Random mixtures of polycyclic aromatic hydrocarbon spectra match interstellar infrared emission

The mid-infrared (IR; 5-15~$\mu$m) spectrum of a wide variety of astronomical objects exhibits a set of broad emission features at 6.2, 7.7, 8.6, 11.3 and 12.7 $\mu$m. About 30 years ago it was proposed that these signatures are due to emission from a family of UV heated nanometer-sized carbonaceous molecules known as polycyclic aromatic hydrocarbons (PAHs), causing them to be referred to as aromatic IR bands (AIBs). Today, the acceptance of the PAH model is far from settled, as the identification of a single PAH in space has not yet been successful and physically relevant theoretical models involving ``true'' PAH cross sections do not reproduce the AIBs in detail. In this paper, we use the NASA Ames PAH IR Spectroscopic Database, which contains over 500 quantum-computed spectra, in conjunction with a simple emission model, to show that the spectrum produced by any random mixture of at least 30 PAHs converges to the same 'kernel'-spectrum. This kernel-spectrum captures the essence of the PAH emission spectrum and is highly correlated with observations of AIBs, strongly supporting PAHs as their source. Also, the fact that a large number of molecules are required implies that spectroscopic signatures of the individual PAHs contributing to the AIBs spanning the visible, near-infrared, and far infrared spectral regions are weak, explaining why they have not yet been detected. An improved effort, joining laboratory, theoretical, and observational studies of the PAH emission process, will support the use of PAH features as a probe of physical and chemical conditions in the nearby and distant Universe

Neutral carbon and CO in 76 (U)LIRGs and starburst galaxy centers A method to determine molecular gas properties in luminous galaxies

We present fluxes in both neutral carbon [CI] lines at the centers of 76 galaxies with FIR luminosities between 10^{9} and 10^{12} L(o) obtained with Herschel-SPIRE and with ground-based facilities, along with the J=7-6, J=4-3, J=2-1 12CO and J=2-1 13CO line fluxes. We investigate whether these lines can be used to characterize the molecular ISM of the parent galaxies in simple ways and how the molecular gas properties define the model results. In most starburst galaxies, the [CI]/13CO flux ratio is much higher than in Galactic star-forming regions, and it is correlated to the total FIR luminosity. The [CI](1-0)/CO(4-3), the [CI](2-1) (2-1)/CO(7-6), and the [CI] (2-1)/(1-0) flux ratios are also correlated, and trace the excitation of the molecular gas. In the most luminous infrared galaxies (LIRGs), the ISM is fully dominated by dense and moderately warm gas clouds that appear to have low [C]/[CO] and [13CO]/[12CO] abundances. In less luminous galaxies, emission from gas clouds at lower densities becomes progressively more important, and a multiple-phase analysis is required to determine consistent physical characteristics. Neither the CO nor the [CI] velocity-integrated line fluxes are good predictors of H2 column densities in individual galaxies, and X(CI) conversion factors are not superior to X(CO) factors. The methods and diagnostic diagrams outlined in this paper also provide a new and relatively straightforward means of deriving the physical characteristics of molecular gas in high-redshift galaxies up to z=5, which are otherwise hard to determine

The excitation of near-infrared H2 emission in NGC 253

Because of its large angular size and proximity to the Milky Way, NGC 253, an archetypal starburst galaxy, provides an excellent laboratory to study the intricacies of this intense episode of star formation. We aim to characterize the excitation mechanisms driving the emission in NGC 253. Specifically we aim to distinguish between shock excitation and UV excitation as the dominant driving mechanism, using Br\gamma, H_2 and [FeII] as diagnostic emission line tracers. Using SINFONI observations, we create linemaps of Br\gamma, [FeII]_{1.64}, and all detected H_2 transitions. By using symmetry arguments of the gas and stellar gas velocity field, we find a kinematic center in agreement with previous determinations. The ratio of the 2-1 S(1) to 1-0 S(1) H_2 transitions can be used as a diagnostic to discriminate between shock and fluorescent excitation. Using the 1-0 S(1)/2-1 S(1) line ratio as well as several other H_2 line ratios and the morphological comparison between H_2 and Br\gamma and [FeII], we find that excitation from UV photons is the dominant excitation mechanisms throughout NGC 253. We employ a diagnostic energy level diagram to quantitatively differentiate between mechanisms. We compare the observed energy level diagrams to PDR and shock models and find that in most regions and over the galaxy as a whole, fluorescent excitation is the dominant mechanism exciting the H_2 gas. We also place an upper limit of the percentage of shock excited H_2 at 29%. We find that UV radiation is the dominant excitation mechanism for the H_2 emission. The H_2 emission does not correlate well with Br\gamma but closely traces the PAH emission, showing that not only is H_2 fluorescently excited, but it is predominately excited by slightly lower mass stars than O stars which excite Br\gamma, such as B stars

