Leaching of selected PGMs : a thermodynamic and electrochemical study employing less aggressive lixiviants

Includes abstract.Includes bibliographical references (leaves 74-79).Historically the platinum group metals (PGMs) have been, and are still being dissolved by means of rather aggressive methods, e.g. aqua regia. Limited research has been conducted into the dissolution of the PGMs using different oxidizing agents. The dissolution of gold on the other hand has been afforded extensive research, and numerous papers and review articles have been published on the subject. The last number of years has seen the biggest application by volume of the PGMs as part of autocatalysts towards the degradation of harmful motor vehicle exhaust gases. This has subsequently sparked research into the recovery of specifically platinum, palladium, and rhodium from spent autocatalysts. Currently pyrometallurgical recovery of PGMs is being employed predominantly. A hydrometallurgical process on the other hand is, based on current technology, still a rather aggressive process that makes for high maintenance costs and an unpleasant environment. Gold has traditionally been dissolved by making use of cyanide, which is still the major route for gold dissolution. Due to environmental concerns lixiviants such as thiosulphate (S2O3 2-), thiourea (H2NCSNH2), and thiocyanate (SCN-) are gaining acceptance due to them being more environmentally friendly and giving good recoveries. These ‘softer’ alternatives have however not been tested on the PGMs. It is therefore the aim of this study to obtain an improved understanding of the leaching of the PGMs using lixiviants less aggressive than aqua-regia. These lixiviants include (i) SCN-, (ii) S2O3 2-, (iii) H2NCSNH2, and (iv) AlCl3/HCl. A thermodynamic study highlighted the fact that thermodynamic data for platinum-, palladium- and rhodium complexes are basically non-existent. To therefore obtain a clearer thermodynamic understanding of the leaching of the platinum group metals by means of these alternative lixiviants, future detailed speciation and thermodynamic investigations need to be conducted. An exploratory electrochemical investigation focusing on open circuit potentials and potentiodynamic scans, showed AlCl3 / HCl / NaOCl to be a good candidate for the leaching of the platinum group metals followed by SCN- / Fe3+ and CS(NH2)2 / Fe3+. Actual leach results, employing virgin autocatalysts as sample material, again highlighted the potential of AlCl3 / HCl / NaOCl as being a good lixiviant system. The surprise package, however, has been SCN- / Fe3+ that rendered very good results for Pd and Pt

Distant red galaxies in the Hubble Ultra Deep Field

We take advantage of the Hubble Ultra Deep Field (UDF) data to study the restframe optical and ultra violet (UV) morphologies of the novel population of Distant Red Galaxies (DRGs). Six galaxies with J-Ks > 2.3 are found to Ks=21.5, five of which have photometric redshifts z_phot > 2, corresponding to a surface density of 0.9/arcmin^2. The surface brightness distributions of the z_phot > 2 galaxies are better represented by exponential disks than R^{1/4}-laws. Two of the z_phot > 2 galaxies are extended, while three have compact morphologies. The restframe optical morphology of the z_phot > 2 galaxies is quite different from the restframe UV morphology: all the galaxies have red central components which dominate in the NICMOS H_{160}-band images, and distinct off-center blue features which show up in (and often dominate) the ACS images. The mean measured effective radius of the z_phot > 2 galaxies is =1.9+/-1.4 kpc, similar (within the errors) to the mean size of LBGs at similar redshifts. All the DRGs are resolved in the ACS images, while four are resolved in the NICMOS images. Two of the z_phot > 2 galaxies are bright X-ray sources and hence host AGN. The diverse restframe optical and UV morphological properties of DRGs derived here suggest that they have complex stellar populations, consisting of both evolved populations that dominate the mass and the restframe optical light, and younger populations, which show up as patches of star formation in the restframe UV light; in many ways resembling the properties of normal local galaxies. This interpretation is supported by fits to the broadband SEDs, which for all five z_phot > 2 are best represented by models with extended star formation histories and substantial amounts of dust.Comment: Accepted for publication in APJ

The Relation between Dynamical Mass-to-light Ratio and Color for Massive Quiescent Galaxies out to z ~ 2 and Comparison with Stellar Population Synthesis Models

Article / Letter to editorSterrewach

The Detection of a Red Sequence of Massive Field Galaxies at z~2.3 and its Evolution to z~0

