Ultra-deep radio surveys are an invaluable probe of dust-obscured star formation, but require a clear understanding of the relative contribution from radio active galactic nuclei (AGNs) to be used to ...their fullest potential. We study the composition of the Jy radio population detected in the Karl G. Jansky Very Large Array COSMOS-XS survey based on a sample of 1540 sources detected at 3 GHz over an area of ∼350 arcmin2. This ultra-deep survey consists of a single pointing in the well-studied COSMOS field at both 3 and 10 GHz and reaches rms sensitivities of 0.53 and 0.41 Jy beam−1, respectively. We find multiwavelength counterparts for 97% of radio sources, based on a combination of near-UV/optical to sub-millimeter data, and through a stacking analysis at optical/near-IR wavelengths we further show that the sources lacking such counterparts are likely to be high-redshift in nature (typical z ∼ 4−5). Utilizing the multiwavelength data over COSMOS, we identify AGNs through a variety of diagnostics and find these to make up 23.2 1.3% of our sample, with the remainder constituting uncontaminated star-forming galaxies. However, more than half of the AGNs exhibit radio emission consistent with originating from star formation, with only 8.8 0.8% of radio sources showing a clear excess in radio luminosity. At flux densities of ∼30 Jy at 3 GHz, the fraction of star formation-powered sources reaches ∼90%, and this fraction is consistent with unity at even lower flux densities. Overall, our findings imply that ultra-deep radio surveys such as COSMOS-XS constitute a highly effective means of obtaining clean samples of star formation-powered radio sources.
Active galactic nuclei: what’s in a name? Padovani, P.; Alexander, D. M.; Assef, R. J. ...
The Astronomy and astrophysics review,
08/2017, Letnik:
25, Številka:
1
Journal Article
Recenzirano
Active galactic nuclei (AGN) are energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies, and present unique observational signatures that cover the full ...electromagnetic spectrum over more than twenty orders of magnitude in frequency. The rich phenomenology of AGN has resulted in a large number of different “flavours” in the literature that now comprise a complex and confusing AGN “zoo”. It is increasingly clear that these classifications are only partially related to intrinsic differences between AGN and primarily reflect variations in a relatively small number of astrophysical parameters as well the method by which each class of AGN is selected. Taken together, observations in different electromagnetic bands as well as variations over time provide complementary windows on the physics of different sub-structures in the AGN. In this review, we present an overview of AGN multi-wavelength properties with the aim of painting their “big picture” through observations in each electromagnetic band from radio to
γ
-rays as well as AGN variability. We address what we can learn from each observational method, the impact of selection effects, the physics behind the emission at each wavelength, and the potential for future studies. To conclude, we use these observations to piece together the basic architecture of AGN, discuss our current understanding of unification models, and highlight some open questions that present opportunities for future observational and theoretical progress.
