Spectral types of southern ULIRGs

Optical spectral classification of southern ultraluminous infrared galaxies

Jong Chul Lee, Ho Seong Hwang, Myung Gyoon Lee, Minjin Kim, and Sang Chul Kim
Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea
CEA Saclay/Service D’Astrophysique, F-91191 Gif-sur-Yvette, France
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, USA
Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea
Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.E-mail: jclee@astro.snu.ac.krE-mail: mglee@astro.snu.ac.kr
Accepted 2011 January 27. Received 2011 January 26; in original form 2010 December 7
Abstract

We present a study of the optical spectral properties of 115 ultraluminous infrared galaxies (ULIRGs) in the southern sky. Using the optical spectra obtained at CTIO 4 m and provided by the 2dF Galaxy Redshift Survey and the 6dF Galaxy Survey, we measure emission line widths and fluxes for spectral classification. We determine the spectral types of ULIRGs with H measurement using the standard diagnostic diagrams. For ULIRGs without H measurement, we determine their spectral types using the plane of flux ratio between [O iii] and H versus [O iii] line width based on our new empirical criterion. This criterion is efficient to distinguish active galactic nuclei (AGNs) from non-AGN galaxies with completeness and reliability of about 90 per cent. The sample of 115 ULIRGs is found to consist of 8 broad-line AGNs, 49 narrow-line AGNs, and 58 non-AGNs. The AGN fraction is on average 50 per cent and increases with infrared luminosity and IRAS 2560 m colour, consistent with previous studies. The IRAS 2560 m colour distributions are significantly different between AGN and non-AGN ULIRGs, while their IRAS 60100 m colour distributions are similar.

keywords:
galaxies: active – galaxies: general – galaxies: starburst – infrared: galaxies
pagerange: Optical spectral classification of southern ultraluminous infrared galaxiesReferencespubyear: 2010

1 Introduction

Ultraluminous infrared galaxies (ULIRGs) with infrared luminosity at 8–1000 m greater than 10 (Soifer et al. 1987) are extremely energetic objects in the universe. Although they contribute little to the infrared luminosity density in the local universe due to small numbers, they become cosmologically important at z  1 (e.g., Le Floc’h et al. 2005; Magnelli et al. 2009). Their enormous infrared luminosity comes from dust heated by hot young stars (starburst), a supermassive black hole rapidly accreting matter (active galactic nucleus, AGN), or a mixture of these two (see Sanders & Mirabel 1996 and Lonsdale et al. 2006 for a review). These starburst and/or AGN activities can be triggered by tidal interactions between galaxies and associated shocks (e.g., Bushouse 1987; Liu & Kennicutt 1995; Barnes 2004). In fact, numerous observational and theoretical studies suggested that ULIRGs are mergers of gas-rich disk galaxies (e.g., Clements et al. 1996; Mihos & Hernquist 1996; Veilleux et al. 2002; Younger et al. 2009; Hwang et al. 2010a), and evolve into quasars (e.g., Sanders et al. 1988; Dasyra et al. 2006; Hopkins et al. 2006; Yuan et al. 2010) or intermediate-mass elliptical galaxies (e.g., Genzel et al. 2001; Tacconi et al. 2002).

To better understand the origin and evolution of ULIRGs, it is essential to find out what their primary energy source is. A standard spectroscopic method to distinguish between starburst and AGN is to use so-called BPT diagrams (Baldwin et al. 1981), which are based on optical emission line ratios sensitive to the photoionization source. This optical diagnostic was revised by Veilleux & Osterbrock (1987) and an alternative scheme was proposed by Kewley et al. (2006). In heavily obscured galaxies like ULIRGs, additional observations at other wavelengths are helpful to characterize their dominant energy source (e.g., X-ray: Franceschini et al. 2003, Teng et al. 2009; infrared: Risaliti et al. 2006, Farrah et al. 2007, Imanishi et al. 2010; radio: Nagar et al. 2003, Sajina et al. 2008). However, it is still difficult to determine the relative contribution of starburst and AGN within individual galaxies.

ULIRGs were discovered in large numbers by the Infrared Astronomical Satellite (IRAS; Neugebauer et al. 1984) and the number of ULIRGs increased with the advent of wide-field galaxy redshift surveys (Goto 2005; Pasquali et al. 2005; Cao et al. 2006; Hwang et al. 2007, 2010a; Hou et al. 2009). However, previous studies based on optical spectra of the large sample are limited to the Sloan Digital Sky Survey (SDSS; York et al. 2000), which mainly covers the northern hemisphere. The optical spectral properties of ULIRGs in the southern hemisphere remain to be studied.

In this study, we present the analysis of optical spectra for about a hundred southern ULIRGs in the catalogue given by Hwang et al. (2007). The structure of this paper is as follows. The survey data and our observations are described in Section 2. Section 3 explains the procedures to analyse these data and to classify the ULIRGs. In Section 4, the results of our study are discussed, and are compared with those of previous studies. We summarize and conclude in Section 5. Throughout, we adopt = 70 km s Mpc and a flat CDM cosmology with density parameters = 0.3 and = 0.7.

2 Observations and data

Hwang et al. (2007) identified 324 ULIRGs by cross-correlating the IRAS Faint Source Catalogue Version 2 (Moshir et al. 1992) with the spectroscopic catalogues of galaxies in the Fourth Data Release of SDSS (Adelman-McCarthy et al. 2006.), the Final Data Release of the 2dF Galaxy Redshift Survey (2dFGRS; Colless et al. 2001), and the Second Data Release of the 6dF Galaxy Survey (6dFGS; Jones et al. 2004, 2005). The spectral types of ULIRGs in the SDSS among them were determined in Hou et al. (2009). In this study, we focus on 198 ULIRGs that were not covered by the SDSS but covered by the 2dFGRS and 6dFGS. We used the optical spectra of these ULIRGs available online111For 2dFGRS, http://www.mso.anu.edu.au/2dFGRS/
For 6dFGS, http://www-wfau.roe.ac.uk/6dFGS/
.

