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Register a new userMolecular gas in radio galaxies in dense Mpc-scale environments at $z=0.4-2.6$
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Gianluca Castignani
Publication date
2018
fields
Physics
and research's language is
English
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We investigate the role of dense Mpc-scale environment in processing molecular gas in distant Low luminosity radio galaxies (LLRGs) in galaxy (proto-)clusters. We have selected within the COSMOS and DES surveys a sample of five LLRGs at $z=0.4-2.6$ that show evidence of ongoing star formation on the basis of their far-infrared emission. We have assembled and modeled the far-infrared-to-ultraviolet spectral energy distributions (SEDs) of the LLRGs. We have observed the sources with the IRAM-30m telescope to search for CO emission. We have then searched for dense Mpc-scale overdensities associated with the LLRGs using photometric redshifts of galaxies and the Poisson Probability Method, that we have upgraded using the wavelet-transform ($mathit{w}$PPM), to characterize the overdensity in the projected space. Color-color and color-magnitude plots have been derived for the fiducial cluster members. We set upper limits to the CO emission of the LLRGs, at $z=0.39, 0.61, 0.91, 0.97$, and $2.6$. For the most distant radio source, COSMOS-FRI 70 at $z=2.6$, a hint of CO(7$rightarrow$6) emission is found at 2.2$sigma$. The upper limits found for the molecular gas content $M({rm H}_2)/M_star<0.11$, 0.09, 1.8, 1.5, and 0.29, respectively, and depletion time $tau_{rm dep}lesssim(0.2-7)$ Gyr of the five LLRGs are overall consistent with the values of main sequence field galaxies. Our SED modeling implies large stellar masses for the LLRGs, in the range $log(M_star/M_odot)=10.9-11.5$, while the associated Mpc-scale overdensities show a complex morphology. The color-color and color-magnitude plots suggest that the LLRGs are consistent with being star forming and on the high-luminosity tail of the red sequence. The present study increases the limited statistics of distant cluster core galaxies with CO observations. The radio galaxies of this work are excellent targets for ALMA and JWST.
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We investigate the role of the environment in processing molecular gas in radio galaxies (RGs). We observed five RGs at $z=0.4-2.6$ in dense Mpc-scale environment with the IRAM-30m telescope. We set four upper-limits and report a tentative CO(7$rightarrow$6) detection for COSMOS-FRI 70 at $z=2.63$, which is the most distant brightest cluster galaxy (BCG) candidate detected in CO. We speculate that the cluster environment might have played a role in preventing the refueling via environmental mechanisms such as galaxy harassment, strangulation, ram-pressure, or tidal stripping. The RGs of this work are excellent targets for ALMA as well as next generation telescopes such as the James Webb Space Telescope.
We study the molecular gas properties of two star-forming galaxies separated by 6 kpc in the projected space and belonging to a galaxy cluster selected from the Irac Shallow Cluster Survey, at a redshift $z=1.2$, i.e., $sim2$ Gyr after the cosmic star formation density peak. This work describes the first CO detection from $1<z<1.4$ star forming cluster galaxies with no reported clear evidence of AGN. We exploit observations taken with the NOEMA interferometer at $sim3$ mm to detect CO(2-1) line emission from the two selected galaxies, unresolved by our observations. Based on the CO(2-1) spectrum we estimate a total molecular gas mass $M({rm H_2})=(2.2^{+0.5}_{-0.4})times10^{10}$ $M_odot$ and dust mass $M_{rm dust}<4.2times10^8~M_odot$ for the two blended sources. The two galaxies have similar stellar masses and a large relative velocity of $sim$400 km/s estimated from the CO(2-1) line width. These findings tend to privilege a scenario where both sources contribute to the observed CO(2-1). By using the archival Spitzer MIPS flux at 24$mu$m we estimate an ${rm SFR(24mu m)}=(28^{+12}_{-8})~M_odot$/yr for each of the two galaxies. Assuming that the two sources equally contribute to the observe CO(2-1) our analysis yields a depletion time scale $tau_{rm dep}=(3.9^{+1.4}_{-1.8})times10^8$ yr, and a molecular gas to stellar mass ratio $0.17pm0.13$ for each of two sources, separately. Our results are in overall agreement with those of other distant cluster galaxies. The two target galaxies have molecular gas mass and depletion time that are marginally compatible with, but smaller than those of main sequence field galaxies, suggesting that the molecular gas has not been refueled enough. Higher resolution and higher frequency observations will enable us to spatially resolve the two sources and possibly distinguish between different gas processing mechanisms.