Concatenation of convolutional and block codes Final report

Comparison of concatenated and sequential decoding systems and convolutional code structural propertie

Molecular gas heating in Arp 299

Understanding the heating and cooling mechanisms in nearby (Ultra) luminous infrared galaxies can give us insight into the driving mechanisms in their more distant counterparts. Molecular emission lines play a crucial role in cooling excited gas, and recently, with Herschel Space Observatory we have been able to observe the rich molecular spectrum. CO is the most abundant and one of the brightest molecules in the Herschel wavelength range. CO transitions are observed with Herschel, and together, these lines trace the excitation of CO. We study Arp 299, a colliding galaxy group, with one component harboring an AGN and two more undergoing intense star formation. For Arp 299 A, we present PACS spectrometer observations of high-J CO lines up to J=20-19 and JCMT observations of $^{13}$CO and HCN to discern between UV heating and alternative heating mechanisms. There is an immediately noticeable difference in the spectra of Arp 299 A and Arp 299 B+C, with source A having brighter high-J CO transitions. This is reflected in their respective spectral energy line distributions. We find that photon-dominated regions (PDRs) are unlikely to heat all the gas since a very extreme PDR is necessary to fit the high-J CO lines. In addition, this extreme PDR does not fit the HCN observations, and the dust spectral energy distribution shows that there is not enough hot dust to match the amount expected from such an extreme PDR. Therefore, we determine that the high-J CO and HCN transitions are heated by an additional mechanism, namely cosmic ray heating, mechanical heating, or X-ray heating. We find that mechanical heating, in combination with UV heating, is the only mechanism that fits all molecular transitions. We also constrain the molecular gas mass of Arp 299 A to 3e9 Msun and find that we need 4% of the total heating to be mechanical heating, with the rest UV heating

A comparison of spectral element and finite difference methods using statically refined nonconforming grids for the MHD island coalescence instability problem

A recently developed spectral-element adaptive refinement incompressible magnetohydrodynamic (MHD) code [Rosenberg, Fournier, Fischer, Pouquet, J. Comp. Phys. 215, 59-80 (2006)] is applied to simulate the problem of MHD island coalescence instability (MICI) in two dimensions. MICI is a fundamental MHD process that can produce sharp current layers and subsequent reconnection and heating in a high-Lundquist number plasma such as the solar corona [Ng and Bhattacharjee, Phys. Plasmas, 5, 4028 (1998)]. Due to the formation of thin current layers, it is highly desirable to use adaptively or statically refined grids to resolve them, and to maintain accuracy at the same time. The output of the spectral-element static adaptive refinement simulations are compared with simulations using a finite difference method on the same refinement grids, and both methods are compared to pseudo-spectral simulations with uniform grids as baselines. It is shown that with the statically refined grids roughly scaling linearly with effective resolution, spectral element runs can maintain accuracy significantly higher than that of the finite difference runs, in some cases achieving close to full spectral accuracy.Comment: 19 pages, 17 figures, submitted to Astrophys. J. Supp

Shock compression of feldspars

Hugoniot data for oligoclase and microcline to 670 and 580 kb and release adiabat data for oligoclase were obtained by means of the inclined mirror and immersed-foil-reflected-light techniques, respectively. Oligoclase and microcline have Hugoniot elastic limits in the range of 40–55 and 80–85 kb. These limits increase slightly with increasing driving shock pressure. Above the elastic limit, extending to ∼300 and ∼400 kb, transition regions of anomalously high compression are observed for microcline and oligoclase. These data and the data of McQueen, Marsh, and Fritz for albitite and anorthosite indicate that at successively higher shock pressures within this region, the feldspars gradually transform to a high-pressure, high-density polymorph. This polymorph probably corresponds to the rutile-like hollandite structure obtained in high-pressure quenching experiments by Kume, Matsumoto, and Koizumi (in KAlGe_3O_8) and by Ringwood, Reid, and Wadsley (in KAlSo_3O_8, microcline). In the hollandite structure germanium or silicon is in octahedral coordination with oxygen. The zero-pressure density and the Birch-Murnaghan equation of state parameters for the adiabat and isotherm are calculated for the high-pressure polymorphs of oligoclase, microcline, anorthosite, and albitite. The release adiabat centered at 180 kb indicates that at this shock pressure some (∼15%) of the hollandite phase forms but apparently reverts to a lower-density phase on pressure release. Release adiabat curves centered at 272 and 417 kb and calculated postshock temperatures indicate that material of feldspar composition recovered from meteorite and laboratory impacts is converted to the hollandite structure upon shock compression; upon pressure release this material probably reverts to the low-density maskelynite form

Parents' involvement in child care: do parental and work identities matter?

