A new Luminous Variable in M33

We present a new luminous star in M33 located in the nuclear region. The star shows strong FeII and [FeII] forest, hydrogen emissions in the spectrum, as well as nebular lines. TiII and SiII lines were detected in absorption, their radial velocity shifted by ~ -30km/s relative to emission lines. The star is variable over seven years with 0.5 mag variations over a year. We studied its spectral energy distribution together with five confirmed Luminous Blue Variables and Var A in M33 using homogeneous data and methods. We found the star's bolometric luminosity to be log(L/Lsun)~6.27, a surface temperature of T~16000K and black body temperatures of two dust components of T~900 and 420K. The new star has properties intermediate between the LBVs and VarA (probable cool hypergiant). In the same time it has a hot photosphere, LBV-like luminosity and an extensive circumstellar material (strong [CaII] lines). In these seven luminous variables in M33 we find the total range of the hot component luminosities is 1.0 dex, but that of the dust componets is 2.0 dex. We conclude that the dust phenomenon in the luminous variables is temporary and variable, and that dust activity may follow strong eruptions.Comment: 5 pages, 6 figures; MNRAS Letter accepte

The Tully-Fisher Relation and H_not

The use of the Tully-Fisher (TF) relation for the determination of the Hubble Constant relies on the availability of an adequate template TF relation and of reliable primary distances. Here we use a TF template relation with the best available kinematical zero-point, obtained from a sample of 24 clusters of galaxies extending to cz ~ 9,000 km/s, and the most recent set of Cepheid distances for galaxies fit for TF use. The combination of these two ingredients yields H_not = 69+/-5 km/(s Mpc). The approach is significantly more accurate than the more common application with single cluster (e.g. Virgo, Coma) samples.Comment: 10 pages, including 2 figures and 1 table; uses AAS LaTex. Submitted to ApJ Letter

A warped disk model for M33 and the 21-cm line width in spiral galaxies

To determine the actual HI distribution and the velocity field in the outermost disk of the spiral galaxy M33, a tilted-ring model is fitted to 21-cm line data taken with the Arecibo Telescope. Since M33 is one of the main calibrators for the extragalactic distance scale derived through the Tully-Fisher relation, the outer disk warping is of interest for a correct determination and deprojection of the galaxy's line width. Even though our best model predicts small effects on the observed line width of M33, we show that similar outer disk warping in galaxies oriented differently along our line of sight could affect the widths considerably. Therefore there may be systematic effects in the determination of the rotation velocities and dynamic masses of spiral galaxies, whose exact value depends also on which method is used for measuring the galaxy's total line width.Comment: 27 pages, ps files only, ApJ in pres

HI distribution and kinematics of NGC 1569

We present WSRT observations of high sensitivity and resolution of the neutral hydrogen in the starburst dwarf galaxy NGC 1569. Assuming a distance of 2.2 Mpc, we find a total HI mass of 1.3 * 10^8 M_sun to be distributed in the form of a dense, clumpy ridge surrounded by more extended diffuse HI containing a few additional discrete features, such as a Western HI Arm and an HI bridge reaching out to a small counterrotating companion cloud. About 10% by mass of all HI in NGC 1569 is at unusually high velocities. Some of this HI may be associated with the mass outflow evident from H-alpha measurements, but some may also be associated with NGC 1569's HI companion and intervening HI bridge, in which case, infall rather than outflow might be the cause of the discrepant velocities. No indication of a large bubble structure was found in position-velocity maps of the high-velocity HI. The galaxy as a whole is in modest overall rotation, but the HI gas lacks any sign of rotation within 60'' (0.6 kpc) from the center, i.e. over most of the optical galaxy. Here, turbulent motions resulting from the starburst appear to dominate over rotation. In the outer disk, the rotational velocities reach a maximum of 35 \pm 6 km/s, but turbulent motion remains significant. Thus, starburst effects are still noticeable in the outer HI disk, although they are no longer dominant beyond 0.6 kpc. Even excluding the most extreme high-velocity HI clouds, NGC 1569 still has an unusually high mean HI velocity dispersion of sigma_v=21.3 km/s, more than double that of other dwarf galaxies.Comment: Figure 11a,b and Figure 14 separately in jpg forma

