We report on a sensitive search for redshifted H$alpha$ line-emission from three high-metallicity damped Ly$alpha$ absorbers (DLAs) at $z approx 2.4$ with the Near-infrared Integral Field Spectrometer (NIFS) on the Gemini-North telescope, assisted by
the ALTtitude conjugate Adaptive optics for the InfraRed (ALTAIR) system with a laser guide star. Within the NIFS field-of-view, $approx 3.22 times 2.92$ corresponding to $approx 25$ kpc $ times 23$ kpc at $z=2.4$, we detect no statistically significant line-emission at the expected redshifted H$alpha$ wavelengths. The measured root-mean-square noise fluctuations in $0.4$ apertures are $1-3times10^{-18}$ erg s$^{-1}$ cm$^{-2}$. Our analysis of simulated, compact, line-emitting sources yields stringent limits on the star-formation rates (SFRs) of the three DLAs, $< 2.2$~M$_{odot}$ yr$^{-1}$ ($3sigma$) for two absorbers, and $< 11$~M$_{odot}$ yr$^{-1}$ ($3sigma$) for the third, at all impact parameters within $approx 12.5$~kpc to the quasar sightline at the DLA redshift. For the third absorber, the SFR limit is $< 4.4$~M$_odot$ yr$^{-1}$ for locations away from the quasar sightline. These results demonstrate the potential of adaptive optics-assisted, integral field unit searches for galaxies associated with high-$z$ DLAs.
We report Karl G. Jansky Very Large Array (VLA) absorption spectroscopy in four methanol (CH$_3$OH) lines in the $z = 0.88582$ gravitational lens towards PKS1830-211. Three of the four lines have very different sensitivity coefficients $K_mu$ to chan
ges in the proton-electron mass ratio $mu$; a comparison between the line redshifts thus allows us to test for temporal evolution in $mu$. We obtain a stringent statistical constraint on changes in $mu$ by comparing the redshifted 12.179 GHz and 60.531 GHz lines, $[Delta mu/mu] leq 1.1 times 10^{-7}$ ($2sigma$) over $0 < z leq 0.88582$, a factor of $approx 2.5$ more sensitive than the best earlier results. However, the higher signal-to-noise ratio (by a factor of $approx 2$) of the VLA spectrum in the 12.179 GHz transition also indicates that this line has a different shape from that of the other three CH$_3$OH lines (at $> 4sigma$ significance). The sensitivity of the above result, and that of all earlier CH$_3$OH studies, is thus likely to be limited by unknown systematic errors, probably arising due to the frequency-dependent structure of PKS1830-211. A robust result is obtained by combining the three lines at similar frequencies, 48.372, 48.377 and 60.531 GHz, whose line profiles are found to be in good agreement. This yields the $2sigma$ constraint $[Delta mu/mu] lesssim 4 times 10^{-7}$, the most stringent current constraint on changes in $mu$. We thus find no evidence for changes in the proton-electron mass ratio over a lookback time of $approx 7.5$ Gyrs.
We report a deep search for redshifted HI 21 cm emission from three damped and sub-damped Lyman-$alpha$ absorbers (DLAs) at $z approx 0.1$ with the Green Bank Telescope (GBT). No evidence for a redshifted HI 21 cm emission signal was obtained in the
GBT spectra of two absorbers, with the data on the third rendered unusable by terrestrial interference. The non-detections of HI 21 cm emission yield strong constraints on the HI masses of the associated galaxies, M$_{rm HI} < 2.3 times 10^9 times (Delta V/100)^{1/2}$ M$_odot$ for the sub-DLA at $z = 0.0830$ towards J1553+3548, and M$_{rm HI} < 2.7 times 10^9 times (Delta V/100)^{1/2}$ M$_odot$ for the DLA at $z = 0.0963$ towards J1619+3342, where $Delta V$ is the HI 21 cm line width, in km s$^{-1}$. This continues the trend of low HI masses found in all low-$z$ DLAs and sub-DLAs that have been searched for redshifted HI 21 cm emission. Low-redshift absorbers with relatively low HI column densities, $lesssim few times 10^{20}$ cm$^{-2}$, thus do not typically arise in massive gas-rich galaxies.
