UGCA Research Articles

Hubble Space Telescope Observations of Tadpole Galaxies Kiso3867, SBS0, SBS1, and UM461

Tadpole galaxies are metal-poor dwarfs with typically one dominant star-forming region, giving them a head–tail structure when inclined. A metallicity drop in the head suggests that gas accretion with even lower metallicity stimulated the star formation. Here we present multiband Hubble Space Telescope WFC3 and Advanced Camera for Surveys images of four nearby (<25 Mpc) tadpoles, SBS0, SBS1, Kiso3867, and UM461, selected for their clear metallicity drops shown in previous spectroscopic studies. The properties of the star complexes and compact clusters are measured. Each galaxy contains from three to 10 young stellar complexes with 103–105 M ⊙ of stars ∼3–10 Myr old. Between the complexes, the disk has a typical age of ∼3 Gyr. Numerous star clusters cover the galaxies, both inside and outside the complexes. The combined cluster mass function, made by normalizing the masses and counts before stacking, is a power law with a slope of −1.12 ± 0.14 on a log–log plot and the combined distribution function of cluster lifetime decays with age as t −0.65±0.24. A comparison between the summed theoretical Lyman continuum (LyC) emission from all the clusters, given their masses and ages, is comparable to or exceeds the LyC needed to excite the observed Hα in some galaxies, suggesting LyC absorption by dust or undetected gas in the halo, or perhaps galaxy escape.

Total ALFALFA Neutral Hydrogen Fluxes for Extended Sources

Abstract A procedure is presented to improve on measurement of total H i fluxes for extended sources in the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey of neutral hydrogen sources in the nearby universe. A number of tests of the procedure are detailed, and we verify that we recover all of the flux measured with much larger telescope beams. Total fluxes are reported for all sources (1) exceeding 10 Jy km s−1 in the α.100 catalog, or (2) with Uppsala General Catalog diameters 3.0 arcmin or more, or (3) ALFALFA pipeline isophotal ellipse area more than 3.0 times the Arecibo beam. Total fluxes are also provided for a number of confused pairs and small groups including one or more of those high-flux sources. These data should be of use in baryonic Tully–Fisher studies and other applications where the measurement of the total reservoir of neutral atomic gas is important.

Star formation and gas in the minor merger UGC 10214

Minor mergers play a crucial role in galaxy evolution. UGC 10214 (the Tadpole galaxy) is a prime example of this process in which a dwarf galaxy has interacted with a large spiral galaxy ∼250 Myr ago and produced a perturbed disc and a giant tidal tail. We used a multi-wavelength dataset that partly consists of new observations (Hα, HI, and CO) and partly of archival data to study the present and past star formation rate (SFR) and its relation to the gas and stellar mass at a spatial resolution down to 4 kpc. UGC 10214 is a high-mass (stellar mass M⋆ = 1.28 × 1011 M⊙) galaxy with a low gas fraction (Mgas/M⋆ = 0.24), a high molecular gas fraction (MH2/MHI = 0.4), and a modest SFR (2–5 M⊙ yr−1). The global SFR compared to its stellar mass places UGC 10214 on the galaxy main sequence (MS). The comparison of the molecular gas mass and current SFR gives a molecular gas depletion time of about ∼2 Gyr (based on Hα), comparable to those of normal spiral galaxies. Both from a comparison of the Hα emission, tracing the current SFR, and far-ultraviolet (FUV) emission, tracing the recent SFR during the past tens of Myr, and also from spectral energy distribution fitting with CIGALE, we find that the SFR has increased by a factor of about 2–3 during the recent past. This increase is particularly noticeable in the centre of the galaxy where a pronounced peak of the Hα emission is visible. A pixel-to-pixel comparison of the SFR, molecular gas mass, and stellar mass shows that the central region has had a depressed FUV-traced SFR compared to the molecular gas and the stellar mass, whereas the Hα-traced SFR shows a normal level. The atomic and molecular gas distribution is asymmetric, but the position-velocity diagram along the major axis shows a pattern of regular rotation. We conclude that the minor merger has most likely caused variations in the SFR in the past that resulted in a moderate increase of the SFR, but it has not perturbed the gas significantly so that the molecular depletion time remains normal.