The optical spectra of 2dFGRS galaxies were taken with the Two-degree Field (2dF) multi-object spectrograph on the Anglo-Australian Telescope. This spectrograph has 140 m diameter fibres corresponding to 2″.16 at the plate centre and 1″.99 at the edge and covers 3600–8000 Å with a spectral resolution of 9 Å. The 6dFGS galaxies were observed with the Six-degree Field (6dF) multi-object spectrograph having 6″.7 diameter fibres on the United Kingdom Schmidt Telescope. The original grating in the 6dFGS spectrograph spans 4000–8400 Åand gives a resolution of 5–12 Å. The improved grating, used after 2002 October, spans 3900–7500 Å with a resolution of 4.9–6.6 Å. It is noted that the 2dFGRS and 6dFGS spectra are not properly flux-calibrated, but the flux ratio of adjacent lines is still useful (to be discussed in Section 4).

In addition, we conducted optical spectroscopy of 15 ULIRGs in the survey sample plus one ULIRG, IRAS 090223615, in Sanders et al. (2003) at the Cerro Tololo Inter-American Observatory (CTIO). The total number of our ULIRG sample with spectra was increased to 199. The additional ULIRGs were observed on 2008 February 20–21 with the Ritchey-Chretien spectrograph and the Loral 3K CCD at the CTIO 4-m telescope (0″.5 pixel). A slit with a width of 1″.5 (a slightly larger than the seeing size) was adopted, and was positioned in the east-west direction (P.A. = 90 °). We used 316 lines mm grating to cover the spectral range 4500–10500 Å with a resolution of 5.6 Å ( 2.8 pixels). Three exposures were taken for each object and the integration times ranged from 360 to 3000 seconds depending on its brightness.

Data reduction was performed using the IRAF package. This involved bias subtraction, flat fielding, sky subtraction, wavelength and flux calibration. Galaxy spectra were extracted using an aperture width corresponding to a constant linear scale of 5 kpc at the redshift of each galaxy. A He-Ne-Ar lamp and standard star Hiltner 600 (Massey et al. 1988) taken nightly were used for wavelength and flux calibration, respectively. In Fig. 1, we display 13 spectra with median signal-to-noise ratio (S/N) per pixel for the continuum greater than 3. Typical emission lines (H, [O iii], H+[N ii], and [S ii]) are clearly seen in most spectra.

Among the sample of 199 ULIRGs considered in this study, we analyse the spectra of 115 ULIRGs with continuum S/N 3. In the case that there are more than one spectrum for the same object, the spectrum with a higher S/N of H flux is chosen for the final classification. If H is not available from any spectra, the spectrum with a higher S/N of H flux has priority.

Figure 1: Optical spectra of the ULIRGs observed at CTIO. These spectra are shifted to the rest-frame.                                      
Figure 2: The diagnostic diagrams for ULIRGs in the Sample A. CTIO, 2dFGRS, and 6dFGS samples are represented by circle, square, and triangle, respectively. The solid, dotted, and dashed lines indicate the extreme starburst (Kewley et al. 2001), pure star formation (Kauffmann et al. 2003), and Seyfert-LINER (Kewley et al. 2006) lines, respectively. NLAGN represents narrow-line AGN.

3 Analysis

3.1 Emission-line measurements

After transforming the spectrum to the rest-frame and subtracting the local continuum defined by a linear fit around emission-lines, we measure the line widths and fluxes via Gaussian profile fit using the MPFIT/IDL package based on the Levenberg-Marquardt method (Markwardt 2009). A single Gaussian is used to obtain the line width of [O iii]. For the line fluxes, we fit the [O i] line, [S ii] doublet, H+[N ii] and H+[O iii] line complexes with one Gaussian, two Gaussians, four Gaussians, and six Gaussians, respectively. The widths of the [N ii] doublet are kept the same and the height ratio of [N ii] to [N ii] is fixed to 1/3, as required by the energy level structure of the [N ii] ion (Osterbrock & Ferland 2006). The [O iii] doublet is fitted in the same way but each of the [O iii] lines is modelled with two Gaussians because [O iii] line often has a blue, asymmetric wing that is perhaps from outflows of gas with opaque clouds (e.g., Heckman et al 1981; Greene & Ho 2005). Some AGN host galaxies show both narrow and broad Balmer lines (e.g., Osterbrock & Mathews 1986; Hao et al. 2005) and the narrow Balmer lines can have extended bases even in star-forming galaxies, which are probably due to Wolf-Rayet stars (e.g., Osterbrook & Cohen 1982; Brinchmann et al. 2008). To take this into account, we fit each Balmer line with two Gaussians. If the H+[N ii] line complex cannot be decomposed into individual lines, the line fluxes are not kept. Meanwhile, the decomposition of [S ii] doublet is not important because we are interested only in the total flux of the lines. Uncertainties in the line measurements, comes from MPFIT routine, are typically 10–30 per cent.

3.2 Corrections

We correct the line fluxes for Galactic extinction using the foreground reddening maps provided by Schlegel et al. (1998) and the extinction law of Cardelli et al. (1989).

Balmer emission lines can be strongly affected by stellar absorption features. We remove the stellar absorption effects following the method discussed in Hopkins et al. (2003): , where is the stellar absorption-corrected line flux, is the measured line flux after foreground reddening correction, is the equivalent width of the line, and is a correction factor. We adopted values (2.6 and 3.2 Å for H and H, respectively) of Sc type galaxies (Miller & Owen 2002).

If both H and H are measured, we can correct the line fluxes for internal extinction using the Balmer decrement and the extinction curve with an assumption of an intrinsic H/H line ratio of 2.85 for star-forming galaxies and 3.1 for AGN galaxies (Osterbrock & Ferland 2006). We do not apply this correction when the observed ratio is smaller than the theoretical value.

An observed line width is the convolution of intrinsic line width and instrumental response. Since the instrumental resolution of each survey is significantly different, the observed line width should be corrected for a fair comparison. To correct the full width at half maximum (FWHM) of [O iii] line using the quadrature method, we adopt 250 km s as a finite resolution of CTIO spectra. Then we determine that the resolutions are roughly 450, 600, and 400 km s for the 2dFGRS, original 6dFGS, and improved 6dFGS grating, respectively, by considering that the [O iii] lines from different spectra have the same intrinsic width. Note that corrected line widths below 100 km s should be treated with caution due to their large uncertainties ( 200 km s).

3.3 Spectral classification

Figure 3: log ([O iii]/H) vs. FWHM diagram for the Sample B ULIRGs, which are denoted by open symbols. The ULIRGs already classified in the diagnostic diagrams are overplotted by filled symbols (black: Seyfert 2 plus LINER; dark-grey: composite; light-grey: star-forming). Circles, squares, and triangles represent CTIO, 2dFGRS, and 6dFGS ULIRGs, respectively. Stars and diamonds represent ULIRGs from the 1 Jy sample (Veilleux et al. 1999; Yuan et al. 2010) and the SDSS sample (Hou et al. 2009), respectively. The dashed and solid lines show the criteria of Zakamska et al. (2003) and our new boundary separating narrow-line AGNs (NLAGNs) and non-AGNs. In this diagram, [O iii]/H line ratios are corrected only for Galactic extinction and stellar absorption.

