The Evolving Faint-End of the Luminosity Function

We investigate the evolution of the faint-end slope of the luminosity function, $\alpha$, using semi-analytical modeling of galaxy formation. In agreement with observations, we find that the slope can be fitted well by $\alpha (z) =a+b z$, with a=-1.13 and b=-0.1. The main driver for the evolution in $\alpha$ is the evolution in the underlying dark matter mass function. Sub-L_* galaxies reside in dark matter halos that occupy a different part of the mass function. At high redshifts, this part of the mass function is steeper than at low redshifts and hence $\alpha$ is steeper. Supernova feedback in general causes the same relative flattening with respect to the dark matter mass function. The faint-end slope at low redshifts is dominated by field galaxies and at high redshifts by cluster galaxies. The evolution of $\alpha(z)$ in each of these environments is different, with field galaxies having a slope b=-0.14 and cluster galaxies b=-0.05. The transition from cluster-dominated to field-dominated faint-end slope occurs roughly at a redshift $z_* \sim 2$, and suggests that a single linear fit to the overall evolution of $\alpha(z)$ might not be appropriate. Furthermore, this result indicates that tidal disruption of dwarf galaxies in clusters cannot play a significant role in explaining the evolution of $\alpha(z)$ at z< z_*. In addition we find that different star formation efficiencies a_* in the Schmidt-Kennicutt-law and supernovae-feedback efficiencies $\epsilon$ generally do not strongly influence the evolution of $\alpha(z)$.Comment: 4 pages, replaced with version accepted to ApJL, minor changes to figure

Compact Lyman-alpha Emitting Candidates at z~2.4 in Deep Medium-band HST WFPC2 Images

Medium-band imaging with HST/WFPC2 in the F410M filter has previously revealed a population of compact Lyman-alpha emission objects around the radio galaxy 53W002 at z~2.4. We report detections of similar objects at z~2.4 in random, high-latitude HST parallel observations of three additional fields, lending support to the idea that they constitute a widespread population at these redshifts. The three new fields contain 18 Lyman-alpha candidates, in contrast to the 17 detected in the deeper exposure of the single WFPC2 field around 53W002. We find substantial differences in the number of candidates from field to field, suggesting that significant large-scale structure is already present in the galaxy distribution at this cosmic epoch. The likely existence of z~2.4 sub-galactic clumps in several random fields shows that these objects may have been common in the early universe and strengthens the argument that such objects may be responsible for the formation of a fraction of the luminous present-day galaxies through hierarchical merging.Comment: Uses slightly modified AASTeX preprint style file (included). Contains 22 pages, including 5 figures and 2 tables. Accepted for the December issue of the Astronomical Journa

Impact of Tactical and Strategic Weather Avoidance on Separation Assurance

The ability to keep flights away from weather hazards while maintaining aircraft-to-aircraft separation is critically important. The Advanced Airspace Concept is an automation concept that implements a ground-based strategic conflict resolution algorithm for management of aircraft separation. The impact of dynamic and uncertain weather avoidance on this concept is investigated. A strategic weather rerouting system is integrated with the Advanced Airspace Concept, which also provides a tactical weather avoidance algorithm, in a fast time simulation of the Air Transportation System. Strategic weather rerouting is used to plan routes around weather in the 20 minute to two-hour time horizon. To address forecast uncertainty, flight routes are revised at 15 minute intervals. Tactical weather avoidance is used for short term trajectory adjustments (30 minute planning horizon) that are updated every minute to address any weather conflicts (instances where aircraft are predicted to pass through weather cells) that are left unresolved by strategic weather rerouting. The fast time simulation is used to assess the impact of tactical weather avoidance on the performance of automated conflict resolution as well as the impact of strategic weather rerouting on both conflict resolution and tactical weather avoidance. The results demonstrate that both tactical weather avoidance and strategic weather rerouting increase the algorithm complexity required to find aircraft conflict resolutions. Results also demonstrate that tactical weather avoidance is prone to higher airborne delay than strategic weather rerouting. Adding strategic weather rerouting to tactical weather avoidance reduces total airborne delays for the reported scenario by 18% and reduces the number of remaining weather violations by 13%. Finally, two features are identified that have proven important for strategic weather rerouting to realize these benefits; namely, the ability to revise reroutes and the use of maneuvers that start far ahead of encountering a weather cell when rerouting around weather

Hubble Space Telescope images of submillimeter sources: large, irregular galaxies at high redshift

We present new Hubble Space Telescope STIS, high-resolution optical imaging of a sample of 13 submillimeter (submm) luminous galaxies, for which the optical emission has been pinpointed either through radio-1.4 GHz or millimeter interferometry. We find a predominance of irregular and complex morphologies in the sample, suggesting that mergers are likely common for submm galaxies. The component separation in these objects are on average a factor two larger than local galaxies with similarly high bolometric luminosities. The sizes and star formation rates of the submm galaxies are consistent with the maximal star formation rate densities of 20 Msun kpc^{-2} in local starburst galaxies (Lehnert & Heckman 1996). We derive quantitative morphological information for the optical galaxies hosting the submm emission; total and isophotal magnitudes, Petrosian radius, effective radius, concentration, aspect ratio, surface brightness, and asymmetry. We compare these morphological indices with those of other galaxies lying within the same STIS images. Most strikingly, we find ~70% of the submm galaxies to be extraordinarily large and elongated relative to the field population, regardless of optical magnitude. Comparison of the submm galaxy morphologies with those of optically selected galaxies at z~2-3 reveal the submm galaxies to be a morphologically distinct population, with generally larger sizes, higher concentrations and more prevalent major-merger configurations.Comment: 16 pages, 6 figures, scheduled for ApJ, v599, Dec10, 2003. Minor edits. For version with higher resolution figures, see http://www.submm.caltech.edu/~schapman/ms_v3.ps.g

