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Register a new userExtremely Low Molecular Gas Content in the Vicinity of a Red Nugget Galaxy at $z=1.91$
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We present Atacama Large Millimeter/submillimeter Array (ALMA) Band 5 observations of a galaxy at $z=1.91$, GDS24569, in search of molecular gas in its vicinity via the [C I] $^3$P$_1$-$^3$P$_0$ line. GDS24569 is a massive ($log M_*/M_odot=11$) passively evolving galaxy, and characterized by compact morphology with an effective radius of $sim0.5$ kpc. We apply two blind detection algorithms to the spectral data cubes, and find no promising detection in or around GDS24569 out to projected distance of $sim320$ kpc, while a narrow tentative line ($4.1 sigma$) is identified at $+1200$ km/s by one of the algorithms. From the non-detection of [C I], we place a $3sigma$ upper limit on molecular hydrogen mass, $sim 7.1 times 10^9 M_odot$, which converts to an extremely low gas-to-stellar mass fraction, $< 5 %$. We conduct a spectral energy distribution modeling by including optical-to-far-infrared data, and find a considerably high ($sim0.1%$) dust-to-stellar mass ratio, $sim10$-$100times$ higher than those of local early-type galaxies. In combination with a previous result of an insufficient number of surrounding satellite galaxies, it is suggested that GDS24569 is unlikely to experience significant size evolution via satellite mergers. We discuss possible physical mechanisms that quenched GDS24569.
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One of the greatest challenges to theoretical models of massive galaxy formation is the regulation of star formation at early times. The relative roles of molecular gas expulsion, depletion, and stabilization are uncertain as direct observational constraints of the gas reservoirs in quenched or quenching galaxies at high redshift are scant. We present ALMA observations of CO(2-1) in a massive ($log M_{star}/M_{odot}=11.2$), recently quenched galaxy at $z=1.522$. The optical spectrum of this object shows strong Balmer absorption lines, which implies that star formation ceased $sim$0.8 Gyr ago. We do not detect CO(2-1) line emission, placing an upper limit on the molecular $mathrm{H_2}$ gas mass of 1.1$times10^{10},M_{odot}$. The implied gas fraction is $f_{rm{H_2}}{equiv M_{H_2}/M_{star}}<7%$, $sim10times$ lower than typical star forming galaxies at similar stellar masses at this redshift, among the lowest gas fractions at this specific star formation rate at any epoch, and the most stringent constraint on the gas contents of a $z>1$ passive galaxy to date. Our observations show that the depletion of $mathrm{H_2}$ from the interstellar medium of quenched objects can be both efficient and fairly complete, in contrast to recent claims of significant cold gas in recently quenched galaxies. We explore the variation in observed gas fractions in high-$z$ galaxies and show that galaxies with high stellar surface density have low $f_{rm{H_2}}$, similar to recent correlations between specific star formation rate and stellar surface density.
The standard AGN-galaxy co-evolutionary scenario predicts a phase of deeply buried supermassive black hole growth coexisting with a starburst (SB) before feedback phenomena deplete the cold molecular gas reservoir of the galaxy and an optically luminous QSO is revealed (SB-QSO evolutionary sequence). The aim of this work is to measure the cold gas reservoir of three highly obscured QSOs to test if their gas fraction is similar to that of sub-millimeter galaxies (SMGs), as expected by some models, and place these measurements in the context of the SB-QSO framework. We target CO(1-0) transition in BzK4892, a Compton Thick (CT) QSO at z=2.6, CO(1-0) in BzK8608 and CO(2-1) in CDF153, two highly obscured QSOs at z=2.5 and z=1.5, respectively. For all these targets, we place 3$sigma$ upper limits on the CO, with $L_{CO} < (1.5div 2.8)times 10^{10}$ K km/s pc$^2$. We also compare the molecular gas conditions of our targets with those of other systems at z>1, considering normal star forming galaxies and SMGs, unobscured and obscured AGN from the literature. For the AGN samples, we provide an updated and (almost) complete collection of targets with CO follow-up. BzK4892 displays a high star formation efficiency (SFE$=L_{IR}/L_{CO}>410$ L$_{odot}$/(K km s$^{-1}$ pc$^2$)) and a gas fraction $f_{gas}<0.1$. Less stringent constraints are derived for the other two targets ($f_{gas}<0.5$ and SFE$>10$). From the comparison with literature data, we found that a) obscured AGN at z>1 are associated with higher SFE and lower $f_{gas}$ with respect to star forming galaxies; b) mildly and highly obscured active galaxies have comparable gas fractions; c) the SFE of CT and obscured AGN are similar to those of unobscured AGN. Within the SB-QSO framework, these findings could be consistent with a scenario where feedback can impact the host galaxy already from the early phases of the SB-QSO sequence.