Table 1. Basic information and spectral types of 115 southern ULIRGs.

IRAS name RA (J2000) Dec (J2000) z sample class sub. known
F000503259 00 07 34.6 32 43 03 0.285 0.144 0.222 0.758 12.12 2dFGRS X
F000913905 00 11 42.3 38 49 15 0.253 0.125 0.316 0.756 12.14 6dFGS N
F001843331 00 20 57.7 33 14 28 0.238 0.120 0.334 0.613 12.10 2dFGRS X
F003183137 00 34 16.0 31 21 04 0.284 0.167 0.257 0.563 12.18 6dFGS X
F003352732 00 35 59.2 27 15 42 0.068 0.632 4.294 3.207 12.01 2dFGRS X Co Co(5)
F004562904 00 48 03.5 28 48 38 0.110 0.141 2.598 3.377 12.23 2dFGRS X Co Co(5)
F004822721 00 50 40.0 27 04 42 0.129 0.182 1.134 1.839 12.03 2dFGRS X Co Co(5)
F005693108 00 59 22.3 30 52 27 0.344 0.055 0.239 0.463 12.34 6dFGS N
F010042237 01 02 51.2 22 21 50 0.117 0.660 2.287 1.790 12.24 6dFGS N Co(5)
F010093241 01 03 19.8 32 25 37 0.256 0.123 0.291 0.750 12.12 2dFGRS N
F011602551 01 18 24.2 25 36 19 0.237 0.138 0.452 1.153 12.23 2dFGRS X
F011644740 01 18 36.5 47 25 04 0.235 0.219 0.487 0.674 12.25 CTIO X SF
F012125025 01 23 20.1 50 09 29 0.201 0.077 0.415 0.717 12.02 6dFGS N
F012340447 01 25 56.0 04 31 57 0.156 0.162 0.711 1.108 12.01 6dFGS X
F013583300 01 38 05.3 32 45 29 0.197 0.138 0.835 1.079 12.31 2dFGRS X SF(1)
F013793203 01 40 15.3 31 48 18 0.202 0.146 0.686 0.929 12.25 6dFGS N
F014972906 01 52 04.0 28 51 18 0.183 0.052 0.588 1.278 12.08 2dFGRS X Co
F015692939 01 59 13.1 29 24 37 0.140 0.143 1.734 1.514 12.29 2dFGRS N Co(5)
F020380816 02 06 18.7 08 02 32 0.220 0.326 0.351 0.525 12.04 6dFGS N
F020682000 02 09 09.0 19 46 30 0.253 0.190 0.278 0.647 12.09 6dFGS N
F021301948 02 15 23.5 19 34 18 0.191 0.196 0.553 0.565 12.10 6dFGS N
F023564628 02 37 29.3 46 15 48 0.206 0.090 1.034 1.738 12.45 2dFGRS X
F023613233 02 38 15.2 32 20 34 0.198 0.073 0.741 1.821 12.26 2dFGRS X SF
F023644751 02 38 13.6 47 38 06 0.097 0.175 2.794 4.953 12.15 6dFGS N
F023841744 02 40 46.4 17 31 52 0.308 0.072 0.269 1.344 12.28 6dFGS X
F024371145 02 46 07.8 11 32 42 0.270 0.148 0.208 0.710 12.03 6dFGS B S1
F025954714 03 01 18.4 47 02 17 0.245 0.063 0.268 0.484 12.04 CTIO X Co
F030002719 03 02 10.6 27 07 29 0.221 0.113 0.918 2.039 12.47 2dFGRS N
F031303119 03 15 04.6 31 08 02 0.258 0.103 0.252 0.438 12.06 2dFGRS X
F032593105 03 27 59.3 30 54 50 0.261 0.065 0.249 0.559 12.07 2dFGRS X
F034834704 03 49 54.4 46 55 10 0.301 0.098 0.156 0.542 12.02 CTIO X SF
F034851827 03 50 45.7 18 18 27 0.174 0.164 0.580 0.822 12.03 6dFGS N
F035692535 03 59 00.9 25 26 44 0.220 0.071 0.613 0.598 12.28 6dFGS X
F040565722 04 06 41.9 57 14 38 0.267 0.079 0.259 0.453 12.11 6dFGS X
F042793035 04 29 55.7 30 28 54 0.232 0.114 0.311 0.839 12.04 6dFGS X
F044891026 04 51 19.5 10 21 22 0.231 0.090 0.357 1.767 12.10 6dFGS N
F045052958 04 52 30.7 29 53 34 0.285 0.189 0.650 0.765 12.58 6dFGS B S1 S1(4)
F050202941 05 04 00.8 29 36 57 0.154 0.102 1.932 2.059 12.43 6dFGS X Co(5)
F051563024 05 17 31.9 30 21 14 0.171 0.103 1.162 1.402 12.31 6dFGS N S2(5)
F053483204 05 36 44.9 32 02 20 0.174 0.224 0.623 0.712 12.06 CTIO N S2
F061584017 06 17 29.8 40 18 57 0.221 0.083 0.311 0.707 12.00 6dFGS N
F082740147 08 30 00.6 01 57 05 0.250 0.149 0.594 0.622 12.40 CTIO X SF
F084112501 08 43 18.8 25 11 58 0.134 0.262 1.655 1.572 12.23 6dFGS N
F090611248 09 08 35.1 13 01 01 0.073 0.191 3.634 5.316 12.01 CTIO X Co SF(2)
F090901349 09 11 27.9 14 01 40 0.171 0.091 0.845 1.553 12.17 6dFGS X
F092480128 09 27 23.9 01 41 19 0.324 0.102 0.221 0.667 12.25 CTIO B S1
F095210400 09 54 37.2 04 15 12 0.237 0.108 0.387 0.600 12.16 2dFGRS N
F100770034 10 10 16.6 00 19 31 0.182 0.127 0.513 0.834 12.02 2dFGRS X SF
F102982300 10 32 11.7 23 15 41 0.285 0.153 0.377 1.583 12.35 6dFGS N
F104792808 10 50 18.9 28 24 01 0.191 0.327 1.002 1.151 12.36 6dFGS B S1
F105112723 10 53 33.3 27 39 05 0.159 0.319 0.896 1.351 12.13 CTIO N S2
F105333534 10 55 40.2 35 50 06 0.190 0.096 0.556 1.153 12.09 6dFGS X Co
F105493702 10 57 18.5 37 18 25 0.216 0.077 0.348 1.265 12.02 6dFGS N
F110532413 11 07 47.0 24 29 25 0.225 0.092 0.314 2.103 12.02 6dFGS N
F110933353 11 11 45.4 34 09 35 0.231 0.102 0.318 0.894 12.05 6dFGS X SF
F110950238 11 12 02.5 02 54 18 0.106 0.418 3.249 2.531 12.30 2dFGRS X Co Co(5)
F112042154 11 22 58.0 22 11 02 0.248 0.114 0.264 1.263 12.04 6dFGS X
F113000522 11 32 41.4 05 39 41 0.230 0.164 0.668 1.610 12.37 2dFGRS X
F114512128 11 47 39.0 21 45 06 0.219 0.182 0.375 0.859 12.07 6dFGS B S1
F121312809 12 15 42.4 28 26 19 0.360 0.102 0.325 1.811 12.52 6dFGS N
F124323138 12 45 57.2 31 54 42 0.423 0.154 0.256 0.718 12.61 6dFGS N
F124522032 12 47 53.6 20 48 24 0.209 0.203 0.383 0.988 12.03 6dFGS X
F132692251 13 29 40.8 23 07 10 0.290 0.114 0.382 1.410 12.37 6dFGS X

