arXiv subjects

Chris Carilli

Publications and source records attributed to Chris Carilli.

At least 37 records · Page 2Linked to original sources

arXiv2020

The Evolution of the Baryons Associated with Galaxies Averaged over Cosmic Time and Space

We combine the recent determination of the evolution of the cosmic density of molecular gas (H_2) using deep, volumetric surveys, with previous estimates of the cosmic density of stellar mass, star formation rate and atomic gas (HI), to constrain the evolution of baryons associated with galaxies averaged over cosmic time and space. The cosmic HI and H_2 densities are roughly equal at z~1.5. The H_2 density then decreases by a factor 6^{+3}_{-2} to today's value, whereas the HI density stays approximately constant. The stellar mass density is increasing continuously with time and surpasses that of the total gas density (HI and H_2) at redshift z~1.5. The growth in stellar mass cannot be accounted for by the decrease in cosmic H_2 density, necessitating significant accretion of additional gas onto galaxies. With the new H_2 constraints, we postulate and put observational constraints on a two step gas accretion process: (i) a net infall of ionized gas from the intergalactic/circumgalactic medium to refuel the extended HI reservoirs, and (ii) a net inflow of HI and subsequent conversion to H_2 in the galaxy centers. Both the infall and inflow rate densities have decreased by almost an order of magnitude since z~2. Assuming that the current trends continue, the cosmic molecular gas density will further decrease by about a factor of two over the next 5 Gyr, the stellar mass will increase by approximately 10%, and cosmic star formation activity will decline steadily toward zero, as the gas infall and accretion shut down.

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arXiv2020

The ALMA Spectroscopic Survey in the HUDF: Multi-band constraints on line luminosity functions and the cosmic density of molecular gas

We present a CO and atomic fine-structure line luminosity function analysis using the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). ASPECS consists of two spatially-overlapping mosaics that cover the entire ALMA 3mm and 1.2mm bands. We combine the results of a line candidate search of the 1.2mm data cube with those previously obtained from the 3mm cube. Our analysis shows that $\sim$80% of the line flux observed at 3mm arises from CO(2-1) or CO(3-2) emitters at $z$=1-3 (`cosmic noon'). At 1.2mm, more than half of the line flux arises from intermediate-J CO transitions ($J_{\rm up}$=3-6); $\sim12$% from neutral carbon lines; and $< 1$% from singly-ionized carbon, [CII]. This implies that future [CII] intensity mapping surveys in the epoch of reionization will need to account for a highly significant CO foreground. The CO luminosity functions probed at 1.2mm show a decrease in the number density at a given line luminosity (in units of $L'$) at increasing $J_{\rm up}$ and redshift. Comparisons between the CO luminosity functions for different CO transitions at a fixed redshift reveal sub-thermal conditions on average in galaxies up to $z\sim 4$. In addition, the comparison of the CO luminosity functions for the same transition at different redshifts reveals that the evolution is not driven by excitation. The cosmic density of molecular gas in galaxies, $ρ_{\rm H2}$, shows a redshift evolution with an increase from high redshift up to $z\sim1.5$ followed by a factor $\sim 6$ drop down to the present day. This is in qualitative agreement with the evolution of the cosmic star-formation rate density, suggesting that the molecular gas depletion time is approximately constant with redshift, after averaging over the star-forming galaxy population.

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arXiv2020

The ALMA Spectroscopic Survey in the HUDF: Constraining the Molecular Content at $\log{(M_*/M_\odot)} \sim 9.5$ with CO stacking of MUSE detected $z\sim1.5$ Galaxies

