Strongly star-forming rotating disks in a complex merging system atz= 4.7 as revealed by ALMA

Abstract

We performed a kinematical analysis of the [CII] line emission of the BR 1202-0725 system at z ∼ 4.7 using ALMA science verification observations. The most prominent sources of this system are a quasar (QSO) and a submillimeter galaxy (SMG), separated by a projected distance of about ∼24 kpc and characterized by very high star formation rates, higher than ∼1000 M yr −1 . However, the ALMA observations reveal that these galaxies apparently have undisturbed rotating disks, which is at variance with the commonly accepted scenario in which strong star formation activity is induced by a major merger. We also detected faint components which, after spectral deblending, were spatially resolved from the main QSO and SMG emissions. The relative velocities and positions of these components are compatible with orbital motions within the gravitational potentials generated by the QSO host galaxy and the SMG, suggesting that they are smaller galaxies in interaction or gas clouds in accretion flows of tidal streams. Moreover, we did not find any clear spectral evidence for outflows caused by active galactic nuclei or stellar feedback. This suggests that the high star formation rates might be induced by interactions or minor mergers with these companions, which do not affect the large-scale kinematics of the disks, however. Alternatively, the strong star formation may be fueled by the accretion of pristine gas from the host halo. Our kinematical analysis also indicates that the QSO and the SMG have similar dynamical masses, mostly in the form of molecular gas, and that the QSO host galaxy and the SMG are seen close to face-on with slightly different disk inclinations: the QSO host galaxy is seen almost face-on (i ∼ 15 • ), while the SMG is seen at higher inclinations (i ∼ 25 • ). Finally, the ratio between the black hole mass of the QSO, obtained from new X-shooter spectroscopy, and the dynamical mass of the host galaxy is similar to value found in very massive local galaxies, suggesting that the evolution of black hole galaxy relations is probably better studied with dynamical than with stellar host galaxy masses.

High-resolution C+imaging of HDF850.1 reveals a merging galaxy atz= 5.185

“…This means that large, settled-down, 10 kpc-scale galaxy disks cannot be in place much before z = 2. Our size measurements of the HDF 850.1 north and south objects are consistent with these ideas, as are the recent measurements of kpc-sized rotating molecular disks in BR1202-07-NW and SE at z = 4.7 (Salomé et al 2012;Carniani et al 2013). The infalling gas does have time to establish rotational support in kpc-scale disks by z = 5, but not in the order-of-magnitude larger disks that may be in place by z = 2.…”

Dusty star-forming galaxies at high redshift

“…Walter et al (2009) derived a source size of ∼ 1.5 kpc, and suggested that the galaxy was undergoing an Eddington-limited starburst event. Similar compact sizes were derived for two z ∼ 4 quasar host galaxies by Gallerani et al (2012) and Carniani et al (2013), who also detected companion galaxies potentially due to merge with the quasar host. Carniani et al (2013) and Wang et al (2013a) displayed the power of using ALMA to examine the dynamics of [CII]-emitting gas in quasar host galaxies, and showed that a number of galaxies in their sample potentially exhibited rotating gas disks.…”

“…Similar compact sizes were derived for two z ∼ 4 quasar host galaxies by Gallerani et al (2012) and Carniani et al (2013), who also detected companion galaxies potentially due to merge with the quasar host. Carniani et al (2013) and Wang et al (2013a) displayed the power of using ALMA to examine the dynamics of [CII]-emitting gas in quasar host galaxies, and showed that a number of galaxies in their sample potentially exhibited rotating gas disks. This all said, there have been a number of notable non-detections of [CII] at high-z that have called into question the reliability of [CII] as a gas tracer in high-z galaxies.…”

“…Luminous quasar host galaxies (e.g. Maiolino et al, 2005;Iono et al, 2006;Maiolino et al, 2009;Walter et al, 2009;Wagg et al, 2010;Stacey et al, 2010;Gallerani et al, 2012;Carniani et al, 2013), SMGs (e.g. Ivison et al, 2010d;Stacey et al, 2010;Cox et al, 2011;De Breuck et al, 2011;Swinbank et al, 2012;Wagg et al, 2012;George et al, 2013;Huynh et al, 2013) and normal star-forming galaxies (e.g.…”

The Dust and [C ii] Morphologies of Redshift ∼4.5 Sub-millimeter Galaxies at ∼200 pc Resolution: The Absence of Large Clumps in the Interstellar Medium at High-redshift

“…It is therefore a good tracer of the gas dynamics in high-redshift galaxies (e.g., Carniani et al 2013;Capak et al 2015). Only ALESS 73.1 and UDS 47.0 have lowresolution observations from Cycle 0/1 deep enough to allow us to search for possible velocity gradients.…”