HUBBLE SPACE TELESCOPE OBSERVATIONS OF ACCRETION-INDUCED STAR FORMATION IN THE TADPOLE GALAXY KISO 5639

ABSTRACT The tadpole galaxy Kiso 5639 has a slowly rotating disk with a drop in metallicity at its star-forming head, suggesting that star formation was triggered by the accretion of metal-poor gas. We present multi-wavelength Hubble Space Telescope Wide Field Camera 3 images of UV through I band plus Hα to search for peripheral emission and determine the properties of various regions. The head has a mass in young stars of and an ionization rate of s−1, equivalent to ∼2100 O9-type stars. There are four older star-forming regions in the tail, and an underlying disk with a photometric age of ∼1 Gyr. The mass distribution function of 61 star clusters is a power law with a slope of −1.73 ± 0.51. Fourteen young clusters in the head are more massive than , suggesting a clustering fraction of 30%–45%. Wispy filaments of Hα emission and young stars extend away from the galaxy. Shells and holes in the head H ii region could be from winds and supernovae. Gravity from the disk should limit the expansion of the H ii region, although hot gas might escape through the holes. The star formation surface density determined from Hα in the head is compared to that expected from likely pre-existing and accreted gas. Unless the surface density of the accreted gas is a factor of ∼3 or more larger than what was in the galaxy before, the star formation rate has to exceed the usual Kennicutt–Schmidt rate by a factor of .

A MULTIWAVELENGTH STUDY OF TADPOLE GALAXIES IN THE HUBBLE ULTRA DEEP FIELD

Multiwavelength data are essential in order to provide a complete picture of galaxy evolution and to inform studies of galaxies' morphological properties across cosmic time. Here we present the results of a multiwavelength investigation of the morphologies of galaxies at intermediate redshift (0.314 < z < 3.175) in the Hubble Ultra Deep Field. These galaxies were previously selected from deep Hubble Space Telescope (HST) F775W data based on their distinct asymmetric knot-plus-tail morphologies. Here we use deep Wide Field Camera 3 near-infrared imaging in addition to the HST optical data in order to study the rest-frame UV/optical morphologies of these galaxies across the redshift range 0.3 < z < 3.2. This study reveals that the majority of these galaxies do retain their general asymmetric morphology in the rest-frame optical over this redshift range, if not the distinct shape. The average stellar mass of tadpole galaxies is lower than that of field galaxies, with the effect being slightly greater at higher redshift within the errors. Estimated from spectral energy distribution fits, the average age of tadpole galaxies is younger than that of field galaxies in the lower-redshift bin, and the average metallicity is lower (whereas the specific star formation rate for tadpoles is roughly the same as field galaxies across the redshift range probed here). These average effects combined support the conclusion that this subset of galaxies is in an active phase of assembly, either late-stage merging or cold gas accretion causing localized clumpy star formation.