Table 1. – Continued.

IRAS name RA (J2000) Dec (J2000) z sample class sub. known
F132700331 13 29 40.7 03 46 59 0.221 0.306 0.954 0.797 12.48 2dFGRS N
F133051739 13 33 15.2 17 55 00 0.148 0.392 1.164 1.044 12.17 CTIO N S2 S2(5)
F133061644 13 33 21.5 17 00 22 0.231 0.208 0.298 0.752 12.02 6dFGS X
F133352612 13 36 22.1 26 27 30 0.125 0.139 1.402 2.101 12.09 CTIO X Co Co(5)
F133790256 13 40 33.4 03 11 42 0.218 0.235 0.728 1.032 12.35 2dFGRS X
F135313422 13 56 06.6 34 37 02 0.220 0.135 0.380 0.777 12.08 6dFGS N
F140213139 14 05 02.1 31 54 17 0.202 0.178 0.568 1.124 12.17 6dFGS N
F140902850 14 11 59.2 29 05 01 0.212 0.197 0.507 0.807 12.17 CTIO N S2
F141210126 14 14 45.7 01 40 53 0.150 0.239 1.394 2.073 12.26 2dFGRS X Co S2(5)
F142072002 14 23 31.5 20 15 47 0.173 0.208 0.850 1.082 12.18 6dFGS N S2(1)
F142483644 14 27 51.7 36 58 03 0.208 0.122 0.450 1.499 12.10 6dFGS X
F142542655 14 28 19.9 27 08 49 0.253 0.206 0.534 0.809 12.37 6dFGS N S2(1)
F143481447 14 37 37.2 15 00 20 0.082 0.495 6.870 7.068 12.39 6dFGS X SF Co(5)
F145441302 14 57 09.5 13 14 55 0.254 0.265 0.309 0.761 12.13 CTIO B S1
F151301958 15 15 55.5 20 09 17 0.108 0.388 1.916 2.299 12.09 6dFGS N S2(3)
F160900139 16 11 40.9 01 47 06 0.134 0.264 3.609 4.874 12.57 6dFGS X Co Co(3)
F161590402 16 18 36.3 04 09 42 0.211 0.299 0.979 1.768 12.45 6dFGS N
F194663649 19 49 55.5 36 42 06 0.093 0.316 2.425 3.378 12.05 6dFGS X SF
F195486237 19 59 18.9 62 29 18 0.351 0.081 0.365 0.823 12.55 6dFGS X
F200235253 20 06 08.1 52 44 48 0.238 0.089 0.312 1.454 12.07 6dFGS X
F200661630 20 09 27.7 16 22 06 0.163 0.146 0.639 1.285 12.00 6dFGS X
F201812244 20 21 03.9 22 35 22 0.184 0.199 0.568 0.885 12.07 6dFGS N
F202483204 20 27 59.1 31 54 54 0.203 0.119 0.440 2.673 12.06 6dFGS X
F202704237 20 30 24.9 42 27 24 0.242 0.102 0.287 0.646 12.05 6dFGS N
F202736558 20 31 50.8 65 48 22 0.349 0.155 0.430 1.151 12.61 6dFGS N
F205421832 20 57 03.6 18 20 43 0.298 0.126 0.322 0.934 12.33 6dFGS N
F205514250 20 58 27.3 42 38 57 0.042 1.906 12.780 9.948 12.06 6dFGS X SF Co(5)
F210161900 21 04 29.8 18 48 17 0.230 0.230 0.305 1.025 12.03 6dFGS N
F213561015 21 38 20.2 10 01 57 0.206 0.159 0.460 0.550 12.09 6dFGS N
F213672405 21 39 36.6 23 51 51 0.234 0.102 0.383 0.565 12.15 6dFGS X SF
F214353648 21 46 31.9 36 34 54 0.160 0.149 0.665 0.999 12.01 6dFGS N
F214882819 21 51 41.4 28 05 14 0.234 0.135 0.301 0.677 12.04 2dFGRS X
F215424050 21 57 21.3 40 36 03 0.301 0.093 0.253 0.531 12.23 6dFGS B S1
F215554235 21 58 37.5 42 21 33 0.181 0.132 0.833 0.987 12.22 6dFGS X
F220583501 22 08 48.8 34 46 38 0.173 0.162 0.561 1.375 12.01 2dFGRS N
F222062715 22 23 29.4 26 59 59 0.131 0.159 1.754 2.333 12.23 2dFGRS X Co Co(5)
F223012822 22 32 56.7 28 07 17 0.244 0.123 0.312 0.925 12.10 2dFGRS N
F224234707 22 45 20.2 46 52 03 0.200 0.150 0.451 0.693 12.05 6dFGS B S1
F225213929 22 54 56.5 39 13 14 0.261 0.135 0.283 0.545 12.13 6dFGS N
F225462637 22 57 23.8 26 21 23 0.163 0.166 0.752 1.362 12.08 2dFGRS X SF(1)
F225603501 22 58 46.7 34 45 44 0.171 0.141 0.593 0.795 12.02 2dFGRS X Co
F230463454 23 07 21.3 34 38 41 0.208 0.093 0.937 1.313 12.41 2dFGRS X
F231285919 23 15 46.5 59 03 14 0.044 1.590 10.800 10.990 12.03 6dFGS X SF
F231420611 23 16 49.3 05 55 13 0.346 0.158 0.263 0.405 12.39 6dFGS N
F231850328 23 21 05.9 03 12 03 0.246 0.228 0.308 0.385 12.10 6dFGS N
F232061222 23 23 14.4 12 06 29 0.249 0.255 0.246 0.689 12.01 2dFGRS N
F232420357 23 26 49.1 03 41 18 0.189 0.275 0.454 0.566 12.00 6dFGS X
F232535415 23 28 06.0 53 58 26 0.129 0.214 2.296 3.493 12.34 6dFGS N LI
F235162420 23 54 13.0 24 04 05 0.154 0.283 0.834 1.240 12.06 6dFGS X
F235292119 23 55 33.8 21 02 49 0.428 0.156 0.327 0.627 12.73 6dFGS N
F235593009 23 58 31.0 29 52 18 0.342 0.152 0.237 0.463 12.33 2dFGRS X
090223615 09 04 12.8 36 27 02 0.059 1.154 11.470 11.080 12.26 CTIO X SF

