D. J. Pisano

THE H I DISTRIBUTION AND DYNAMICS IN TWO LATE-TYPE BARRED SPIRAL GALAXIES : NGC 925 AND NGC 1744

We present H I observations of NGC 925 and NGC 1744, two late-type barred spiral galaxies that are quite comparable in mass and optical morphology. NGC 925 is a very asymmetric galaxy with a strong spiral arm in the south and flocculent arms in the north. The rotation curves of the east and west sides of the galaxy are consistent with this asymmetry as well. In addition, NGC 925s dynamical center may be slightly offset (215, or ~1 kpc) from its optical center. The ratio of the H I to optical diameter of NGC 925 is small compared with other galaxies. We detected a ~107 M⊙ H I cloud and streamer, with little or no stellar component, apparently interacting with NGC 925. While the interaction between NGC 925 and the cloud may be responsible for the observed asymmetries, given the weakness of the interaction we postulate that NGC 925 has suffered other gravitational encounters over the past few gigayears. NGC 1744, on the other hand, is an unperturbed galaxy having a textbook example of a velocity field for a disk galaxy. It has a symmetric, albeit weak, spiral structure. Both NGC 925 and NGC 1744 have small streaming motions (~10 km s-1) associated with the bar and spiral arms.

The Gas Content and Kinematics of Nearby Blue Compact Galaxies: Implications for Studies at Intermediate and High Redshift

We present Arecibo 21 cm spectroscopy, Keck HIRES Hβ spectroscopy, and WIYN R-band imaging of 11 nearby blue compact galaxies with effective B-band surface brightnesses of SBe = 19.4–21.2 mag arcsec^2 and effective radii of R_ eff = 0.6–1.9 kpc. This sample was selected to test the reliability of mass estimates derived using optical emission line widths, particularly for the blue compact star-forming galaxies observed at intermediate redshifts (0.1 < z < 1). In addition, we also measure the H I content and gas depletion timescales for the nearby blue compact galaxies in an attempt to infer the present nature and possible future evolution of their intermediate-redshift analogs. We detected H I in 10 of 11 sample galaxies. They have H I masses of 0.3–4 × 10^9 M_⊙, H I line widths, W_20, of 133–249 km s^-1, dynamical masses of 0.5 to 5 × 10^10 M_⊙, gas depletion timescales, τgas, of 0.3–7 Gyr, H I mass fractions of 0.01–0.58, and mass-to-light ratios of 0.1–0.8. These values span the range of values typical of nearby H II galaxies, irregulars, and spirals. Despite the restricted morphological selection, our sample of galaxies is quite heterogeneous in terms of H I content, dynamical mass, and gas depletion timescale. Therefore, these galaxies have a variety of evolutionary paths and should look very different from each other in 5 Gyr. Those with high masses and gas depletion timescales are likely to retain their interstellar medium for future star formation, while the lower mass objects with small gas depletion timescales may be undergoing their last major event of star formation. Hence, the fading of intermediate-redshift, luminous, blue compact galaxies into NGC 205-type spheroidals is a viable evolutionary scenario, but only for the least massive, most gas-poor objects in this sample. The most consistent characteristic of our morphologically selected sample is that the ratios of H II line widths to H I 21 cm line widths, Script Script R = W20(H )/W20(H ), are systematically less than unity, with an average value of Script R = 0.66 ± 0.16, similar to findings for local H II galaxies. The simplest explanation for this result is that the ionized gas is more centrally concentrated than the neutral gas within the gravitational potential. We find that Script R is a function of line width, such that smaller line width galaxies have smaller values of Script R. Correcting optical line widths by this factor not only raises the derived masses of these galaxies, but also makes them consistent with the local luminosity–line width (Tully-Fisher) relation as well. If this ratio applies to intermediate-redshift galaxies, then the masses of intermediate-redshift, blue compact galaxies can be obtained from optical line widths after applying a small correction factor, and the proposed luminosity evolution of the Tully-Fisher relation is much smaller and more gradual than suggested by studies using optical emission line width measurements.