The existence of massive galaxies with strongly suppressed star formation at z~2.3, identified in a previous paper, suggests that a red sequence may already be in place beyond z=2. In order to test this hypothesis, we study the rest-frame U-B color distribution of massive galaxies at 2<z<3. The sample is drawn from our near-infrared spectroscopic survey for massive galaxies. The color distribution shows a statistically significant (>3 sigma) red sequence, which hosts ~60% of the stellar mass at the high-mass end. The red-sequence galaxies have little or no ongoing star formation, as inferred from both emission-line diagnostics and stellar continuum shapes. Their strong Balmer breaks and their location in the rest-frame U-B, B-V plane indicate that they are in a post-starburst phase, with typical ages of ~0.5-1.0 Gyr. In order to study the evolution of the red sequence, we compare our sample with spectroscopic massive galaxy samples at 0.02<z<0.045 and 0.6<z<1.0. The rest-frame U-B color reddens by ~0.25 mag from z~2.3 to the present at a given mass. Over the same redshift interval, the number and stellar mass density on the high-mass end (>10^11 Msol) of the red sequence grow by factors of ~8 and ~6, respectively. We explore simple models to explain the observed evolution. Passive evolution models predict too strong d(U-B), and produce z~0 galaxies that are too red. More complicated models that include aging, galaxy transformations, and red mergers can explain both the number density and color evolution of the massive end of the red sequence between z~2.3 and the present.Comment: Accepted for publication in the Astrophysical Journa

Stellar Kinematics of z ~ 2 Galaxies and the Inside-out Growth of Quiescent Galaxies

Using stellar kinematics measurements, we investigate the growth of massive, quiescent galaxies from z ~{} 2 to today. We present X-Shooter spectra from the UV to NIR and dynamical mass measurements of five quiescent massive ({gt}10$^{11}$ M $_{⊙}$) galaxies at z ~{} 2. This triples the sample of z {gt} 1.5 galaxies with well-constrained ({$delta$}{$σ$} {lt} 100 km s$^{-1}$) velocity dispersion measurements. From spectral population synthesis modeling we find that these galaxies have stellar ages that range from 0.5 to 2 Gyr, with no signs of ongoing star formation. We measure velocity dispersions (290-450 km s$^{-1}$) from stellar absorption lines and find that they are 1.6-2.1 times higher than those of galaxies in the Sloan Digital Sky Survey at the same mass. Sizes are measured using GALFIT from Hubble Space Telescope Wide Field Camera 3 H $_{160}$ and UDS K-band images. The dynamical masses correspond well to the spectral energy distribution based stellar masses, with dynamical masses that are ~{}15% higher. We find that M $_{*}$/M $_{dyn}$ may decrease slightly with time, which could reflect the increase of the dark matter fraction within an increasing effective radius. We combine different stellar kinematic studies from the literature and examine the structural evolution from z ~{} 2 to z ~{} 0: we confirm that at fixed dynamical mass, the effective radius increases by a factor of ~{}2.8, and the velocity dispersion decreases by a factor of ~{}1.7. The mass density within one effective radius decreases by a factor of ~{}20, while within a fixed physical radius (1 kpc) it decreases only mildly (factor of ~{}2). When we allow for an evolving mass limit by selecting a population of galaxies at fixed number density, a stronger size growth with time is found (factor of ~{}4), velocity dispersion decreases by a factor of ~{}1.4, and interestingly, the mass density within 1 kpc is consistent with no evolution. This finding suggests that massive quiescent galaxies at z ~{} 2 grow inside out, consistent with the expectations from minor mergers

Stellar Kinematics of z ~ 2 Galaxies and the Inside-out Growth of Quiescent Galaxies