Column descriptions: (1) Object name in the IRAS catalogue. (2-3) Right ascension and declination in units of and ° ′ ″, respectively. (4) Redshift. (5-7) The IRAS flux density at 25, 60, and 100 m [Jy]. (8) Infrared luminosity. (1-8) Basic information taken from Hwang et al. (2007) except 090223615, for which information was taken from Sanders et al. (2003). (9) Adopted spectrum. (10) Spectral class in this study (Bbroad-line AGN, Nnarrow-line AGN, Xnon-AGN). (11) Subclass in this study (S1=Seyfert 1, S2=Seyfert 2, LI=LINER, Co=composite, SF=star-forming galaxy). (12) Subclass from previous studies. Numbers in parentheses are references (1=Allen et al. 1991, 2=Duc et al. 1997, 3=Kim et al. 1998, 4=Low et al. 1988, 5=Yuan et al. 2010).

The spectral types of these objects are different between this study and previous studies.

This object was classified as a LINER by Kim et al. (1998) using the Veilleux & Osterbrock (1987) scheme.

Table 2. Line information of the Sample A ULIRGs.

IRAS name quality EW FWHM        
F003352732 3333333 50 5. 81 559 23 0.06 0.10 0.44 0.06 0.66 0.10 0.99 0.10
F004562904 3333333 78 4. 75 394 16 0.25 0.03 0.33 0.02 0.48 0.04 1.23 0.09
F004822721 2323320 40 5. 44 445 76 0.01 0.19 0.23 0.06 0.49 0.30 0.86 0.22
F011644740 1103322 8.5 86 10. 88 489 168 0.04 0.45 0.71 0.16 0.75 0.25
F014972906 3203300 47 4. 09 526 74 0.06 0.23 0.18 0.12
F015692939 3333333 97 5. 58 636 8 0.32 0.03 0.02 0.02 0.28 0.06 0.63 0.03
F023613233 3303300 72 5. 14 507 21 0.03 0.10 0.54 0.10
F025954714 2303321 3.9 25 5. 69 493 45 0.12 0.18 0.17 0.06 0.46 0.29
F034834704 1203311 6.5 72 4. 78 485 64 0.10 0.35 0.39 0.10 0.59 0.35
F053483204 1323322 16.5 99 4. 24 1351 38 0.59 0.22 0.21 0.06 0.48 0.21 0.96 0.25
F082740147 3323333 20.4 236 8. 18 573 88 0.31 0.08 0.80 0.07 0.68 0.11 1.18 0.20
F090611248 1103332 13.4 36 7. 72 338 145 0.29 0.38 0.30 0.10 0.43 0.23
F100770034 2222300 66 4. 55 410 56 0.03 0.25 0.44 0.21 0.93 0.30
F105112723 2323332 12.5 25 2. 26 837 10 0.99 0.13 0.01 0.07 0.01 0.17 0.57 0.19
F105333534 2103300 201 1. 71 0.06 0.46 0.32 0.17
F110933353 2102200 36 1. 31 636 157 0.27 0.40 0.56 0.31
F110950238 3232322 54 5. 39 409 40 0.07 0.28 0.24 0.26 0.40 0.29 0.85 0.30
F133051739 3333330 128.5 228 6. 15 1229 9 0.96 0.08 0.27 0.05 0.57 0.04 0.98 0.06
F133352612 2323321 9.0 58 5. 66 418 52 0.12 0.20 0.28 0.20 0.39 0.38 0.96 0.37
F140902850 3303332 33.3 161 6. 76 710 12 0.91 0.08 0.42 0.09 0.53 0.18
F141210126 2103332 68 10. 40 615 51 0.07 0.41 0.16 0.13 0.46 0.18
F143481447 3333333 99 4. 68 0.01 0.11 0.55 0.09 0.62 0.16 1.02 0.20
F160900139 2103300 27 2. 13 627 176 0.32 0.40 0.05 0.17
F194663649 2103300 15 3. 78 143 120 0.37 0.43 0.42 0.10
F205514250 3333333 60 3. 92 318 13 0.17 0.03 0.53 0.04 0.44 0.05 1.07 0.06
F213672405 3303300 91 1. 23 0.10 0.11 0.52 0.13
F220583501 1302300 19 13. 34 407 20 0.97 0.17 0.09 0.33
F222062715 2203300 43 6. 97 217 76 0.06 0.19 0.36 0.05
F225603501 2202322 51 3. 78 274 66 0.15 0.32 0.42 0.29 0.48 0.28
F231285919 3302222 52 2. 57 360 19 0.33 0.17 0.69 0.41 0.60 0.31
F232535415 3223300 40 2. 04 733 99 0.07 0.30 0.01 0.06 0.79 0.23
090223615 3333333 154.7 164 5. 67 547 11 0.06 0.04 0.48 0.16 0.58 0.16 1.43 0.19

Column descriptions: (1) Object name in the IRAS catalogue. (2) Line flux qualities at H, [O iii], [O i], H, [N ii], [S ii], and [S ii] (0=unmeasurable, 1=low, 2=moderate, 3=high). (3) Absolute flux of H [10 ergs s cm]. The values from uncalibrated spectra are not presented. (4) Equivalent width of H [Å]. (5) Observed H/H line ratio. (6) [O iii] line width after instrumental resolution correction and its uncertainty [km s]. Line widths below 100 km s are not shown. (7-10) Logarithms of the line ratios and their uncertainties. The ratios are corrected for Galactic extinction and stellar absorption. The internal extinction correction is applied, if available.

