arXiv subjects

Michael A. K. Gross

Publications and source records attributed to Michael A. K. Gross.

8 recordsLinked to original sources

arXiv2010

Planning and Executing Airborne Astronomy Missions for SOFIA

SOFIA is a 2.5 meter airborne infrared telescope, mounted in a Boeing 747SP aircraft. Due to the large size of the telescope, only a few degrees of azimuth are available at the telescope bearing. This means the heading of the aircraft is fundamentally associated with the telescope's observation targets, and the ground track necessary to enable a given mission is highly complex and dependent on the coordinates, duration, and order of observations to be performed. We have designed and implemented a Flight Management Infrastructure (FMI) product in order to plan and execute such missions in the presence of a large number of external constraints (e.g. restricted airspace, international boundaries, elevation limits of the telescope, aircraft performance, winds at altitude, and ambient temperatures). We present an overview of the FMI, including the process, constraints and basic algorithms used to plan and execute SOFIA missions.

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arXiv2010

Pointing the SOFIA Telescope

SOFIA is an airborne, gyroscopically stabilized 2.5m infrared telescope, mounted to a spherical bearing. Unlike its predecessors, SOFIA will work in absolute coordinates, despite its continually changing position and attitude. In order to manage this, SOFIA must relate equatorial and telescope coordinates using a combination of avionics data and star identification, manage field rotation and track sky images. We describe the algorithms and systems required to acquire and maintain the equatorial reference frame, relate it to tracking imagers and the science instrument, set up the oscillating secondary mirror, and aggregate pointings into relocatable nods and dithers.

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arXiv2000

Hot Dark Matter in Cosmology

Cosmological dark matter in the form of neutrinos with masses of up to a few electron volts is known as hot dark matter. After an historical review of the subject, this article considers constraints on hot dark matter from current data on neutrino oscillations and on cosmology. The atmospheric neutrino oscillation data imply a lower limit on the HDM contribution to the cosmological density $Ω_ν\gsim 0.001$. The possible improvement of low-$Ω_m$ flat ($Λ$CDM) cosmological models with the addition of light neutrinos appears to be rather limited, but$Λ$CDM models with $Ω_ν\lsim 0.1$ may be consistent with presently available data. Data expected soon may permit detection of such a hot dark matter contribution, or alternatively provide stronger upper limits on $Ω_ν$ and neutrino masses.

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arXiv1998

Cosmological Implications of Lyman-Break Galaxy Clustering

We review our analysis of the clustering properties of ``Lyman-break'' galaxies (LBGs) at redshift z~3, previously discussed in Wechsler et al (1998). We examine the likelihood of spikes found by Steidel et al (1998) in the redshift distribution of LBGs, within a suite of models for the evolution of structure in the Universe. Using high-resolution dissipationless N-body simulations, we analyze deep pencil-beam surveys from these models in the same way that they are actually observed, identifying LBGs with the most massive dark matter halos. We find that all the models (with SCDM as a marginal exception) have a substantial probability of producing spikes similar to those observed, because the massive halos are much more clumped than the underlying matter -- i.e., they are biased. Therefore, the likelihood of such a spike is not a good discriminator among these models. The LBG correlation functions are less steep than galaxies today (gamma~1.4), but show similar or slightly longer correlation lengths. We have extened this analysis and include a preliminary comparison to the new data presented in Adelberger et al (1998). We also discuss work in progress, in which we use semi-analytic models to identify Lyman-break galaxies within dark-matter halos.

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arXiv1998

Cold + Hot Dark Matter After Super-Kamiokande

The recent atmospheric neutrino data from Super-Kamiokande provide strong evidence of neutrino oscillations and therefore of non-zero neutrino mass. These data imply a lower limit on the hot dark matter (i.e., light neutrino) contribution to the cosmological density $Ω_ν>~ 0.001$ --- almost as much as that of all the stars in the universe --- and permit higher $Ω_ν$. The ``standard'' COBE-normalized critical-matter-density (i.e., $Ω_m=1$) Cold Dark Matter (CDM) model has too much power on small scales. But adding to CDM neutrinos with mass of about 5 eV, corresponding to $Ω_ν\approx 0.2$, results in a much improved fit to data on the nearby galaxy and cluster distribution. Indeed, the resulting Cold + Hot Dark Matter (CHDM) cosmological model is arguably the most successful $Ω_m=1$ model for structure formation. However, other recent data has begun to make the case for $Ω_m <~ 0.6$ fairly convincing. In light of all this new data, we reconsider whether cosmology still provides evidence favoring neutrino mass of a few eV in flat models with cosmological constant $Ω_Λ= 1 - Ω_m$. We find that the possible improvement of the low-$Ω_m$ flat (LCDM) cosmological models with the addition of light neutrinos appears to be rather limited.

