Caroline E. Simpson

High-resolution Mass Models of Dwarf Galaxies from LITTLE THINGS

We present high-resolution rotation curves and mass models of 26 dwarf galaxies from LITTLE THINGS. LITTLE THINGS is a high-resolution Very Large Array HI survey for nearby dwarf galaxies in the local volume within 11 Mpc. The rotation curves of the sample galaxies derived in a homogeneous and consistent manner are combined with Spitzer archival 3.6 micron and ancillary optical U, B, and V images to construct mass models of the galaxies. We decompose the rotation curves in terms of the dynamical contributions by baryons and dark matter halos, and compare the latter with those of dwarf galaxies from THINGS as well as Lambda CDM SPH simulations in which the effect of baryonic feedback processes is included. Being generally consistent with THINGS and simulated dwarf galaxies, most of the LITTLE THINGS sample galaxies show a linear increase of the rotation curve in their inner regions, which gives shallower logarithmic inner slopes alpha of their dark matter density profiles. The mean value of the slopes of the 26 LITTLE THINGS dwarf galaxies is alpha =-0.32 +/- 0.24 which is in accordance with the previous results found for low surface brightness galaxies (alpha = -0.2 +/- 0.2) as well as the seven THINGS dwarf galaxies (alpha =-0.29 +/- 0.07). However, this significantly deviates from the cusp-like dark matter distribution predicted by dark-matter-only Lambda CDM simulations. Instead our results are more in line with the shallower slopes found in the Lambda CDM SPH simulations of dwarf galaxies in which the effect of baryonic feedback processes is included. In addition, we discuss the central dark matter distribution of DDO 210 whose stellar mass is relatively low in our sample to examine the scenario of inefficient supernova feedback in low mass dwarf galaxies predicted from recent Lambda SPH simulations of dwarf galaxies where central cusps still remain.

A Comparative Study of Star-forming and Quiescent Dwarf Galaxies

We present the results from a comparative study of the atomic hydrogen (H I) and optical properties of a sample of 16 dwarf galaxies, chosen to investigate the effects of star formation on the properties of low-mass systems. The violent star formation bursts believed to occur in these low-mass systems suggest a possible connection between the actively star-forming blue compact dwarfs (BCDs), and the quiescent low surface brightness dwarfs (LSBDs). It has been suggested that LSBDs, upon undergoing a burst of star formation, will evolve into BCDs and then back into LSBDs when the star formation slows or stops as the H I column density falls below the critical threshold necessary to support it. We have examined the location and kinematics of H I in eight BCDs and eight LSBDs of similar H I masses and a range of color indices to investigate this evolutionary sequence. The starburst episodes in these low-mass galaxies should lead to (1) a dispersal/depletion of the H I seen in the eight LSB dwarfs and (2) more centrally concentrated and agitated H I in the eight BCDs. The results of this project indicate that the quiescent LSBD galaxies have more diffuse H I distributions and often show a ringlike structure, while the active galaxies have more highly centrally concentrated H I reservoirs. The bluer, more recently active systems of both types also have higher internal H I velocity dispersions, indicating that energy has been pumped into the interstellar medium of these galaxies. These observations are consistent with an evolutionary scheme wherein the H I reservoirs in these galaxies take on different characteristics depending upon their star formation histories.

DDO 88: A GALAXY-SIZED HOLE IN THE INTERSTELLAR MEDIUM

We present an H I and optical study of the gas-rich dwarf irregular galaxy DDO 88. Although the global optical and H I parameters of DDO 88 are normal for its morphological type, it hosts a large (3 kpc diameter) and unusually complete ring of enhanced H I emission. The normal appearance of this galaxy in the optical and the outer regions of the H I give no hint of the presence of the striking H I ring in the inner regions. The gas ring is located at approximately one-third of the total H I radius and one-half the optically defined Holmberg radius, and contains 30% of the total H I of the galaxy. The ring surrounds a central depression in the H I distribution. If the H I ring and central depression in the gas were formed by the energy input from winds and supernova explosions of massive stars formed in a starburst, as is thought often to be the case, the star-forming event would have formed 0.1%–1% of the total stellar mass of the galaxy. However, the UBV colors in the H I hole are not bluer than the rest of the galaxy, as would be expected if an unusual star-forming event had taken place there recently, although there is an old (~1–3 Gyr), red cluster near the center of the hole that is massive enough to have produced the hole in the H I. An age estimate for the ring is uncertain, however, because it is not observed to be expanding. An expansion model produces a lower estimate of 0.5 Gyr, but the presence of faint star formation regions associated with the ring indicates a much younger age. We also estimate that the ring could have dispersed by now if it is older than 0.5 Gyr. This implies that the ring is younger than 0.5 Gyr. A younger age would indicate that the red cluster did not produce the hole and ring. Therefore, uncertainties prevent us from concluding that the cluster and the H I hole are definitely related. If this ring and the depression in the gas that it surrounds were not formed by stellar winds and supernovae, this would indicate that some other, currently unidentified, mechanism is operating.