Table 3. Line information of the Sample B ULIRGs.

IRAS name quality FWHM   IRAS name quality FWHM  
F000503259 33 378 41 0.11 0.14 F124323138 12 701 66 0.74 0.39
F000913905 12 621 102 0.78 0.40 F124522032 33 185 53 0.56 0.13
F001843331 11 472 141 0.43 0.38 F132692251 11 384 93 0.05 0.51
F003183137 22 0.26 0.30 F132700331 33 1146 8 0.93 0.14
F005693108 12 0.84 0.52 F133061644 12 175 48 0.53 0.34
F010042237 32 850 28 0.55 0.16 F133790256 23 310 58 0.02 0.16
F010093241 33 786 13 1.09 0.07 F135313422 23 760 23 0.90 0.19
F011602551 33 250 54 0.15 0.09 F140213139 12 575 87 0.31 0.37
F012125025 22 908 134 0.13 0.39 F142072002 12 448 91 0.56 0.53
F012340447 22 0.17 0.20 F142483644 12 331 90 0.34 0.41
F013583300 22 368 18 0.25 0.09 F142542655 11 711 61 0.62 0.45
F013793203 23 1176 27 0.77 0.19 F151301958 12 1283 75 0.72 0.47
F020380816 13 277 28 0.94 0.38 F161590402 11 833 134 0.07 0.43
F020682000 22 0.52 0.25 F195486237 11 335 119 0.18 0.51
F021301948 12 657 83 0.27 0.29 F200235253 12 376 135 0.03 0.35
F023564628 33 569 45 0.13 0.15 F200661630 32 134 102 0.28 0.30
F023644751 33 508 40 0.43 0.10 F201812244 33 882 12 0.56 0.12
F023841744 12 248 66 0.07 0.43 F202483204 12 359 120 0.29 0.54
F030002719 23 664 17 0.92 0.02 F202704237 12 696 78 1.13 0.37
F031303119 11 281 83 0.22 0.47 F202736558 13 1402 70 1.07 0.30
F032593105 31 256 36 0.01 0.32 F205421832 23 418 24 0.58 0.22
F034851827 33 657 11 0.92 0.12 F210161900 12 863 165 0.03 0.31
F035692535 23 157 37 0.17 0.19 F213561015 23 902 12 0.50 0.20
F040565722 32 0.27 0.18 F214353648 13 557 25 0.76 0.18
F042793035 23 116 37 0.36 0.20 F214882819 33 346 13 0.33 0.09
F044891026 13 525 53 0.44 0.19 F215554235 33 123 32 0.09 0.14
F050202941 33 154 46 0.01 0.15 F223012822 32 1083 113 0.02 0.19
F051563024 23 990 28 0.76 0.17 F225213929 13 775 79 0.31 0.40
F061584017 22 767 52 0.28 0.28 F225462637 23 194 37 0.10 0.16
F084112501 31 908 149 0.46 0.41 F230463454 33 309 23 0.17 0.10
F090901349 12 0.07 0.52 F231420611 22 452 51 0.50 0.38
F095210400 12 845 94 0.22 0.40 F231850328 33 956 32 0.42 0.15
F102982300 23 1490 71 0.99 0.24 F232061222 33 451 5 0.46 0.15
F105493702 12 491 112 0.22 0.28 F232420357 22 0.20 0.38
F110532413 13 521 55 0.89 0.44 F235162420 21 0.39 0.43
F112042154 22 391 87 0.02 0.36 F235292119 12 634 106 0.50 0.51
F113000522 32 0.45 0.26 F235593009 11 485 71 0.19 0.54
F121312809 23 713 40 0.47 0.17

Column descriptions: (1, 5) Object name in the IRAS catalogue. (2, 6) Line flux qualities at H and [O iii] (0=unmeasurable, 1=low, 2=moderate, 3=high). (3, 7) [O iii] line width after instrumental resolution correction and its uncertainty [km s]. Line widths below 100 km s are not presented. (4, 8) Logarithm of [O iii]/H line ratio and its uncertainty. The ratio is corrected only for Galactic extinction and stellar absorption.

We use the emission lines with flux uncertainty 60% for spectral classification. The qualities of line fluxes with uncertainty 20%, 20–40%, and 40–60% are referred to hereafter as high, moderate, and low, respectively (see Tables 2 and 3).

In the sample of 115 ULIRGs, 8 ULIRGs have broad Balmer lines, FWHM of broad component of Balmer lines 2000 km s and height of the broad component 3 times the local rms of the continuum-subtracted spectra. These are considered to be broad-line AGN galaxies222In Table 1, we use the term “Seyfert 1 galaxies” to indicate broad-line AGNs, which is commonly used in previous studies..

There are 32 narrow emission-line ULIRGs for which more than two line ratios ([O iii]/H and at least one of [N ii]/H, [S ii]/H, and [O i]/H) were measured (hereafter Sample A). In Fig. 2, we show the diagnostic diagrams for these ULIRGs and divide them into star-forming, (starburst-AGN) composite, and narrow-line AGN galaxies based on their loci in the diagrams. Star-forming galaxies lie below the pure star formation line (Kauffmann et al. 2003) in the [N ii]/H diagram and lie below the extreme starburst line (Kewley et al. 2001) in other diagrams. Composite galaxies lie between the extreme starburst line and the pure star formation line in the [N ii]/H diagram. Narrow-line AGNs lie above the extreme starburst line in all three diagrams. Whenever possible, narrow-line AGNs are subdivided into Seyfert 2 and low ionization narrow emission-line region (LINER) galaxies. Seyfert 2 galaxies lie above the Seyfert-LINER classification lines (Kewley et al. 2006) in the [S ii]/H and [O i]/H diagrams, whereas LINERs lie below the lines. For ambiguous galaxies that are classified as one type in two diagrams but another type in the remaining diagram, we adopt the types that are given in the first two diagrams.