We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at z = 4.567, in order to better constrain the nature of the high-redshift end of the submillimeter-selected galaxy ...(SMG) population. We secured 12CO molecular line detections for the J = 2 →1 and J = 5 →4 transitions using the Karl G. Jansky Very Large Array (VLA) and the NOrthern Extended Millimeter Array (NOEMA) interferometer. The broad (FWHM ~ 750 km s−1) and tentative double-peaked profiles of the two 12CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk, as previously revealed from CII 158 μm line observations. Based on the 12CO(2 →1) emission line, we derived L′CO=(3.4±0.6)×1010 K km s−1 pc2 $L'_{\rm{CO}}=(3.4\pm0.6)\times10^{10}\textrm{\,K\,km\,s}^{-1}\textrm{\,pc}^{2}$L′CO=(3.4±0.6)×1010 K km s−1 pc2 , which yields a molecular gas mass of MH2(αCO/4.3)=(1.5±0.3)×1011 M⊙ $M_{\textrm{H}_2} (\alpha_{\textrm{CO}}/4.3)=(1.5\pm0.3)\times 10^{11} \, M_{\odot}$MH2(αCO/4.3)=(1.5±0.3)×1011 M⊙ and unveils a gas-rich system with μgas(αCO/4.3)≡MH2/M⋆=3.3±0.7 $\mu_{\textrm{gas}}(\alpha_{\textrm{CO}}/4.3)\equiv M_{\textrm{H}_2}/M_{\star}=3.3\pm0.7$μgas(αCO/4.3)≡MH2/M⋆=3.3±0.7 . The extreme star formation efficiency of AzTEC/C159, parametrized by the ratio LIR/L′CO=(216±80) L⊙ (K km s−1 pc2)−1 $L_{\rm{IR}}/L'_{\rm{CO}}=(216\pm80)\, {L}_{\odot}\textrm{\,(K\,km\,s}^{-1}\textrm{\,pc}^{2})^{-1}$LIR/L′CO=(216±80) L⊙ (K km s−1 pc2)−1 , is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies and SMGs. Likewise, the 12CO(5 →4)/CO(2 →1) line brightness temperature ratio of r52 = 0.55 ± 0.15 is consistent with high-excitation conditions as observed in SMGs. Based on mass budget considerations, we constrained the value for the L′CO $L'_{\text{CO}}$L′CO – H2 mass conversion factor in AzTEC/C159, that is, αCO=3.9−1.3+2.7 M⊙ K−1 km−1 s pc−2 $\alpha_{\text{CO}}=3.9^{+2.7}_{-1.3}{\,M}_{\odot}\textrm{\,K}^{-1}\textrm{\,km}^{-1}\textrm{\,s\,pc}^{-2}$αCO=3.9−1.3+2.7 M⊙ K−1 km−1 s pc−2 , which is consistent with a self-gravitating molecular gas distribution as observed in local star-forming disk galaxies. Cold gas streams from cosmological filaments might be fueling a gravitationally unstable gas-rich disk in AzTEC/C159, which breaks into giant clumps and forms stars as efficiently as in merger-driven systems and generates high gas excitation. These results support the evolutionary connection between AzTEC/C159-like systems and massive quiescent disk galaxies at z ~ 2.
The VLA-COSMOS Large Project is described and Its scientific objective is discussed. We present a catalog of similar to 3600 radio sources found in the 2 deg super(2) COSMOS field at 1.4 GHz. The ...observations in the VLA A and C configuration resulted in a resolution of 1.5" x 1.4" and a mean rms noise of similar to 10.5 (15) mu Jy beam super(-1) in the central 1 (2) deg super(2). Eighty radio sources are clearly extended consisting of multiple components, and most of them appear to be double-lobed radio galaxies. The astrometry of the catalog has been thoroughly tested, and the uncertainty in the relative and absolute astrometry are 130 and <55 mas, respectively.
Over the past decade, several works have used the ratio between total (rest 8−1000
μ
m) infrared and radio (rest 1.4 GHz) luminosity in star-forming galaxies (
q
IR
), often referred to as the ...infrared-radio correlation (IRRC), to calibrate the radio emission as a star formation rate (SFR) indicator. Previous studies constrained the evolution of
q
IR
with redshift, finding a mild but significant decline that is yet to be understood. Here, for the first time, we calibrate
q
IR
as a function of
both
stellar mass (
M
⋆
) and redshift, starting from an
M
⋆
-selected sample of > 400 000 star-forming galaxies in the COSMOS field, identified via (
NUV
−
r
)/(
r
−
J
) colours, at redshifts of 0.1 <
z
< 4.5. Within each (
M
⋆
,
z
) bin, we stacked the deepest available infrared/sub-mm and radio images. We fit the stacked IR spectral energy distributions with typical star-forming galaxy and IR-AGN templates. We then carefully removed the radio AGN candidates via a recursive approach. We find that the IRRC evolves primarily with
M
⋆
, with more massive galaxies displaying a systematically lower
q
IR
. A secondary, weaker dependence on redshift is also observed. The best-fit analytical expression is the following:
q
IR
(
M
⋆
,
z
) = (2.646 ± 0.024) × (1 +
z
)
( − 0.023 ± 0.008)
–(0.148 ± 0.013) × (log
M
⋆
/
M
⊙
− 10). Adding the UV dust-uncorrected contribution to the IR as a proxy for the total SFR would further steepen the
q
IR
dependence on
M
⋆
. We interpret the apparent redshift decline reported in previous works as due to low-
M
⋆
galaxies being progressively under-represented at high redshift, as a consequence of binning only in redshift and using either infrared or radio-detected samples. The lower IR/radio ratios seen in more massive galaxies are well described by their higher observed SFR surface densities. Our findings highlight the fact that using radio-synchrotron emission as a proxy for SFR requires novel
M
⋆
-dependent recipes that will enable us to convert detections from future ultra-deep radio surveys into accurate SFR measurements down to low-
M
⋆
galaxies with low SFR.
