Galaxy formation and evolution

Abstract:

We present Atacama Large Millimeter/Submillimeter Array observations of the rest-frame far-infrared (FIR) dust continuum emission of six bright Lyman-break galaxies (LBGs) at z ≃ 7. One LBG is detected (5.2σ at peak emission), whilst the others remain individually undetected at the 3σ level. The average FIR luminosity of the sample is found to be LFIR≃2×1011L⊙⁠, corresponding to an obscured star formation rate (SFR) that is comparable to that inferred from the unobscured UV emission. In comparison to the infrared excess (IRX=LFIR/LUV⁠)–β relation, our results are consistent with a Calzetti-like attenuation law (assuming a dust temperature of T = 40–50 K). We find a physical offset of 3kpc between the dust continuum emission and the rest-frame UV light probed by Hubble Space Telescope imaging for galaxy ID65666 at z=7.17+0.09−0.06⁠. The offset is suggestive of an inhomogeneous dust distribution, where 75 per cent of the total star formation activity (SFR≃70M⊙/yr⁠) of the galaxy is completely obscured. Our results provide direct evidence that dust obscuration plays a key role in shaping the bright end of the observed rest-frame UV luminosity function at z ≃ 7, in agreement with cosmological galaxy formation simulations. The existence of a heavily obscured component of galaxy ID65666 indicates that dusty star-forming regions, or even entire galaxies, that are ‘UV dark’ are significant even in the z ≃ 7 galaxy population.

Abstract:

We present new CO(2–1) observations of three low-z (d ∼350 Mpc) ultra-luminous infrared galaxy (ULIRG) systems (six nuclei) observed with the Atacama large millimeter/submillimeter array (ALMA) at high spatial resolution (∼500 pc). We detect massive cold molecular gas outflows in five out of six nuclei (Mout ∼ (0.3 − 5) × 108 Mo). These outflows are spatially resolved with deprojected effective radii between 250 pc and 1 kpc although high-velocity molecular gas is detected up to Rmax ∼ 0.5 − 1.8 kpc (1 − 6 kpc deprojected). The mass outflow rates are 12 − 400 Mo yr−1 and the inclination corrected average velocity of the outflowing gas is 350 − 550 km s−1 (vmax = 500 − 900 km s−1 ). The origin of these outflows can be explained by the strong nuclear starbursts although the contribution of an obscured active galactic nucleus cannot be completely ruled out. The position angle (PA) of the outflowing gas along the kinematic minor axis of the nuclear molecular disk suggests that the outflow axis is perpendicular to the disk for three of these outflows. Only in one case is the outflow PA clearly not along the kinematic minor axis, which might indicate a different outflow geometry. The outflow depletion times are 15 − 80 Myr. These are comparable to, although slightly shorter than, the starformation (SF) depletion times (30 − 80 Myr). However, we estimate that only 15 − 30% of the outflowing molecular gas will escape the gravitational potential of the nucleus. The majority of the outflowing gas will return to the disk after 5 − 10 Myr and become available to form new stars. Therefore, these outflows will not likely completely quench the nuclear starbursts. These star-forming powered molecular outflows would be consistent with being driven by radiation pressure from young stars (i.e., momentum-driven) only if the coupling between radiation and dust increases with increasing SF rates. This can be achieved if the dust optical depth is higher in objects with higher SF. This is the case in at least one of the studied objects. Alternatively, if the outflows are mainly driven by supernovae (SNe), the coupling efficiency between the interstellar medium and SNe must increase with increasing SF levels. The relatively small sizes (<1 kpc) and dynamical times (<3 Myr) of the cold molecular outflows suggests that molecular gas cannot survive longer in the outflow environment or that it cannot form efficiently beyond these distances or times. In addition, the ionized and hot molecular phases have been detected for several of these outflows, so this suggests that outflowing gas can experience phase changes and indicates that the outflowing gas is intrinsically multiphase, likely sharing similar kinematics, but different mass and, therefore, different energy and momentum contributions.

Abstract:

We present the results from our Atacama Large Millimeter/submillimeter Array (ALMA) imaging observations of the CO(7-6), [C i] 370 μm (hereafter [C i]), and [N ii] 205 μm (hereafter [N ii]) lines and their underlying continuum emission of BRI 1335-0417, an infrared bright quasar at z = 4.407. At the achieved resolutions of ∼1.″1 to 1.″2 (or 7.5-8.2 kpc), the continuum at 205 and 372 μm (rest frame), the CO(7-6), and the [C i] emissions are at best barely resolved whereas the [N ii] emission is well resolved with a beam-deconvolved major axis of 1.″3(±0.″3) or 9(±2) kpc. As a warm dense gas tracer, the CO(7-6) emission shows a more compact spatial distribution and a significantly higher peak velocity dispersion than the other two lines that probe lower density gas, a picture favoring a merger-Triggered star formation (SF) scenario over an orderly rotating SF disk. The CO(7-6) data also indicate a possible QSO-driven gas outflow that reaches a maximum line-of-sight velocity of 500-600 km s-1. The far-infrared (FIR) dust temperature (Tdust) of 41.5 K from a graybody fit to the continuum agrees well with the average Tdustinferred from various line luminosity ratios. The resulting LCO(7-6)/LFIRluminosity ratio is consistent with that of local luminous infrared galaxies powered predominantly by SF. The LCO(7-6)-inferred SF rate is 5.1(±1.5) × 103Moyr-1. The system has an effective star-forming region of kpc in diameter and a molecular gas reservoir of ∼5 × 1011Mo.