Sc Galaxies Research Articles

Near-Infrared Adaptive Optics Imaging of the Central Regions of Nearby S[CLC]c[/CLC] Galaxies. II. NGC 247 and NGC 2403

J, H, and K' images obtained with the Canada-France-Hawaii Telescope adaptive optics system are used to investigate the star-forming histories of the central regions of the Sc galaxies NGC 247 and NGC 2403. The brightest resolved red stars within 15'' of the nucleus of each galaxy are red supergiants (RSGs), indicating that the central few hundred parsecs of these galaxies experienced star formation within the last ~0.1 Gyr. While there are indications of galaxy-to-galaxy differences in the recent (t ≤ 0.1 Gyr) star-forming histories, a comparison of the K luminosity functions of stars near the centers of NGC 2403 and M33 indicates that, when averaged over gigayear timescales, the star-forming histories of the inner disks of these galaxies have been remarkably similar. This is consistent with suggestions that the long-term evolution of disks is defined by local characteristics such as mass density. It is demonstrated that NGC 247 and NGC 2403, like M33, harbor nuclear star clusters with stellar contents that differ from the surrounding central light concentrations, which in turn have near-infrared spectral energy distributions that are similar to old stellar systems. The nucleus of NGC 2403 is significantly bluer than that of the other two galaxies. If the nuclei of NGC 247, NGC 2403, and M33 are subject to similar amounts of extinction, then this indicates that NGC 2403 harbors the youngest nuclear star cluster in the sample. The K-band surface brightnesses near the centers of NGC 247 and NGC 2403 are 1–2 mag arcsec-2 lower than in M33. Finally, it is noted that young or intermediate-age nuclear star clusters are a common occurrence in nearby spirals, indicating that nuclear star formation in these objects is either continuous or episodic on timescales of 0.1–1 Gyr. This is consistent with models that have been proposed to explain the properties of the Galactic center.

Seeing Galaxies through Thick and Thin. IV. The Superposed Spiral Galaxies of NGC 3314

The superposed pair of spiral galaxies making up NGC 3314 offers a unique opportunity to trace the dust properties in a spiral galaxy. We analyze multicolor Hubble Space Telescope imaging, supported by ground-based near-IR imaging and fiber-array spectroscopy, to measure dust extinction in the foreground Sc galaxy NGC 3314A, which is backlit by the Sb system NGC 3314B. The superposition allows us to measure extinctions over a wide range of galactocentric radii in the foreground galaxy from 0.4–4.5 kpc. In the outer half of the disk, the extinction is strongly localized in discrete dust lanes, including some patches whose galactic setting is clear only because of associated Hα emission at the foreground velocity. These dust features show an extinction curve with a slope close to the Galactic mean (R = 3.5 ± 0.3) over a range in galactocentric radius from 1.6 to 3.8 kpc, with no radial trend. Using the I-K color of the background nucleus, we derive an extinction of AI = 3.3 through the disk at a projected distance of 400 pc from the nucleus of NGC 3314A. The extinction in even the inner disk of NGC 3314A is quite patchy, since background Hα emission is detected from all parts of the system. Local anticorrelations between foreground and background line emission demonstrate that the dust is concentrated in star-forming regions, as has been found for the blue light in several systems. The colors of the dust lanes in NGC 3314A that are projected only partially against the background disk indicate that the dust scale height in the foreground disk is substantially smaller than that of the stars. The color-intensity behavior of the net light in these regions tracks the predictions of a thin-layer model closely.

The Continuing Radio Evolution of SN 1970G

Using the Very Large Array, we have detected radio emission from the site of SN 1970G in the Sc galaxy M101. These observations are 31 years after the supernova event, making SN 1970G the longest monitored radio supernova. With flux densities of 0.12 +/- 0.020 mJy at 6 cm and 0.16 +/- 0.015 mJy at 20 cm, the spectral index of -0.24 +/- 0.20 appears to have flattened somewhat when compared with the previously reported value of -0.56 +/- 0.11, taken in 1990. The radio emission at 20 cm has decayed since the 1990 observations with a power-law index of beta_20cm = -0.28 +/- 0.13. We discuss the radio properties of this source and compare them to those of other Type II radio supernovae.

The Fading Radio Emission from SN 1961V: Evidence for a Type II Peculiar Supernova?