We present the methodology for ``blind millimetre-wave surveys for redshifted molecular absorption in the CO/HCO$^+$ rotational lines. The frequency range $30-50$ GHz appears optimal for such surveys, providing sensitivity to absorbers at $z gtrsim 0
.85$. It is critical that the survey is ``blind, i.e. based on a radio-selected sample, including sources without known redshifts. We also report results from the first large survey of this kind, using the Q-band receiver on the Green Bank Telescope (GBT) to search for molecular absorption towards 36 sources, 3 without known redshifts, over the frequency range $39.6 - 49.5$ GHz. The GBT survey has a total redshift path of $Delta z approx 24$, mostly at $0.81 < z < 1.91$, and a sensitivity sufficient to detect equivalent ${rm H_2}$ column densities $gtrsim 3 times 10^{21}$ cm$^{-2}$ in absorption at $5sigma$ significance (using the CO-to-${rm H_2}$ and HCO$^+$-to-${rm H_2}$ conversion factors of the Milky Way). The survey yielded no confirmed detections of molecular absorption, yielding the $2sigma$ upper limit $n(z=1.2) < 0.15$ on the redshift number density of molecular gas at column densities $N({rm H_2}) gtrsim 3 times 10^{21}$ cm$^{-2}$.
We report results from a programme aimed at investigating the temperature of neutral gas in high-redshift damped Lyman-$alpha$ absorbers (DLAs). This involved (1) HI 21cm absorption studies of a large DLA sample, (2) VLBI studies to measure the low-f
requency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallicities and velocity widths. Including literature data, our sample consists of 37 DLAs with estimates of the spin temperature $T_s$ and the covering factor. We find a strong $4sigma$) difference between the $T_s$ distributions in high-z (z>2.4) and low-z (z<2.4) DLA samples. The high-z sample contains more systems with high $T_s$ values, $gtrsim 1000$ K. The $T_s$ distributions in DLAs and the Galaxy are also clearly (~$6sigma$) different, with more high-$T_s$ sightlines in DLAs than in the Milky Way. The high $T_s$ values in the high-z DLAs of our sample arise due to low fractions of the cold neutral medium. For 29 DLAs with metallicity [Z/H] estimates, we confirm the presence of an anti-correlation between $T_s$ and [Z/H], at $3.5sigma$ significance via a non-parametric Kendall-tau test. This result was obtained with the assumption that the DLA covering factor is equal to the core fraction. Monte Carlo simulations show that the significance of the result is only marginally decreased if the covering factor and the core fraction are uncorrelated, or if there is a random error in the inferred covering factor. We also find evidence for redshift evolution in DLA $T_s$ values even for the z>1 sub-sample. Since z>1 DLAs have angular diameter distances comparable to or larger than those of the background quasars, they have similar efficiency in covering the quasars. Low covering factors in high-z DLAs thus cannot account for the observed redshift evolution in spin temperatures. (Abstract abridged.)
We report a Plateau de Bure interferometer search for CII-158$mu$m emission from HCM6A, a lensed Lyman-$alpha$ emitter (LAE) at $z = 6.56$. Our non-detections of CII-158$mu$m line emission and 1.2mm radio continuum emission yield $3sigma$ limits of L
$_{rm CII} < 6.4 times 10^7 times (Delta V/100 km s^{-1})^{1/2}$ L$_odot$ on the CII-158$mu$m line luminosity and S$_{rm 1.2mm} < 0.68$ mJy on the 1.2mm flux density. The local conversion factor between L$_{rm CII}$ and star formation rate (SFR) yields an SFR $< 4.7$ M$_odot$ yr$^{-1}$, $approx 2$ times lower than that inferred from the ultraviolet (UV) continuum, suggesting that the local factor may not be applicable in high-$z$ LAEs. The non-detection of 1.2mm continuum emission yields a total SFR $< 28$ M$_odot$/yr; any obscured star formation is thus within a factor of two of the visible star formation. Our best-fit model to the rest-frame UV/optical spectral energy distribution of HCM6A yields a stellar mass of $1.3 times 10^9$ M$_odot$ and an SFR of ~10 M$_odot$/yr, with negligible dust obscuration. We fortuitously detect CO J=3-2 emission from a $z=0.375$ galaxy in the foreground cluster Abell370, obtaining a CO line luminosity of L$^prime ({rm CO}) > (8.95 pm 0.79) times 10^8$ K km s$^{-1}$ pc$^2$, and a molecular gas mass of M$({rm H_2}) > (4.12 pm 0.36) times 10^9$ M$_odot$, for a CO-to-H$_2$ conversion factor of 4.6 M$_odot$ (K km s$^{-1}$ pc$^2$)$^{-1}$.