We report molecular gas mass estimates obtained from a stacking analysis of CO line emission in the ALMA Spectroscopic Survey (ASPECS) using the spectroscopic redshifts from the optical integral field spectroscopic survey by the Multi Unit Spectroscopic Explorer (MUSE) of the {\it Hubble} Ultra Deep Field (HUDF). Stacking was performed on subsets of the sample of galaxies classified by their stellar mass and position relative to the main-sequence relation (on, above, below). Among all the CO emission lines, from \cotwoone to CO(6-5), with redshifts accessible via the ASPECS Band~3 and the MUSE data, \cotwoone provides the strongest constraints on the molecular gas content. We detect \cotwoone emission in galaxies down to stellar masses of $\log{(M_*/M_\odot)}=10.0$. Below this stellar mass, we present a new constraint on the molecular gas content of $z\sim1.5$ main-sequence galaxies by stacking based on the MUSE detections. We find that the molecular gas mass of main-sequence galaxies continuously decreases with stellar mass down to $\log{(M_*/M_\odot)}\approx9.0$. Assuming a metallicity-based CO--to--$\rm H_2$ conversion factor, the molecular gas-to-stellar mass ratio from $\log{(M_*/M_\odot)}\sim9.0$ to $\sim10.0$ does not seem to decrease as fast as for $\log{(M_*/M_\odot)}>10.0$, which is in line with simulations and studies at lower redshift. The inferred molecular gas density $ρ{\rm (H_2)}=(0.49\pm0.09)\times10^8\,{\rm M_\odot\,Mpc^{-3}}$ of MUSE-selected galaxies at $z\sim1.5$ is comparable with the one derived in the HUDF with a different CO selection. Using the MUSE data we recover most of the CO emission in our deep ALMA observations through stacking, demonstrating the synergy between volumetric surveys obtained at different wavebands.

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arXiv2020

A Wideband Polarization Study of Cygnus A with the JVLA. I: The Observations and Data

We present results from deep, wideband, high spatial and spectral resolution observations of the nearby luminous radio galaxy Cygnus A with the Jansky Very Large Array. The high surface brightness of this source enables detailed polarimetric imaging, providing images at $0.75''$, spanning 2 - 18 GHz, and at 0.30$''$ (6 - 18 GHz). The fractional polarization from 2000 independent lines of sight across the lobes decreases strongly with decreasing frequency, with the eastern lobe depolarizing at higher frequencies than the western lobe. The depolarization shows considerable structure, varying from monotonic to strongly oscillatory. The fractional polarization in general increases with increasing resolution at a given frequency, as expected. However, there are numerous lines of sight with more complicated behavior. We have fitted the $0.3''$ images with a simple model incorporating random, unresolved fluctuations in the cluster magnetic field to determine the high resolution, high-frequency properties of the source and the cluster. From these derived properties, we generate predicted polarization images of the source at lower frequencies, convolved to 0.75$\arcsec$. These predictions are remarkably consistent with the observed emission. The observations are consistent with the lower-frequency depolarization being due to unresolved fluctuations on scales $\gtrsim$ 300 - 700 pc in the magnetic field and/or electron density superposed on a partially ordered field component. There is no indication in our data of the location of the depolarizing screen or the large-scale field, either, or both of which could be located throughout the cluster, or in a boundary region between the lobes and the cluster.

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arXiv2020

Detection of Cosmic Structures using the Bispectrum Phase. I. Mathematical Foundations

Many low-frequency radio interferometers are aiming to detect very faint spectral signatures from structures at cosmological redshifts, particularly of neutral Hydrogen using its characteristic 21 cm spectral line. Due to the very high dynamic range needed to isolate these faint spectral fluctuations from the very bright foregrounds, spectral systematics from the instrument or the analysis, rather than thermal noise, are currently limiting their sensitivity. Failure to achieve a spectral calibration with fractional inaccuracy $\lesssim 10^{-5}$ will make the detection of the critical cosmic signal unlikely. The bispectrum phase from interferometric measurements is largely immune to this calibration issue. We present a basis to explore the nature of bispectrum phase in the limit of small spectral fluctuations. We establish that they measure the intrinsic dissimilarity in the transverse structure of the cosmic signal relative to the foregrounds, expressed as rotations in the underlying phase angle. Their magnitude is related to the strength of the cosmic signal relative to the foregrounds. Using a range of sky models, we detail the behavior of bispectrum phase fluctuations using standard Fourier-domain techniques and find it comparable to existing approaches, with a few key differences. Mode-mixed foreground contamination is more pronounced than in existing approaches because the bispectrum phase is a product of three individual interferometric phases. The multiplicative coupling of foregrounds in the bispectrum phase fluctuations results in the mixing of foreground signatures with that of the cosmic signal. We briefly outline a variation of this approach to avoid extensive mode-mixing. Despite its limitations, the interpretation of results using bispectrum phase is possible with forward-modeling. Importantly, it is an independent and a viable alternative to existing approaches.

