Robust scaling in fusion science: case study for the L-H power threshold

In regression analysis for deriving scaling laws in the context of fusion studies, standard regression methods are usually applied, of which ordinary least squares (OLS) is the most popular. However, concerns have been raised with respect to several assumptions underlying OLS in its application to fusion data. More sophisticated statistical techniques are available, but they are not widely used in the fusion community and, moreover, the predictions by scaling laws may vary significantly depending on the particular regression technique. Therefore we have developed a new regression method, which we call geodesic least squares regression (GLS), that is robust in the presence of significant uncertainty on both the data and the regression model. The method is based on probabilistic modeling of all variables involved in the scaling expression, using adequate probability distributions and a natural similarity measure between them (geodesic distance). In this work we revisit the scaling law for the power threshold for the L-to-H transition in tokamaks, using data from the multi-machine ITPA databases. Depending on model assumptions, OLS can yield different predictions of the power threshold for ITER. In contrast, GLS regression delivers consistent results. Consequently, given the ubiquity and importance of scaling laws and parametric dependence studies in fusion research, GLS regression is proposed as a robust and easily implemented alternative to classic regression techniques

The elusive HI-> H2 transition in high-z damped Lyman-alpha systems

We study the H2 molecular content in high redshift damped Lyman-alpha systems (DLAs) as a function of the HI column density. We find a significant increase of the H2 molecular content around log N(HI) (cm^-2)~21.5-22, a regime unprobed until now in intervening DLAs, beyond which the majority of systems have log N(H2) > 17. This is in contrast with lines of sight towards nearby stars, where such H2 column densities are always detected as soon as log N(HI)>20.7. This can qualitatively be explained by the lower average metallicity and possibly higher surrounding UV radiation in DLAs. However, unlike in the Milky Way, the overall molecular fractions remain modest, showing that even at a large N(HI) only a small fraction of overall HI is actually associated with the self-shielded H2 gas. Damped Lyman-alpha systems with very high-N(HI) probably arise along quasar lines of sight passing closer to the centre of the host galaxy where the gas pressure is higher. We show that the colour changes induced on the background quasar by continuum (dust) and line absorption (HI Lyman and H2 Lyman & Werner bands) in DLAs with log N(HI)~22 and metallicity ~1/10 solar is significant, but not responsible for the long-discussed lack of such systems in optically selected samples. Instead, these systems are likely to be found towards intrinsically fainter quasars that dominate the quasar luminosity function. Colour biasing should in turn be severe at higher metallicities.Comment: accepted for publication in A&A letter

[O II] nebular emission from Mg II absorbers: Star formation associated with the absorbing gas

We present nebular emission associated with 198 strong Mg II absorbers at 0.35 $\le z \le$ 1.1 in the fibre spectra of quasars from the Sloan Digital Sky Survey. Measured [O II] luminosities (L$_{[O II]}$) are typical of sub-L$^{\star}$ galaxies with derived star formation rate (uncorrected for fibre losses and dust reddening) in the range of 0.5-20 ${\rm M_\odot\ yr^{-1}}$. Typically less than$\sim$3equivalent width,$W_{2796}$$\ge$2 \AA, show L$_{[O II]} \ge 0.3$L$^{\star}_{[O II]}$. The detection rate is found to increase with increasing$W_{2796}$and$z$. No significant correlation is found between$W_{2796}$and L$_{[O II]}$even when we restrict the samples to narrow$z$-ranges. A strong correlation is seen between L$_{[O II]}$and$z$. While this is expected from the luminosity evolution of galaxies, we show finite fibre size plays a very crucial role in this correlation. The measured nebular line ratios (like [O III]/[O II] and [O III]/H$\beta$) and their$z$evolution are consistent with those of galaxies detected in deep surveys. Based on the median stacked spectra, we infer the average metallicity (log Z$\sim$8.3), ionization parameter (log$q$$\sim$7.5) and stellar mass (log (M/M$_\odot$)$\sim$9.3). The Mg II systems with nebular emission typically have$W_{2796}$$\ge 2$\AA, Mg II doublet ratio close to 1 and W(Fe II$\lambda$2600)/$W_{2796}$$\sim 0.5$as often seen in damped Ly$\alpha$and 21-cm absorbers at these redshifts. This is the biggest reported sample of [O II] emission from Mg II absorbers at low impact parameters ideally suited for probing various feedback processes at play in$z\le 1$ galaxies.Comment: 18 Pages, 18 Figures, 4 Tables (Accepted for the Publication in MNRAS main-journal