Abstract
Attempts to trace star formation with rest-frame UV/optical observations at redshifts
z
> 2 are affected by the presence of potentially substantial, yet uncertain, dust attenuation. Recent ...studies have demonstrated the existence of a population of galaxies that are virtually invisible in the observed optical/near-infrared (NIR) due to dust obscuration, but which could contribute substantially to the star formation history at 2 <
z
< 6. Here, we make use of ultradeep 3 GHz Karl G. Jansky Very Large Array observations from the COSMOS-XS survey to investigate the contribution 20of radio-selected “optically dark” galaxies (undetected to a depth of
K
S
∼ 25.9 mag) to the cosmic star formation rate density (SFRD). We identify 19 such “optically dark” sources and utilize recent deblended far-infrared photometry to determine photometric redshifts based on IR and radio information for 11 of them. Through stacking, we infer that the remaining eight sources reside predominantly at high redshift (
z
> 4). Therefore, we conservatively assume these sources lie between
z
= 2 and
z
= 5. We derive the radio luminosity function (LF) for the sample with and without “optically dark” sources by fixing the faint and bright end shape of the radio LF to the local values and allowing for luminosity evolution. By integrating both LFs, we estimate the contribution of the “optically dark” galaxies to the radio SFRD to be
∼
15
−
7
+
7
%
at
z
∼ 5. This is consistent with constraints from NIR-dark and UV-dark sources, while being in disagreement with some estimates using
H
-dropouts. This result implies that “optically dark” sources play a nonnegligible role at high redshift.
In the context of the VLA-COSMOS Deep project, additional VLA A array observations at 1.4 GHz were obtained for the central degree of the COSMOS field and combined with the existing data from the ...VLA-COSMOS Large project. A newly constructed Deep mosaic with a resolution of 2.''5 was used to search for sources down to 4{sigma} with 1{sigma} {approx} 12 {mu}Jy beam{sup -1} in the central 50' x 50'. This new catalog is combined with the catalog from the Large project (obtained at 1.''5 x 1.''4 resolution) to construct a new Joint catalog. All sources listed in the new Joint catalog have peak flux densities of {>=}5{sigma} at 1.''5 and/or 2.''5 resolution to account for the fact that a significant fraction of sources at these low flux levels are expected to be slightly resolved at 1.''5 resolution. All properties listed in the Joint catalog, such as peak flux density, integrated flux density, and source size, are determined in the 2.''5 resolution Deep image. In addition, the Joint catalog contains 43 newly identified multi-component sources.
