broad He i and He ii lines are not unusually strong in the high-state spectra of such objects 14,15 (see Supplementary Information for details). As far as we are aware, the variability observed in F01004-2237, in which the broad helium emission lines dominate the high-state spectrum, is without precedent for an AGN. Moreover, the He ii λ4686/Hβ and He i λ5876/Hβ ratios measured for the broad, variable emission lines in F01004-2237 are significantly higher than those measured for even the innermost, highest ionization zones of typical AGN broad line regions (He ii λ4686/Hβ ≈ 1; He i λ5876/Hβ ≈ 0.5–0.6) 16,17 , as represented by extreme red and blue wings of the emission line profiles.
Although not typical of AGN, unusually strong and variable broad He i and He ii lines have been observed in some tidal disruption events (TDEs)
The cause of the strong helium lines detected in the optical spectra of TDEs is the subject of debate. Recently, it has been proposed that, even in the case of solar abundances, the ratio of the helium to hydrogen lines might be substantially enhanced if the emitting gas is both matter-bounded and has a sufficiently high density that the Balmer lines of hydrogen are optically thick 16,24 . In the case of F01004-2237, this explanation is favoured over the alternative that the He/H abundance ratio is substantially super-solar in the tidal debris 25 . This is because such an enhanced helium abundance would require the disrupted star to be sufficiently massive (>10M ⊙ ) that it had converted much of its hydrogen to helium over the <10-Myr lifetime of the nuclear star cluster; the tidal distruption of such a massive star is expected to be extremely rare for a typical initial mass function.
The detection of one event in a sample of just 15 ULIRGs over a period of ~10 years suggests that the rate of TDEs in such objects is orders of magnitude higher than the 10−5–10−4 TDE yr−1 galaxy−1 deduced for the field galaxy population
Several mechanisms could enhance the rate of TDEs in ULIRGs, including concentrated nuclear star formation leading to high densities of stars close to the central SMBHs 27 , the formation of close black hole binaries comprising the SMBHs of the progenitor galaxies 28 , and black hole recoils that might follow the coalescence of such binaries 29 . We note that the disruptive effect of a SMBH binary on the debris stream of the TDE 30 might also help to explain the fact that the flare observed in F01004-2237 is unusually prolonged.
The simultaneous detection of a TDE, a massive young stellar population and an AGN in the nucleus of a ULIRG provides striking evidence of the close proximity of star formation and growing SMBHs at the centres of starburst galaxies. The TDE flare in F01004-2237 has required the consumption of a total mass of 0.02–0.61M ⊙ by its SMBH (see Methods). This corresponds to an average mass accretion rate of M ⊙ yr−1, assuming a TDE rate in the range yr−1. If this rate were maintained for the ~100-Myr timescale typical of starbursts in ULIRGs, the SMBH in F01004-2237 would grow in mass by between 2 × 104 M ⊙ and 6.1 × 106 M ⊙ (0.1–25%) owing to TDEs alone. Although not sufficient to trigger a luminous, quasar-like episode of AGN activity, the integrated photoionizing effect of the frequent TDE flares would be capable of sustaining lower-level LINER- or Seyfert-like narrow emission-line activity in ULIRGs in periods of relative quiescence, when the rates of direct gas accretion onto the SMBH are low.
We have estimated the absolute magnitude of the peak of the flare in F01004-2237 by assuming a cosmology with H 0 = 73.0 km s−1 Mpc−1, Ω m = 0.27, Ω λ = 0.73, which results in a luminosity distance of D L = 523 Mpc for the redshift of F01004-2237 (z = 0.117835). The spectral energy distribution (SED) of the flare is unknown, but if we assume that it follows the Rayleigh–Jeans tail of a hot black body (temperature T > 20,000 K), the K-correction is 0.24 magnitudes. Applying this K-correction and a Galactic extinction correction of A V = 0.05 mag, we derive an absolute magnitude for the peak of the flare of M V = −20.1 mag. However, this is likely to represent a lower limit on the luminosity, since we have not corrected for intrinsic dust extinction.
To calculate the bolometric luminosity associated with the flare, it is necessary to assume a bolometric correction factor (BCF) to convert between the V-band monochromatic luminosity and the bolometric luminosity. This BCF depends on the (unknown) SED of the flare. Assuming that the SED of the flare in F01004-2237 is similar to that of other TDEs with multi-wavelength photometry and follows a black body with temperature in the range 10–50 kK, the BCF will be in the range 1.75 < L bol/νL V < 60. For comparison, typical AGN have L bol/νL V ≈ 8. Considering the full range of likely black body temperatures, the peak bolometric luminosity of the TDE flare in F01004-2237 falls in the range 4 × 1043 < L bol(peak) < 1.4 × 1045 erg s−1, and performing a simple trapezium-rule integration of the light curve, the total energy associated with the flare up to the end of 2015 was in the range 3 × 1051 < E flare < 1.1 × 1053 erg.
The mass consumed by the black hole in order to produce the flare is M flare = E flare/(c 2 η), where η is the efficiency. Therefore, for a typical SMBH accretion disk efficiency of η = 0.1, the total mass consumed so far to produce the TDE flare observed in F01004-2237 is in the range 0.02 < M flare < 0.61M ⊙ .
The data used to make the photometric light curve presented in Fig. 2 are available from the CSS data release 2 website (http://nunuku.caltech.edu/cgi-bin/getcssconedb_release_img.cgi); the data that support Fig. 1 within this paper and other findings of this study are available from the corresponding author upon reasonable request.
How to cite this article: Tadhunter, C. et al. A tidal disruption event in the nearby ultra-luminous infrared galaxy F01004-2237. Nat. Astron. 1, 0061 (2017).
The William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canaria. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555, these observations are associated with program no. 8190. This project made use of data obtained by the Catalina Sky Survey. C.T., R.S., M.R. and P.C. acknowledge financial support from the UK Science and Technology Facilities Council. We thank J. Maund for discussions about the possibility of a supernova origin for the flare.
Supplementary Figures 1–2, Supplementary Tables 1–2 and Supplementary