25 research outputs found
Resolving the Interstellar Medium at the Peak of Cosmic Star Formation
The interstellar medium feeds both the formation of stars and the growth of
black holes, making it a key ingredient in the evolution of galaxies. With the
advent of the Atacama Large Millimeter/ submillimeter Array (ALMA), we can now
probe the interstellar medium within high-redshift galaxies in increasingly
exquisite detail. Our recent ALMA observations map the molecular gas and dust
continuum emission in sub-millimetre-selected galaxies on 1-5 kpc scales,
revealing significant differences in how the gas, dust continuum, and existing
stellar emission are distributed within the galaxies. This study demonstrates
the power of ALMA to shed new light on the structure and kinematics of the
interstellar medium in the early Universe, suggesting that the interpretation
of such observations is more complex than typically assumed.Comment: 4 pages, 5 figures, published on The Messenger, No. 173 (September
2018
How does the radio enhancement of broad absorption line quasars relate to colour and accretion rate?
The origin of radio emission in different populations of radio-quiet quasars is relatively unknown, but recent work has uncovered various drivers of increased radio-detection fraction. In this work, we pull together three known factors: optical colour (g - i), C IV distance (a proxy for L/LEdd), and whether or not the quasar contains broad absorption lines (BALQSOs) which signify an outflow. We use SDSS (Sloan Digital Sky Survey) DR14 spectra along with the LOFAR Two Metre Sky Survey Data Release 2 and find that each of these properties have an independent effect. BALQSOs are marginally more likely to be radio-detected than non-BALQSOs at similar colours and L/LEdd, moderate reddening significantly increases the radio-detection fraction and the radio detection increases with L/LEdd above a threshold for all populations. We test a widely used simple model for radio wind shock emission and calculate energetic efficiencies that would be required to reproduce the observed radio properties. We discuss interpretations of these results concerning radio-quiet quasars more generally. We suggest that radio emission in BALQSOs is connected to a different physical origin than the general quasar population since they show different radio properties independent of colour and C IV distance
LOFAR/H-ATLAS: The low-frequency radio luminosity - star-formation rate relation
Radio emission is a key indicator of star-formation activity in galaxies, but
the radio luminosity-star formation relation has to date been studied almost
exclusively at frequencies of 1.4 GHz or above. At lower radio frequencies the
effects of thermal radio emission are greatly reduced, and so we would expect
the radio emission observed to be completely dominated by synchrotron radiation
from supernova-generated cosmic rays. As part of the LOFAR Surveys Key Science
project, the Herschel-ATLAS NGP field has been surveyed with LOFAR at an
effective frequency of 150 MHz. We select a sample from the MPA-JHU catalogue
of SDSS galaxies in this area: the combination of Herschel, optical and
mid-infrared data enable us to derive star-formation rates (SFRs) for our
sources using spectral energy distribution fitting, allowing a detailed study
of the low-frequency radio luminosity--star-formation relation in the nearby
Universe. For those objects selected as star-forming galaxies (SFGs) using
optical emission line diagnostics, we find a tight relationship between the 150
MHz radio luminosity () and SFR. Interestingly, we find that a single
power-law relationship between and SFR is not a good description of
all SFGs: a broken power law model provides a better fit. This may indicate an
additional mechanism for the generation of radio-emitting cosmic rays. Also, at
given SFR, the radio luminosity depends on the stellar mass of the galaxy.