Gas-Rich Companions of Isolated Galaxies

We have used the VLA to search for gaseous remnants of the galaxy formation process around six extremely isolated galaxies. We found two distinct H I clouds around each of two galaxies in our sample (UGC 9762 and UGC 11124). These clouds are rotating and appear to have optical counterparts, strongly implying that they are typical dwarf galaxies. The companions are currently weakly interacting with the primary galaxy but look to be in stable orbits as they have dynamical friction timescales of 5–100 Gyr. In one case, (UGC 11124N), we see ongoing accretion of the companion onto the primary galaxy. The small mass ratio involved in this interaction means that the resulting merger will be a minor one. The companion does, however, contain enough gas that the merger will represent a significant infusion of fuel to drive future star formation in UGC 11124 while building up the mass of the disk.

An H I/Optical Atlas of Isolated Galaxies

We present an H I and optical survey of 41 extremely isolated galaxies in an attempt to search for the gaseous remnants of the galaxy formation process, predicted to exist by hierarchical models of galaxy formation. By observing extremely isolated, quiescent, nonpeculiar galaxies we minimize the chances that any extragalactic H I found will be another galaxy, tidal debris, or ejecta from a galactic fountain or superwind. We have obtained new and archival data from the VLA and ATCA in a search for H I clouds down to M ~ 107 M☉ around these galaxies. We found 13 H I-rich companions around 10 of the 41 galaxies surveyed. Optical imaging finds spatially coincident stars associated with all 13 companions. We find that the isolated galaxies have properties fairly similar to those of field galaxies, while the companions are similar to dwarf irregular galaxies. The presence of stars in all 13 companions suggests that H I clouds without stars discovered by other authors around field galaxies are not primordial and most likely have either a tidal or ejecta origin.

The State of Galaxy Formation in the Local Universe

We present the analysis of the H I survey of isolated galaxies described in the first two papers in this series. This survey found 13 gas-rich companions around 10 of the 41 galaxies observed. In this paper, we present numerous pieces of evidence that the number of companions detected is less than expected by cold dark matter models of galaxy formation or H I observations of the population of galaxies in the field. We discuss the implications of these data for galaxy formation and the evolution of the properties of these companions. A statistical analysis of the observed distribution of the ensemble of companions with respect to the primary galaxies implies that, in general, these companions are on circular orbits and not eccentric orbits. While individual companions may be on eccentric orbits, the majority of the orbits are circular and, therefore, fairly stable. In fact, on the basis of the observed separations, both spatial and in velocity space, only those companions currently interacting with the galaxy they orbit should be accreted in the next few Gyr, and only half of the companies will be accreted in less than a Hubble time. Of the close companions, only UGC 260A and UGC 11152A appear to be significantly affecting the morphology of the primary galaxy. The remaining companions have too little mass and are too far away from the main galaxies to have any current effect on their morphology. None of these companions are massive enough to destroy the disk of the isolated galaxy when they are accreted, but instead will represent a minor merger that will supply a large amount of H I to fuel future star formation in these galaxies and produce small effects on the morphology. All of which implies that we are seeing nothing more than the tail end of the galaxy formation process. We have also compared the cumulative velocity distribution function of our companions with the predictions of cold dark matter and other galaxy formation models. Our data are only consistent with cold dark matter models when generous, but not unreasonable, correction factors are applied for nondetections of gas-poor galaxies. Models that suppress observable satellite galaxies, such as warm dark matter models or those that prevent neutral gas from accumulating in dark matter halos, would better explain our observed population of companions. Finally, we compare the H I mass function (MF) for companions to isolated galaxies with the H I MF from the work of others. Our H I MF is consistent with a variety of faint-end slopes up to α ~ -1.3. Such a slope argues against the Blitz et al. model for high-velocity clouds.

Structure and Star Formation in NGC 925

We present the results from an optical study of the stellar and star formation properties of NGC 925 using the WIYN 3.5 m telescope. Images in B, V, R, and Hα reveal a galaxy that is fraught with asymmetries. From isophote fits we discover that the bar center is not coincident with the center of the outer isophotes or with the dynamical center (from Paper I in this series). Cuts across the spiral arms reveal that the northern arms are distinctly different from the southern arm. The southern arm not only appears more coherent, but the peaks in stellar and Hα emission are found to be coincident with those of the H I distribution, while no such consistency is present in the northern disk. We also examine the gas surface density criterion for massive star formation in NGC 925, and we find that its behavior is more consistent with that for irregular galaxies than with late-type spirals. In particular, star formation persists beyond the radius at which the gas surface density falls below the predicted critical value for star formation for late-type spirals. Such properties are characteristic of Magellanic spirals but are present at a less dramatic level in NGC 925, a late-type spiral.