Extreme emission-line galaxies out toz~ 1 in zCOSMOS

We present a thorough characterization of a large sample of 183 extreme emission-line galaxies (EELGs) at redshift 0.11 < z < 0.93 selected from the 20k zCOSMOS Bright Survey because of their unusually large emission line equivalent widths. We use multiwavelength COSMOS photometry, HST-ACS I-band imaging and optical zCOSMOS spectroscopy to derive the main global properties of EELGs, such as sizes, masses, SFRs, reliable metallicities from both "direct" and "strong-line" methods. The EELGs are compact (R_50 ~ 1.3 kpc), low-mass (log(M*/Msol)~7-10) galaxies forming stars at unusually high specific SFR (log(sSFR/yr) up to ~ -7) compared to main sequence SFGs of the same stellar mass and redshift. At UV wavelengths, the EELGs are luminous and show high surface brightness and include strong Ly$\alpha$ emitters, as revealed by GALEX spectroscopy. We show that zCOSMOS EELGs are high-ionization, low-metallicity systems, with median 12+log(O/H)=8.16, including a handful of extremely metal-deficient galaxies (<10% solar). While ~80% of the EELGs show non-axisymmetric morphologies, including clumpy and tadpole galaxies, we find that ~29% of them show additional low surface-brightness features, which strongly suggest recent or ongoing interactions. As star-forming dwarfs in the local Universe, EELGs are most often found in relative isolation. While only very few EELGs belong to compact groups, almost one third of them are found in spectroscopically confirmed loose pairs or triplets. We conclude that EELGs are galaxies caught in a transient and probably early period of their evolution, where they are efficiently building-up a significant fraction of their present-day stellar mass in an ongoing galaxy-wide starburst. Therefore, the EELGs constitute an ideal benchmark for comparison studies between low- and high-redshift low-mass star-forming galaxies.

Star Formation in Tadpole Galaxies

Tadpole Galaxies look like a star forming head with a tail structure to the side. They are also named cometaries. In a series of recent works we have discovered a number of issues that lead us to consider them extremely interesting targets. First, from images, they are disks with a lopsided starburst. This result is rmly established with long slit spectroscopy in a nearby representative sample. They rotate with the head following the rotation pattern but displaced from the rotation center. Moreover, in a search for extremely metal poor (XMP) galaxies, we identied tadpoles as the dominant shapes in the sample - nearly 80% of the local XMP galaxies have a tadpole morphology. In addition, the spatially resolved analysis of the metallicity shows the remarkable result that there is a metallicity drop right at the position of the head. This is contrary to what intuition would say and dicult to explain if star formation has happened from gas processed in the disk. The result could however be understood if the star formation is driven by pristine gas falling into the galaxy disk. If conrmed, we could be unveiling, for the rst time, cool ows in action in our nearby world. The tadpole class is relatively frequent at high redshift - 10% of resolvable galaxies in the Hubble UDF but less than 1% in the local Universe. They are systems that could track cool ows and test models of galaxy formation.

METALLICITY INHOMOGENEITIES IN LOCAL STAR-FORMING GALAXIES AS A SIGN OF RECENT METAL-POOR GAS ACCRETION

We measure the oxygen metallicity of the ionized gas along the major axis of seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14, show 0.5 dex metallicity decrements in inner regions with enhanced star-formation activity. This behavior is similar to the metallicity drop observed in a number of local tadpole galaxies by Sanchez Almeida et al. (2013) and interpreted as showing early stages of assembling in disk galaxies, with the star formation sustained by external metal-poor gas accretion. The agreement with tadpoles has several implications: (1) it proves that galaxies other than the local tadpoles present the same unusual metallicity pattern. (2) Our metallicity inhomogeneities were inferred using the direct method, thus discarding systematic errors usually attributed to other methods. (3) Taken together with the tadpole data, our findings suggest a threshold around one tenth the solar value for the metallicity drops to show up. Although galaxies with clear metallicity drops are rare, the physical mechanism responsible for them may sustain a significant part of the star-formation activity in the local Universe. We argue that the star-formation dependence of the mass-metallicity relationship, as well as other general properties followed by most local disk galaxies, are naturally interpreted as side effects of pristine gas infall. Alternatives to the metal poor gas accretion are examined too.