Objects which were not classified as SFGs have higher 150-MHz radio luminosity
than would be expected given their SFR, implying an important role for
low-level active galactic nucleus activity.Comment: 21 pages, 12 figures, accepted for publication in MNRA
A panchromatic view of infrared quasars: excess star formation and radio emission in the most heavily obscured systems
To understand the active galactic nuclei (AGNs) phenomenon and their impact on the evolution of galaxies, a complete AGN
census is required; however, finding heavily obscured AGNs is observationally challenging. Here we use the deep and extensive
multiwavelength data in the COSMOS field to select a complete sample of 578 infrared (IR) quasars (LAGN,IR > 1045 erg s−1)
at z < 3, with minimal obscuration bias, using detailed UV-to-far-IR spectral energy distribution (SED) fitting. We complement
our SED constraints with X-ray and radio observations to further investigate the properties of the sample. Overall, 322 of the
IR quasars are detected by Chandra and have individual X-ray spectral constraints. From a combination of X-ray stacking
and L2−10 kev – L6 μm analyses, we show that the majority of the X-ray faint and undetected quasars are heavily obscured
(many are likely Compton thick), highlighting the effectiveness of the mid-IR band to find obscured AGNs. We find that
355 (≈61 per cent) IR quasars are obscured (NH > 1022 cm−2) and identify differences in the average properties between the
obscured and unobscured quasars: (1) obscured quasars have star formation rates ≈3 times higher than unobscured systems
for no significant difference in stellar mass and (2) obscured quasars have stronger radio emission than unobscured systems,
with a radio-loudness parameter ≈ 0.2 dex higher. These results are inconsistent with a simple orientation model but in general
agreement with either extreme host-galaxy obscuration towards the obscured quasars or a scenario where obscured quasars are
an early phase in the evolution of quasars
Obscuration beyond the nucleus: infrared quasars can be buried in extreme compact starbursts
In the standard quasar model, the accretion disk obscuration is due to the
canonical dusty torus. Here, we argue that a substantial part of the quasar
obscuration can come from the interstellar medium (ISM) when the quasars are
embedded in compact starbursts. We use an obscuration-unbiased sample of 578
infrared (IR) quasars at and archival ALMA submillimeter host
galaxy sizes to investigate the ISM contribution to the quasar obscuration. We
calculate SFR and ISM column densities for the IR quasars and a control sample
of submillimeter galaxies (SMGs) not hosting quasar activity and show that: (1)
the quasar obscured fraction is constant up to , and then increases towards higher SFR, suggesting that the ISM
obscuration plays a significant role in starburst host galaxies, and (2) at
, the SMGs and IR quasars have
similarly compact submillimeter sizes () and,
consequently, the ISM can heavily obscure the quasar, even reaching
Compton-thick () levels in extreme cases.
Based on our results, we infer that of the IR quasars with
are obscured solely by the ISM.Comment: Accepted for publication in MNRAS Letter
Resolving the ISM at the Peak of Cosmic Star Formation with ALMA: The Distribution of CO and Dust Continuum in z ∼ 2.5 Submillimeter Galaxies
We use Atacama Large Millimeter Array (ALMA) observations of four submillimeter galaxies (SMGs) at z ~ 2–3 to investigate the spatially resolved properties of the interstellar medium (ISM) at scales of 1–5 kpc (0farcs1–0farcs6). The velocity fields of our sources, traced by the 12CO(J = 3–2) emission, are consistent with disk rotation to the first order, implying average dynamical masses of ~3 × 1011 within two half-light radii. Through a Bayesian approach we investigate the uncertainties inherent to dynamically constraining total gas masses. We explore the covariance between the stellar mass-to-light ratio and CO-to-H2 conversion factor, α CO, finding values of for dark matter fractions of 15%. We show that the resolved spatial distribution of the gas and dust continuum can be uncorrelated to the stellar emission, challenging energy balance assumptions in global SED fitting. Through a stacking analysis of the resolved radial profiles of the CO(3–2), stellar, and dust continuum emission in SMG samples, we find that the cool molecular gas emission in these sources (radii ~5–14 kpc) is clearly more extended than the rest-frame ~250 μm dust continuum by a factor >2. We propose that assuming a constant dust-to-gas ratio, this apparent difference in sizes can be explained by temperature and optical depth gradients alone. Our results suggest that caution must be exercised when extrapolating morphological properties of dust continuum observations to conclusions about the molecular gas phase of the interstellar medium (ISM)
ALMA Reveals Potential Evidence for Spiral Arms, Bars, and Rings in High-redshift Submillimeter Galaxies
We present subkiloparsec-scale mapping of the 870 μm ALMA continuum emission in six luminous
(LIR ∼ 5 × 1012 Le) submillimeter galaxies (SMGs) from the ALESS survey of the Extended Chandra Deep
Field South. Our high-fidelity 0 07-resolution imaging (∼500 pc) reveals robust evidence for structures with
deconvolved sizes of 0.5–1 kpc embedded within (dominant) exponential dust disks. The large-scale
morphologies of the structures within some of the galaxies show clear curvature and/or clump-like structures
bracketing elongated nuclear emission, suggestive of bars, star-forming rings, and spiral arms. In this
interpretation, the ratio of the “ring” and “bar” radii (1.9 ± 0.3) agrees with that measured for such features in
