Far-Ultraviolet & X-ray Observations of VV 114: Feedback in a Local Analog to Lyman Break Galaxies

We have analyzed FUSE, XMM, and Chandra observations of VV 114, a local galaxy merger with strong similarities to typical high-redshift Lyman Break Galaxies (LBGs). Diffuse thermal X-ray emission encompassing VV114 has been observed by Chandra and XMM. This region of hot (kT~0.59 keV) gas has an enhanced alpha to iron element ratio relative to solar abundances and follows the same relation as typical starbursts between its properties (luminosity, size, and temperature) and those of the starburst galaxy (star formation rate, dust temperature, galaxy mass). These results are consistent with the X-ray gas having been produced by shocks driven by a galactic superwind. The FUSE observations of VV 114 show strong, broad interstellar absorption lines with a pronounced blueshifted component(similar to what is seen in LBGs). This implies an outflow of material moving at 300-400 km/s relative to VV 114. The properties of the strong OVI absorption line are consistent with radiative cooling at the interface between the hot outrushing gas seen in X-rays and the cooler material seen in the other outflowing ions in the FUSE data. We show that the wind in VV114 has not created a ``tunnel'' that enables more than a small fraction (< few percent) of the ionizing photons from VV114 to escape into the IGM. Taken together, these data provide a more complete physical basis for understanding the outflows that seem to be generic in LBGs. This will lead to improved insight into the role that such outflows play in the evolution of galaxies and the inter-galactic medium.Comment: 33 pages, 13 figure

Discovery of a Nearby Low-Surface-Brightness Spiral Galaxy

During the course of a search for compact, isolated gas clouds moving with anomalous velocities in or near our own Galaxy (Braun and Burton 1998 A&A, in press), we have discovered, in the data of the Leiden/Dwingeloo survey (Hartmann and Burton 1997, Atlas of Galactic Neutral Hydrogen, CUP) of Galactic hydrogen, the HI signature of a large galaxy, moving at a recession velocity of 282 km/s, with respect to our Galaxy. Deep multicolor and spectroscopic optical observations show the presence of star formation in scattered HII regions; radio HI synthesis interferometry confirms that the galaxy is rich in HI and has the rotation signature of a spiral galaxy; a submillimeter observation failed to detect the CO molecule. The radio and optical evidence combined suggest its classification as a low-surface-brightness spiral galaxy. It is located in close spatial and kinematic proximity to the galaxy NGC 6946. The newly-discovered galaxy, which we call Cepheus 1, is at a distance of about 6 Mpc. It is probably to be numbered amongst the nearest few LSB spirals.Comment: 13 page LaTeX, requires aastex, 4 GIF figures. Accepted for publication in the AJ, January 199

Far Ultraviolet and H-alpha Imaging of Nearby Spirals: The OB Stellar Population in the Diffuse Ionized Gas

(Abridged) We have compared H-alpha and far ultraviolet (FUV) images of 10 nearby spirals, with the goal of understanding the contribution of field OB stars to the ionization of the diffuse ionized gas (DIG) in spiral galaxies. The FUV images were obtained by the Ultraviolet Imaging Telescope (UIT) and the H-alpha images were obtained using various ground-based telescopes. In all of the galaxies, the F_H-alpha/F_UIT flux ratio is lower in the DIG than in the HII regions. This is likely an indication that the mean spectral type for OB stars in the field is later than that in HII regions. Comparison of the N_Lyc/L_UIT ratio with models of evolving stellar populations shows that the stellar population in the DIG is consistent with either an older single burst population or a steady state model with constant star formation and an initial mass function (IMF) slope steeper than alpha=2.35. We compared the F_H\alpha/F_UIT ratio in the DIG of these galaxies with that in M33. If the mean spectral types of stars in HII regions and in the DIG are the same as in M33, and the difference in extinction between DIG and HII regions is constant among galaxies, then the analysis suggests that field stars are important sources of ionization in most galaxies, and may be the dominant source in some galaxies.Comment: 29 pages, 8 figures, 6 tables. Accepted for publication in Ap

Escape of Lyman alpha Emission in the Starburst Galaxy Mkn 357 - a Wind's Far Side