We present a detailed study of an estimator of the HI column density, based on a combination of HI 21cm absorption and HI 21cm emission spectroscopy. This isothermal estimate is given by $N_{rm HI,ISO} = 1.823 times 10^{18} int left[ tau_{rm tot} tim
es {rm T_B} right] / left[ 1 - e^{-tau_{rm tot}} right] {rm dV}$, where $tau_{rm tot}$ is the total HI 21cm optical depth along the sightline and ${rm T_B}$ is the measured brightness temperature. We have used a Monte Carlo simulation to quantify the accuracy of the isothermal estimate by comparing the derived $N_{rm HI,ISO}$ with the true HI column density $N_{rm HI}$. The simulation was carried out for a wide range of sightlines, including gas in different temperature phases and random locations along the path. We find that the results are statistically insensitive to the assumed gas temperature distribution and the positions of different phases along the line of sight. The median value of the ratio of the true H{sc i} column density to the isothermal estimate, $N_{rm HI}/{N_{rm HI, ISO}}$, is within a factor of 2 of unity while the 68.2% confidence intervals are within a factor of $approx 3$ of unity, out to high HI column densities, $le 5 times 10^{23}$,cm$^{-2}$ per 1 km s$^{-1}$ channel, and high total optical depths, $le 1000$. The isothermal estimator thus provides a significantly better measure of the HI column density than other methods, within a factor of a few of the true value even at the highest columns, and should allow us to directly probe the existence of high HI column density gas in the Milky Way.
We report results from a deep search for redshifted HI 21cm absorption from eight damped Lyman-$alpha$ absorbers (DLAs) detected in our earlier optical survey for DLAs towards radio-loud quasars. HI 21cm absorption was detected from the $z = 2.192$ D
LA towards TXS2039+187, only the sixth case of such a detection at $z > 2$, while upper limits on the HI 21cm optical depth were obtained in six other DLAs at $z > 2$. Our detection of HI 21cm absorption in the eighth system, at $z = 2.289$ towards TXS0311+430, has been reported earlier. We also present high spatial resolution images of the background quasars at frequencies close to the redshifted HI 21cm line frequency, allowing us to estimate the covering factor of each DLA, and then determine its spin temperature $T_s$. For three non-detections of HI 21cm absorption, we obtain strong lower limits on the spin temperature, $T_s gtrsim 790$ K, similar to the bulk of the high-$z$ DLA population; three other DLAs yield weak lower limits, $T_s > 140-400$ K. However, for the two DLAs with detections of HI 21cm absorption, the derived spin temperatures are both low $T_s = (160 pm 35) times (f/0.35)$ K for the $z = 2.192$ DLA towards TXS2039+187 and $T_s = (72 pm 18) times (f/0.52)$ K for the $z = 2.289$ DLA towards TXS0311+430. These are the first two DLAs at $z > 1$ with $T_s$ values comparable to those obtained in local spiral galaxies. Based on the observed correlations between spin temperature and metallicity and velocity spread and metallicity in DLAs, we suggest that the hosts of the two absorbers are likely to be massive, high-metallicity galaxies.
We report Westerbork Synthesis Radio Telescope and Arecibo Telescope observations of the redshifted satellite OH-18cm lines at $z sim 0.247$ towards PKS1413+135. The conjugate nature of these lines, with one line in emission and the other in absorpti
on, but with the same shape, implies that the lines arise in the same gas. The satellite OH-18cm line frequencies also have different dependences on the fine structure constant $alpha$, the proton-electron mass ratio $mu = m_p/m_e$, and the proton gyromagnetic ratio $g_p$. Comparisons between the satellite line redshifts in conjugate systems can hence be used to probe changes in $alpha$, $mu$, and $g_p$, with few systematic effects. The technique yields the expected null result when applied to Cen.A, a nearby conjugate satellite system. For the $z sim 0.247$ system towards PKS1413+135, we find, on combining results from the two telescopes, that $[Delta G/G] = (-1.18 pm 0.46) times 10^{-5}$ (weighted mean), where $G = g_p [mu alpha^2]^{1.85}$; this is tentative evidence (with $2.6 sigma$ significance, or at 99.1% confidence) for a smaller value of $alpha$, $mu$, and/or $g_p$ at z~0.247, i.e. at a lookback time of ~2.9 Gyrs. If we assume that the dominant change is in $alpha$, this implies $[Delta alpha /alpha ] = (-3.1 pm 1.2) times 10^{-6}$. We find no evidence that the observed offset might be produced by systematic effects, either due to observational or analysis procedures, or local conditions in the molecular cloud.
We report results from a deep high-frequency search for pulsars within the central parsec of Sgr A* using the Green Bank Telescope. The observing frequency of 15 GHz was chosen to maximize the likelihood of detecting normal pulsars (i.e. with periods
of $sim 500$,ms and spectral indices of $sim -1.7$) close to Sgr A*, that might be used as probes of gravity in the strong-field regime; this is the highest frequency used for such pulsar searches of the Galactic Center to date. No convincing candidate was detected in the survey, with a $10sigma$ detection threshold of $sim 10 mu$Jy achieved in two separate observing sessions. This survey represents a significant improvement over previous searches for pulsars at the Galactic Center and would have detected a significant fraction ($gtrsim 5%) of the pulsars around Sgr A*, if they had properties similar to those of the known population. Using our best current knowledge of the properties of the Galactic pulsar population and the scattering material toward Sgr A*, we estimate an upper limit of 90 normal pulsars in orbit within the central parsec of Sgr A*.