Using the Very Large Array (VLA), we have detected radio emission from the site of SN 1961V in the Sc galaxy NGC 1058. With a peak flux density of 0.063 ± 0.008 mJy beam-1 at 6 cm and 0.147 ± 0.026 mJy beam-1 at 18 cm, the source is nonthermal, with a spectral index of -0.79 ± 0.23. Within errors, this spectral index is the same value reported for previous VLA observations taken in 1984 and 1986. The radio emission at both wavelengths has decayed since the mid-1980s observations with power-law indices of β20 cm = -0.69 ± 0.23 and β6 cm = -1.75 ± 0.16. We discuss the radio properties of this source and compare them with those of Type II radio supernovae and luminous blue variables.

Global instabilities in disks of spiral galaxies: Comparison of theory and observations for galaxies NGC 488, NGC 1566, and NGC 6503

Abstract r and 2D nonlinear simulations of spiral structure in the galaxies NGC 488, NGC 1566 and NGC 6503. The observed velocity dispersions and the rotation curves have been used to model background axisymmetric equilibria in these galaxies. The stability properties of the galactic disks are governed by the disk masses and by the ratio of the disk's radial and the vertical velocity dispersions. The massive disk of the galaxy NGC 488 has a few unstable modes with the two-armed spiral being the most unstable one. The nonlinear 2D numerical simulations show that spiral pattern in NGC 488 evolves into a patchy two-armed spiral structure. Disks of Sc galaxies NGC 1566 and NGC 6503 are unstable towards modes m = 2, and m = 3 when the ratio of the vertical to the radial velocity dispersion is about 0.8–1.0. The two-armed spiral is a dominant mode in galaxy NGC 1566, while the m = 3 mode prevails in the disk of NGC 6503. Theoretical spiral arms predicted for NGC 1566 and NGC 6503 are shorter compared to the obs...

Statistical Properties of the Emission in Mixed-Morphology (E + S) Pairs. II. MIR/FIR Results

We present a statistical analysis of mid/far-IR emission properties for a mixed morphology (E/S0 + S/I) sample of galaxy pairs based on a new co-addition of the IRAS database. The sample is large and diverse enough (~130 pairs) to permit discrimination of pairs by component morphologies and interaction classes. It samples a large enough volume of space to enable a nonparametric estimate of the far-infrared luminosity function (FIRLF). We find average factors of 3 and 5 enhancement in FIR and 25 μm luminosities, respectively, for the late-type components relative to an isolated galaxy control sample. This is interpreted as the MIR/FIR signature of interaction induced star formation. A more detailed study reveals (1) a strong correlation between the IR luminosity and interaction class, (2) a poor anticorrelation between IR luminosity and projected separation in pairs where 25 μm seems to be more sensitive than FIR, (3) an increase in the ratio of present to past star formation (IR luminosities scaled to LB luminosity) from early to late type spiral pair components, and (4) a high-mass (M ≥ 10 M⊙) star formation rate (SFR) higher in paired Sc spirals than that in isolated Sc galaxies by a factor ~2–3.

ENEAR Redshift-Distance Survey: Cosmological Constraints

We present an analysis of the ENEAR sample of peculiar velocities of field and cluster elliptical galaxies, obtained with Dn-σ distances. We use the velocity correlation function ψ1(r) to analyze the statistics of the field object's velocities, while the analysis of the cluster data is based on the estimate of their rms peculiar velocity Vrms. The results are compared with predictions from cosmological models using linear theory. The statistics of the model velocity field is parameterized by the amplitude η8 = σ8Ω and by the shape parameter Γ of the cold dark matter-like power spectrum. This analysis is performed in redshift space, so as to circumvent the need to address corrections due to inhomogeneous Malmquist bias and to the redshift cutoff adopted in the sample selection. From the velocity correlation statistics, we obtain η8 = 0.51 for Γ = 0.25 at the 2 σ level for one interesting fitting parameter. This result agrees with that obtained from a similar analysis of the SFI I-band Tully-Fisher (TF) survey of field Sc galaxies. Even though less constraining, a consistent result is obtained by comparing the measured Vrms of clusters with linear theory predictions. For Γ = 0.25, we find η8 = 0.63 at 1 σ. Again, this result agrees, within the uncertainties, with that obtained from the SCI cluster sample based on TF distances. Overall, our results point toward a statistical concordance of the cosmic flows traced by spiral and early-type galaxies, with galaxy distances estimated using TF and Dn-σ distance indicators, respectively.