There are 75 narrow emission-line ULIRGs without measurable H line mainly because H falls outside the spectral coverage (hereafter Sample B). These galaxies could not be classified in the diagnostic diagrams so that we attempt to classify them in flux ratio between [O iii] and H lines versus [O iii] line width diagram as demonstrated in Fig. 3. Zakamska et al. (2003) used the criteria of  ([O iii]/H 0.3 and FWHM  400 km s to select narrow-line AGN galaxies (dashed line). Using our Sample A and two large, homogeneous samples in the literature (the IRAS 1 Jy ULIRGs: Kim & Sanders 1998; Veilleux et al. 1999; Yuan et al. 2010; the SDSS ULIRGs: Hou et al. 2009), we determine a new boundary to separate narrow-line AGN galaxies from the others (solid line) with high completeness and reliability as far as possible:  ([O iii]/H 0.13 (FWHM/100 km s 0.85. If we regard AGNs classified in the diagnostic diagrams as genuine AGNs, the application of our new boundary provides 89% completeness (among the total 88 AGNs, 78 AGNs are found inside this boundary) and 89% reliability (among 88 objects in the boundary, 78 objects are AGNs), while 68% (60/88) completeness and 90% (60/67) reliability using the criteria of Zakamska et al. The galaxies outside the boundary are mostly star-forming or composite galaxies, but they are not separable from each other in this diagram. They are referred to as non-AGN galaxies.

Our 115 southern ULIRGs contain 8 broad-line AGNs, 49 narrow-line AGNs including four Seyfert 2 and one LINER galaxies, and 58 non-AGNs including thirteen composite and twelve star-forming galaxies. The spectral classification results (and their basic information) are presented in Table 1. Detailed line information of Samples A and B is listed in Tables 2 and 3, respectively.

4 Discussion

4.1 Reliability of our spectral classification

Table 4. AGN fraction as a function of infrared properties

sample all [12.0, 12.15) [12.15, 12.4) [12.4, 13.0)
This study 50% of 115 48% of 61 50% of 40 57% of 14
Kim & Sanders (1998) 45% of 108 38% of 40 39% of 49 79% of 19
Hou et al. (2009) 55% of 209 43% of 90 51% of 81 89% of 36
sample all [1.4, 0.9) [0.9, 0.5) [0.5, 0.1)
This study 59% of 37 40% of 10 47% of 15  92% of 12
Kim & Sanders (1998) 56% of 59 29% of 28 76% of 25 100% of  6
Hou et al. (2009) 53% of 45 33% of 18 38% of 13  93% of 14
sample all [0.7, 0.3) [0.3, 0.1) [0.1, 0.3)
This study 40% of  70 36% of 11 33% of 36 52% of 23
Kim & Sanders (1998) 45% of 108 33% of 51 56% of 57
Hou et al. (2009) 43% of 106 58% of 19 39% of 61 46% of 24

The small aperture spectroscopy could not always contain enough light of an extended source to determine its spectral type. For reliable classification, it is suggested to use an aperture covering more than 20% of the galaxy light. The minimum aperture covering fraction of 20% corresponds to 2.1 kpc (see fig. 6 in Kewley et al. 2005). All spectra of ULIRGs in this study satisfy this condition. Therefore, aperture-related effects on our classification are expected to be negligible.

In the 2dFGRS and 6dFGS spectra, the flux for individual line can be unreliable because these spectra are not properly flux-calibrated. Nevertheless, the flux ratio between lines with similar wavelengths is still useful (e.g., Mouhcine et al. 2005; Owers et al. 2007). To ensure this, in Fig. 4, we compare [O iii]/H line ratios derived from both calibrated (i.e., CTIO or SDSS) and uncalibrated (i.e., 2dFGRS or 6dFGS) spectra. It shows that two measurements agree well within the errors. The measurements between the 2dFGRS and 6dFGS spectra also agree well.

Among the 115 ULIRGs, there are 22 objects whose spectral types have been previously determined in the literature. The spectral types for 19 out of 22 ULIRGs determined in this study are consistent with those in the literature. If we compare ULIRGs whose subclasses (see columns 11 and 12 in Table 1) were determined, 9 out of 13 ULIRGs show a good agreement. Although there are three ULIRGs with different types (or four ULIRGs with different subclasses), all of these ULIRGs are composite galaxies either in this study or in the previous studies.

Figure 4: Comparison of [O iii]/H line ratio between two spectra. In the left panel, the measurements from CTIO-6dFGS, CTIO-2dFGRS, SDSS-6dFGS, and SDSS-2dFGRS are denoted by squares, diamonds, triangles, and inverse triangles, respectively. In the right panel, the measurements from 6dFGS-2dFGRS are denoted by circles. Only objects with S/Ns of both spectra 3 are presented. The one-to-one relation (solid line) is overplotted.

4.2 Dependence of optical properties on infrared parameters

In our sample, 50% (57/115) of the ULIRGs are found to host AGN. The AGN fraction depends on the infrared luminosity and the IRAS flux density ratios (hereafter IRAS colours) as follows: 48% for ULIRGs with 12.0 12.15, 50% for [12.15, 12.4), and 57% for [12.4, 13.0). 40% for ULIRGs with 1.4 0.9, 47% for [0.9, 0.5), and 92% for [0.5, 0.1). 36% for ULIRGs with 0.7 0.3, 33% for [0.3, 0.1), and 52% for [0.1, 0.3). The ULIRGs with a flux upper limit at 25 (100) m are not included in the calculations for IRAS 2560 (60100) m colour dependence. These results are summarized in Table 4 together with those from the 1 Jy (Kim & sanders 1998) and SDSS (Hou et al. 2009) samples. Note that no ULIRGs in the 1 Jy sample have colours of 0.3 due to the selection criteria in Kim & Sanders (1998). The AGN fractions for each sample are comparable in the sense that the differences are less than 1.6 by assuming Poisson errors. In all samples, there is a tendency for the ULIRGs with higher infrared luminosity, warmer IRAS 2560 m colour to be more AGN-like. These findings are consistent with those in previous works (e.g., Veilleux et al. 1999; Kewley et al. 2001; Goto 2005; Cao et al. 2006; Hou et al. 2009; Yuan et al. 2010).

