We discuss two mistreatments of damped Lya (DLA) kinematic analysis that were first performed by Haehnelt, Steinmetz, & Rauch (1998; hereafter HSR98) and have recently been repeated by Hong et al. (2010; arXiv:1008.4242v1, arXiv:1008.4242v2; hereafte
r H10). Each mistreatment led to the improper excising of simulated absorption profiles. Specifically, their analyses are strictly biased against DLA sightlines that have low HI column density log NHI < 20.5, very high NHI values, and (for all NHI) sightlines with low velocity width Dv (<30 km/s for HSR98; <[20-30] km/s for H10). None of these biases exist in the observational analysis. We suspect these mistreatments compromise the results that followed. Hopefully this posting will prevent their repetition in the future.
We probe the physical conditions in high redshift galaxies, specifically, the Damped Lyman-alpha Systems (DLAs) using neutral carbon (CI) fine structure lines and molecular hydrogen (H2). We report five new detections of CI and analyze the CI in an a
dditional 2 DLAs with previously published data. We also present one new detection of H2 in a DLA. We present a new method of analysis that simultaneously constrains emph{both} the volume density and the temperature of the gas, as opposed to previous studies that a priori assumed a gas temperature. We use only the column density of CI measured in the fine structure states and the assumption of ionization equilibrium in order to constrain the physical conditions in the gas. We present a sample of 11 CI velocity components in 6 DLAs and compare their properties to those derived by the global CII* technique. The resulting median values for this sample are: <n(HI)> = 69 cm^{-3}, <T> = 50 K, and <log(P/k)> = 3.86 cm^{-3} K, with standard deviations, sigma_{n(HI)} = 134 cm^{-3}, sigma_T = 52 K, and sigma_{log(P/k)} = 3.68 cm^{-3} K. This can be compared with the integrated median values for the same DLAs : <n(HI)> = 2.8 cm^{-3}, <T> = 139 K, and <log(P/k)> = 2.57 cm^{-3} K, with standard deviations sigma_{n(HI)} = 3.0 cm^{-3}, sigma_T = 43 K, and sigma_{log(P/k)} = 0.22 cm^{-3} K. Interestingly, the pressures measured in these high redshift CI clouds are similar to those found in the Milky Way. We conclude that the CI gas is tracing a higher-density, higher-pressure region, possibly indicative of post-shock gas or a photodissociation region on the edge of a molecular cloud. We speculate that these clouds may be direct probes of the precursor sites of star formation in normal galaxies at high redshift.
We present the first direct evidence for cold gas in a high redshift DLA galaxy. We measured several multiplets of weak neutral carbon (CI) transitions in order to perform a curve of growth analysis. A delta chi-squared test constrains the best fit D
oppler parameter, b = 0.33_{-0.04}^{+0.05} km/s, and logN(CI) = 13.30 +- 0.2 cm^-2. This Doppler parameter constrains the kinetic temperature of the gas to T <= 78 K (T <= 115 K, 2 sigma). We used the associated CI fine structure lines to constrain the volume density of the gas, n(HI) ~ 40 - 200 cm^-3 (2 sigma), resulting in a lower limit on the cloud size of approximately 0.1 - 1 parsec. While it is difficult to determine the metallicity of the cold component, the absence of Cr II indicates that the cold cloud suffers a high level of dust depletion. Additionally, the large amount of Lyman and Werner-band molecular hydrogen absorption (log N(H2)_{total} = 19.88 cm^-2, f_{H_2} >= 0.06) with an excitation temperature of T_{ex} = 46 K as determined by the rotational J = 0 and J = 1 states, is consistent with the presence of cold gas. We propose that this cloud may be gravitationally confined and may represent a transition gas-phase from primarily neutral atomic gas, to a colder, denser molecular phase that will eventually host star formation.
