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Register a new userHigh resolution observations of a starburst at z=0.223: resolved CO(1-0)
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F. Combes
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We present the results of CO(1-0) emission mapping with the IRAM interferometer, at sim 1 arcsec, resolution, of the z=0.223 ultra-luminous starburst IRAS 11582+3020. This galaxy was selected from an IRAM-30m survey of 30 galaxies at moderate redshift (z sim 0.2-0.6) to explore galaxy evolution and in particular the star formation efficiency, in the redshift range filling the gap between local and very high-z objects. The CO emission is kinematically resolved, and about 50% of the total emission found in the 27 arcsec (97 kpc) single dish beam is not recovered by the interferometer. This indicates that some extended emission may be present on large scales (typically 7-15 arcsec). The FIR-to-CO luminosity ratio follows the trend between local and high-z ultra-luminous starbursts.
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Using the Australia Telescope Compact Array we have detected CO(1-0) and CO(5-4) from TNJ0924-2201 at z=5.2, the most distant radio galaxy known to date. This is the second highest redshift detection of CO published so far. The CO(1-0) is 250-400 km/sec wide with a peak flux density of 520 +- 115 microJy/beam whilst the CO(5-4) line emission is 200-300 km/sec wide with a peak flux density of 7.8 +- 2.7 mJy/beam. Both transitions are spatially unresolved but there is marginal evidence for spatial offsets between the CO and the host galaxy; the CO(1-0) is located 28 +- 11 kpc (4.5 +- 1.7 arcsec) north of the radio galaxy whilst the CO(5-4) is located 18 +- 8 kpc (2.8 +- 1.2 arcsec) south of the radio galaxy. Higher spatial resolution observations are required to determine the reality of these offsets. Our result is the second detection of CO in a high redshift galaxy without pre-selection based on a massive dust content.
High-redshift, dust-obscured galaxies -- selected to be luminous in the radio but relatively faint at 850um -- appear to represent a different population from the ultra-luminous submillimeter- (submm-) bright population. They may be star-forming galaxies with hotter dust temperatures or they may have lower far-infrared luminosities and larger contributions from obscured active galactic nuclei (AGN). Here we present observations of three z~2 examples of this population, which we term submm-faint radio galaxies (SFRGs) in CO(3-2) using the IRAM Plateau de Bure Interferometer to study their gas and dynamical properties. We estimate the molecular gas mass in each of the three SFRGs (8.3x10^{9} M_odot, <5.6x10^{9} M_odot and 15.4x10^{9} M_odot, respectively) and, in the case of RG163655, a dynamical mass by measurement of the width of the CO(3-2) line (8x10^{10} csc^2i M_odot). While these gas masses are substantial, on average they are 4x lower than submm-selected galaxies (SMGs). Radio-inferred star formation rates (<SFR_radio>=970 M_odotyr) suggest much higher star-formation efficiencies than are found for SMGs, and shorter gas depletion time scales (~11 Myr), much shorter than the time required to form their current stellar masses (~160 Myr; ~10^{11} M_odot). By contrast, SFRs may be overestimated by factors of a few, bringing the efficiencies in line with those typically measured for other ultraluminous star-forming galaxies and suggesting SFRGs are more like ultraviolet- (UV-)selected star-forming galaxies with enhanced radio emission. A tentative detection of rga at 350um suggests hotter dust temperatures -- and thus similar gas-to-dust mass fractions -- as the SMGs. We conclude that SFRGs radio luminosities are larger than would naturally scale from local ULIRGs given their gas masses or gas fractions.
We investigate the prevalence of galactic-scale outflows in post-starburst (PSB) galaxies at high redshift ($1 < z < 1.4$), using the deep optical spectra available in the UKIDSS Ultra Deep Survey (UDS). We use a sample of $sim40$ spectroscopically confirmed PSBs, recently identified in the UDS field, and perform a stacking analysis in order to analyse the structure of strong interstellar absorption features such as Mg ii ($lambda2800$ Ang.). We find that for massive ($M_* > 10^{10}rm,M_{odot}$) PSBs at $z > 1$, there is clear evidence for a strong blue-shifted component to the Mg ii absorption feature, indicative of high-velocity outflows ($v_{rm out}sim1150pm160rm,km,s^{-1}$) in the interstellar medium. We conclude that such outflows are typical in massive PSBs at this epoch, and potentially represent the residual signature of a feedback process that quenched these galaxies. Using full spectral fitting, we also obtain a typical stellar velocity dispersion $sigma_*$ for these PSBs of $sim200rm,km,s^{-1}$, which confirms they are intrinsically massive in nature (dynamical mass $M_{rm d}sim10^{11}rm,M_{odot}$). Given that these high-$z$ PSBs are also exceptionally compact ($r_{rm e}sim1$--$2rm,kpc$) and spheroidal (Sersic index $nsim3$), we propose that the outflowing winds may have been launched during a recent compaction event (e.g. major merger or disc collapse) that triggered either a centralised starburst or active galactic nuclei (AGN) activity. Finally, we find no evidence for AGN signatures in the optical spectra of these PSBs, suggesting they were either quenched by stellar feedback from the starburst itself, or that if AGN feedback is responsible, the AGN episode that triggered quenching does not linger into the post-starburst phase.