LOCAL TADPOLE GALAXIES: DYNAMICS AND METALLICITY

Tadpole galaxies, with a bright peripheral clump on a faint tail, are morphological types unusual in the nearby universe but very common early on. Low mass local tadpoles were identified and studied photometrically in a previous work, which we complete here analyzing their chemical and dynamical properties. We measure Halpha velocity curves of seven local tadpoles, representing 50% of the initial sample. Five of them show evidence for rotation (sim 70%), and a sixth target hints at it. Often the center of rotation is spatially offset with respect to the tadpole head (3 out of 5 cases). The size and velocity dispersion of the heads are typical of giant HII regions, and three of them yield dynamical masses in fair agreement with their stellar masses as inferred from photometry. In four cases the velocity dispersion at the head is reduced with respect to its immediate surroundings. The oxygen metallicity estimated from [NII]6583/Halpha often shows significant spatial variations across the galaxies (sim 0.5 dex), being smallest at the head and larger elsewhere. The resulting chemical abundance gradients are opposite to the ones observed in local spirals, but agrees with disk galaxies at high redshift. We interpret the metallicity variation as a sign of external gas accretion (cold-flows) onto the head of the tadpole. The galaxies are low metallicity outliers of the mass-metallicity relationship. In particular, two of the tadpole heads are extremely metal poor, with a metallicity smaller than a tenth of the solar value. These two targets are also very young (ages smaller than 5 Myr). All these results combined are consistent with the local tadpole galaxies being disks in early stages of assembling, with their star formation sustained by accretion of external metal poor gas.

Abstract 348: Dietary Quercetin Prevents Warfarin-Induced Vascular Calcification in Rats

Medial arterial calcification contributes to the development of isolated systolic hypertension, the most frequent type of blood pressure elevation in the elderly; complicates a variety of prevalent disorders such as diabetes and renal disease; and can develop in response to warfarin - the staple in anticoagulant therapy to millions of people. Our in vitro studies have shown that the bioflavonoid quercetin effectively prevents warfarin-induced vascular calcification in VSMCs and identified b-catenin signaling as a specific target of this effect. Here, we have tested in vivo the association of b-catenin signaling with warfarin-induced calcification and the efficacy of quercetin in preventing warfarin-induced aortic calcium accrual. Vitamin K-warfarin treatment was used for 4 weeks to induce arterial calcification in male rats. Molecular analysis demonstrates activation of b-catenin signaling in the calcified areas and osteoblast-like transformation in vascular cells. Dietary quercetin (10 mg/kg body weight) (Quercegen Pharma, Newton, MA) efficiently prevents warfarin-induced calcification (3.04+/-0.45 ug Ca in vitamin K-warfarin animals vs 0.77+/-0.08 ug Ca in quercetin supplementation per mg dry arterial tissue (n=5; p&lt;0.05). In parallel, quercetin inhibits b-catenin signaling and phenotypic transformation in vascular smooth muscle analyzed by expression of transcription factor Runx2, collagen type I and osteocalcin. Further, quercetin diet enhances expression of VSMC markers smooth muscle actin, myosin heavy chain, sm22a and calponin, suggesting a stabilized smooth muscle phenotype of these cells. No toxic or systemic effects of quercetin treatment were observed, identifying this bioflavonoid as a putative therapeutic for vascular calcification.

LOCAL TADPOLE GALAXIES

Tadpole galaxies have a giant star-forming region at the end of an elongated intensity distribution. Here we use SDSS data to determine the ages, masses, and surface densities of the heads and tails in 14 local tadpoles selected from the Kiso and Michigan surveys of UV-bright galaxies, and we compare them to tadpoles previously studied in the Hubble Ultra Deep Field. The young stellar mass in the head scales linearly with restframe galaxy luminosity, ranging from ~10^5 M_solar at galaxy absolute magnitude U=-13 mag to 10^9 M_solar at U=-20 mag. The corresponding head surface density increases from several M_solar pc^{-2} locally to 10-100 M_solar pc^{-2} at high redshift, and the star formation rate per unit area in the head increases from ~0.01 M_solar yr^{-1} kpc^{-2} locally to ~1 M_solar yr^{-1} kpc^{-2} at high z. These local values are normal for star-forming regions, and the increases with redshift are consistent with other cosmological star formation rates, most likely reflecting an increase in gas abundance. The tails in the local sample look like bulge-free galaxy disks. Their photometric ages decrease from several Gyr to several hundred Myr with increasing z, and their surface densities are more constant than the surface densities of the heads. The far outer intensity profiles in the local sample are symmetric and exponential. We suggest that most local tadpoles are bulge-free galaxy disks with lopsided star formation, perhaps from environmental effects such as ram pressure or disk impacts, or from a Jeans length comparable to half the disk size.