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arXiv2020

The ALMA Spectroscopic Survey in the HUDF: The Cosmic Dust and Gas Mass Densities in Galaxies up to $z\sim3$

Using the deepest 1.2 mm continuum map to date in the Hubble Ultra Deep Field obtained as part of the ALMA Spectroscopic Survey (ASPECS) large program, we measure the cosmic density of dust and implied gas (H$_{2}+$H I) mass in galaxies as a function of look-back time. We do so by stacking the contribution from all $H$-band selected galaxies above a given stellar mass in distinct redshift bins, $ρ_{\rm dust}(M_\ast>M,z)$ and $ρ_{\rm gas}(M_\ast>M,z)$. At all redshifts, $ρ_{\rm dust}(M_\ast>M,z)$ and $ρ_{\rm gas}(M_\ast>M,z)$ grow rapidly as $M$ decreases down to $10^{10}\,M_\odot$, but this growth slows down towards lower stellar masses. This flattening implies that at our stellar mass-completeness limits ($10^8\,M_\odot$ and $10^{8.9}\,M_\odot$ at $z\sim0.4$ and $z\sim3$), both quantities converge towards the total cosmic dust and gas mass densities in galaxies. The cosmic dust and gas mass densities increase at early cosmic time, peak around $z\sim2$, and decrease by a factor $\sim4$ and 7, compared to the density of dust and molecular gas in the local universe, respectively. The contribution of quiescent galaxies -- i.e., with little on-going star-formation -- to the cosmic dust and gas mass densities is minor ($\lesssim10\%$). The redshift evolution of the cosmic gas mass density resembles that of the star-formation rate density, as previously found by CO-based measurements. This confirms that galaxies have relatively constant star-formation efficiencies (within a factor $\sim2$) across cosmic time. Our results also imply that by $z\sim0$, a large fraction ($\sim90\%$) of dust formed in galaxies across cosmic time has been destroyed or ejected to the intergalactic medium.

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arXiv2019

The ALMA Spectroscopic Survey in the HUDF: Constraining cumulative CO emission at $1 \lesssim z \lesssim 4$ with power spectrum analysis of ASPECS LP data from 84 to 115 GHz

We present a power spectrum analysis of the ALMA Spectroscopic Survey Large Program (ASPECS LP) data from 84 to 115 GHz. These data predominantly probe small-scale fluctuations ($k=10$-$100$ h Mpc$^{-1}$) in the aggregate CO emission in galaxies at $1 \lesssim z \lesssim 4$. We place an integral constraint on CO luminosity functions (LFs) in this redshift range via a direct measurement of their second moments in the three-dimensional (3D) auto-power spectrum, finding a total CO shot noise power $P_{\textrm{CO,CO}}(k_{\textrm{CO(2-1)}}) \leq 1.9\times10^2$ $μ$K$^2$ (Mpc h$^{-1}$)$^3$. This upper limit ($3σ$) is consistent with the observed ASPECS CO LFs in Decarli et al. 2019, but rules out a large space in the range of $P_{\textrm{CO,CO}}(k_{\textrm{CO(2-1)}})$ inferred from these LFs, which we attribute primarily to large uncertainties in the normalization $Φ_*$ and knee $L_*$ of the Schechter-form CO LFs at $z > 2$. Also, through power spectrum analyses of ASPECS LP data with 415 positions from galaxies with available optical spectroscopic redshifts, we find that contributions to the observed mean CO intensity and shot noise power of MUSE galaxies are largely accounted for by ASPECS blind detections, though there are $\sim20$% contributions to the CO(2-1) mean intensity due to sources previously undetected in the blind line search. Finally, we sum the fluxes from individual blind CO detections to yield a lower limit on the mean CO surface brightness at 99 GHz of $\langle T_{\textrm{CO}} \rangle = 0.55\pm0.02$ $μ$K, which we estimate represents $68$-$80$% of the total CO surface brightness at this frequency.