Using stellar kinematics measurements, we investigate the growth of massive, quiescent galaxies from z ~{} 2 to today. We present X-Shooter spectra from the UV to NIR and dynamical mass measurements of five quiescent massive ({gt}10$^{11}$ M $_{⊙}$) galaxies at z ~{} 2. This triples the sample of z {gt} 1.5 galaxies with well-constrained ({$delta$}{$σ$} {lt} 100 km s$^{-1}$) velocity dispersion measurements. From spectral population synthesis modeling we find that these galaxies have stellar ages that range from 0.5 to 2 Gyr, with no signs of ongoing star formation. We measure velocity dispersions (290-450 km s$^{-1}$) from stellar absorption lines and find that they are 1.6-2.1 times higher than those of galaxies in the Sloan Digital Sky Survey at the same mass. Sizes are measured using GALFIT from Hubble Space Telescope Wide Field Camera 3 H $_{160}$ and UDS K-band images. The dynamical masses correspond well to the spectral energy distribution based stellar masses, with dynamical masses that are ~{}15% higher. We find that M $_{*}$/M $_{dyn}$ may decrease slightly with time, which could reflect the increase of the dark matter fraction within an increasing effective radius. We combine different stellar kinematic studies from the literature and examine the structural evolution from z ~{} 2 to z ~{} 0: we confirm that at fixed dynamical mass, the effective radius increases by a factor of ~{}2.8, and the velocity dispersion decreases by a factor of ~{}1.7. The mass density within one effective radius decreases by a factor of ~{}20, while within a fixed physical radius (1 kpc) it decreases only mildly (factor of ~{}2). When we allow for an evolving mass limit by selecting a population of galaxies at fixed number density, a stronger size growth with time is found (factor of ~{}4), velocity dispersion decreases by a factor of ~{}1.4, and interestingly, the mass density within 1 kpc is consistent with no evolution. This finding suggests that massive quiescent galaxies at z ~{} 2 grow inside out, consistent with the expectations from minor mergers

Faint, Evolving Radio AGN in SDSS Luminous Red Galaxies

We detect and study the properties of faint radio AGN in Luminous Red Galaxies (LRGs). The LRG sample comprises 760,000 objects from a catalog of LRG photometric redshifts constructed from the Sloan Digital Sky Survey (SDSS) imaging data, and 65,000 LRGs from the SDSS spectroscopic sample. These galaxies have typical 1.4 GHz flux densities in the 10s-100s of microJy, with the contribution from a low-luminosity AGN dominating any contribution from star formation. To probe the radio properties of such faint objects, we employ a stacking technique whereby FIRST survey image cutouts at each optical LRG position are sorted by the parameter of interest and median-combined within bins. We find that median radio luminosity scales with optical luminosity (L_opt) as L_1.4 GHz ~ L_opt^(beta), where beta appears to decrease from beta ~ 1 at z = 0.4 to beta ~ 0 at z = 0.7, a result which could be indicative of AGN cosmic downsizing. We also find that the overall LRG population, which is dominated by low-luminosity AGN, experiences significant cosmic evolution between z = 0.2 and z = 0.7. This implies a considerable increase in total AGN heating for these massive ellipticals with redshift. By matching against the FIRST catalog, we investigate the incidence and properties of LRGs associated with double-lobed (FR I/II) radio galaxies. (Abridged)Comment: 12 pages, 9 figures, Accepted by A

Spatially resolved kinematics in the central 1 kpc of a compact star-forming galaxy at z=2.3 from ALMA CO observations

We present high spatial resolution (FWHM$\sim$0.14'') observations of the CO($8-7$) line in GDS-14876, a compact star-forming galaxy at $z=2.3$ with total stellar mass of $\log(M_{\star}/M_{\odot})=10.9$. The spatially resolved velocity map of the inner $r\lesssim1$~kpc reveals a continous velocity gradient consistent with the kinematics of a rotating disk with $v_{\rm rot}(r=1\rm kpc)=163\pm5$ km s$^{-1}$ and $v_{\rm rot}/\sigma\sim2.5$. The gas-to-stellar ratios estimated from CO($8-7$) and the dust continuum emission span a broad range, $f^{\rm CO}_{\rm gas}=M_{\rm gas}/M_{\star}=13-45\%$ and $f^{\rm cont}_{\rm gas}=50-67\%$, but are nonetheless consistent given the uncertainties in the conversion factors. The dynamical modeling yields a dynamical mass of$\log(M_{\rm dyn}/M_{\odot})=10.58^{+0.5}_{-0.2}$ which is lower, but still consistent with the baryonic mass, $\log$(M$_{\rm bar}$= M$_{\star}$ + M$^{\rm CO}_{\rm gas}$/M$_{\odot}$)$=11.0$, if the smallest CO-based gas fraction is assumed. Despite a low, overall gas fraction, the small physical extent of the dense, star-forming gas probed by CO($8-7$), $\sim3\times$ smaller than the stellar size, implies a strong concentration that increases the gas fraction up to $f^{\rm CO, 1\rm kpc}_{\rm gas}\sim 85\%$ in the central 1 kpc. Such a gas-rich center, coupled with a high star-formation rate, SFR$\sim$ 500 M$_{\odot}$ yr$^{-1}$, suggests that GDS-14876 is quickly assembling a dense stellar component (bulge) in a strong nuclear starburst. Assuming its gas reservoir is depleted without replenishment, GDS-14876 will quickly ($t_{\rm depl}\sim27$ Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.Comment: Accepted for Publication in ApJL. Kinematic maps are shown in Figures 2 and