Bias Properties of Extragalactic Distance Indicators. IX. Absolute Luminosity and Line‐Width Distributions for Galaxy Types along the Hubble Sequence Based on Distance‐limited Samples from the Revised Shapley‐Ames Catalog

ABSTRACTType‐specific luminosity functions on the M0,iB(T) magnitude system of the Revised Shapley‐Ames Catalog (RSA) are derived for distance‐limited samples of galaxies for all Hubble types and van den Bergh luminosity classes listed in the RSA. The distributions of 21 cm line widths are derived for the Sa to Sd spirals and the Sm/Im types in the same samples. The distance‐limited subsamples have been separated from the complete flux‐limited listings in the RSA using properties of Spaenhauer diagrams.Each type‐specific luminosity function is bounded on the bright and faint ends. All galaxies in the sample that have an appreciable spiral pattern are brighter than M0,iB(T) = -15 (H0 = 50). There are no dwarf spirals in this sample. Galaxies on the exponentially rising faint end of “general” luminosity functions in the recent literature called “spirals” are not galaxies with spiral arms but rather are star‐forming galaxies of Sm and Im types. The distinction is crucial in predictions of the morphological mix expected at faint apparent magnitudes and large redshifts. One of the purposes of this paper is to begin to provide the necessary morphological resolution to adjudicate the high‐redshift suggestions for an appreciable morphological evolution with look‐back time.The brightest and fastest rotating galaxies along the present‐day Hubble sequence are large‐bulge Sa types. The faintest and slowest rotators are Sd and Sm/Im galaxies. The luminosity function of E galaxies is appreciably fainter than for all spirals earlier than Sc II–III. S0 galaxies that have pronounced disk characteristics are fainter than those with “intermediate” (I) and/or “subtle” (S) S0 characteristics.Bias in the slope and zero point of the Tully‐Fisher (TF) relation in flux‐limited samples compared with distance‐limited subsamples is demonstrated using Sb and Sbc galaxies, similar to that shown for Sc galaxies in Paper VII. A fine structure in the TF correlation that varies with Hubble type and van den Bergh luminosity class is suggested for Sb and Sbc galaxies, also similar to that set out for Sc galaxies in Paper VII. The sense is that at fixed absolute magnitude the later luminosity classes (i.e., larger L numbers) for a fixed Hubble type have higher rotational velocities by ∼4% per luminosity L number. Said differently, the Sb, Sbc, and Sc galaxies with less developed spiral arms rotate faster at a given absolute magnitude. A larger sample is required to prove or disprove the trend seen in the present data.

Star Formation: The Current Frontier

ASTRONOMY Star formation occurs by the collapse of a cloud of dust and gas. Spiral galaxies, which formed from collapsed clouds, have a dense nucleus or bulge dominated by older stars formed by an early, rapid burst of star formation. The spiral arms consist of gas, dust, and younger stars swirling outward away from the center. Many processes may trigger star formation, including gravitational interactions or shock waves from supernovae, but distinguishing between these processes is difficult without gaining more information about the composition and age distributions of stars in the galaxies. Davidge has begun a survey in the near-infrared wavelength range of the Sc galaxies, the most “open” class of spiral galaxies, with the Canada-France-Hawaii Telescope (CFHT) adaptive optics bonnette (AOB) to determine the stellar structure of the centers of these galaxies and understand star formation processes. This initial study focuses on M33, the Triangulum galaxy, which is the third largest spiral galaxy in the Local Group (after the Milky Way and Andromeda). He has determined that there was a burst of star formation near the nucleus of M33 that occurred between 1 to 3 billion years ago. He also noted a recent episode of star formation in the nucleus, which is somewhat unexpected because M33 does not have a supermassive black hole at its center that might trigger star formation. Finally, he notes that the source of the gas for the recent star formation episode cannot be derived from the disk, which is chemically distinct from the young stars, so the collapsing gas is probably concentrated in the center. These results suggest that star formation is complex and variable, but more observations are needed to distinguish between the possible mechanisms that initiate the collapse.— LR Astron. J. 119 , 748 (2000).