Detection of 21-cm, H2 and Deuterium absorption at z>3 along the line-of-sight to J1337+3152

We report the detection of 21-cm and H2 absorption lines in the same DLA system (log N(HI)=21.36+-0.10) at zabs=3.17447 towards SDSSJ133724+315254 (z=3.174). We estimate the spin temperature of the gas to be, Ts~600 K, intermediate between the expected values for cold and warm neutral media. This suggests that the HI absorption originates from a mixture of different phases. The total molecular fraction is low, f=10^-7, and H2 rotational level populations are not in equilibrium. The average abundance of the alpha-elements is, [S/H]=-1.45. N and Fe are found underabundant with respect to alpha-elements by ~1.0 dex and ~0.5 dex respectively. Using photoionization models we conclude that the gas is located more than 270 kpc away from the QSO. While the position of 21-cm absorption line coincides with the H2 velocity profile, their centroid are shifted by 2.7+-1.0 km/s from each other. However, the position of the strongest metal absorption component matches the position of the 21-cm absorption line within 0.5 km/s. From this, we constrain the variation of the combination of fundamental constants x=alpha^2 Gp/mu, Delta x/x=-(1.7+-1.7)x10^-6. This system is unique as we can at the same time have an independent constrain on mu using H2 lines. However only Werner band absorption lines are seen and the range of sensitivity coefficients is too narrow to provide a stringent constraint: Delta mu/mu <= 4.0x10^-4. The VLT/UVES spectrum reveals another DLA at zabs=3.16768 with log N(HI)=20.41+-0.15 and low metallicity, [Si/H]=-2.68+-0.11. We derive log N(DI)/N(HI)=-(4.93+-0.15) in this system. This is a factor of two smaller than the value expected from the best fitted value of Omega_b from the WMAP 5 yr data. This confirms the presence of astration of deuterium even at very low metallicity. [abridged]Comment: 14 pages, 11 figures, 4 tables, accepted for publication in MNRA

Neutral atomic-carbon QSO absorption-line systems at z>1.5: Sample selection, HI content, reddening, and 2175 A extinction feature

We present the results of a search for cold gas at high redshift along QSO lines-of-sight carried out without any a priori assumption on the neutral atomic-hydrogen (HI) content of the absorbers. To do this, we systematically looked for neutral-carbon (CI) 1560,1656 transition lines in low-resolution QSO spectra from the SDSS database. We built up a sample of 66 CI absorbers with redshifts 1.5<z<3.1 and equivalent widths 0.1<W_r(1560)<1.7 A. The completeness limit of our survey is W_r,lim(1560)~0.4 A. CI systems stronger than that are more than one hundred-times rarer than DLAs at z_abs=2.5. The number of CI systems per unit redshift increases significantly below z=2. We suggest that the CI absorbers are closely related to the process of star formation and the production of dust in galaxies. We derive the HI content of the CI systems and find that a majority of them are sub-DLAs with N(HI)~10^20 atoms cm^-2. The dust content of these absorbers is yet significant as seen from the redder optical colours of the background QSOs and their reddened SEDs. The overall N(HI) distribution of CI systems is relatively flat however. As a consequence, among the CI systems classifying as DLAs there is a probable excess of strong DLAs with log N(HI)>21 compared to systematic DLA surveys. We study empirical relations between W_r(CI), N(HI), E(B-V) and the strength of the 2175 A extinction feature, the latter being detected in about 30% of the CI absorbers. We show that the 2175 A feature is weak compared to Galactic lines-of-sight exhibiting the same amount of reddening. This is probably the consequence of current or past star formation in the vicinity of the CI systems. We also find that the strongest CI systems tend to have the largest amounts of dust and that the metallicity of the gas and its molecular fraction is likely to be high in a large number of cases.Comment: Accepted for publication in Astronomy & Astrophysics Main Journal on 20 April 201