Ultradeep Infrared Array Camera Observations of sub-L* z~7 and z~8 Galaxies in the Hubble Ultra Deep Field: the Contribution of Low-Luminosity Galaxies to the Stellar Mass Density and Reionization

We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame optical) fluxes of 14 newly WFC3/IR-detected z=7 z_{850}-dropout galaxies and 5 z=8 Y_{105}-dropout galaxies. The WFC3/IR depth and spatial resolution allow accurate removal of contaminating foreground light, enabling reliable flux measurements at 3.6 micron and 4.5 micron. None of the galaxies are detected to [3.6]=26.9 (AB, 2 sigma), but a stacking analysis reveals a robust detection for the z_{850}-dropouts and an upper limit for the Y_{105}-dropouts. We construct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxes and fit stellar population synthesis models to derive mean redshifts, stellar masses, and ages. For the z_{850}-dropouts, we find z=6.9^{+0.1}_{-0.1}, (U-V)_{rest}=0.4, reddening A_V=0, stellar mass M*=1.2^{+0.3}_{-0.6} x 10^9 M_sun (Salpeter IMF). The best-fit ages ~300Myr, M/L_V=0.2, and SSFR=1.7Gyr^{-1} are similar to values reported for luminous z=7 galaxies, indicating the galaxies are smaller but not younger. The sub-L* galaxies observed here contribute significantly to the stellar mass density and under favorable conditions may have provided enough photons for sustained reionization at 7<z<11. In contrast, the z=8.3^{+0.1}_{-0.2} Y_{105}-dropouts have stellar masses that are uncertain by 1.5 dex due to the near-complete reliance on far-UV data. Adopting the 2 sigma upper limit on the M/L(z=8), the stellar mass density to M_{UV,AB} < -18 declines from rho*(z=7)=3.7^{+1.0}_{-1.8} x 10^6 M_sun Mpc^{-3} to rho*(z=8) < 8 x 10^5 M_sun Mpc^{-3}, following (1+z)^{-6} over 3<z<8. Lower masses at z=8 would signify more dramatic evolution, which can be established with deeper IRAC observations, long before the arrival of the James Webb Space Telescope.Comment: 6 pages, 3 figures, 2 tables, emulateapj, accepted for publication in ApJ

Welcome to the Twilight Zone: The Mid-Infrared Properties of Poststarburst Galaxies

We investigate the optical and Wide-field Survey Explorer (WISE) colors of "E+A" identified post-starburst galaxies, including a deep analysis on 190 post-starbursts detected in the 2{\mu}m All Sky Survey Extended Source Catalog. The post-starburst galaxies appear in both the optical green valley and the WISE Infrared Transition Zone (IRTZ). Furthermore, we find that post-starbursts occupy a distinct region [3.4]-[4.6] vs. [4.6]-[12] WISE colors, enabling the identification of this class of transitioning galaxies through the use of broad-band photometric criteria alone. We have investigated possible causes for the WISE colors of post-starbursts by constructing a composite spectral energy distribution (SED), finding that mid-infrared (4-12{\mu}m) properties of post-starbursts are consistent with either 11.3{\mu}m polycyclic aromatic hydrocarbon emission, or Thermally Pulsating Asymptotic Giant Branch (TP-AGB) and post-AGB stars. The composite SED of extended post- starburst galaxies with 22{\mu}m emission detected with signal to noise >3 requires a hot dust component to produce their observed rising mid-infrared SED between 12 and 22{\mu}m. The composite SED of WISE 22{\mu}m non-detections (S/N<3), created by stacking 22{\mu}m images, is also flat, requiring a hot dust component. The most likely source of this mid-infrared emission of these E+A galaxies is a buried active galactic nucleus. The inferred upper limit to the Eddington ratios of post-starbursts are 1e-2 to 1e-4, with an average of 1e-3. This suggests that AGNs are not radiatively dominant in these systems. This could mean that including selections able to identify active galactic nuclei as part of a search for transitioning and post-starburst galaxies would create a more complete census of the transition pathways taken as a galaxy quenches its star formation.Comment: 13 pages, 11 figures, accepted for publication in the Astrophysical Journa