Evidence for extended, obscured starbursts in submm galaxies

We compare high-resolution optical and radio imaging of 12 luminous submm galaxies at z=2.2+/-0.2 observed with HST and the MERLIN and VLA at comparable spatial resolution, 0.3" (2kpc). The radio emission traces the likely far-infrared morphology of these dusty, luminous galaxies. In ~30% of the sample the radio appears unresolved, suggesting that the emission is compact: either an obscured AGN or nuclear starburst. However, in the majority, ~70% (8/12), the radio emission is resolved by MERLIN/VLA on scales of ~1" (10 kpc). For these galaxies the radio morphologies are broadly similar to their restframe UV emission seen by HST. We discuss the probable mechanisms for the extended emission and conclude that their luminous radio and submm emission arises from a large, spatially-extended starburst. The median SFRs are 1700Mo/yr occuring within a ~40kpc^2 region, giving a star formation density of 45Mo/yr/kpc^2. Such vigorous and extended starbursts appear to be uniquely associated with the submm population. A more detailed comparison of the distribution of UV and radio emission shows that the broad similarities on large scales are not carried through to smaller scales, where there is rarely a one-to-one correspondance. We interpret this as resulting from highly structured internal obscuration, suggesting that the vigorous activity is producing wind-blown channels through the obscuration in these galaxies. If correct this underlines the difficulty of using UV morphologies to understand structural properties of this population and also may explain the surprising frequency of Ly-alpha emission in their spectra. [Abridged]Comment: 7 pages, 3 figure

NICMOS Imaging of the Dusty Microjansky Radio Source VLA J123642+621331 at z = 4.424

We present the discovery of a radio galaxy at a likely redshift of z = 4.424 in one of the flanking fields of the Hubble Deep Field. Radio observations with the VLA and MERLIN centered on the HDF yielded a complete sample of microjansky radio sources, of which about 20% have no optical counterpart to I < 25 mag. In this Letter, we address the possible nature of one of these sources, through deep HST NICMOS images in the F110W (J) and F160W (H) filters. VLA J123642+621331 has a single emission line at 6595-A, which we identify with Lyman-alpha at z = 4.424. We argue that this faint (H = 23.9 mag), compact (r = 0.2 arcsec), red (I - K = 2.0) object is most likely a dusty, star-forming galaxy with an embedded active nucleus.Comment: Accepted for publication in Astrophysical Journal Letters. 11 pages, 4 figures, uses aastex v5.0 and psfi

Morphological number-count and redshift distributions to I < 26 from the Hubble Deep Field: Implications for the evolution of Ellipticals, Spirals and Irregulars

We combine the photometric redshift data of Fernandez-Soto et al. (1997) with the morphological data of Odewahn et al. (1996) for all galaxies with I < 26.0 detected in the Hubble Deep Field. From this combined catalog we generate the morphological galaxy number-counts and corresponding redshift distributions and compare these to the predictions of high normalization zero- and passive- evolution models. From this comparison we conclude the following: (1) E/S0s are seen in numbers and over a redshift range consistent with zero- or minimal passive- evolution to I = 24. Beyond this limit fewer E/S0s are observed than predicted implying a net negative evolutionary process --- luminosity dimming, disassembly or masking by dust --- at I > 24. (2) Spiral galaxies are present in numbers consistent with zero- evolution predictions to I = 22. Beyond this magnitude some net- positive evolution is required. Although the number-counts are consistent with the passive-evolution predictions to I=26.0 the redshift distributions favor number AND luminosity evolution. (3) There is no obvious explanation for the late-type/irregular class and this category requires further subdivision. While a small fraction of the population lies at low redshift (i.e. true irregulars), the majority lie at redshifts, 1 < z < 3. At z > 1.5 mergers are frequent and, taken in conjunction with the absence of normal spirals at z > 2, the logical inference is that they represent the progenitors of normal spirals forming via hierarchical merging.Comment: Accepted for publication in ApJ Letters, colour plates available from http://www.phys.unsw.edu.au/~spd/bib.htm

The Axis Ratio Distribution of Local and Distant Galaxies

Surface photometry from 16 HST/WFPC2 fields in the I(F814W) filter is used to derive the distribution of apparent axis ratios for galaxies in progressively fainter magnitude intervals for I<25. We assess the systematic and accidental errors in ellipticity measurements as a function of image resolution and signal-to-noise ratio, and statistically correct for the effect of cosmological surface brightness dimming on our isophotal measurements. The axis ratio distribution for the local galaxy population was computed using logR measurements for 1569 RC3 galaxies with Bt<13 mag. Nonparametric tests are used to show that our distant samples, in the redshift range 0.1<z<1.5, are not statistically different from the local sample. We present image montages of galaxies selected randomly from different axis ratio and apparent magnitude ranges and discuss the evolutionary consequences of the lack of a strong difference between the ellipticity distributions in near and far data sets.Comment: LaTex, 35 pages, 8 figures, accepted for Dec97 A