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arXiv2019

The z=7.54 Quasar ULAS J1342+0928 Is Hosted by a Galaxy Merger

We present ALMA 0.28"x0.20" (1.4kpc x 1.0kpc) resolution observations of the [CII] 158um line and underlying dust continuum in the host galaxy of the most distant quasar currently known, ULAS J1342+0928 at z=7.5413. Both the [CII] and continuum are detected and spatially resolved. The [CII] line is ~1.5 times more extended than the continuum emission, showing an elongated and complex structure of approximately 3.2kpc x 6.4kpc. Two separate peaks are clearly seen (>6sigma each) in three 100 km/s width [CII] channel maps. The velocity field of the [CII] gas does not show evidence of a coherent rotation field but rather chaotic motion, reminiscent of an on-going merger.

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arXiv2019

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Evolution of the molecular gas in CO-selected galaxies

We analyze the interstellar medium properties of a sample of sixteen bright CO line emitting galaxies identified in the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS) Large Program. This CO$-$selected galaxy sample is complemented by a couple of additional CO line emitters in the UDF that are identified based on their MUSE optical spectroscopic redshifts. The ASPECS CO$-$selected galaxies cover a larger range of star-formation rates and stellar masses compared to literature CO emitting galaxies at $z>1$ for which scaling relations have been established previously. Most of ASPECS CO-selected galaxies follow these established relations in terms of gas depletion timescales and gas fractions as a function of redshift, as well as the star-formation rate-stellar mass relation (`galaxy main sequence'). However, we find that $\sim30\%$ of the galaxies (5 out of 16) are offset from the galaxy main sequence at their respective redshift, with $\sim12\%$ (2 out of 16) falling below this relationship. Some CO-rich galaxies exhibit low star-formation rates, and yet show substantial molecular gas reservoirs, yielding long gas depletion timescales. Capitalizing on the well-defined cosmic volume probed by our observations, we measure the contribution of galaxies above, below, and on the galaxy main sequence to the total cosmic molecular gas density at different lookback times. We conclude that main sequence galaxies are the largest contributor to the molecular gas density at any redshift probed by our observations (z$\sim$1$-$3). The respective contribution by starburst galaxies above the main sequence decreases from z$\sim$2.5 to z$\sim$1, whereas we find tentative evidence for an increased contribution to the cosmic molecular gas density from the passive galaxies below the main sequence.

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arXiv2019

An ALMA multi-line survey of the interstellar medium of the redshift 7.5 quasar host galaxy J1342+0928

We use ALMA observations of the host galaxy of the quasar ULAS-J1342+0928 at z=7.54 to study the dust continuum and far infrared lines emitted from its interstellar medium. The Rayleigh-Jeans tail of the dust continuum is well sampled with eight different spectral setups, and from a modified black body fit we obtain an emissivity coefficient of beta=1.85+-0.3. Assuming a standard dust temperature of 47 K we derive a dust mass of Mdust=0.35x10^8 Msol and a star formation rate of 150+-30 Msol/yr. We have >4sigma detections of the [CII]_158, [OIII]_88 and [NII]_122 atomic fine structure lines and limits on the [CI]_369, [OI]_146 and [NII]_205 emission. We also report multiple limits of CO rotational lines with Jup >= 7, as well as a tentative 3.3sigma detection of the stack of four CO lines (Jup=11, 10, 8, 7). We find line deficits that are in agreement with local ultra luminous infrared galaxies. Comparison of the [NII]_122 and [CII]_158 lines indicates that the [CII]_158 emission arises predominantly from the neutral medium, and we estimate that the photo-disassociation regions in J1342+0928 have densities <~5x10^4cm^-3. The data suggest that ~16% of hydrogen is in ionized form and that the HII regions have high electron densities of n_e>180 cm^-3. Our observations favor a low gas-to-dust ratio of <100 and a metallicity of the interstellar medium comparable to the Solar value. All the measurements presented here suggest that the host galaxy of J1342+0928 is highly enriched in metal and dust, despite being observed just 680 Myr after the Big Bang.