We report on an attempt to accurately wavelength calibrate four nights of data taken with the Keck HIRES spectrograph on QSO PHL957, for the purpose of determining whether the fine structure constant was different in the past. Using new software and
techniques, we measured the redshifts of various Ni II, Fe II, Si II, etc. lines in a damped Ly-alpha system at z=2.309. Roughly half the data was taken through the Keck iodine cell which contains thousands of well calibrated iodine lines. Using these iodine exposures to calibrate the normal Th-Ar Keck data pipeline output we found absolute wavelength offsets of 500 m/s to 1000 m/s with drifts of more than 500 m/s over a single night, and drifts of nearly 2000 m/s over several nights. These offsets correspond to an absolute redshift of uncertainty of about Delta z=10^{-5} (Delta lambda= 0.02 Ang), with daily drifts of around Delta z=5x10^{-6} (Delta lambda =0.01 Ang), and multiday drifts of nearly Delta z=2x10^{-5} (0.04 Ang). The causes of the wavelength offsets are not known, but since claimed shifts in the fine structure constant would result in velocity shifts of less than 100 m/s, this level of systematic uncertainty makes may make it difficult to use Keck HIRES data to constrain the change in the fine structure constant. Using our calibrated data, we applied both our own fitting software and standard fitting software to measure (Delta alpha)/alpha, but discovered that we could obtain results ranging from significant detection of either sign, to strong null limits, depending upon which sets of lines and which fitting method was used. We thus speculate that the discrepant results on (Delta alpha)/alpha reported in the literature may be due to random fluctuations coming from under-estimated systematic errors in wavelength calibration and fitting procedure.
We present new results on the frequency distribution of projected HI column densities f(N,X), total comoving covering fraction, and integrated mass densities rho_HI of high redshift, HI `disks from a survey of damped Lya systems (DLAs) in the Sloan D
igital Sky Survey, Data Release 5. For the full sample spanning z=2.2 to 5 [738 DLAs], f(N,X) is well fitted by a double power-law with a break column density N_d = 10^(21.55 +/- 0.04) and low/high-end exponents alpha = -2.00 +/- 0.05, -6.4^{+1.1}_{-1.6}. The shape of f(N,X) is invariant during this redshift interval and also follows the projected surface density distribution of present-day HI disks as inferred from 21cm observations. We conclude that HI gas has been distributed in a self-similar fashion for the past 12Gyr. The normalization of f(N,X), in contrast, decreases by a factor of two during the ~2Gyr interval from z=4 to 2.2 giving corresponding decreases in both the total covering fraction and rho_HI. At z~2, these quantities match the present-day values suggesting no evolution during the past ~10Gyr. We argue that the evolution at early times is driven by `violent processes that removes gas from nearly half the galaxies at z~3 establishing the antecedants of current early-type galaxies. The perceived constancy of rho_HI, meanwhile, implies that HI gas is a necessary but insufficient pre-condition for star formation and that the global star-formation rate is driven by the accretion and condensation of fresh gas from the intergalactic medium.
We report evidence for a bimodality in damped Ly systems (DLAs). Using [C II] 158 mu cooling rates, lc, we find a distribution with peaks at lc=10^-27.4 and 10^-26.6 ergs s^-1 H^-1 separated by a trough at lc^crit ~= lc < 10^-27.0 ergs s^-1 H^-1. We
divide the sample into low cool DLAs with lc < lc^crit and high cool DLAs with lc > lc^crit and find the Kolmogorv-Smirnov probabilities that velocity width, metallicity, dust-to-gas ratio, and Si II equivalent width in the two subsamples are drawn from the same parent population are small. All these quantities are significantly larger in the high cool population, while the H I column densities are indistinguishable in the two populations. We find that heating by X-ray and FUV background radiation is insufficient to balance the cooling rates of either population. Rather, the DLA gas is heated by local radiation fields. The rare appearance of faint, extended objects in the Hubble Ultra Deep Field rules out in situ star formation as the dominant star-formation mode for the high cool population, but is compatible with in situ star formation as the dominant mode for the low cool population. Star formation in the high cool DLAs likely arises in Lyman Break galaxies. We investigate whether these properties of DLAs are analogous to the bimodal properties of nearby galaxies. Using Si II equivalent width as a mass indicator, we construct bivariate distributions of metallicity, lc, and areal SFR versus the mass indicators. Tentative evidence is found for correlations and parallel sequences, which suggest similarities between DLAs and nearby galaxies. We suggest that the transition-mass model provides a plausible scenario for the bimodality we have found. As a result, the bimodality in current galaxies may have originated in DLAs.
