Interferometric Observations of Powerful Co Emission from Three Submillimeter Galaxies

We report IRAM millimeter interferometry of three –3.4 Submillimeter Common-User Bolometric Array z ∼ 2.4 deep field galaxies. Our CO line observations confirm the rest-frame UV/optical redshifts, thus more than doubling the number of confirmed published redshifts of the faint submillimeter population and proving their high-z nature. In all three sources our measurements of the intrinsic gas and dynamical mass are large (10–10 M,). In at least two cases the data show that the submillimeter sources are part of an interacting system. Together with recent information gathered in the X-ray, optical, and radio bands, our observations support the interpretation that the submillimeter population, at least the radio-detected ones, consists of gas-rich (gas–to–dynamical mass ratio ∼0.5) and massive interacting starburst/active galactic nucleus systems. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation The extragalactic far-IR/submillimeter background is probably dominated by luminous and ultraluminous infrared galaxies (LIRGs/ULIRGs: –10 L,) at (e.g., Smail, 11.5 L ∼ 10 z 1 IR Ivison, & Blain 1997; Bertoldi et al. 2002; Scott et al. 2002; Cowie, Barger, & Kneib 2002). However, far-IR/submillimeter sources in most cases have relatively poorly known positions and frequently have only weak counterparts in the rest-frame UV and optical (Smail et al. 2000, 2002; Dannerbauer et al. 2002). As a result, redshifts have thus far been confirmed with CO interferometry for only two of the ∼100 detected systems (Blain et al. 2002). Recently a subgroup of the authors have obtained optical spectroscopic redshifts for a number of sources detected with the Submillimeter Common-User Bolometric Array (SCUBA) camera at 850 mm, to a large extent aided by more precise positions derived from deep 1.4 GHz Very Large Array observations of the same fields. Here we report the first results on the millimeter CO line follow-up of these submillimeter sources. We believe that these observations mark a sensitive breakthrough in the notoriously difficult study of the faint far-IR/submillimeter galaxy population. The observations were carried out between late summer 2002 and winter 2003 with the IRAM Plateau de Bure interferometer, consisting of six 15 m diameter telescopes. We used the compact 1 Institut de Radio Astronomie Millimétrique (IRAM), 300 Rue de la Piscine, Domaine Universitaire de Grenoble, St. Martin d’Hères F-38406, France; [email protected]. 2 Max-Planck Institut für extraterrestrische Physik, Giessenbachstrasse Postfach 1312, Garching D-85741, Germany; [email protected]. 3 Department of Physics, University of California at Berkeley, 366 LeConte Hall, Berkeley, CA 94720-7300. 4 Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK; [email protected]. 5 Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, Bonn D53121, Germany; [email protected]. 6 Department of Astronomy, California Institute of Technology, MS 10524, 1201 East California Boulevard, Pasadena, CA 91125; awb@astro .caltech.edu, [email protected]. 7 Institut d’Astrophysique Spatiale, Université de Paris Sud, Bâtiment 121, Orsay F-91405, France; [email protected]. 8 Institute for Astronomy, Royal Observatory, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK; [email protected]. 9 Institut d’Astrophysique de Paris, CNRS, Université de Paris 6, Paris, France; [email protected]. 10 Space Infrared Telescope Facility Science Center, California Institute of Technology, Mail Code 314-6, Pasadena, CA 91125; [email protected]. D configuration and for follow-up observations of two of the sources the more extended BC configurations. The correlator was configured for CO line and continuum observations to simultaneously cover 580 MHz in the 3 and 1.3 mm bands. The frequency settings were adjusted for all three sources to optimize the CO line centering in the bandpass. SMMJ 04431 0210 was observed between 2002 September and 2003 February in D and BC configurations for a total integration time of 22 hr. SMMJ 09431 4700 was observed in D configuration only, in 2002 November, for 13 hr. SMMJ 16368 4057 was observed between 2002 September and 2003 February for 24 hr. All sources were observed in very good observing conditions. Passband calibration used one or more bright quasars. Phase and amplitude variations within each track were calibrated out by interleaving reference observations of nearby quasars every 20 minutes. The overall flux scale for each epoch was set on MWC 349. Figures 1 and 2 show the CO spectra and maps. The derived properties are listed in Tables 1 and 2. We adopt a flat, L-cosmology with km s 1 Mpc . To conQ p 0.3 H p 70 M 0 vert CO luminosities to gas masses, including a 37% correction for helium, we adopt, under the assumption of constant brightness temperature for the lowest rotational transitions from (1– 0) to (4–3), a factor of M,/(K km s 1 pc) p 0.2a a p 0.8 (Galactic), as derived from observations of ULIRGs z ∼ 0.1 (Downes & Solomon 1998). The gas masses are probably uncertain by a factor of at least 2. We estimate dynamical masses from (M,) p . 