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arXiv2019

ALMA and HST kiloparsec-scale imaging of a quasar-galaxy merger at $z\approx 6.2$

We present kpc-scale ALMA and HST imaging of the quasar PJ308-21 at $z$=$6.2342$, tracing dust, gas (via the [CII] 158 $μ$m line) and young stars. At a resolution of $\sim0.3''$ ($\approx1.7$ kpc), the system is resolved over $>4''$ ($>$20 kpc). In particular, it features a main component, identified to be the quasar host galaxy, centered on the accreting supermassive black hole; and two other extended components on the West and East side, one redshifted and the other blueshifted relative to the quasar. The [CII] emission of the entire system stretches over $>$1500 km/s along the line of sight. All the components of the system are observed in dust, [CII], and rest-frame UV emission. The inferred [CII] luminosities [(0.9-4.6)$\times 10^9$ L$_\odot$], dust luminosities [(0.15-2.6)$\times10^{12}$ L$_\odot$], and rest-frame UV luminosities [(6.6-15)$\times10^{10}$ L$_\odot$], their ratios, and the implied gas/dust masses and star formation rates [11-290 M$_\odot$ yr$^{-1}$] are typical of high-redshift star-forming galaxies. A toy model of a single satellite galaxy that is tidally stripped by the interaction with the quasar host galaxy can account for the observed velocity and spatial extent of the two extended components. An outflow interpretation of the unique features in PJ308-21 is not supported by the data. PJ308-21 is thus one of the earliest galaxy mergers imaged at cosmic dawn.

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arXiv2019

The Radio Universe at Low Surface Brightness: Feedback & accretion in the circumgalactic medium

Massive galaxies at high-z are known to co-evolve with their circumgalactic medium (CGM). If we want to truly understand the role of the CGM in the early evolution of galaxies and galaxy-clusters, we need to fully explore the multi-phase nature of the CGM. We present two novel science cases that utilize low-surface-brightness observations in the radio regime to better understand the CGM around distant galaxies. At the lowest temperatures, observations of widespread molecular gas are providing evidence for the cold baryon cycle that grows massive galaxies. At the highest temperatures, observations of the Sunyaev-Zeldovich Effect are starting to reveal the effect of quasar feedback onto the hot gas in the CGM. We discuss the critical role that radio interferometers with compact configurations in the millimeter regime will play over the next decade in understanding the crucial role of the multi-phase CGM in galaxy evolution.

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arXiv2019

The ALMA Spectroscopic Survey in the HUDF: the molecular gas content of galaxies and tensions with IllustrisTNG and the Santa Cruz SAM