Dynamical and chemical evolution of NGC1569

Blue Compact Dwarf and Dwarf Irregular galaxies are generally believed to be unevolved objects, due to their blue colors, compact appearance and large gas fractions. Many of these objects show an ongoing intense burst of star formation or have experienced it in the recent past. By means of 2-D hydrodynamical simulations, coupled with detailed chemical yields originating from SNeII, SNeIa, and intermediate-mass stars, we study the dynamical and chemical evolution of model galaxies with structural parameters similar to NGC1569, a prototypical starburst galaxy. A burst of star formation with short duration is not able to account for the chemical and morphological properties of this galaxy. The best way to reproduce the chemical composition of this object is by assuming long-lasting episodes of star formation and a more recent burst, separated from the previous episodes by a short quiescent period. The last burst of star formation, in most of the explored cases, does not affect the chemical composition of the galaxy, since the enriched gas produced by young stars is in a too hot phase to be detectable with the optical spectroscopy. Models assuming the infall of a big cloud towards the center of the galaxy reproduce the chemical composition of the NGC1569, but the pressure exercised by the cloud hampers the expansion of the galactic wind, at variance with what observed in NGC1569.Comment: 16 pages, 10 figures, A&A accepte

Gemini GMOS/IFU spectroscopy of NGC 1569 - I: Mapping the properties of a young star cluster and its environment

[Abridged] We present Gemini-North GMOS/IFU observations of a young star cluster (cluster 10) and its environment near the centre of the dwarf irregular starburst galaxy NGC 1569. This forms part of a larger and on-going study of the formation and collimation mechanisms of galactic winds, including three additional IFU pointings in NGC 1569 covering the base of the galactic wind which are analysed in a companion paper. The good spatial- and spectral-resolution of these GMOS/IFU observations, covering 4740-6860 A, allow us to probe the interactions between clusters and their environments on small scales. Combining our GMOS spectrum with HST imaging, we find that cluster 10 is composed of two very close components with ages of 5-7 Myr and <5 Myr, and a combined mass of 7 +/- 5 x 10^3 Msun. A detailed analysis of the H_alpha emission line profile shapes across the whole field-of-view shows them to be composed of a bright narrow feature (intrinsic FWHM ~ 50 km/s) superimposed on a fainter broad component (FWHM < 300 km/s). By mapping the properties of each individual component, we investigate the small-scale structure and properties of the ionized ISM, including reddening, excitation and electron densities, and for the first time find spatial correlations between the line component properties. We discuss in detail the possible mechanisms that could give rise to the two components and these correlations, and conclude that the most likely explanation for the broad emission is that it is produced in a turbulent mixing layer on the surface of the cool gas clumps embedded within the hot, fast-flowing cluster winds. We conclude we are sampling well within the outer bounding shocks of the expanding superbubbles and within the outflow 'energy injection zone'.Comment: 22 pages, 14 figures, 4 tables, accepted to MNRA

Simulating Supersonic Turbulence in Galaxy Outflows

We present three-dimensional, adaptive mesh simulations of dwarf galaxy out- flows driven by supersonic turbulence. Here we develop a subgrid model to track not only the thermal and bulk velocities of the gas, but also its turbulent velocities and length scales. This allows us to deposit energy from supernovae directly into supersonic turbulence, which acts on scales much larger than a particle mean free path, but much smaller than resolved large-scale flows. Unlike previous approaches, we are able to simulate a starbursting galaxy modeled after NGC 1569, with realistic radiative cooling throughout the simulation. Pockets of hot, diffuse gas around individual OB associations sweep up thick shells of material that persist for long times due to the cooling instability. The overlapping of high-pressure, rarefied regions leads to a collective central outflow that escapes the galaxy by eating away at the exterior gas through turbulent mixing, rather than gathering it into a thin, unstable shell. Supersonic, turbulent gas naturally avoids dense regions where turbulence decays quickly and cooling times are short, and this further enhances density contrasts throughout the galaxy- leading to a complex, chaotic distribution of bubbles, loops and filaments as observed in NGC 1569 and other outflowing starbursts.Comment: 22 pages, 13 figures, MNRAS, in pres

The Stellar and Gas Kinematics of the LITTLE THINGS Dwarf Irregular Galaxy NGC 1569