Observations using the 7 mm receiver system on the Australia Telescope Compact Array have revealed large reservoirs of molecular gas in two high-redshift radio galaxies: HATLAS J090426.9+015448 (z = 2.37) and HATLAS J140930.4+003803 (z = 2.04). Optically the targets are very faint, and spectroscopy classifies them as narrow-line radio galaxies. In addition to harbouring an active galactic nucleus the targets share many characteristics of sub-mm galaxies. Far-infrared data from Herschel-ATLAS suggest high levels of dust (>10^9 M_solar) and a correspondingly large amount of obscured star formation (~1000 M_solar / yr). The molecular gas is traced via the J = 1-0 transition of 12CO, its luminosity implying total H_2 masses of (1.7 +/- 0.3) x 10^11 and (9.5 +/- 2.4) x 10^10 (alpha_CO/0.8) M_solar in HATLAS J090426.9+015448 and HATLAS J140930.4+003803 respectively. Both galaxies exhibit molecular line emission over a broad (~1000 km/s) velocity range, and feature double-peaked profiles. We interpret this as evidence of either a large rotating disk or an on-going merger. Gas depletion timescales are ~100 Myr. The 1.4 GHz radio luminosities of our targets place them close to the break in the luminosity function. As such they represent `typical z > 2 radio sources, responsible for the bulk of the energy emitted at radio wavelengths from accretion-powered sources at high redshift, and yet they rank amongst the most massive systems in terms of molecular gas and dust content. We also detect 115 GHz rest-frame continuum emission, indicating a very steep high-radio-frequency spectrum, possibly classifying the targets as compact steep spectrum objects.
Understanding the relationship between the formation and evolution of galaxies and their central super massive black holes (SMBH) is one of the main topics in extragalactic astrophysics. Links and feedback may reciprocally affect both black hole and galaxy growth. Observations of the CO line at redshifts of 2-4 are crucial to investigate the gas mass, star formation activity and accretion onto SMBHs, as well as the effect of AGN feedback. Potential correlations between AGN and host galaxy properties can be highlighted by observing extreme objects. Despite their luminosity, hyper-luminous QSOs at z=2-4 are still little studied at mm wavelengths. We targeted CO(3-2) in ULAS J1539+0557, an hyper-luminos QSO (Lbol> 10^48 erg/s) at z=2.658, selected through its unusual red colors in the UKIDSS Large Area Survey (ULAS). We find a molecular gas mass of 4.1+-0.8 10^10 Msun, and a gas fraction of 0.4-0.1, depending mostly on the assumed source inclination. We also find a robust lower limit to the star-formation rate (SFR=250-1600 Msun/yr) and star-formation efficiency (SFE=25-350 Lsun/(K km s-1 pc2) by comparing the observed optical-near-infrared spectral energy distribution with AGN and galaxy templates. The black hole gas consumption timescale, M(H_2)/dM(accretion)/dt, is ~160 Myr, similar or higher than the gas consumption timescale. The gas content and the star formation efficiency are similar to those of other high-luminosity, highly obscured QSOs, and at the lower end of the star-formation efficiency of unobscured QSOs, in line with predictions from AGN-galaxy co-evolutionary scenarios. Further measurements of the (sub)-mm continuum in this and similar sources are mandatory to obtain a robust observational picture of the AGN evolutionary sequence.
We have used the Atacama Large Millimeter/submillimeter Array (ALMA) to carry out a search for CO (3$-$2) or (4$-$3) emission from the fields of 12 high-metallicity ([M/H]~$geq -0.72$,dex) damped Lyman-$alpha$ absorbers (DLAs) at $z approx 1.7-2.6$. We detected CO emission from galaxies in the fields of five DLAs (two of which have been reported earlier), obtaining high molecular gas masses, $rm M_{mol} approx (1.3 - 20.7) times (alpha_{rm CO}/4.36) times 10^{10} ; M_odot$. The impact parameters of the CO emitters to the QSO sightline lie in the range $b approx 5.6-100$~kpc, with the three new CO detections having $b lesssim 15$~kpc. The highest CO line luminosities and inferred molecular gas masses are associated with the highest-metallicity DLAs, with [M/H]~$gtrsim -0.3$,dex. The high inferred molecular gas masses may be explained by a combination of a stellar mass-metallicity relation and a high molecular gas-to-stars mass ratio in high-redshift galaxies; the DLA galaxies identified by our CO searches have properties consistent with those of emission-selected samples. None of the DLA galaxies detected in CO emission were identified in earlier optical or near-IR searches and vice-versa; DLA galaxies earlier identified in optical/near-IR searches were not detected in CO emission. The high ALMA CO and C[{sc ii}]~158$mu$m detection rate in high-$z$, high-metallicity DLA galaxies has revolutionized the field, allowing the identification of dusty, massive galaxies associated with high-$z$ DLAs. The H{sc i}-absorption criterion identifying DLAs selects the entire high-$z$ galaxy population, including dusty and UV-bright galaxies, in a wide range of environments.
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