Figure 5: Infrared luminosity (left), IRAS 2560 m colour (middle), and IRAS 60100 m colour (right) distributions of the combined ULIRG sample. The distributions of non-AGN, narrow-line AGN, and broad-line AGN ULIRGs are demonstrated by solid, hatched, and shaded histograms, respectively. The ULIRGs with a flux upper limit at 25 (100) m are not included in the middle (right) panel. The histograms are normalized to the total numbers of each spectral type, which are represented in the upper-left corner (upper: non-AGN; middle: narrow-line AGN; lower: broad-line AGN). The values in the upper-right corner show the probability that the two types of ULIRGs are drawn from the same population, given by the K-S test (Small values mean that the distributions of two types are significantly different.). The upper, middle, and lower values are calculated between non-AGNs and narrow-line AGNs, narrow-line AGNs and broad-line AGNs, and broad-line AGNs and non-AGNs, respectively.
Figure 6: Relations between optical and infrared properties of narrow emission-line objects in the combined ULIRG sample. [O iii]/H line ratio vs. infrared luminosity (upper-left), [O iii]/H line ratio vs. the IRAS 2560 m colour (upper-middle), [O iii]/H line ratio vs. the IRAS 60100 m colour (upper-right), FWHM vs. infrared luminosity (lower-left), FWHM vs. the IRAS 2560 m colour (lower-middle) and FWHM vs. the IRAS 60100 m colour (lower-right) diagrams are presented. The symbol shapes are the same as in Fig. 3. The filled and open symbols are narrow-line AGNs and non-AGNs, respectively. The values in each panel indicate Spearman’s rank correlation coefficient between two properties. The upper, middle, and lower values are calculated from narrow-line AGN plus non-AGN, narrow-line AGN, and non-AGN ULIRGs, respectively. In the middle (right) panels, the ULIRGs which have a flux upper limit at 25 (100) m are not used in the calculations and they are represented by dots.

In Fig. 5, we present infrared luminosity, IRAS 2560 m colour, and IRAS 60100 m colour distributions of the combined sample (432 ULIRGs) including this study, Kim & Sanders (1998), and Hou et al. (2009). Based on the Kolmogorov-Smirnov (K-S) test, we find that the infrared luminosity distribution of broad-line AGN ULIRGs is significantly different from those of non-AGN and narrow-line AGN ULIRGs, while those of non-AGN and narrow-line AGN ULIRGs are not so different. We also find that the three types of ULIRGs are significantly different from each other in IRAS 2560 m colour distribution but they are indistinguishable in IRAS 60100 m colour distribution. In IRAS 2560 m colour distribution, the significant difference between AGN and non-AGN ULIRGs strengthens that mid-infrared colour is a good indicator of AGN activity in infrared galaxies (e.g., de Grijp et al. 1985; Neff & Hutchings 1992). The little differences in IRAS 60100 m colour distribution reinforce that star formation dominates the emission of AGN in the far-infrared regime (e.g., Elbaz et al. 2010; Hatziminaoglou et al. 2010; Hwang et al. 2010b; Shao et al. 2010). On the other hand, narrow-line AGN ULIRGs have in general lower infrared luminosity and cooler IRAS 2560 m colour than broad-line AGN ULIRGs do. If we assume that the infrared emission in galaxies is emitted isotropically, and does not depend on viewing angle (e.g., Mulchaey et al. 1994; Schartmann et al. 2008; Gandhi et al. 2009), the difference between narrow-line AGN and broad-line AGN ULIRGs seems not to be compatible with the predictions of the orientation-dependent unification model of AGNs in which narrow-line and broad-line AGNs are intrinsically same objects observed from different angles (Antonucci 1993). This conflict can be explained if narrow-line AGN ULIRGs host a central engine deeply buried by extended, dusty regions of star formation as proposed by several authors (e.g., Genzel et al 1998; Gerssen et al. 2004).

In Fig. 6, we show the relations between optical ([O iii]/H line ratio or [O iii] line width) and infrared (infrared luminosity, IRAS 2560 m colour or IRAS 60100 m colour) properties of the combined ULIRG sample. Broad-line AGN ULIRGs are not plotted because their line widths and fluxes are not properly measured in many cases333 The spectra of broad-line AGNs should be analysed with more sophisticated methods of galaxy modelling (e.g., Kim et al. 2006).. Since the spectral types are determined by the optical emission lines, narrow-line AGNs and non-AGNs are well separated along the y-axis. As expected, clear correlations are found only in the middle panels (see the values of Spearman’s rank correlation coefficients). We checked these relations using various infrared colours from the AKARI all-sky survey point source catalogues444 http://www.ir.isas.jaxa.jp/AKARI/Observation/PSC/Public/ but we could not draw any meaningful results due to the small number of data points.

5 Summary

We studied optical spectral properties of 115 southern ULIRGs using the spectra obtained from our CTIO observations, 2dFGRS, and 6dFGS. For ULIRGs with H measurement, we classified them in the standard diagnostic diagrams. We classified the other ULIRGs using the [O iii]/H line ratio against [O iii] line width diagram with an empirically determined criterion. Main results are summarized below.

  1. Our new criterion,  ([O iii]/H 0.13 (FWHM /100 km s 0.85, is successful to separate AGN ULIRGs from non-AGN ULIRGs with completeness and reliability of about 90%.

  2. In our sample of the 115 ULIRGs, there are 8 broad-line AGNs, 49 narrow-line AGNs, and 58 non-AGNs. The AGN fraction is 50% and changes as a function of infrared luminosity and IRAS 2560 m colour. These results are consistent with those in previous studies.

  3. Using the combined ULIRG sample, we show that the colour distributions of AGN and non-AGN ULIRGs are significantly different in IRAS 2560 m colour and are indistinguishable in IRAS 60100 m colour. These results support that mid-infrared colour is sensitive to AGN activity, whereas far-infrared colour is dominated by star formation.

  4. We also show that broad-line AGN ULIRGs differ from narrow-line AGN ULIRGs in infrared luminosity and IRAS 2560 m colour. This presents a challenge to the simple unification model of AGNs.

Acknowledgments

We thank the CTIO staff for their help on the observations. We also thank L. G. Hou for providing the [O iii] line width data of the SDSS ULIRGs. We are grateful to anonymous referee whose comments helped to improve the original manuscript. This work was supported by Mid-career Research Program through NRF grant funded by the MEST (No.2010-0013875). H.S.H acknowledges the support of the Centre National d’Etudes Spatiales (CNES). S.C.K is a member of the Dedicated Researchers for Extragalactic AstronoMy (DREAM) team in Korea Astronomy and Space Science Institute (KASI).

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