The H I chronicles of little things BCDs II: The origin of IC 10's H I structure

In this paper we analyze Very Large Array (VLA) telescope and Green Bank Telescope (GBT) atomic hydrogen (H I) data for the LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, The H I Nearby Galaxy Survey; https://science.nrao.edu/science/surveys/littlethings) blue compact dwarf galaxy IC 10. The VLA data allow us to study the detailed H I kinematics and morphology of IC 10 at high resolution while the GBT data allow us to search the surrounding area at high sensitivity for tenuous H I. IC 10s H I appears highly disturbed in both the VLA and GBT H I maps with a kinematically distinct northern H I extension, a kinematically distinct southern plume, and several spurs in the VLA data that do not follow the general kinematics of the main disk. We discuss three possible origins of its H I structure and kinematics in detail: a current interaction with a nearby companion, an advanced merger, and accretion of intergalactic medium. We find that IC 10 is most likely an advanced merger or a galaxy undergoing accretion.

THE H I CHRONICLES OF LITTLE THINGS BCDs: EVIDENCE FOR EXTERNAL PERTURBATIONS IN THE MORPHOLOGY AND KINEMATICS OF HARO 29 AND HARO 36

We analyze high angular and velocity resolution H I line data of two LITTLE THINGS blue compact dwarfs (BCDs): Haro 29 and Haro 36. Both of these BCDs are disturbed morphologically and kinematically. Haro 29s H I data reveal a kinematic major axis that is offset from the optical major axis, and a disturbed outer H I component, indicating that Haro 29 may have had a past interaction. Position-velocity diagrams of Haro 36 indicate that it has two kinematically separate components at its center and a likely tidal tail in front of the galaxy. We find that Haro 36 most likely had an interaction in the past, is currently interacting with an unknown companion, or is a merger remnant.

DDO 43: A Prototypical Dwarf Irregular Galaxy?

We present sensitive and high-resolution 21 cm observations of the dwarf irregular (Im) galaxy DDO 43, in conjunction with optical broadband and narrowband images in U, B, V, and Hα. The observations are used to examine the relationship of its H I morphology and kinematics to past and present star formation. Optically, it is a small (R25 = 990 pc), faint (MB of -14.0) dwarf Im with a slightly boxy shape. In H I, DDO 43 has an extended (R/RH = 2.8) gas envelope. There is a high-density ridge associated with the optical body of the galaxy containing several higher density knots and lower density holes. The largest hole is ~850 × 530 pc. No expansion is detected, so it must be relatively old. The largest and potentially oldest (7–70 Myr) of the six identified star clusters is located at the western edge of the hole. Four of the other clusters are located near high-density peaks. There are several H II regions, most (but not all) of which are associated with peaks in the H I surface density. The overall star formation rate is average for its type. In many ways, DDO 43 is a very typical dwarf Im galaxy. Its H I morphology is consistent with a history of episodes of localized star formation that create holes and shells in the interstellar medium, some of which can overlap. These features are located within the area of solid-body rotation in the galaxy; the lack of shear in these small systems allows such structures to persist for long periods of time.

Neutral Hydrogen in Arp 158

We present 21 cm observations of Arp 158. We have performed a study of the neutral hydrogen (H I) to help us understand the overall formation and evolution of this system. This is a disturbed system with distinct optical knots connected by a linear structure embedded in luminous material. There is also a diffuse spray to the southeast. The H I seems to be made up of three distinct, kinematically separate systems. Arp 158 bears a certain optical resemblance to NGC 520 (Arp 157), which has been identified as a mid-stage merger. From our 21 cm observations of Arp 158, we also see a comparable H I content with NGC 520. These similarities suggest that Arp 158 is also an intermediate-stage merger.