Dependence of Spiral Galaxy Distribution on Viewing Angle in RC3

The normalized inclination distributions are presented for the spiral galaxies in RC3. The results show that, except for the bin of 81°-90°, in which the apparent minor isophotal diameters that are used to obtain the inclinations are affected by the central bulges, the distributions for Sa, Sab, Scd and Sd are well consistent with the Monte-Carlo simulation of random inclinations within 3-σ, and Sb and Sbc almost, but Sc is different. One reason for the difference between the real distribution and the Monte-Carlo simulation of Sc may be that some quite inclined spirals, the arms of which are inherently loosely wound on the galactic plane and should be classified to Sc galaxies, have been incorrectly classified to the earlier ones, because the tightness of spiral arms which is one of the criteria of the Hubble classification in RC3 is different between on the galactic plane and on the tangent plane of the celestial sphere. Our result also implies that there might exist biases in the luminosity functions of individual Hubble types if spiral galaxies are only classified visually.

Neutral Hydrogen in HCG 95

AbstractWe have observed Hɪ emission and radio continuum emission from the compact group of galaxies HCG 95 with the Very Large Array (VLA)1. Two continuum sources coincide in with galaxies in this group: HCG 95 B (3.9 mJy) and HCG95C (6 mJy). Hɪ emission and absorption was detected in galaxy HCG 95 C. In addition we detected two so far unknown dwarf galaxies by their Hɪ emission within 3.5 arcmin of the group center. We did not detect galaxy b (with ç = 8000 kms−1 it is obviously a foreground object) and galaxy d — an edge-on Sc galaxy. This group definitely is Hɪ deficient compared with the average Hɪ content expected for spiral galaxies of the same luminosity and type. The first-ranked elliptical galaxy HCG 95 A might be responsible for the observed Hɪ deficiency in this group.

Bias Properties of Extragalactic Distance Indicators. VIII. \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $H_{0}$ \end{document} from Distance–limited Luminosity Class and Morphological Type–Specific Luminosity Functions for Sb, Sbc, and Sc Galaxies Calibrated Using Cepheids

Relative, reduced to absolute, magnitude distributions are obtained for Sb, Sbc, and Sc galaxies in the flux-limited Revised Shapley-Ames Catalog (RSA2) for each van den Bergh luminosity class (L), within each Hubble type (T). The method to isolate bias-free subsets of the total sample is via Spaenhauer diagrams, as in previous papers of this series. The distance-limited type and class-specific luminosity functions are to numbers of galaxies per unit volume (105 Mpc3), rather than being left as relative functions, as in Paper V. The functions are calculated using kinematic absolute magnitudes, based on an arbitrary trial value of H0 = 50. Gaussian fits to the individual functions are listed for each T and L subclass. As in Paper V, the data can be freed from the T and L dependencies by applying a correction of 0.23T + 0.5L to the individual absolute magnitudes. Here, T = 3 for Sb, 4 for Sbc, and 5 for Sc galaxies, and the L values range from 1 to 6 as the luminosity class changes from I to III-IV. The total luminosity function, obtained by combining the volume-normalized Sb, Sbc, and Sc individual luminosity functions, each corrected for the T and L dependencies, has an rms dispersion of 0.67 mag, similar to much of the Tully-Fisher parameter space. Absolute calibration of the trial kinematic absolute magnitudes is made using 27 galaxies with known T and L that also have Cepheid distances. This permits the systematic correction to the H0 = 50 kinematic absolute magnitudes of 0.22 ± 0.12 mag, giving The Cepheid are based on the Madore/Freedman Cepheid period-luminosity (PL) zero point that requires (m - M)0 = 18.50 for the LMC. Using the modern LMC modulus of (m - M)0 = 18.58 requires a 4% decrease in H0, giving a final value of H0 = 53 ± 7 (external) by this method. These values of H0, based here on the method of luminosity functions, are in good agreement with (1) H0 = 55 ± 5 by Theureau and coworkers from their bias-corrected Tully-Fisher method of normalized distances for field galaxies; (2) H0 = 56 ± 4 from the method through the Virgo Cluster, as corrected to the global kinematic frame (Tammann and coworkers); and (3) H0 = 58 ± 5 from Cepheid-calibrated Type Ia supernovae (Saha and coworkers). Our value here also disagrees with the final value from the NASA Key Project group value of H0 = 70 ± 7. Analysis of the total flux-limited sample of Sb, Sbc, and Sc galaxies in the RSA2 by the present method, but uncorrected for selection bias, would give an incorrect value of H0 = 71 using the same Cepheid calibration. The effect of the bias is pernicious at the 30% level; either it must be corrected by the methods in the papers of this series, or the data must be restricted to the distance-limited subset of any sample, as is done here.