2 2 4 M sin i 4 # 10 Dv R dyn FWHM Here we assume that the gas emission comes from a rotating disk of outer radius R (in units of kiloparsecs) observed at inclination angle i. In a merger model the dynamical masses would be a factor of 2 larger (Genzel et al. 2003). The numerical constant incorporates a factor of 2.4 between observed FWHM velocity width of the line emission, (in units of km s ), DvFWHM and the product of rotation velocity and . This factor is sin i estimated from model disks taking into account local line broadening, beam, and spectral smearing. We deduce IR luminosities from the 850 mm continuum flux densities S by adopting a modified gray-body model ( K) with proT p 40 portional dependent emissivity, such that in the range from 1.5 n to 3.5 LIR (L,) S850, with S850 in units of 12 z p 2 p 1.9 # 10 mJy (Blain et al. 2002). These luminosities are uncertain by a factor of 2–3 since dust temperature and emissivity law may L114 NERI ET AL. Vol. 597 Fig. 1.—CO spectra of the three SCUBA sources. The LSR velocity scale is with respect to the CO redshift listed in Table 1. Optical redshifts (arrows, horizontal bars are uncertainties) are from Frayer et al. 2003 (Ha [N ii], left panel), Ledlow et al. 2002 (Lya, center), and Smail et al. 2003 (UV photospheric Seyfert 2), respectively. The rms noise per 20 MHz channel is 0.7, 0.9, and 0.6 mJy for the three sources, respectively (left to right), in the spectral region where the frequency settings were overlapping, and increases to about 20% toward the edges of the bandpass. Overplotted on the CO (3–2) spectrum of SMMJ 16368 4057 is the CO (7–6) spectrum scaled down to 1/10 of its flux density. The rms noise per 40 MHz channel is here 2.2 mJy. Fig. 2.—Velocity-integrated natural-weighted CO maps of the three SCUBA sources, superposed on gray-scale images of the optical emission. The contours are in units of 2 j of the noise level and are 0.26, 0.31, and 0.20 Jy km s 1 in the three panels (left to right), respectively. The synthesized beams are (left to right, shown as hatched ellipses) 5 .6 # 3 .3 at position angle 23 (east of north), 6 .6 # 3 .6 at 108 , and 3 .3 # 2 .6 at 79 . The three underlying images are in the K band (left panel: Frayer et al. 2003; right panel: Smail et al. 2003) and in the I band (center panel: Ledlow et al. 2002). The asterisk (left image) is the position of the ERO N4 (uncertainty 0 .5; see text); the filled squares (black and light) are the millimeter continuum positions (center and right). The edge-on spiral galaxy 2 southeast of the CO source is the source N1 in the foreground cluster at redshift . In the center panel, the positions of the two radio z p 0.18 sources H6 and H7 are denoted by arrows. The stronger optical and radio source H6 is a narrow-line Seyfert 1 galaxy at redshift (Lya). The CO (4– z p 3.349 3) emission from N2 850.4 remains largely unresolved. vary from source to source (Blain, Barnard, & Chapman 2003). In the following, all linear sizes, masses, and luminosities are corrected for the foreground lensing factors in Table 2. SMMJ 04431 0210 ( ) was originally found in the z p 2.51 SCUBA Lens Survey ( mJy; Smail et al. 1997, 2002). S p 7.2 850 It is located behind the cluster MS 0440 02. Smail z p 0.18 et al. (1999) identified the submillimeter source with the K p extremely red object (ERO) N4 about 3 northwest of an 19.4 edge-on cluster spiral galaxy N1 (N4: R K p 6.3; Frayer et al. 2003). Frayer et al. deduced a redshift of z p 2.5092 from Ha/[N ii]/[O iii] line emission. The rest-frame 0.0008 optical line ratios suggest that N4 is a composite starburst/ narrow-line active galactic nucleus (AGN). We adopt the foreground lens magnification of 4.4 deduced by Smail et al. (1999). Our BCD configuration data show a strong CO (3–2) line centered at , 17 ( 17) km s 1 redz p 2.5094 0.0002 ward of the nominal redshift of the Ha line. The line width is FWHM p km s , somewhat smaller than that of 350 60 Ha (520 km s ). The integrated CO line flux corresponds to a total gas mass of M,. Most of the CO line emission 9 8 # 10 comes from within 1 . The CO emission centroid is 1 .1 southwest of the near-IR position of N4, as determined by a new astrometric solution of the near-IR/radio astrometry that we obtained by comparing USNO stars with radio sources in the field [N4: R.A. p 044307 .25, decl. p 02 10 24 .4 (J2000.0); the uncertainty is 0 .5]. This new position of N4 is 2 .2 east and 0 .6 south of the position reported by Smail et al. (1999). The 1.3 mm continuum data show a marginally significant detection (1.1 mJy, 3.7 j) near the position of N4. We set a limit to the CO (7–6) emission of ≤0.8 Jy km s 1 (2 j). For comparison, the Ha emission exhibits a velocity gradient of ≥400 km s 1 over about 1 and along the slit at a position angle (P.A.) of 14 , that is, at about 50 relative to the direction of the extended CO emission. The centroid of the Ha emission is on N4. It thus appears that the rest-frame submillimeter and optical observations sample a similar region (size ∼1 ) but with some differences in the spatial structure in the two wavelength ranges. SMMJ 04431 0210 has by far the lowest intrinsic IR luminosity ( L,) and gas/dynamic 12 3 # 10 mass of our three galaxies. We deduce an upper limit to the dynamical mass of (M,) for a source diameter 9 2 4.5 # 10 sin i of ≤1 . In terms of luminosity, gas, and dynamical mass, SMMJ 04431 0210 thus resembles local ULIRGs. SMMJ 09431 4700 ( ) was first identified by Cowie z p 3.35 No. 2, 2003 OBSERVATIONS OF POWERFUL CO EMISSION L115