The H i Chronicles of LITTLE THINGS BCDs. III. Gas Clouds in and around Mrk 178, VII Zw 403, and NGC 3738

In most blue compact dwarf (BCD) galaxies, it remains unclear what triggers their bursts of star formation. We study the HI of three relatively isolated BCDs, Mrk 178, VII Zw 403, and NGC 3738, in detail to look for signatures of star formation triggers, such as gas cloud consumption, dwarf-dwarf mergers, and interactions with companions. High angular and velocity resolution atomic hydrogen (H I) data from the Very Large Array (VLA) dwarf galaxy HI survey, Local Irregulars That Trace Luminosity Extremes, The HI Nearby Galaxy Survey (LITTLE THINGS), allows us to study the detailed kinematics and morphologies of the BCDs in HI. We also present high sensitivity HI maps from the NRAO Green Bank Telescope (GBT) of each BCD to search their surrounding regions for extended tenuous emission or companions. The GBT data do not show any distinct galaxies obviously interacting with the BCDs. The VLA data indicate several possible star formation triggers in these BCDs. Mrk 178 likely has a gas cloud impacting the southeast end of its disk or it is experiencing ram pressure stripping. VII Zw 403 has a large gas cloud in its foreground or background that shows evidence of accreting onto the disk. NGC 3738 has several possible explanations for its stellar morphology and H I morphology and kinematics: an advanced merger, strong stellar feedback, or ram pressure stripping. Although apparently isolated, the HI data of all three BCDs indicate that they may be interacting with their environments, which could be triggering their bursts of star formation.

PROPERTIES OF THE DWARF IRREGULAR GALAXY DDO 169

This paper investigates the H i, optical, and dark matter properties of the dwarf irregular galaxy DDO 169. The galaxy shows signs of being tidally disrupted. Evidence of this can easily be seen in the galaxys velocity field. We also show that DDO 169, which was thought to have a companion galaxy, does not. On obtaining a satisfactory result with the edited calibrator data, the gain, phase, and bandpass calibrations were determined. The calibration was then applied to the channel-zero data set and was checked for problems. The few that were identified were edited out, and the calibration was then applied to the line data set. The (u, v) data were examined very carefully to see whether anyfurthereditingwasnecessarybeforebeingtransformedinto images. The C-configuration source line data showed signs of solar contamination.Hence,allfluxvaluesabove1Jywereeditedfrom the C-configuration source data. After each configuration data set for the galaxy was individu- allycalibratedandedited,thecontinuumemissionwassubtracted. This was done by making linear fits to the (u, v) data set in the H i lineYfree channels. A fast Fourier transform was then ap- plied to the individual data sets to produce a cube (� , � , v )o f im- ages. During the imaging process, each channel in the cube was CLEANed (Hogbom 1974; Clark 1980) to reduce the effects of sidelobes produced by non-Gaussian features of the synthesized

The Nature of Turbulence in the LITTLE THINGS Dwarf Irregular Galaxies

We present probability density functions and higher order (skewness and kurtosis) analyses of the galaxy-wide and spatially-resolved HI column density distributions in the LITTLE THINGS sample of dwarf irregular galaxies. This analysis follows that of Burkhart et al. (2010) for the Small Magellanic Cloud. About 60% of our sample have galaxy-wide values of kurtosis that are similar to that found for the Small Magellanic Cloud, with a range up to much higher values, and kurtosis increases with integrated star formation rate. Kurtosis and skewness were calculated for radial annuli and for a grid of 32 pixel X 32 pixel kernels across each galaxy. For most galaxies, kurtosis correlates with skewness. For about half of the galaxies, there is a trend of increasing kurtosis with radius. The range of kurtosis and skewness values is modeled by small variations in the Mach number close to the sonic limit and by conversion of HI to molecules at high column density. The maximum HI column densities decrease with increasing radius in a way that suggests molecules are forming in the weak field limit, where H_2 formation balances photodissociation in optically thin gas at the edges of clouds.