Neutral Gas Distribution and Kinematics of the Nearly Face-on Spiral Galaxy NGC 1232

We have analyzed high velocity resolution H I synthesis observations of the nearly face-on Sc galaxy NGC 1232. The neutral gas distribution extends well beyond the optical extent of the galaxy. As expected, local peaks in the H I column density are associated with the spiral arms. Further, the H I column density drops precipitously near the center of the galaxy. Closed contours in the velocity field suggest either that the system is warped or that the rotation curve declines. The velocity dispersion is approximately constant throughout the system, with a median value of 9.9 ± 1.8 km s-1. When corrected for rotational broadening, there is no indication of a radial trend in the neutral gas velocity dispersion in this galaxy.

A Kinematic Link Between Boxy Bulges, Stellar Bars, and Nuclear Activity in NGC 3079 and NGC 4388

We present direct kinematic evidence for bar streaming in two active galaxies with boxy stellar bulges. The Hawaii Imaging Fabry-Perot Interferometer was used on the Canada-France-Hawaii 3.6-m telescope and the University of Hawaii 2.2-m telescope to derive the two-dimensional velocity field of the line-emitting gas in the disks of the Sc galaxy NGC 3079 and the Sb galaxy NGC 4388. In contrast to previous work based on long-slit data, the detection of the bar potential from the Fabry-Perot data does not rely on the existence of inner Lindblad resonances or strong bar-induced shocks. Simple kinematic models which approximate the intrinsic gas orbits as nonintersecting, inclined elliptical annuli that conserve angular momentum characterize the observed velocity fields. In NGC 3079, bar streaming motions with moderately eccentric orbits (e = b/a approx. 0.7) aligned along PA = 130 deg. intrinsic to the disk (PA = 97 deg. on the sky) are detected out to R(sub b) = 3.6 kpc. The orbits become increasingly circular beyond that radius (e = 1 at R(sub d) approx. = 6 kpc). The best model for NGC 4388 includes highly eccentric orbits (e approx. 0.3) for R(sub) less than or equal to 1.5 kpc which are aligned along PA = 135 deg. intrinsic to the disk (PA = 100 deg. on the sky). The observed spiral arms are produced by having the orbits become increasingly circular from the ends of the bar to the edge of the disk (R(sub d) approx. = 5 kpc), and the intrinsic bar PA shifting from 135 deg. to 90 deg.. Box-shaped bulges in both NGC 3079 and NGC 4388 are confirmed using new near-infrared images to reduce dust obscuration. Morphological analysis of starlight in these galaxies is combined with the gas kinematics derived from the Fabry-Perot spectra to test evolutionary models of stellar bars that involve transitory boxy bulges, and to quantify the importance of such bars in fueling active nuclei. Our data support the evolutionary bar models, but fail to prove convincingly that the stellar bars in NGC 3079 and NGC 4388 directly trigger or sustain the nuclear activity.

Deep CCD Surface Photometry of the Edge-on Spiral NGC 4244

We have obtained deep surface photometry of the edge-on spiral galaxy NGC 4244. Our data reliably reach 27.5 R magnitude arcsec^{-2}, a significant improvement on our earlier deep CCD surface photometry of other galaxies. NGC 4244 is a nearby Scd galaxy whose total luminosity is approximately one magnitude fainter than the peak of the Sc luminosity function. We find that it has a simple structure: a single exponential disk, with a scale height h_Z = 246 +/- 2 pc, a scale length h_R = 1.84 +/- 0.02 kpc and a disk cutoff at a radius R(max) = 10.0 kpc (5.4 scale lengths). We confirm a strong cutoff in the stellar disk at R(max), which happens over only 1 kpc. We do not see any statistically significant evidence for disk flaring with radius. Unlike the more luminous Sc galaxies NGC 5907 and M 33, NGC 4244 does not show any evidence for a second component, such as a thick disk or halo, at mu(R) < 27.5 magnitude arcsec^{-2}.