The current study draws on identity theory to explore mothers' and fathers' involvement in childcare. It examined the relationships between the salience and centrality of individuals’ parental and work-related identities and the extent to which they are involved in various forms of childcare. A sample of 148 couples with at least one child aged 6 years or younger completed extensive questionnaires. As hypothesized, the salience and centrality of parental identities were positively related to mothers' and fathers' involvement in childcare. Moreover, maternal identity salience was negatively related to fathers' hours of childcare and share of childcare tasks. Finally, work hours mediated the negative relationships between the centrality of work identities and time invested in childcare, and gender moderated this mediation effect. That is, the more central a mother's work identity, the more hours she worked for pay and the fewer hours she invested in childcare. These findings shed light on the role of parental identities in guiding behavioral choices, and attest to the importance of distinguishing between identity salience and centrality as two components of self-structure

Radiative and mechanical feedback into the molecular gas of NGC 253

Starburst galaxies are undergoing intense periods of star formation. Understanding the heating and cooling mechanisms in these galaxies can give us insight to the driving mechanisms that fuel the starburst. Molecular emission lines play a crucial role in the cooling of the excited gas. With SPIRE on the Herschel Space Observatory we have observed the rich molecular spectrum towards the central region of NGC 253. CO transitions from J=4-3 to 13-12 are observed and together with low-J line fluxes from ground based observations, these lines trace the excitation of CO. By studying the CO excitation ladder and comparing the intensities to models, we investigate whether the gas is excited by UV radiation, X-rays, cosmic rays, or turbulent heating. Comparing the $^{12}$CO and $^{13}$CO observations to large velocity gradient models and PDR models we find three main ISM phases. We estimate the density, temperature,and masses of these ISM phases. By adding $^{13}$CO, HCN, and HNC line intensities, we are able to constrain these degeneracies and determine the heating sources. The first ISM phase responsible for the low-J CO lines is excited by PDRs, but the second and third phases, responsible for the mid to high-J CO transitions, require an additional heating source. We find three possible combinations of models that can reproduce our observed molecular emission. Although we cannot determine which of these are preferable, we can conclude that mechanical heating is necessary to reproduce the observed molecular emission and cosmic ray heating is a negligible heating source. We then estimate the mass of each ISM phase; $6\times 10^7$ M$_\odot$ for phase 1 (low-J CO lines), $3\times 10^7$ M$_\odot$ for phase 2 (mid-J CO lines), and $9\times 10^6$ M$_\odot$ for phase 3 (high-J CO lines) for a total system mass of $1\times10^{8}$ M$_\odot$

The Herschel Comprehensive (U)LIRG Emission Survey (HERCULES): CO Ladders, Fine Structure Lines, and Neutral Gas Cooling

(Ultra) luminous infrared galaxies ((U)LIRGs) are objects characterized by their extreme infrared (8-1000 μm) luminosities (L_(LIRG) > 10^(11) L_☉ and L_(ULIRG) > 10^(12) L_☉). The Herschel Comprehensive ULIRG Emission Survey (PI: van der Werf) presents a representative flux-limited sample of 29 (U)LIRGs that spans the full luminosity range of these objects (10^(11) L_☉ ≤ L_(IR) ≤ 10^(13) L_☉). With the Herschel Space Observatory, we observe [C II] 157 μm, [O I] 63 μm, and [O I] 145 μm line emission with Photodetector Array Camera and Spectrometer, CO J = 4-3 through J = 13-12, [C I] 370 μm, and [C I] 609 μm with SPIRE, and low-J CO transitions with ground-based telescopes. The CO ladders of the sample are separated into three classes based on their excitation level. In 13 of the galaxies, the [O I] 63 μm emission line is self absorbed. Comparing the CO excitation to the InfraRed Astronomical Satellite 60/100 μm ratio and to far infrared luminosity, we find that the CO excitation is more correlated to the far infrared colors. We present cooling budgets for the galaxies and find fine-structure line flux deficits in the [C II], [Si II], [O I], and [C I] lines in the objects with the highest far IR fluxes, but do not observe this for CO 4 ≤ J_(upp) ≤ 13. In order to study the heating of the molecular gas, we present a combination of three diagnostic quantities to help determine the dominant heating source. Using the CO excitation, the CO J = 1-0 linewidth, and the active galactic nucleus (AGN) contribution, we conclude that galaxies with large CO linewidths always have high-excitation CO ladders, and often low AGN contributions, suggesting that mechanical heating is important