Three-dimensional spectroscopy of local luminous compact blue galaxies: kinematic maps of a sample of 22 objects: 3D spectroscopy of local Luminous Compact Blue Galaxies

We use three-dimensional optical spectroscopy observations of a sample of 22 local luminous compact blue galaxies (LCBGs) to create kinematic maps. By means of these, we classify the kinematics of these galaxies into three different classes: rotating disc (RD), perturbed rotation (PR) and complex kinematics (CK). We find 48 per cent are RDs, 28 per cent are PRs and 24 per cent are CKs. RDs show rotational velocities that range between similar to 50 and similar to 200 km s(-1), and dynamical masses that range between similar to 1 x 10(9) and similar to 3 x 10(10) M(circle dot). We also address the following two fundamental questions through the study of the kinematic maps: (i) What processes are triggering the current starburst in LCBGs? We search our maps of the galaxy velocity fields for signatures of recent interactions and close companions that may be responsible for the enhanced star formation in our sample. We find that 5 per cent of objects show evidence of a recent major merger, 10 per cent of a minor merger and 45 per cent of a companion. This argues in favour of ongoing interactions with close companions as a mechanism for the enhanced star formation activity in these galaxies. (ii) What processes may eventually quench the current starbust in LCBGs? Velocity and velocity width maps, together with emission line ratio maps, can reveal signatures of active galactic nuclei (AGNs) activity or supernova (SN)-driven galactic winds that could halt the current burst. We find only 5 per cent of objects with clear evidence of AGN activity and 27 per cent with kinematics consistent with SN-driven galactic winds. Therefore, a different mechanism may be responsible for quenching the star formation in LCBGs. Finally, from our analysis, we find that the velocity widths of RDs, rather than accounting exclusively for the rotational nature of these objects, may account as well for other kinematic components and may not be good tracers of their dynamical masses.

3D Spectroscopy of Local Luminous Compact Blue Galaxies: Kinematic Maps of a Sample of 22 Objects

We use three-dimensional optical spectroscopy observations of a sample of 22 local luminous compact blue galaxies (LCBGs) to create kinematic maps. By means of these, we classify the kinematics of these galaxies into three different classes: rotating disc (RD), perturbed rotation (PR) and complex kinematics (CK). We find 48 per cent are RDs, 28 per cent are PRs and 24 per cent are CKs. RDs show rotational velocities that range between similar to 50 and similar to 200 km s(-1), and dynamical masses that range between similar to 1 x 10(9) and similar to 3 x 10(10) M(circle dot). We also address the following two fundamental questions through the study of the kinematic maps: (i) What processes are triggering the current starburst in LCBGs? We search our maps of the galaxy velocity fields for signatures of recent interactions and close companions that may be responsible for the enhanced star formation in our sample. We find that 5 per cent of objects show evidence of a recent major merger, 10 per cent of a minor merger and 45 per cent of a companion. This argues in favour of ongoing interactions with close companions as a mechanism for the enhanced star formation activity in these galaxies. (ii) What processes may eventually quench the current starbust in LCBGs? Velocity and velocity width maps, together with emission line ratio maps, can reveal signatures of active galactic nuclei (AGNs) activity or supernova (SN)-driven galactic winds that could halt the current burst. We find only 5 per cent of objects with clear evidence of AGN activity and 27 per cent with kinematics consistent with SN-driven galactic winds. Therefore, a different mechanism may be responsible for quenching the star formation in LCBGs. Finally, from our analysis, we find that the velocity widths of RDs, rather than accounting exclusively for the rotational nature of these objects, may account as well for other kinematic components and may not be good tracers of their dynamical masses.

HI Properties of Luminous Blue Compact Galaxies in the Local Universe

We present Arecibo 21 cm spectroscopy and Keck echelle Hβ spectroscopy of 11 local galaxies selected to resemble the blue, compact starforming galaxies which are observed at intermediate redshifts. Although measurement of the HI properties of our sample does not allow strong constraints to be placed on the evolutionary descendants of our sample, we can infer that their counterparts in the intermediate and high-redshift universe are likely to be similarly low mass and gas rich. We have found that the ratios of optical Balmer linewidths to HI 21 cm linewidths are systematically less than unity, with an average value of 0.66 ± 0.16. This fraction correlates strongly with the Hβ linewidth, concentration index and α equivalent width. A combination of these parameters could be used to apply correction factors which enable robust measurements of the masses of similar galaxies at cosmological distances.