In order to understand the formation and evolution of dIm galaxies, one needs to understand their three-dimensional structure. We present measurements of the stellar velocity dispersion in NGC 1569, a nearby post-starburst dIm galaxy. The stellar vertical velocity dispersion, $\sigma_{\rm z}$, coupled with the maximum rotational velocity derived from \ion{H}{1} observations, $V_{\rm max}$, gives a measure of how kinematically hot the galaxy is, and, therefore, indicates its structure. We conclude that the stars in NGC 1569 are in a thick disk with a $V_{\rm max} / \sigma_{\rm z}$ = 2.4 $\pm$ 0.7. In addition to the structure, we analyze the ionized gas kinematics from \ion{O}{3} observations along the morphological major axis. These data show evidence for outflow from the inner starburst region and a potential expanding shell near supermassive star cluster (SSC) A. When compared to the stellar kinematics, the velocity dispersion of the stars increase in the region of SSC A supporting the hypothesis of an expanding shell. The stellar kinematics closely follow the motion of the gas. Analysis of high resolution \ion{H}{1} data clearly reveals the presence of an \ion{H}{1} cloud that appears to be impacting the eastern edge of NGC 1569. Also, an ultra-dense \ion{H}{1} cloud can be seen extending to the west of the impacting \ion{H}{1} cloud. This dense cloud is likely the remains of a dense \ion{H}{1} bridge that extended through what is now the central starburst area. The impacting \ion{H}{1} cloud was the catalyst for the starburst, thus turning the dense gas into stars over a short timescale, $\sim$ 1 Gyr. We performed a careful study of the spectral energy distribution using infrared, optical, and ultraviolet photometry producing a state-of-the-art mass model for the stellar disk. This mass modeling shows that stars dominate the gravitational potential in the inner 1 kpc.Comment: 49 pages, 25 figures, accepted in A

The Metallicity-Luminosity Relation, Effective Yields, and Metal Loss in Spiral and Irregular Galaxies

I present results on the correlation between galaxy mass, luminosity, and metallicity for a sample of spiral and irregular galaxies having well-measured abundance profiles, distances, and rotation speeds. Additional data for low surface brightness galaxies from the literature are also included for comparison. These data are combined to study the metallicity-luminosity and metallicity-rotation speed correlations for spiral and irregular galaxies. The metallicity luminosity correlation shows its familiar form for these galaxies, a roughly uniform change in the average present-day O/H abundance of about a factor 100 over 11 magnitudes in B luminosity. However, the O/H - V(rot) relation shows a change in slope at a rotation speed of about 125 km/sec. At faster V(rot), there appears to be no relation between average metallicity and rotation speed. At lower V(rot), the metallicity correlates with rotation speed. This change in behavior could be the result of increasing loss of metals from the smaller galaxies in supernova-driven winds. This idea is tested by looking at the variation in effective yield, derived from observed abundances and gas fractions assuming closed box chemical evolution. The effective yields derived for spiral and irregular galaxies increase by a factor of 10-20 from V(rot) approximately 5 km/sec to V(rot) approximately 300 km/sec, asympotically increasing to approximately constant y(eff) for V(rot) > 150 km/sec. The trend suggests that galaxies with V(rot) < 100-150 km/sec may lose a large fraction of their SN ejecta, while galaxies above this value tend to retain metals.Comment: 40 pages total, including 7 encapsulated postscript figures. Accepted for publication in 20 Dec 2002 Ap

Neutral hydrogen observations of spiral and irregular galaxies

This thesis describes hydrogen-line observations using the Cambridge Half-Mile telescope. The initial chapters describe observations of two nearby galaxies af late spiral type, namely M33 and NGC 2403. The severe warping of the HI layer in the outer parts of M33 was clearly seen, and the origin of this phenomenon is discussed. Sensitive low-resolution maps reveal only a very slight warp in NGC 2403, while maps at the full resolution allowed the dynamical parameters to be investigated and showed several features associated with spiral structure. The later chapters describe observations of irregular galaxies, many belonging to the class IrII which was of particular interest. NGC 1569, a dwarf irregular galaxy, has been the subject of interest due to its prominent Ha [Greek letter alpha] filaments. Observations reveal that the overall HI distribution to be that expected of a disc in normal rotation, although there are several unusual features. Arp 205 consists of an IrII galaxy (NGC 3448) and a dwarf companion less than one diameter away. A model for the HI dynamics is presented which suggests that there has been a planar tidal interaction between these two galaxies. NGG 2805, NGC 2814, NGG 2820 and 10 2458 form a compact group of galaxies of which NGC 2805 is the brightest. HI was detected in all except NGC 2814, and there is good evidence for a HI bridge linking NGC 2820 and IC 2458. It is postulated that the HI bridge and disturbed nature of this group are the result of a gravitational interaction. Less detailed observations of other irregular galaxies are also presented in this thesis, including NGC 3432 and NGG 3310