Submillimeter Galaxies at z ~ 2: Evidence for Major Mergers and Constraints on Lifetimes, IMF, and CO-H2 Conversion Factor

By L. Tacconi, R. Genzel, I. Smail, R. Neri, S. Chapman, R. Ivison, A. Blain, P. Cox, A. Omont, F. Bertoldi and 10 more. 2008

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We report subarcsecond resolution IRAM PdBI millimeter CO interferometry of four z ∼ 2 submillimeter galaxies (SMGs), and sensitive CO(3-2) flux limits toward three z ∼ 2 UV/optically selected star-forming galaxies. The new data reveal for the first time spatially resolved CO gas kinematics in the observed SMGs. Two of the SMGs show double or multiple morphologies, with complex, disturbed gas motions. The other two SMGs exhibit CO velocity gradients of ~500 km s−1 across ≤0.2″ (1.6 kpc) diameter regions, suggesting that the star-forming gas is in compact, rotating disks. Our data provide compelling evidence that these SMGs represent extreme, short-lived “maximum” star-forming events in highly dissipative mergers of gas-rich galaxies. The resulting high-mass surface and volume densities of SMGs are similar to those of compact quiescent galaxies in the same redshift range and much higher than those in local spheroids. From the ratio of the comoving volume densities of SMGs and quiescent galaxies in the same mass and redshift ranges, and from the comparison of gas exhaustion timescales and stellar ages, we estimate that the SMG phase duration is about 100 Myr. Our analysis of SMGs and optically/UV selected high-redshift star-forming galaxies supports a “universal” Chabrier IMF as being valid over the star-forming history of these galaxies. We find that the 12CO luminosity to total gas mass conversion factors at z ∼ 2-3 are probably similar to those assumed at z ∼ 0. The implied gas fractions in our sample galaxies range from 20% to 50%.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