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS) provides new constraints for galaxy formation models on the molecular gas properties of galaxies. We compare results from ASPECS to predictions from two cosmological galaxy formation models: the IllustrisTNG hydrodynamical simulations and the Santa Cruz semi-analytic model (SC SAM). We explore several recipes to model the H$_2$ content of galaxies, finding them to be consistent with one another, and take into account the sensitivity limits and survey area of ASPECS. For a canonical CO-to-H$_2$ conversion factor of $α_{\rm CO} = 3.6\,\rm{M}_\odot/(\rm{K}\,\rm{km/s}\,\rm{pc}^{2})$ the results of our work include: (1) the H$_2$ mass of $z>1$ galaxies predicted by the models as a function of their stellar mass is a factor of 2-3 lower than observed; (2) the models do not reproduce the number of H$_2$-rich ($M_{\rm H2} > 3\times 10^{10}\,\rm{M}_\odot$) galaxies observed by ASPECS; (3) the H$_2$ cosmic density evolution predicted by IllustrisTNG (the SC SAM) is in tension (only just agrees) with the observed cosmic density, even after accounting for the ASPECS selection function and field-to-field variance effects. The tension between models and observations at $z>1$ can be alleviated by adopting a CO-to-H$_2$ conversion factor in the range $α_{\rm CO} = 2.0 - 0.8\,\rm{M}_\odot/(\rm{K}\,\rm{km/s}\,\rm{pc}^{2})$. Additional work on constraining the CO-to-H$_2$ conversion factor and CO excitation conditions of galaxies through observations and theory will be necessary to more robustly test the success of galaxy formation models.

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arXiv2019

The ALMA Spectroscopic Survey in the HUDF: CO luminosity functions and the molecular gas content of galaxies through cosmic history

We use the results from the ALMA large program ASPECS, the spectroscopic survey in the Hubble Ultra Deep Field (HUDF), to constrain CO luminosity functions of galaxies and the resulting redshift evolution of $ρ$(H$_2$). The broad frequency range covered enables us to identify CO emission lines of different rotational transitions in the HUDF at $z>1$. We find strong evidence that the CO luminosity function evolves with redshift, with the knee of the CO luminosity function decreasing in luminosity by an order of magnitude from $\sim$2 to the local universe. Based on Schechter fits, we estimate that our observations recover the majority (up to $\sim$90%, depending on the assumptions on the faint end) of the total cosmic CO luminosity at $z$=1.0-3.1. After correcting for CO excitation, and adopting a Galactic CO-to-H$_2$ conversion factor, we constrain the evolution of the cosmic molecular gas density $ρ$(H$_2$): this cosmic gas density peaks at $z\sim1.5$ and drops by factor of $6.5_{-1.4}^{+1.8}$ to the value measured locally. The observed evolution in $ρ$(H$_2$) therefore closely matches the evolution of the cosmic star formation rate density $ρ_{\rm SFR}$. We verify the robustness of our result with respect to assumptions on source inclusion and/or CO excitation. As the cosmic star formation history can be expressed as the product of the star formation efficiency and the cosmic density of molecular gas, the similar evolution of $ρ$(H$_2$) and $ρ_{\rm SFR}$ leaves only little room for a significant evolution of the average star formation efficiency in galaxies since $z\sim 3$ (85% of cosmic history).

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arXiv2019

The ALMA Spectroscopic Survey in the HUDF: Nature and physical properties of gas-mass selected galaxies using MUSE spectroscopy

We discuss the nature and physical properties of gas-mass selected galaxies in the ALMA spectroscopic survey (ASPECS) of the Hubble Ultra Deep Field (HUDF). We capitalize on the deep optical integral-field spectroscopy from the MUSE HUDF Survey and multi-wavelength data to uniquely associate all 16 line-emitters, detected in the ALMA data without preselection, with rotational transitions of carbon monoxide (CO). We identify ten as CO(2-1) at $1 < z < 2$, five as CO(3-2) at $2 < z < 3$ and one as CO(4-3) at $z = 3.6$. Using the MUSE data as a prior, we identify two additional CO(2-1)-emitters, increasing the total sample size to 18. We infer metallicities consistent with (super-)solar for the CO-detected galaxies at $z \le 1.5$, motivating our choice of a Galactic conversion factor between CO luminosity and molecular gas mass for these galaxies. Using deep Chandra imaging of the HUDF, we determine an X-ray AGN fraction of 20% and 60% among the CO-emitters at $z \sim 1.4$ and $z \sim 2.6$, respectively. Being a CO-flux limited survey, ASPECS-LP detects molecular gas in galaxies on, above and below the main sequence (MS) at $z \sim 1.4$. For stellar masses $\ge 10^{10} (10^{10.5})$ M$_{\odot}$, we detect about 40% (50%) of all galaxies in the HUDF at $1 < z < 2$ ($2 < z < 3$). The combination of ALMA and MUSE integral-field spectroscopy thus enables an unprecedented view on MS galaxies during the peak of galaxy formation.

