arXiv subjects

R. E. Ryan Jr

Publications and source records attributed to R. E. Ryan Jr.

9 recordsLinked to original sources

arXiv2018

A Graphical-User Interface for Editing Segmentation Images

I present a graphical-user interface for performing several image operations on segmentation maps. The package is written entirely in IDL, and is provided as source code (for those who may want to develop, link to existing packages, or reappropriate the code base) and eventually as a stand-alone, run-time executable upon request (for those without an IDL license). The software facilitates a number of operations, which are generally tedious without a graphical interface, such as deleting, merging, ungrouping, and drawing regions; erasing and painting individual pixels; and compression, dilation, and erosion of a segmentation image. The segmentation image is displayed with random RGB triplets to ensure adjacent regions are readily discernible, whereas the direct image is shown as an inverted greyscale with controls for brightness range, bias, and contrast with several scaling functions (as similar to ds9). The opacity between the segmentation and direct image is tunable, which gives full control to the image display.

astro-ph.IM↗

arXiv2013

Stellar Populations of Lyman Break Galaxies at z=1-3 in the HST/WFC3 Early Release Science Observations

We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z=1-3 selected using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about 50 sq. arcmin in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z=1-3 are selected using dropout selection criteria similar to high redshift LBGs. The deep multi-band photometry in this field is used to identify best-fit SED models, from which we infer the following results: (1) the photometric redshift estimate of these dropout selected LBGs is accurate to within few percent; (2) the UV spectral slope (beta) is redder than at high redshift (z>3), where LBGs are less dusty; (3) on average, LBGs at z=1-3 are massive, dustier and more highly star-forming, compared to LBGs at higher redshifts with similar luminosities (0.1L*<~L<~2.5L*), though their median values are similar within 1-sigma uncertainties. This could imply that identical dropout selection technique, at all redshifts, find physically similar galaxies; and (4) stellar masses of these LBGs are directly proportional to their UV luminosities with a logarithmic slope of ~0.46, and star-formation rates are proportional to their stellar masses with a logarithmic slope of ~0.90. These relations hold true --- within luminosities probed in this study --- for LBGs from z~1.5 to 5. The star-forming galaxies selected using other color-based techniques show similar correlations at z~2, but to avoid any selection biases, and for direct comparison with LBGs at z>3, a true Lyman break selection at z~2 is essential. The future HST UV surveys, both wider and deeper, covering a large luminosity range are important to better understand LBG properties, and their evolution.

astro-ph.CO↗

arXiv2012

Newborn spheroids at high redshift: when and how did the dominant, old stars in today's massive galaxies form?

We study ~330 massive (M* > 10^9.5 MSun), newborn spheroidal galaxies (SGs) around the epoch of peak star formation (1 10^10.5 MSun, and an age trend becomes evident in this mass regime: SGs with M* > 10^11.5 MSun are ~2 Gyrs older than their counterparts with M* < 10^10.5 MSun. Nevertheless, a smooth downsizing trend with galaxy mass is not observed, and the large scatter in starburst ages indicate that SGs are not a particularly coeval population. Around half of the blue SGs appear not to drive their star formation via major mergers, and those that have experienced a recent major merger, show only modest enhancements (~40%) in their specific star formation rates. Our empirical study indicates that processes other than major mergers (e.g. violent disk instability driven by cold streams and/or minor mergers) likely play a dominant role in building SGs, and creating a significant fraction of the old stellar populations that dominate today's Universe.

astro-ph.CO↗

arXiv2011

iGalFit: An Interactive Tool for GalFit

We present a suite of IDL routines to interactively run GALFIT whereby the various surface brightness profiles (and their associated parameters) are represented by regions, which the User is expected to place. The regions may be saved and/or loaded from the ASCII format used by ds9 or in the Hierarchical Data Format (version 5). The software has been tested to run stably on Mac OS X and Linux with IDL 7.0.4. In addition to its primary purpose of modeling galaxy images with GALFIT, this package has several ancillary uses, including a flexible image display routines, several basic photometry functions, and qualitatively assessing Source Extractor. We distribute the package freely and without any implicit or explicit warranties, guarantees, or assurance of any kind. We kindly ask users to report any bugs, errors, or suggestions to us directly (as opposed to fixing them themselves) to ensure version control and uniformity.

astro-ph.IM↗

arXiv2008

"GiGa": the Billion Galaxy HI Survey -- Tracing Galaxy Assembly from Reionization to the Present