Search for cold gas in z>2 damped Lyman-alpha systems: 21-cm and H_2 absorption

(Abridged) We present the results of a systematic GBT and GMRT survey for 21-cm absorption in a sample of 10 DLAs at 2<z_abs<3.4. Analysis of L-band VLBA images of the background QSOs are also presented. We detect 21-cm absorption in only one DLA (at z_abs = 3.1745 towards J1337+3152). Combining our data with the data from the literature (a sample of 28 DLAs) and assuming the measured core fraction at milliarcsecond scale to represent the gas covering factor, we find that the HI gas in DLAs at z> 2 is predominantly constituted by WNM. The detection rate of 21-cm absorption seems to be higher for systems with higher N(HI) or metallicity. However, no clear correlation is found between the integrated 21-cm optical depth (or spin temperature) and either N(HI), metallicity or velocity spread of the low ionization species. There are 13 DLAs in our sample for which high resolution optical spectra covering the expected wavelength range of H_2 absorption are available. We report the detection of H_2 molecules in the z_abs = 3.3871 21-cm absorber towards J0203+1134 (PKS 0201+113). In 8 cases, neither H_2 nor 21-cm absorption are detected. The lack of 21-cm and H_2 absorption in these systems can be explained if most of the HI in these DLAs originate from low density high temperature gas. In one case we have a DLA with 21-cm absorption not showing H_2 absorption. In two cases, both species are detected but do not originate from the same velocity component. In the remaining 2 cases 21-cm absorption is not detected despite the presence of H_2 with evidence for the presence of cold gas. All this is consistent with the idea that the H_2 components seen in DLAs are compact (with sizes of < 15 pc) and contain only a small fraction (i.e typically <10%) of the total N(HI) measured in the DLAs.Comment: Accepted for publication in MNRA

Dear Professor: A Chronicle of Absences

Dear Professor: A Chronicle of Absences is a collection of over two hundred often involuntarily comical emails in which students excuse themselves for missing class. The result is a satirical yet unexpectedly sympathetic collective portrait of modern-day academia where both students and teachers feel pressured to comply with the impositions of hyper-connectivity

A ~6 Mpc overdensity at z = 2.7 detected along a pair of quasar sight lines: filament or protocluster?

Simulations predict that gas in the intergalactic medium (IGM) is distributed in filamentary structures that connect dense galaxy clusters and form the cosmic web. These structures of predominantly ionized hydrogen are difficult to observe directly due to their lack of emitting regions. We serendipitously detected an overdensity of log N(HI) > 18.0 absorbers at z = 2.69 along the lines of sight toward a pair of background quasars. Three main absorption regions spanning ~2000 km/s (corresponding to 6.4 (h_70)^(-1) Mpc proper) are coincident in the two lines of sight, which are separated by ~90 (h_70)^(-1) kpc transverse proper distance. Two regions have [Fe/H] < -1.9 and correspond to mild overdensities in the IGM gas. The third region is a sub-DLA with [Fe/H] = -1.1 that is probably associated with a galaxy. We discuss the possibility that the lines of sight probe along the length of a filament or intercept a galaxy protocluster

A translucent interstellar cloud at z=2.69: CO, H2 and HD in the line-of-sight to SDSS J123714.60+064759.5

We present the analysis of a sub-DLA system (log N(H^0)=20.0+/-0.15, z_abs=2.69) toward SDSS J123714+064759 (z_em=2.78). Using the VLT/UVES and X-shooter spectrographs, we detect H2, HD and CO molecules in absorption with log N(H2,HD,CO)=(19.21,14.48,14.17). The overall metallicity of the system is super-solar ([Zn/H]=+0.34) and iron is highly depleted ([Fe/Zn]=-1.39), revealing metal-rich and dusty gas. The strongest H2 component does not coincide with the centre of the HI absorption. This implies that the molecular fraction in this component, f=2N(H2)/(2N(H2)+N(H^0)), is larger than the mean molecular fraction =1/4 in the system. This is supported by the detection of Cl^0 associated with this H2-component having N(Cl^0)/N(Cl^+)>0.4. Since Cl^0 is tied up to H2 by charge exchange reactions, this means that the molecular fraction in this component is not far from unity. The size of the molecular cloud is probably smaller than 1pc. Both the CO/H2=10^-5 and CO/C^0~1 ratios for f>0.24 indicate that the cloud classifies as translucent, i.e., a regime where carbon is found both in atomic and molecular form. The corresponding extinction, Av=0.14, albeit lower than the definition of a translucent sightline (based on extinction properties), is high for the observed H^0 column density. This means that intervening clouds with similar local properties but with larger column densities could be missed by current magnitude-limited QSO surveys. The excitation of CO is dominated by radiative interaction with the Cosmic Microwave Background Radiation (CMBR) and we derive Tex(CO)=10.5+0.8-0.6 K when TCMBR(z=2.69)=10.05 K is expected. The astration factor of deuterium -with respect to the primordial D/H ratio- is only about 3. This can be the consequence of accretion of unprocessed gas from the intergalactic medium onto the associated galaxy. [abridged]Comment: 17 pages, 21 figures, 8 tables, accepted for publication in A&