The Luminosity Functions and Size Distributions of HiiRegions in Irregular Galaxies

CCD Hα images were used to study the properties of H II regions in 29 normal Im galaxies and six blue compact dwarf/starburst irregulars. The Hα emission line fluxes were measured and used to construct luminosity functions for each galaxy. For most galaxies, the luminosity function is well represented by a power law. Only two of the 29 Im galaxies have supergiant H II regions, which is probably a result of the small size and small total numbers of H II regions in irregular galaxies. BCDs and starburst galaxies, on the other hand, do not always have supergiant H II regions but may have them even if only a few regions are present. Comparison of the cumulative composite H II luminosity function with that of spiral galaxies shows that the irregulars in our sample do not have an unusually large population of supergiant H II regions relative to the galactic luminosity. Thus, these galaxies will not suffer a disproportionate amount of disruption by the concentration of massive stars within such regions. However, in BCD/starburst galaxies there is an excess of H II regions with luminosities greater than 1038 ergs s-1 compared to Im and Sc galaxies relative to the luminosities of the host galaxies. Most of the H II region luminosity in normal Im galaxies comes from small regions and that in BCD/starburst galaxies from moderately large regions. Over 80% of the H II region luminosity in irregulars is found in complexes of H II regions typically with summed luminosities equivalent to ≤10 Orion nebulae. Two types of luminosity functions were observed: those that exhibit turnover and those that do not. Those that do exhibit turnover have an average slope of -1.5 ± 0.1, and those that do not exhibit turnover tend to be complete to fainter levels, have lower upper luminosity cutoffs, have fewer H II regions, and have shallower slopes of -1.0 ± 0.1. If the luminosity function is universal for a galaxy type, this difference suggests the possibility of a break in the luminosity function corresponding to H II regions ionized by single stars and H II regions ionized by clusters of stars. However, we cannot exclude the possibility that the two groups of galaxies are different in terms of their global star formation properties. Diameters of H II regions were also measured and diameter distributions were constructed for each of the galaxies. The diameter distribution was fitted with an exponential and the characteristic diameter D0 was measured for each galaxy, although this fit did not characterize the distribution very well in most galaxies. The weak correlation between D0 and the absolute blue magnitude of the galaxy suggested by others is confirmed to be very weak.

The [ITAL]I[/ITAL]-Band Tully-Fisher Relation for S[CLC]c[/CLC] Galaxies: 21 Centimeter H [CSC]i[/CSC] Line Data

A compilation of 21 cm line spectral parameters specifically designed for application of the Tully-Fisher (TF) distance method is presented for 1201 spiral galaxies, primarily field Sc galaxies, for which optical I-band photometric imaging is also available. New H I line spectra have been obtained for 881 galaxies. For an additional 320 galaxies, spectra available in a digital archive have been reexamined to allow application of a single algorithm for the derivation of the TF velocity width parameter. A velocity width algorithm is used that provides a robust measurement of rotational velocity and permits an estimate of the error on that width taking into account the effects of instrumental broadening and signal-to-noise. The digital data are used to establish regression relations between measurements of velocity widths using other common prescriptions so that comparable widths can be derived through conversion of values published in the literature. The uniform H I line widths presented here provide the rotational velocity measurement to be used in deriving peculiar velocities via the TF method.

A 180 Kiloparsec Tidal Tail in the Luminous Infrared Merger Arp 299

We present VLA H I observations and University of Hawaii 88 inch (2.2 m) deep optical B- and R-band observations of the IR-luminous merger Arp 299 (= NGC 3690 + IC 694). These data reveal a gas-rich (MH I = 3.3 × 109 M⊙) optically faint (μB 27 mag arcsec-2, μR 26 mag arcsec-2) tidal tail with a length of over 180 kpc. The size of this tidal feature necessitates an old interaction age for the merger (750 Myr since first periapse), which is currently experiencing a very young starburst (20 Myr). The observations reveal a most remarkable structure within the tidal tail: it appears to be composed of two parallel filaments separated by approximately 20 kpc. One of the filaments is gas-rich with little if any starlight, while the other is gas-poor. We believe that this bifurcation results from a warped disk in one of the progenitors. The quantities and kinematics of the tidal H I suggest that Arp 299 results from the collision of a retrograde Sab–Sb galaxy (IC 694) and a prograde Sbc–Sc galaxy (NGC 3690) that occurred 750 Myr ago and will merge into a single object in roughly 60 Myr. We suggest that the present IR-luminous phase in this system is due in part to the retrograde spin of IC 694. Finally, we discuss the apparent lack of tidal dwarf galaxies within the tail.