In this paper, we review the Billion Galaxy Survey that will be carried out at radio--optical wavelengths to micro--nanoJansky levels with the telescopes of the next decades. These are the Low-Frequency Array, the Square Kilometer Array and the Large Synoptic Survey Telescope as survey telescopes, and the Thirty Meter class Telescopes for high spectral resolution+AO, and the James Webb Space Telescope (JWST) for high spatial resolution near--mid IR follow-up. With these facilities, we will be addressing fundamental questions like how galaxies assemble with super-massive black-holes inside from the epoch of First Light until the present, how these objects started and finished the reionization of the universe, and how the processes of star-formation, stellar evolution, and metal enrichment of the IGM proceeded over cosmic time. We also summarize the high-resolution science that has been done thus far on high redshift galaxies with the Hubble Space Telescope (HST). Faint galaxies have steadily decreasing sizes at fainter fluxes and higher redshifts, reflecting the hierarchical formation of galaxies over cosmic time. HST has imaged this process in great structural detail to z<~6. We show that ultradeep radio-optical surveys may slowly approach the natural confusion limit, where objects start to unavoidably overlap because of their own sizes, which only SKA can remedy with HI redshifts for individual sub-clumps. Finally, we summarize how the 6.5 meter James Webb Space Telescope (JWST) will measure first light, reionization, and galaxy assembly in the near--mid-IR.

astro-ph↗

arXiv2007

Surface Brightness Profiles of Composite Images of Compact Galaxies at z~4-6 in the HUDF

The Hubble Ultra Deep Field (HUDF) contains a significant number of B, V and i'-band dropout objects, many of which were recently confirmed to be young star-forming galaxies at z~4-6. These galaxies are too faint individually to accurately measure their radial surface brightness profiles. Their average light profiles are potentially of great interest, since they may contain clues to the time since the onset of significant galaxy assembly. We separately co-add V, i' and z'-band HUDF images of sets of z~4,5 and 6 objects, pre-selected to have nearly identical compact sizes and the roundest shapes. From these stacked images, we are able to study the averaged radial structure of these objects at much higher signal-to-noise ratio than possible for an individual faint object. Here we explore the reliability and usefulness of a stacking technique of compact objects at z~4-6 in the HUDF. Our results are: (1) image stacking provides reliable and reproducible average surface brightness profiles; (2) the shape of the average surface brightness profiles show that even the faintest z~4-6 objects are resolved; and (3) if late-type galaxies dominate the population of galaxies at z~4-6, as previous HST studies have shown, then limits to dynamical age estimates for these galaxies from their profile shapes are comparable with the SED ages obtained from the broadband colors. We also present accurate measurements of the sky-background in the HUDF and its associated 1-sigma uncertainties.

astro-ph↗

arXiv2007

The Evolving Faint-End of the Luminosity Function

We investigate the evolution of the faint-end slope of the luminosity function, $α$, using semi-analytical modeling of galaxy formation. In agreement with observations, we find that the slope can be fitted well by $α(z) =a+b z$, with a=-1.13 and b=-0.1. The main driver for the evolution in $α$ 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 $α$ 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 $α(z)$ 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 $α(z)$ 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 $α(z)$ at z< z_*. In addition we find that different star formation efficiencies a_* in the Schmidt-Kennicutt-law and supernovae-feedback efficiencies $ε$ generally do not strongly influence the evolution of $α(z)$.

astro-ph↗

arXiv2006

Clues to AGN Growth from Optically Variable Objects in the Hubble Ultra Deep Field

We present a photometric search for objects with point-source components that are optically variable on timescales of weeks--months in the Hubble Ultra Deep Field (HUDF) to i'(AB)=28.0 mag. The data are split into four sub-stacks of approximately equal exposure times. Objects exhibiting the signature of optical variability are selected by studying the photometric error distribution between the four different epochs, and selecting 622 candidates as 3.0 sigma outliers from the original catalog of 4644 objects. Of these, 45 are visually confirmed as free of contamination from close neighbors or various types of image defects. Four lie within the positional error boxes of Chandra X-ray sources, and two of these are spectroscopically confirmed AGN. The photometric redshift distribution of the selected variable sample is compared to that of field galaxies, and we find that a constant fraction of ~1% of all field objects show variability over the range of 0.1<z<4.5. Combined with other recent HUDF results, as well as those of recent state-of-the-art numerical simulations, we discuss a potential link between the hierarchical merging of galaxies and the growth of AGN.

astro-ph↗

arXiv2005

Tracing Galaxy Assembly: Tadpole Galaxies in the Hubble Ultra Deep Field

In the Hubble Ultra Deep Field (HUDF) an abundance of galaxies is seen with a knot at one end plus an extended tail, resembling a tadpole. These "tadpole galaxies" appear dynamically unrelaxed--presumably in an early merging state--where tidal interactions likely created the distorted knot-plus-tail morphology. Here we systematically select tadpole galaxies from the HUDF and study their properties as a function of their photometric redshifts. In a companion HUDF variability study, Cohen et al. (2005) revealed a total of 45 variable objects believed to be Active Galactic Nuclei (AGN). Here we show that this faint AGN sample has no overlap with the tadpole galaxy sample, as predicted by theoretical work. The tadpole morphology--combined with the lack of overlap with the variable objects--supports the idea that these galaxies are in the process of an early-stage merger event, i.e., at a stage that likely precedes the "turn-on" of any AGN component and the onset of any point-source variability.

astro-ph↗