TADPOLE GALAXIES IN THE HUBBLE ULTRA DEEP FIELD

Tadpole galaxies have a head-tail shape with a large clump of star formation at the head and a diffuse tail or streak of stars off to one side. We measured the head and tail masses, ages, surface brightnesses, and sizes for 66 tadpoles in the Hubble Ultra Deep Field (UDF), and we looked at the distribution of neighbor densities and tadpole orientations with respect to neighbors. The heads have masses of 10^7-10^8 Msun and photometric ages of ~0.1 Gyr for z~2. The tails have slightly larger masses than the heads, and comparable or slightly older ages. The most obvious interpretation of tadpoles as young merger remnants is difficult to verify. They have no enhanced proximity to other resolved galaxies as a class, and the heads, typically less than 0.2 kpc in diameter, usually have no obvious double-core structure. Another possibility is ram pressure interaction between a gas-rich galaxy and a diffuse cosmological flow. Ram pressure can trigger star formation on one side of a galaxy disk, giving the tadpole shape when viewed edge-on. Ram pressure can also strip away gas from a galaxy and put it into a tail, which then forms new stars and gravitationally drags along old stars with it. Such an effect might have been observed already in the Virgo cluster. Another possibility is that tadpoles are edge-on disks with large, off-center clumps. Analogous lop-sided star formation in UDF clump clusters are shown.

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

In the Hubble Ultra Deep Field (HUDF) an abundance of is seen with a knot at one end plus an extended tail, resembling a tadpole. These 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 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 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.

Remarkable Disk and Off-Nuclear Starburst Activity in the Tadpole Galaxy as revealed by theSpitzer Space Telescope

We present ground-based optical and Spitzer Space Telescope infrared imaging observations of the interacting galaxy UGC 10214, the Tadpole galaxy (z = 0.0310), focusing on the star formation activity in the nuclear, disk, spiral arms, and tidal tail regions. The ground-based optical data set spans a wavelength range between 0.3 and 0.8 μm, the near-IR data set spans 1–2.2 μm, and the Spitzer IR data set spans 3–70 μm. The major findings of this study are that the Tadpole is actively forming stars in the main disk outside of the nucleus and in the tidal plume, with an estimated mean star formation rate of ~2–4 M⊙ yr-1. The most prominent sites of mid-IR emission define a morphology that, combined with the overall morphology of the system, suggests the interaction may belong to the rare class of off-center collisional ring systems that form both shock-induced rings of star formation and tidal plumes. In stark contrast to the disk star formation, the nuclear emission is solely powered by older stars, with little evidence for ongoing star formation at the center of the Tadpole. Extranuclear star formation accounts for >50% of the total star formation in the disk and spiral arms, featuring infrared-bright spots that exhibit strong polycyclic aromatic hydrocarbon (PAH) emission, the band strength of which is comparable to that of late-type star-forming disk galaxies. The tidal tail, which extends 2' (~75 kpc) into the intergalactic medium, is populated by supermassive star clusters, M ~ 106 M⊙, likely triggered by the galaxy-galaxy interaction that has distorted UGC 10214 into its current tadpole shape. The Tadpole is therefore an example of an off-nuclear or tidal-tail starburst, with several large sites of massive star formation in the disk and in the plume, including the most prominent Hubble Space Telescope–revealed cluster, J160616.85+552640.6. The clusters exhibit remarkable IR properties, including exceptionally strong 24 μm emission relative to the underlying starlight, hot dust continuum, and PAH emission, with an estimated current star formation rate of ~0.1–0.4 M⊙ yr-1, representing >10% of the total star formation in the system. We estimate the mass of the largest cluster to be ~ × 106 M⊙ based on the g'-band (0.5 μm) and near-IR (2.2 μm) integrated fluxes in combination with an assumed mass-to-light ratio appropriate to young clusters, or large enough to be classified as a nascent dwarf galaxy or globular cluster.