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arXiv1998

CDM-Variant Cosmological Models - I: Simulations and Preliminary Comparisons

We present two matched sets of five simulations each, covering five presently favored simple modifications to the standard cold dark matter (CDM) scenario. One simulation suite, with a linear box size of 75 Mpc/h, is designed for high resolution and good statistics on the group/poor cluster scale, and the other, with a box size of 300 Mpc/h, is designed for good rich cluster statistics. All runs had 57 million cold particles, and models with massive neutrinos had an additional 113 million hot particles. We consider separately models with massive neutrinos, tilt, curvature, and a nonzero cosmological constant in addition to the standard CDM model. We find that our tilted Omega+Omega_Lambda=1 (TLCDM) model produces too much small-scale power by a factor of ~3, and our open Lambda=0 (OCDM) model also exceeds observed small-scale power by a factor of 2. In addition, we take advantage of the large dynamic range in detectable halo masses our simulations allow to check the shape of the Press-Schechter approximation. We find good fits at cluster masses for delta_c=1.27--1.35 for a Gaussian filter and delta_c=1.57--1.73 for a tophat filter. However, Press-Schechter overpredicts the number density of halos compared to the simulations in the high resolution suite by a weakly cosmology-dependent factor of 1.5--2 at galaxy and group masses, which cannot be fixed by adjusting delta_c within reasonable bounds. An appendix generalizes the spherical collapse model to any isotropic cosmology.

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arXiv1998

Implications of Spikes in the Redshift Distribution of $z\sim3$ Galaxies

We address the high peaks found by Steidel et al (1997) in the redshift distribution of ``Lyman-break'' objects (LBOs) at redshift z~3. The highest spike represents a relative overdensity of 2.6 in the distribution of LBOs in pixels of comoving size ~10Mpc/h. We examine the likelihood of such a spike in the redshift distribution within a suite of models for the evolution of structure in the Universe, including models with Omega=1 (SCDM and CHDM) and with Omega=0.4-0.5 (LCDM and OCDM). Using high-resolution dissipationless N-body simulations, we analyze deep pencil-beam surveys from these models in the same way that they are actually observed, identifying LBOs with the most massive dark matter halos. We find that all the models (with SCDM as a marginal exception) have a substantial probability of producing spikes similar to those observed, because the massive halos are much more clumped than the underlying matter -- i.e., they are biased. Therefore, the likelihood of such a spike is not a good discriminator among these models. We find in these models that the mean biasing parameter b of LBOs with respect to dark matter varies within a range b ~2-5 on a scale of ~10Mpc/h. We also compute the two-body correlation functions of LBOs predicted in these models. The LBO correlation functions are less steep than galaxies today (gamma ~1.4), but show similar or slightly longer correlation lengths.

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arXiv1997

Clusters in Various Cosmological Models: Abundance and Evolution

The combination of measurements of the local abundance of rich clusters of galaxies and its evolution to higher redshift offers the possibility of a direct measurement of $Ω_0$ with little contribution from other cosmological parameters. We investigate the significance of recent claims that this evolution indicates that $Ω_0$ must be small. The most recent cluster velocity dispersion function from a compilation including the ESO Northern Abell Cluster Survey (ENACS) results in a significantly higher normalization for models, corresponding to $σ_8\approx 0.6$ for $Ω_0=1$, compared to the Eke, Cole, & Frenk result of $σ_8=0.52\pm 0.04$. Using the ENACS data for a $z=0$ calibration results in strong evolution in the abundance of clusters, and we find that the velocity dispersion function is consistent with $Ω_0=1$. The results are dependent upon the choice and analysis of low-redshift and high-redshift data, so at present, the data is not good enough to determine $Ω_0$ unambiguously.

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