• THE STELLAR MASS CONTENT OF SUBMILLIMETER-SELECTED GALAXIES (2010, background): “Moreover, high-resolution millimeter observations by Tacconi et al. (2006, 2008) and Bothwell et al. (2010) suggest that the CO-emitting region (R ∼ 1 − 2kpc) in SMGs is frequently smaller than the overall stellar mass and star formation distribution (Chapman et al. 2004; Biggs & Ivison 2008;…”
• The CO-H2 Conversion Factor in Disc Galaxies and Mergers (2011, background): “Empirically, there is a tentative trend thatXCO decreases with increasing galaxy surface density (Tacconi et al. 2008).”
• EVIDENCE FOR WIDE-SPREAD ACTIVE GALACTIC NUCLEUS-DRIVEN OUTFLOWS IN THE MOST MASSIVE z ∼ 1–2 STAR-FORMING GALAXIES (2014, background): “…very dense “cores” may be present among our galaxies and cause FWHMs up to ∼100 km s−1, these are very rare and unlikely to dominate our sample (e.g., Tacconi et al. 2008; Nelson et al. 2014). mass bins (leaving out those stacks with strong detected broad components), and then analyzing the…”
• AN ALMA SURVEY OF SUBMILLIMETER GALAXIES IN THE EXTENDED CHANDRA DEEP FIELD SOUTH: SOURCE CATALOG AND MULTIPLICITY (2013, background): “J (J > 2) CO transitions, which typically report sizes in the range 4–6 kpc (FWHM; Tacconi et al. 2006, 2008; Bothwell et al. 2010; Engel et al. 2010; Bothwell et al. 2012).”
• A MASSIVE MOLECULAR GAS RESERVOIR IN THE z = 5.3 SUBMILLIMETER GALAXY AzTEC-3 (2010, result): “This is comparable to the values found for z&2 SMGs (typically 2×109M⊙ kpc −2; Tacconi et al. 2006, 2008), and about half of the peak gas density of the z=6.42 quasar J1148+5251 (3.5×109M⊙ kpc −2; Riechers et al. 2009).”
• VALIDATION OF THE EQUILIBRIUM MODEL FOR GALAXY EVOLUTION TO z ∼ 3 THROUGH MOLECULAR GAS AND DUST OBSERVATIONS OF LENSED STAR-FORMING GALAXIES (2013, background): “Under such conditions, αCO should scale as ρ 0.5/T (Tacconi et al. 2008; Bolatto et al. 2013).”
• HYDRODYNAMICS OF HIGH-REDSHIFT GALAXY COLLISIONS: FROM GAS-RICH DISKS TO DISPERSION-DOMINATED MERGERS AND COMPACT SPHEROIDS (2010, background): “SMGs are among the most actively star-forming objects at high redshift, and hence generally proposed to be the most actively starbursting phases of major mergers (Tacconi et al. 2008).”
• High molecular gas fractions in normal massive star-forming galaxies in the young Universe (2010, unknown): “In z~0 dwarf galaxies with sub-solar metallicity (probably applicable to most of the lower mass systems in Table 1), there is evidence for additional, extended and CO-faint molecular gas outside of the bright star forming clouds, so that the galaxy-wide CO to H2 conversion factor in these systems is”
• On the frequency, intensity, and duration of starburst episodes triggered by galaxy interactions and mergers (2008, background): “Strong starbursts can occur in some major mergers, and this is consistent with submillimiter galaxies being mostly major mergers (e.g. Tacconi et al. 2008), but, on average, classical merger-driven gas inflows do not seem to be an efficient process, and other mechanisms could 11 The SFR being…”
• Cosmic evolution of submillimeter galaxies and their contribution to stellar mass assembly (2009, background): “Such extreme SFRs likely result from major mergers (e.g. Chapman et al., 2004; Swinbank et al., 2004; Greve et al., 2005; Tacconi et al., 2006, 2008; Younger et al., 2007, 2008; Berciano Alba et al., 2010; Narayanan et al., 2009, 2010) and cannot be sustained for a long period (after a few hundred…”
• PHIBSS: MOLECULAR GAS CONTENT AND SCALING RELATIONS IN z ∼ 1–3 MASSIVE, MAIN-SEQUENCE STAR-FORMING GALAXIES (2012, background): “Such a trend is plausible if larger surface densities correlate with high temperature (see the discussion in Appendix A and Figure 10 of Tacconi et al. 2008, and in Kennicutt & Evans 2012).”
• MAXIMALLY STAR-FORMING GALACTIC DISKS. I. STARBURST REGULATION VIA FEEDBACK-DRIVEN TURBULENCE (2011, background): “Empirically, a systematic trend for α to decrease with increasing Σ is already evident (e.g. Tacconi et al. 2008).”
• IMAGING THE MOLECULAR GAS IN A SUBMILLIMETER GALAXY AT z = 4.05: COLD MODE ACCRETION OR A MAJOR MERGER? (2010, background): “Tacconi et al. (2006; 2008) argue, based on CO imaging of a sample of z ∼ 2 SMGs, that SMGs are predominantly nuclear starbursts, with median sizes < 0.5” (< 4kpc), ’representing extreme, short-lived, maximum star forming events in highly dissipative mergers of gas rich galaxies.’”
• Cold streams in early massive hot haloes as the main mode of galaxy formation (2008, background): “The sBzK/BX/BM galaxies are marked SFG.11 The SMGs respectively brighter and fainter than 5 mJy are marked accordingly.11,10 We see that the overall gas inflow rate is sufficient for the observed SFR, but the small margin implies that the SFR must closely follow the rate of gas supply.”
• Tracing the molecular gas in distant submillimetre galaxies via CO(1-0) imaging with the EVLA (2010, methodology): “This also suggests potential spatial biases in the high-J 12CO emission – which exhibit typical half-light radii of 0.8–2.8 kpc for SMGs (Tacconi et al. 2008) – means they may not trace the true dimensions (or kinematics) of the total molecular gas distribution.”
• COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies – I. Relations between H2, H i, stellar content and structural properties (2011, background): “The CO 1 → 0 transition traces well the entire molecular gas contents of the galaxies at n(H2) > 102 cm−3 (see e.g. the appendix in Tacconi et al. 2008).”
• The Dramatic Size Evolution of Elliptical Galaxies and the Quasar Feedback (2008, background): “Similar star to gas ratios are found in the central regions of sub-mm bright galaxies (Tacconi et al. 2008).”
• MERGERS AND BULGE FORMATION IN ΛCDM: WHICH MERGERS MATTER? (2009, background): “…1985; Sanders & Mirabel 1996) to bright sub-millimeter galaxies at high redshifts (Alexander et al. 2005; Younger et al. 2008b; Shapiro et al. 2008; Tacconi et al. 2008), and properties ranging from their observed kinematics, structural correlations, and clustering link these populations to…”
• Dusty Star Forming Galaxies at High Redshift (2014, background): “Tacconi et al. (2008) simultaneously modeled the CO–H2 conversion factor, stellar masses and IMFs of z ∼ 2 SMGs, and found that the IMFmay have an excess of high-mass stars, with a best fit mass to light (M/L) ratio roughly half that off a standard Kroupa IMF.”