Environmental Extremists in the Virgo Cluster

Many types of galaxy interactions have been posited to occur in clusters, although it remains unclear which processes actually occur, and which ones might help explain the tendency for early type galaxies to inhabit high density environments, or cause the rapid evolution of cluster galaxies (e.g., Dressler et al. 1997). With these questions in mind, we have been conducting an environmental inventory of galaxies in the Virgo Cluster. Our approach is to combine surveys of spirals and S0s with detailed studies of the most interesting and peculiar galaxies. In this paper, we describe two main points. 1.) There is a population of spiral galaxies in the Virgo cluster with the small central light concentrations (bulge-to-disk ratios, or B/D's) characteristic of isolated Sb and Sc galaxies, but global star formation rates lower than those of isolated spirals of any Hubble class (Sa-Sc). These Virgo galaxies are generally classified as “early type” (e.g. Sa), and thus contribute to the morphology-density relationship. 2.) There areseveral typesof environmental interactions occurring in Virgo, including low velocity tidal interactions and mergers, high velocity tidal interactions and collisions, HI accretion, and ICM-ISM stripping. We discuss examples of some of these interactions.

Bias Properties of Extragalactic Distance Indicators. VII. Correlation of Absolute Luminosity and Rotational Velocity for S[CSC]c[/CSC] Galaxies over the Range of Luminosity Class from I to III–IV

A distance-limited subset of the complete flux-limited sample of Sc galaxies in the Revised Shapley-Ames Catalog of Bright Galaxies is isolated by means of separate Spaenhauer diagrams for six individual van den Bergh luminosity class intervals from Sc I + I.2, .3 to Sc III–IV. The distribution functions of kinematic absolute B(220, 50) magnitudes and 21 cm line widths, W20, corrected to edge-on orientation, have been determined for the same six bins of luminosity class. The individual luminosity functions for each luminosity class are bounded on both the bright and faint ends, showing that the present sample includes no dwarf Sc spirals fainter than M(BT)(220, 50) = -18 belonging to luminosity classes I to III–IV, as defined by the regularity of the spiral pattern. Star-forming galaxies with spiral structures as regular as the ones found in these luminosity classes have absolute magnitudes brighter than MB(H = 50) = -18 and 21 cm line widths larger than W20/ sin i = 2vrot(max) = 165 km s-1. Furthermore, the 21 cm line-width distributions move toward smaller rotational velocities as the luminosity classes change from I to III, showing that rotation is a principal parameter determining the regularity of the spiral pattern. Whether it is the only parameter awaits a similar investigation for spirals of all luminosity classes along the complete Hubble sequence. In particular, it has not yet been proved that all Im and Sm galaxies, where, by definition, the spiral arms are either lacking or are semichaotic, have absolute magnitudes that are fainter than MB = -18 and whose 21 cm LWs are smaller than ~165 km s-1, presumably because of smaller mass than the high-luminosity, regular spirals. The Teerikorpi cluster population incompleteness bias is demonstrated again. Here, however, as in Papers II–IV of this series, we use field galaxies to show that the slope and zero point of the Tully-Fisher (T-F) relation are systematically incorrect for flux-limited samples, the error increasing with redshift. The data also support the possibility that the zero point of the T-F relation for Sc galaxies, even in the distance-limited sample, is a function of luminosity class, varying by 0.42 mag between Sc I + I.2, .3 and Sc III galaxies if the slope of the T-F relation for each class is taken to be that of the fundamental calibrating galaxies with Cepheid distances. If true, this is presumably due to differences in the mass distribution and, therefore, in the resulting rotational velocity field as a function of luminosity class even within a fixed Hubble type.