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arXiv2019

The ALMA Spectroscopic Survey in the HUDF: CO emission lines and 3 mm continuum sources

The ALMA SPECtroscopic Survey in the {\it Hubble} Ultra Deep Field is an ALMA large program that obtained a frequency scan in the 3\,mm band to detect emission lines from the molecular gas in distant galaxies. We here present our search strategy for emission lines and continuum sources in the HUDF. We compare several line search algorithms used in the literature, and critically account for the line-widths of the emission line candidates when assessing significance. We identify sixteen emission lines at high fidelity in our search. Comparing these sources to multi-wavelength data we find that all sources have optical/infrared counterparts. Our search also recovers candidates that have lower significance that can be used statistically to derive, e.g. the CO luminosity function. We apply the same detection algorithm to obtain a sample of six 3 mm continuum sources. All of these are also detected in the 1.2 mm continuum with optical/near-infrared counterparts. We use the continuum sources to compute 3 mm number counts in the sub-mJy regime, and find them to be higher by an order of magnitude than expected for synchrotron-dominated sources. However, the number counts are consistent with those derived at shorter wavelengths (0.85--1.3\,mm) once extrapolating to 3\,mm with a dust emissivity index of $β=1.5$, dust temperature of 35\,K and an average redshift of $z=2.5$. These results represent the best constraints to date on the faint end of the 3 mm number counts.

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arXiv2019

The evolution of the cosmic molecular gas density

One of the last missing pieces in the puzzle of galaxy formation and evolution through cosmic history is a detailed picture of the role of the cold gas supply in the star-formation process. Cold gas is the fuel for star formation, and thus regulates the buildup of stellar mass, both through the amount of material present through a galaxy's gas mass fraction, and through the efficiency at which it is converted to stars. Over the last decade, important progress has been made in understanding the relative importance of these two factors along with the role of feedback, and the first measurements of the volume density of cold gas out to redshift 4, (the "cold gas history of the Universe") has been obtained. To match the precision of measurements of the star formation and black-hole accretion histories over the coming decades, a two orders of magnitude improvement in molecular line survey speeds is required compared to what is possible with current facilities. Possible pathways towards such large gains include significant upgrades to current facilities like ALMA by 2030 (and beyond), and eventually the construction of a new generation of radio-to-millimeter wavelength facilities, such as the next generation Very Large Array (ngVLA) concept.

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arXiv2019

Improved 21 cm Epoch Of Reionization Power Spectrum Measurements with a Hybrid Foreground Subtraction and Avoidance Technique

Observations of the 21cm Epoch of Reionization (EoR) signal are dominated by Galactic and extragalactic foregrounds. The need for foreground removal has led to the development of two main techniques, often referred to as "foreground avoidance" and "foreground subtraction." Avoidance is associated with filtering foregrounds in Fourier space, while subtraction uses an explicit foreground model that is removed. Using 1088 hours of data from the 64-element PAPER array, we demonstrate that subtraction of a foreground model prior to delay-space foreground filtering results in a modest but measurable improvement of the performance of the filter. This proof-of-concept result shows that improvement stems from the reduced dynamic range requirements needed for the foreground filter: subtraction of a foreground model reduces the total foreground power, so for a fixed dynamic range, the filter can push towards fainter limits. We also find that the choice of window function used in the foreground filter can have an appreciable affect on the performance near the edges of the observing band. We demonstrate these effects using a smaller 3 hour sampling of data from the MWA, and find that the hybrid filtering and subtraction removal approach provides similar improvements across the band as seen in the case with PAPER-64.

astro-ph.IM↗