We present Atacama Large Millimeter/submillimeter Array observations of CO lines and dust continuum emission of the source RCSGA 032727--132609, a young $z=1.7$ low-metallicity starburst galaxy. The CO(3-2) and CO(6-5) lines, and continuum at rest-frame $450,mu m$ are detected and show a resolved structure in the image plane. We use the corresponding lensing model to obtain a source plane reconstruction of the detected emissions revealing intrinsic flux density of $S_{450,mu m}=23.5_{-8.1}^{+26.8}$ $mu$Jy and intrinsic CO luminosities $L_{rm CO(3-2)}=2.90_{-0.23}^{+0.21}times10^{8}$ ${rm K,km,s^{-1},pc^{2}}$ and $L_{rm CO(6-5)}=8.0_{-1.3}^{+1.4}times10^{7}$ ${rm K,km,s^{-1},pc^{2}}$. We used the resolved properties in the source plane to obtain molecular gas and star-formation rate surface densities of $Sigma_{rm H2}=16.2_{-3.5}^{+5.8},{rm M}_{odot},{rm pc}^{-2}$ and $Sigma_{rm SFR}=0.54_{-0.27}^{+0.89},{rm M}_{odot},{rm yr}^{-1},{rm kpc}^{-2}$ respectively. The intrinsic properties of RCSGA 032727--132609 show an enhanced star-formation activity compared to local spiral galaxies with similar molecular gas densities, supporting the ongoing merger-starburst phase scenario. RCSGA 032727--132609 also appears to be a low--density starburst galaxy similar to local blue compact dwarf galaxies, which have been suggested as local analogs to high-redshift low-metallicity starburst systems. Finally, the CO excitation level in the galaxy is consistent with having the peak at ${rm J}sim5$, with a higher excitation concentrated in the star-forming clumps.
Within the GASP survey, aimed at studying the effect of the ram-pressure stripping on the star formation quenching in cluster galaxies, we analyze here ALMA observations of the jellyfish galaxy JW100. We find an unexpected large amount of molecular gas ($sim 2.5 times 10^{10} M_{odot}$), 30% of which is located in the stripped gas tail out to $sim$35 kpc from the galaxy center. The overall kinematics of molecular gas is similar to the one shown by the ionized gas, but for clear signatures of double components along the stripping direction detected only out to 2 kpc from the disk. The line ratio $r_{21}$ has a clumpy distribution and in the tail can reach large values ($geq 1$), while its average value is low (0.58 with a 0.15 dispersion). All these evidence strongly suggest that the molecular gas in the tail is newly born from stripped HI gas or newly condensed from stripped diffuse molecular gas. The analysis of interferometric data at different scales reveals that a significant fraction ($sim 40%$) of the molecular gas is extended over large scales ($geq 8$ kpc) in the disk, and this fraction becomes predominant in the tail ($sim 70%$). By comparing the molecular gas surface density with the star formation rate surface density derived from the Ha emission from MUSE data, we find that the depletion time on 1 kpc scale is particularly large ($5-10$ Gyr) both within the ram-pressure disturbed region in the stellar disk, and in the complexes along the tail.
Most molecular gas studies of $z > 2.5$ galaxies are of intrinsically bright objects, despite the galaxy population being primarily normal galaxies with less extreme star formation rates. Observations of normal galaxies at high redshift provide a more representative view of galaxy evolution and star formation, but such observations are challenging to obtain. In this work, we present ALMA $rm ^{12}CO(J = 3 rightarrow 2)$ observations of a sub-millimeter selected galaxy group at $z = 2.9$, resulting in spectroscopic confirmation of seven images from four member galaxies. These galaxies are strongly lensed by the MS 0451.6-0305 foreground cluster at $z = 0.55$, allowing us to probe the molecular gas content on levels of $rm 10^9-10^{10} ; M_odot$. Four detected galaxies have molecular gas masses of $rm (0.2-13.1) times 10^{10} ; M_odot$, and the non-detected galaxies have inferred molecular gas masses of $rm < 8.0 times 10^{10} ; M_odot$. We compare these new data to a compilation of 546 galaxies up to $z = 5.3$, and find that depletion times decrease with increasing redshift. We then compare the depletion times of galaxies in overdense environments to the field scaling relation from the literature, and find that the depletion time evolution is steeper for galaxies in overdense environments than for those in the field. More molecular gas measurements of normal galaxies in overdense environments at higher redshifts ($z > 2.5$) are needed to verify the environmental dependence of star formation and gas depletion.
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