Engineering topics

Spaans, Marco

Publications and source records attributed to Spaans, Marco.

6 records

NASA NTRS2017

Herschel Extreme Lensing Line Observations: [CII] Variations in Galaxies at Redshifts z=1-3*

We observed the [C II] line in 15 lensed galaxies at redshifts 1 less than z less than 3 using HIFI on the Herschel Space Observatory and detected 14/15 galaxies at 3sigma or better. High magnifications enable even modestly luminous galaxies to be detected in [C II] with Herschel. The [C II] luminosity in this sample ranges from 8 × 10(exp 7) solar luminosity to 3.7 × 10(exp 9) solar luminosity (after correcting for magnification), confirming that [C II] is a strong tracer of the ISM at high redshifts. The ratio of the [C II] line to the total far-infrared (FIR) luminosity serves as a measure of the ratio of gas to dust cooling and thus the efficiency of the grain photoelectric heating process. It varies between 3.3% and 0.09%. We compare the [C II]/FIR ratio to that of galaxies at z = 0 and at high redshifts and find that they follow similar trends. The [C II]/FIR ratio is lower for galaxies with higher dust temperatures. This is best explained if increased UV intensity leads to higher FIR luminosity and dust temperatures, but gas heating does not rise due to lower photoelectric heating efficiency. The [C II]/FIR ratio shows weaker correlation with FIR luminosity. At low redshifts highly luminous galaxies tend to have warm dust, so the effects of dust temperature and luminosity are degenerate. Luminous galaxies at high redshifts show a range of dust temperatures, showing that [C II]/FIR correlates most strongly with dust temperature. The [C II] to mid-IR ratio for the HELLO sample is similar to the values seen for low-redshift galaxies, indicating that small grains and PAHs dominate the heating in the neutral ISM, although some of the high [CII]/FIR ratios may be due to turbulent heating.

Malhotra, Sangeeta↗

NASA NTRS2014

Herschel Extreme Lensing Line Observations: Dynamics of Two Strongly Lensed Star-Forming Galaxies near Redshift z=2*

We report on two regularly rotating galaxies at redshift z approx. = 2, using high-resolution spectra of the bright [C microns] 158 micrometers emission line from the HIFI instrument on the Herschel Space Observatory. Both SDSS090122.37+181432.3 ("S0901") and SDSSJ120602.09+514229.5 ("the Clone") are strongly lensed and show the double-horned line profile that is typical of rotating gas disks. Using a parametric disk model to fit the emission line profiles, we find that S0901 has a rotation speed of v sin(i) approx. = 120 +/- 7 kms(sup −1) and a gas velocity dispersion of (standard deviation)g < 23 km s(sup −1) (1(standard deviation)). The best-fitting model for the Clone is a rotationally supported disk having v sin(i) approx. = 79 +/- 11 km s(sup −1) and (standard deviation)g 4 kms(sup −1) (1(standard deviation)). However, the Clone is also consistent with a family of dispersion-dominated models having (standard deviation)g = 92 +/- 20 km s(sup −1). Our results showcase the potential of the [C microns] line as a kinematic probe of high-redshift galaxy dynamics: [C microns] is bright, accessible to heterodyne receivers with exquisite velocity resolution, and traces dense star-forming interstellar gas. Future [C microns] line observations with ALMA would offer the further advantage of spatial resolution, allowing a clearer separation between rotation and velocity dispersion.

LENSING↗

NASA NTRS1998

Search for Interstellar Water in the Translucent Molecular Cloud Toward HD 154368

We report an upper limit of 9 x 10(exp 12)/sq cm on the column density of water in the translucent cloud along the line of sight toward HD 154368. This result is based on a search for the C-X band of water near 1240 A carried out using the Goddard High-Resolution Spectrograph (GHRS) of the Hubble Space Telescope. Our observational limit on the water abundance, together with detailed chemical models of translucent clouds and previous measurements of OH along the line of sight, constrains the branching ratio in the dissociative recombination of H3O(+) to form water. We find at the 3 sigma level that no more than 30% of dissociative recombinations of H3O(+) can lead to H2O. The observed spectrum also yielded high-resolution observations of the Mg II doublet at 1239.9 A and 1240.4 A, allowing the velocity structure of the dominant ionization state of magnesium to be studied along the line of sight. The Mg II spectrum is consistent with GHRS observations at lower spectral resolution that were obtained previously but allow an additional velocity component to be identified.