We present high spatial resolution (FWHM$sim$0.14) observations of the CO($8-7$) line in GDS-14876, a compact star-forming galaxy at $z=2.3$ with total stellar mass of $log(M_{star}/M_{odot})=10.9$. The spatially resolved velocity map of the inner $rlesssim1$~kpc reveals a continous velocity gradient consistent with the kinematics of a rotating disk with $v_{rm rot}(r=1rm kpc)=163pm5$ km s$^{-1}$ and $v_{rm rot}/sigmasim2.5$. The gas-to-stellar ratios estimated from CO($8-7$) and the dust continuum emission span a broad range, $f^{rm CO}_{rm gas}=M_{rm gas}/M_{star}=13-45%$ and $f^{rm cont}_{rm gas}=50-67%$, but are nonetheless consistent given the uncertainties in the conversion factors. The dynamical modeling yields a dynamical mass of$log(M_{rm dyn}/M_{odot})=10.58^{+0.5}_{-0.2}$ which is lower, but still consistent with the baryonic mass, $log$(M$_{rm bar}$= M$_{star}$ + M$^{rm CO}_{rm gas}$/M$_{odot}$)$=11.0$, if the smallest CO-based gas fraction is assumed. Despite a low, overall gas fraction, the small physical extent of the dense, star-forming gas probed by CO($8-7$), $sim3times$ smaller than the stellar size, implies a strong concentration that increases the gas fraction up to $f^{rm CO, 1rm kpc}_{rm gas}sim 85%$ in the central 1 kpc. Such a gas-rich center, coupled with a high star-formation rate, SFR$sim$ 500 M$_{odot}$ yr$^{-1}$, suggests that GDS-14876 is quickly assembling a dense stellar component (bulge) in a strong nuclear starburst. Assuming its gas reservoir is depleted without replenishment, GDS-14876 will quickly ($t_{rm depl}sim27$ Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.
We present CO observations of 78 spiral galaxies in local merger pairs. These galaxies representa subsample of a Ks-band selected sample consisting of 88 close major-merger pairs (HKPAIRs), 44 spiral-spiral (S+S) pairs and 44 spiral-elliptical (S+E) pairs, with separation $<20 h^{-1}$ kpc and mass ratio <2.5. For all objects, the star formation rate (SFR) and dust mass were derived from HERSCHEL PACS and SPIRE data, and the atomic gas mass, MHI, from the Green Bank Telescope HI observations. The complete data set allows us to study the relation between the gas (atomic and molecular) mass, dust mass and SFR in merger galaxies. We derive the molecular gas fraction (MH2/M*), molecular-to-atomic gas mass ratio (MH2/MHI), gas-to-dust mass ratio and SFE (=SFR/MH2) and study their dependences on pair type (S+S compared to S+E), stellar mass and the presence of morphological interaction signs. We find an overall moderate enhancements (~2x) in both molecular gas fraction (MH2/M*), and molecular-to-atomic gas ratio (MH2/MHI) for star-forming galaxies in major-merger pairs compared to non-interacting comparison samples, whereas no enhancement was found for the SFE nor for the total gas mass fraction (MHI+MH2)/M*. When divided into S+S and S+E, low mass and high mass, and with and without interaction signs, there is a small difference in SFE, moderate difference in MH2/M*, and strong differences in MH2/MHI between subsamples. For MH2/MHI, the difference between S+S and S+E subsamples is 0.69+-0.16 dex and between pairs with and without interaction signs is 0.53+-0.18 dex. Together, our results suggest (1) star formation enhancement in close major-merger pairs occurs mainly in S+S pairs after the first close encounter (indicated by interaction signs) because the HI gas is compressed into star-forming molecular gas by the tidal torque; (2) this effect is much weakened in the S+E pairs.
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