HST imaging and slitless spectroscopy are used to examine where the strong Lyman alpha emission escapes from the interstellar medium in the starburst galaxy Mkn 357. An H-alpha image shows that the ionized gas is mostly in a global wind, rather than associated with the individual star-forming regions seen in the optical and UV continuum. The Lyman alpha emission comes predominantly from the northwest side of the wind structure spatially, and shows a significant redshift relative to the optical lines. Both of these properties are signatures of seeing the line photons backscattered from the far side of a prolate or bipolar starburst wind, fitting both with escape calculations and evidence for winds in high-redshift galaxies with net Lyman alpha emission. Scattering is most important within this wind itself, rather than involving a surrounding neutral medium, as shown by the decreasing relative redshift of the line peak from 250 to 30 km/s between the center and edge of the detected emission. The Lyman alpha emission exhibits strong asymmetry in comparison with both the starlight and H-alpha structures. These results add to the evidence that kinematics, rather than gas metallicity or dust content, are the dominant effect in determining which galaxies have strong Lyman alpha emission, and that powerful (and perhaps episodic) starbursts are common among Lyman-break galaxies as well as those discovered from Lyman alpha line emission.Comment: Accepted for Astron.J., April 200

The Contribution of Field OB Stars to the Ionization of the Diffuse Ionized Gas in M33

(Abridged) We present a study of the ionizing stars associated with the diffuse ionized gas (DIG) and HII regions in the nearby spiral galaxy M33. We compare our Schmidt H-alpha image to the far-ultraviolet (FUV, 1520A) image from the Ultraviolet Imaging Telescope (UIT). The H-alpha/FUV ratio is higher in HII regions than in the DIG, suggesting an older population of ionizing stars in the DIG. When compared to models of evolving stellar populations, the N(Lyc)/FUV ratio in HII regions is consistent with a young burst, while the DIG ratio resembles an older burst population, or a steady state population built up by constant star formation. The UIT data is complimented with archival FUV and optical images of a small portion of the disk of M33 obtained with WFPC2 on HST. Using the HST FUV and optical photometry, we assign spectral types to the stars observed in DIG and HII regions. The photometry indicates that ionizing stars are present in the DIG. We compare the predicted ionizing flux with the amount required to produce the observed H-alpha emission, and find that field OB stars in the HST images can account for 40% +/- 12% of the ionization of the DIG, while the stars in HII regions can provide 107% +/- 26% of the H-alpha luminosity of the HII regions. We do not find any correlation between leakage of ionizing photons and H-alpha luminosity for the HII regions in these HST fields. If stellar photons alone are responsible for ionizing the DIG, the current results are consistent with no or few ionizing photons escaping from the galaxy.Comment: 30 pages, 7 figures, accepted for publication in Ap

HST morphologies of local Lyman break galaxy analogs I: Evidence for starbursts triggered by merging

Heckman et al. (2005) used the Galaxy Evolution Explorer (GALEX) UV imaging survey to show that there exists a rare population of nearby compact UV-luminous galaxies (UVLGs) that closely resembles high redshift Lyman break galaxies (LBGs). We present HST images in the UV, optical, and Ha, and resimulate them at the depth and resolution of the GOODS/UDF fields to show that the morphologies of UVLGs are also similar to those of LBGs. Our sample of 8 LBG analogs thus provides detailed insight into the connection between star formation and LBG morphology. Faint tidal features or companions can be seen in all of the rest-frame optical images, suggesting that the starbursts are the result of a merger or interaction. The UV/optical light is dominated by unresolved (~100-300 pc) super starburst regions (SSBs). A detailed comparison with the galaxies Haro 11 and VV 114 at z=0.02 indicates that the SSBs themselves consist of diffuse stars and (super) star clusters. The structural features revealed by the new HST images occur on very small physical scales and are thus not detectable in images of high redshift LBGs, except in a few cases where they are magnified by gravitational lensing. We propose, therefore, that LBGs are mergers of gas-rich, relatively low-mass (~10^10 Msun) systems, and that the mergers trigger the formation of SSBs. If galaxies at high redshifts are dominated by SSBs, then the faint end slope of the luminosity function is predicted to have slope alpha~2. Our results are the most direct confirmation to date of models that predict that the main mode of star formation in the early universe was highly collisional.Comment: 32 pages, 15 figures. ApJ In pres