local galaxies. These potential spiral/ring/bar structures would be consistent with the idea of tidal disturbances,
with their detailed properties implying flat inner rotation curves and Toomre-unstable disks (Q < 1). The inferred
one-dimensional velocity dispersions (σr 70–160 km s−1
) are marginally consistent with the limits implied if the
sizes of the largest structures are comparable to the Jeans length. We create maps of the star formation rate density
(ΣSFR) on ∼500 pc scales and show that the SMGs are able to sustain a given (galaxy-averaged) ΣSFR over much
larger physical scales than local (ultra)luminous infrared galaxies. However, on 500 pc scales, they do not exceed
the Eddington limit set by radiation pressure on dust. If confirmed by kinematics, the potential presence of
nonaxisymmetric structures would provide a means for net angular momentum loss and efficient star formation,
helping to explain the very high star formation rates measured in SMGs.We also thank Sharon
Meidt, Arjen van der Wel, Aaron Evans, Francoise Combes,
Rob Ivison, and Karin Sandstrom for useful discussions and
advice. J.H. and M.R. acknowledge support of the VIDI
research program with project No. 639.042.611, which is
(partly) financed by the Netherlands Organization for Scientific
Research (NWO). I.R.S. acknowledges support from STFC
(ST/P000541/1) and the ERC Advanced Grant DUSTYGAL
(321334). E.d.C. gratefully acknowledges the Australian
Research Council for funding support as the recipient of a
Future Fellowship (FT150100079). H.D. acknowledges financial support from the Spanish Ministry of Economy and
Competitiveness (MINECO) under the 2014 Ramón y Cajal
program MINECO RYC-2014-15686. J.L.W. acknowledges
support from an STFC Ernest Rutherford Fellowship (ST/
P004784/1 and ST/P004784/2). This paper makes use of the
following ALMA data: ADS/JAO.ALMA#2016.1.00048.S
and ADS/JAO.ALMA #2012.1.00307.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and
NINS (Japan), together with NRC (Canada), NSC and ASIAA
(Taiwan), and KASI (Republic of Korea), in cooperation with
the Republic of Chile. The Joint ALMA Observatory is
operated by ESO, AUI/NRAO, and NAOJ
A Spatially Resolved Study of Cold Dust, Molecular Gas, H ii Regions, and Stars in the z = 2.12 Submillimeter Galaxy ALESS67.1
We present detailed studies of a z = 2.12 submillimeter galaxy, ALESS67.1, using sub-arcsecond resolution ALMA, adaptive optics-aided VLT/SINFONI, and Hubble Space Telescope (HST)/CANDELS data to investigate the kinematics and spatial distributions of dust emission (870 μm continuum), 12CO(J = 3–2), strong optical emission lines, and visible stars. Dynamical modeling of the optical emission lines suggests that ALESS67.1 is not a pure rotating disk but a merger, consistent with the apparent tidal features revealed in the HST imaging. Our sub-arcsecond resolution data set allows us to measure half-light radii for all the tracers, and we find a factor of 4–6 smaller sizes in dust continuum compared to all the other tracers, including 12CO; also, ultraviolet (UV) and Hα emission are significantly offset from the dust continuum. The spatial mismatch between the UV continuum and the cold dust and gas reservoir supports the explanation that geometrical effects are responsible for the offset of the dusty galaxy on the IRX–β diagram. Using a dynamical method we derive an , consistent with other submillimeter galaxies (SMGs) that also have resolved CO and dust measurements. Assuming a single value we also derive resolved gas and star formation rate surface densities, and find that the core region of the galaxy ( kpc) follows the trend of mergers on the Schmidt–Kennicutt relationship, whereas the outskirts ( kpc) lie on the locus of normal star-forming galaxies, suggesting different star formation efficiencies within one galaxy. Our results caution against using single size or morphology for different tracers of the star formation activity and gas content of galaxies, and therefore argue the need to use spatially resolved, multi-wavelength observations to interpret the properties of SMGs, and perhaps even for galaxies in general
Cosmic evolution of low-excitation radio galaxies in the LOFAR two-meter sky survey deep fields
Abstract Feedback from low-excitation radio galaxies (LERGs) plays a key role in the lifecycle of massive galaxies in the local Universe; their evolution, and the impact of these active galactic nuclei on early galaxy evolution, however, remain poorly understood. We use a sample of 10 481 LERGs from the first data release of the LOFAR Two-meter Sky Survey Deep Fields, covering ∼ 25 deg2, to present the first measurement of the evolution of the radio luminosity function (LF) of LERGs out to z ∼ 2.5; this shows relatively mild evolution. We split the LERGs into those hosted by quiescent and star-forming galaxies, finding a new dominant population of LERGs hosted by star-forming galaxies at high redshifts. The incidence of LERGs in quiescent galaxies shows a steep dependence on stellar-mass out to z ∼ 1.5, consistent with local Universe measurements of accretion occurring from cooling of hot gas haloes. The quiescent-LERGs dominate the LFs at z &lt; 1, showing a strong decline in space density with redshift, tracing that of the available host galaxies, while there is an increase in the characteristic luminosity. The star-forming LERG LF increases with redshift, such that this population dominates the space densities at most radio-luminosities by z ∼ 1. The incidence of LERGs in star-forming galaxies shows a much weaker stellar-mass dependence, and increases with redshift, suggesting a different fuelling mechanism compared to their quiescent counterparts, potentially associated with the cold gas supply present in the star-forming galaxies