We combine observations from the Atacama Large Millimeter/submillimeter Array and the NOrthern Extended Millimeter Array to assess the redshift and to study the star formation conditions in AzTEC2, ...one of the brightest submillimeter galaxies (SMGs) in the COSMOS field ( mJy). Our high-resolution observations confirm that AzTEC2 splits into two components (namely AzTEC2-A and AzTEC2-B) for which we detect C ii and 12CO(5 → 4) line emission, implying a redshift of 4.626 0.001 (4.633 0.001) for AzTEC2-A (AzTEC2-B) and ruling out previous associations with a galaxy at . We use the 12CO(5 → 4) line emission and adopt typical SMG-like gas excitation conditions to estimate the molecular gas mass, which is for AzTEC2-A, and a factor four lower for AzTEC2-B. With the infrared-derived star formation rate of AzTEC2-A ( yr−1) and AzTEC2-B ( yr−1), they both will consume their current gas reservoir within (30-200) Myr. We find evidence of a rotation-dominated C ii disk in AzTEC2-A, with a deprojected rotational velocity of km s−1, velocity dispersion km s−1, and dynamical mass of . We propose that an elevated gas accretion rate from the cosmic web might be the main driver of the intense levels of star formation in AzTEC2-A, which might be further enhanced by gravitational torques induced by its minor companion (AzTEC2-B). These results strengthen the picture whereby the population of single-dish selected SMGs is rather heterogeneous, including a population of pairs of massive, highly active galaxies in a pre-coalescence phase.
Abstract
We present ultradeep, matched-resolution Karl G. Jansky Very Large Array observations at 10 and 3 GHz in the COSMOS field: the COSMOS-XS survey. The final 10 and 3 GHz images cover ∼16 and
...and reach median rms values at the phase center of 0.41 and 0.53
μ
Jy beam
−1
, respectively. Both images have an angular resolution of ∼20. To account for the spectral shape and resolution variations across the broad bands, we image all data with a multiscale, multifrequency synthesis algorithm. We present source catalogs for the 10 and 3 GHz image with 91 and 1498 sources, respectively, above a peak brightness threshold of 5
σ
. We present source counts with completeness corrections included that are computed via Monte Carlo simulations. Our corrected counts at 3 GHz are consistent within the uncertainties with other results at 3 and 1.4 GHz but extend to fainter flux densities than previous direct detections. The 3 GHz number counts exceed the counts predicted by the semiempirical simulations developed in the framework of the SKA Simulated Skies project, consistent with previous
P
(
D
) analyses. Our source counts suggest a steeper luminosity function evolution for faint star-forming sources. The semiempirical Tiered Radio Extragalactic Continuum Simulation predicts this steeper evolution and is in better agreement with our results at 10 and 3 GHz within the expected variations from cosmic variance. In summary, the multiband, matched-resolution COSMOS-XS survey in the COSMOS field provides a high-resolution view of the ultrafaint radio sky that can help guide next-generation radio facilities.
The VLA-COSMOS Large Project has imaged the 2 deg super(2) COSMOS field with a resolution of 1.5 super(image ) and a sensitivity of about 11 muJy (1 capital sigma ), yielding a catalog of image3600 ...radio sources. In this paper we present a further analysis of the VLA-COSMOS Large Project catalog of radio sources aimed to (1) quantify and correct for the effect of bandwidth smearing in the catalog, (2) determine the incompleteness produced by the noise bias and the resolution bias in the new catalog, and (3) derive the radio source counts at 1.4 GHz. The effect of bandwidth smearing on the radio sources in the catalog was quantified comparing the peak and total flux densities in the final mosaic and in each of the individual pointings where the source was closest to the center of the field. We find that the peak flux densities in the original VLA-COSMOS Large Project catalog have to be divided by a factor of about 0.8 or 0.9, depending on the distance from the mosaic center. The completeness of the radio catalog has been tested using samples of simulated radio sources with different angular size distributions. These simulated sources have been added to the radio image and recovered using the same techniques used to produce the radio catalog. The fraction of missed sources as a function of the total flux density is a direct measure of the incompleteness. Finally, we derived the radio source counts down to 60 muJy with unprecedented good statistics. Comparison to the findings of other surveys shows good agreement in the flux density range 0.06-1 mJy confirming the upturn at image0.5 mJy and a possible decline of the source counts below image0.1 mJy.
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