Multicolor Surface Photometry of Lenticular Galaxies. I. The Data

We present multicolor surface and aperture photometry in the B, V, R, and K' bands for a sample of 34 lenticular galaxies from the Uppsala General Catalogue. From surface photometric analysis, we obtain radial profiles of surface brightness, colors, ellipticity, position angle, and the Fourier coefficients that describe the departure of isophotal shapes from a purely elliptical form; we find the presence of dust lanes, patches, and ringlike structure in several galaxies in the sample. We obtain total integrated magnitudes and colors and find that these are in good agreement with the values from the Third Reference Catalogue. Isophotal colors are correlated with each other, following the sequence expected for early-type galaxies. The color gradients in lenticular galaxies are more negative than the corresponding gradients in elliptical galaxies. There is a good correlation between B-V and B-R color gradients, and the mean gradients in the B-V, B-R, and V-K' colors are -0.13 ± 0.06, -0.18 ± 0.06, and -0.25 ± 0.11 mag dex -1 in radius, respectively.

A Constant Bar Fraction out to Redshiftz∼ 1 in the Advanced Camera for Surveys Field of the Tadpole Galaxy

Barlike structures were investigated in a sample of 186 disk galaxies larger than 05 that are in the I-band image of the Tadpole galaxy taken with the Hubble Space Telescope Advanced Camera for Surveys. We found 22 clear cases of barred galaxies, 21 galaxies with small bars that appear primarily as isophotal twists in a contour plot, and 11 cases of peculiar bars in clump-cluster galaxies, which are face-on versions of chain galaxies. The latter bars are probably young, as the galaxies contain only weak interclump emission. Four of the clearly barred galaxies at z ~ 0.8-1.2 have grand-design spirals. The bar fraction was determined as a function of galaxy inclination and compared with the analogous distribution in the local universe. The bar fraction was also determined as a function of galaxy angular size. These distributions suggest that inclination and resolution effects obscure nearly half of the bars in our sample. The bar fraction was also determined as a function of redshift. We found a nearly constant bar fraction of 0.23 ± 0.03 from z ~ 0 to z = 1.1. When corrected for inclination and size effects, this fraction is comparable to the bar fraction in the local universe, ~0.4, which we tabulated for all bar and Hubble types in the Third Reference Catalogue of Bright Galaxies. The average major axis of a barred galaxy in our sample is ~10 kpc after correcting for redshift with a Λ-dominated cold dark matter cosmology. The average exponential scale length is ~2 kpc. These are half the sizes of local barred galaxies and not likely to be influenced much by cosmological dimming, because the high-z galaxies are intrinsically brighter. We conclude that galaxy bars were present in normal abundance at least ~8 Gyr ago (z ~ 1); the bars in clump-cluster galaxies may have formed from gaseous disk instabilities and star formation rather than stellar disk instabilities, and bar dissolution cannot be common during a Hubble time unless the bar formation rate is comparable to the dissolution rate. If galaxy interactions trigger bar formation more than bar destruction, then the higher interaction rate in the past suggests relatively few bars actually dissolved in a Hubble time.