Spaans, Marco↗

NASA NTRS1997

Molecular Hydrogen in Diffuse Interstellar Clouds of Arbitrary Three-Dimensional Geometry

We have constructed three-dimensional models for the equilibrium abundance of molecular hydrogen in diffuse interstellar clouds of arbitrary geometry that are illuminated by ultraviolet radiation. The position-dependent photodissociation rate of H2 in such clouds was computed with a 26 ray approximation to model the attenuation of the incident ultraviolet radiation field by dust and by H2 line absorption. We have applied our modeling technique to the isolated diffuse cloud G236+39, assuming that the cloud has a constant density and that the thickness of the cloud along the line of sight is at every point proportional to the 100 micron continuum intensity measured by IRAS. We find that our model can successfully account for observed variations in the ratio of 100 micron continuum intensity to H I column density, with larger values of that ratio occurring along lines of sight in which the molecular hydrogen fraction is expected to be the largest. Using a standard chi square analysis to assess the goodness of fit of our models, we find (at the 60 a level) that a three-dimensional model is more successful in matching the observational data than a one-dimensional model in which the geometrical extent of the cloud along the line of sight is assumed to be much smaller than its extent in the plane of the sky. If D is the distance to G236 + 39, and given standard assumptions about the rate of grain-catalyzed H2 formation, we find that the cloud has an extent along the line of sight that is 0.9 +/- 0.1 times its mean extent projected onto the plane of the sky and a gas density of 53 +/- 8 (100 pc/D) H nuclei/cc and is illuminated by a radiation field of 1.1 +/- 0.2 (100 pc/D) times the mean interstellar radiation field. The derived 100 micron emissivity per nucleon is 1.13 +/- 0.06 x 10(exp -20) MJy/sr sq cm.

Spaans, Marco↗

NASA NTRS1996

Red Fluorescent Line Emission from Hydrogen Molecules in Diffuse Molecular Clouds

We have modeled the fluorescent pumping of electronic and vibrational emissions of molecular hydrogen (H2) within diffuse molecular clouds that are illuminated by ultraviolet continuum radiation. Fluorescent line intensities are predicted for transitions at ultraviolet, infrared, and red visible wavelengths as functions of the gas density, the visual extinction through the cloud, and the intensity of the incident UV continuum radiation. The observed intensity in each fluorescent transition is roughly proportional to the integrated rate of H2 photodissociation along the line of sight. Although the most luminous fluorescent emissions detectable from ground-based observatories lie at near-infrared wavelengths, we argue that the lower sky brightness at visible wavelengths makes the red fluorescent transitions a particularly sensitive probe. Fabry-Perot spectrographs of the type that have been designed to observe very faint diffuse Ha emissions are soon expected to yield sensitivities that will be adequate to detect H2 vibrational emissions from molecular clouds that are exposed to ultraviolet radiation no stronger than the mean radiation field within the Galaxy. Observations of red H2 fluorescent emission together with cospatial 21 cm H I observations could serve as a valuable probe of the gas density in diffuse molecular clouds.

Neufeld, David A.↗

NASA NTRS1994

Photon-dominated regions around cool stars: The effects of the color temperature of the radiation field

We have investigated the influence of the color temperature of the illuminating radiation field on the chemical and thermal structure of photon-dominated regions (PDRs). We present the results of a study of the photoelectric efficiency of heating by large molecules such as polycyclic aromatic hydrocarbons (PAHs) and very small grains for radiation fields characterized by different effective temperatures. We show that the efficiency for cooler (T(sub eff) approximately = 6000-10,000 K) stars is at most an order of magnitude smaller than that for hotter (T(sub eff) approximately = 20,000-30,000 K) stars. While cooler radiation fields result in less ultraviolet photons capable of heating, the efficiency per absorbed photon is higher, because the grains become less positively charged. We also present detailed calculations of the chemistry and thermal balance for generic PDRs (n(sub 0) approximately = 10(exp 3), G(sub 0) approximately = 10(exp 3)). For cooler radiation fields, the H/H2 and C(+)/C/CO transition layers shift toward the surface of the PDR, because fewer photons are available to photodissociate H2 and CO and to ionize C. The dominant cooling lines are the (C II) 158 micron and the (O I) 63 micron lines for the hotter radiation fields, but cooling by CO becomes dominant for a color temperature of 6000 K or lower. The (C II)/CO and (O I)/CO ratios are found to be very good diagnostics for the color temperature of the radiation field.

Spaans, Marco↗