Chain Galaxies in the Tadpole Advanced Camera for Surveys Field

Colors and magnitudes were determined for 69 chain galaxies, 58 other linear structures, 32 normal edge-on galaxies, and all of their large star formation clumps in the Hubble Space Telescope Advanced Camera for Surveys field of the Tadpole galaxy. Redshifts of 0.5-2 are inferred from comparisons with published color-evolution models. The linear galaxies have no red nuclear bulges like the normal disk galaxies in our field, but the star formation clumps in each have about the same colors and magnitudes. Light profiles along the linear galaxies tend to be flat, unlike the exponential profiles of normal galaxies. Although the most extreme of the linear objects look like beaded filaments, they are all probably edge-on disks that will evolve to late Hubble type galaxies. The lack of an exponential profile is the result of either a dust scale height that is comparable to the stellar scale height or an intrinsically irregular structure. Examples of galaxies that could be face-on versions of linear galaxies are shown. They have an irregular clumpy structure with no central bulge and with clump colors and magnitudes that are comparable to those in the linear galaxies. Radiative transfer solutions to the magnitudes and surface brightnesses of inclined dusty galaxies suggest that edge-on disks should become more prominent near the detection limit for surface brightness. The surface brightness distribution of the edge-on galaxies in this field confirm this selection effect. The star formation regions are much more massive than in modern galaxies, averaging up to ~109 M☉ for kiloparsec scales.

Discovery of Face-on Counterparts of Chain Galaxies in the Tadpole Advanced Camera for Surveys Field

The background field of the Tadpole galaxy image taken with the Hubble Space Telescope Advanced Camera for Surveys contains 87 clump clusters that are probably low-inclination versions of the 69 chain galaxies that we found in a previous paper. This conclusion follows from the similarity in colors and apparent magnitudes of the brightest clumps in the clump clusters and chain galaxies and from the similarity between the distribution of axial ratios for the combined sample and the distribution for normal disk galaxies. These distributions are consistent with chains and clump clusters being disks viewed at random angles with intrinsic axial ratios of 0.1-0.2 or less. The disks appear to be gas-rich galaxies at z = 1-2 that are forming an early generation of stars in a small number of very large star complexes.

VLBA Observations of a Sample of Nearby FR I Radio Galaxies

We observed 17 nearby low luminosity FR I radio galaxies using the NRAO Very Long Baseline Array (VLBA) at 1.67 GHz, as part of a multi-wavelength study of a complete sample of 21 sources selected by radio flux density from the Uppsala General Catalogue of Galaxies. We detected radio emission from all 17 galaxies. At a FWHM resolution of ~ 10x4 mas, five galaxies show only an unresolved radio core, ten galaxies show core-jet structures, and two galaxies show twin-jet structures. Comparing these VLBA images with images previously obtained with the NRAO VLA, we find that all detected VLBA jets are well-aligned on parsec scales with the VLA jets on kilo parsec scales, and that the jet to counter-jet surface brightness ratios, or the sidedness, decreases systematically with increasing distance along the jet. We attribute the sidedness to the Doppler boosting effect and its decline to the deceleration of the jets. We show that a distribution of Lorentz factor centered near Gamma = 5 can reproduce our VLBA detection statistics for core, core-jet and twin-jet sources. We also note that the luminosity per unit length, L_j, of the VLBA jets drops quickly with distance, r, along the jet, approximately as L_j being proportional to r^(-2.0). We discuss three different mechanisms to explain this jet fading: (1) the decrease of Doppler boosting due to jet deceleration, (2) synchrotron losses, and (3) expansion losses in constant velocity but adiabatically spreading jets. Mechanisms (1) and (2) are inconsistent with the observations, while mechanism (3) is consistent with the observations provided the magnetic field lines in the jets are aligned perpendicular to the jet axis. This implies that the deceleration of the jets required by the Unified Scheme does not occur on the tens of parsec scales, but must occur on larger scales.

Arcsecond Positions of UGC Galaxies

We present accurate B1950 and J2000 positions for all confirmed galaxies in the Uppsala General Catalog (UGC). The positions were measured visually from Digitized Sky Survey images with rms uncertainties σ ≤ [(12)2 + (θ/100)2]1/2, where θ is the major-axis diameter. We compared each galaxy measured with the original UGC description to ensure high reliability. The full position list is available in the electronic version only.