Very-wide-field camera. Proposal of Space Astronomy Laboratory for second Spacelab mission

A proposal is made for inclusion of a very wide field camera onboard Spacelab. Its scientific program is outlined: detection and photometry, spectrography, and star and starlike object photography. The optics, receptor, and mechanical structure are described. Scientific and technical constraints are discussed, and a development plan is detailed. The dust contamination of Spacelab using the camera was also studied

Ejection of Matter and Energy from NGC 4258

It has been claimed that the megamaser observations of the nucleus of NGC 4258 show that a massive black hole is present in its center (Miyoshi et al. 1995, Greenhill et al. 1995). We show that the evidence of ejection of gas, radio plasma, and X-ray emitting QSOs from this nucleus all show that the ejection is coming from the center in a curving flow within a cone with angle ~40 degrees, centered at P.A. 100 degrees. This is close to the direction in which the velocities from the megamaser have been measured, so that the evidence taken as a whole suggests that the masering gas also is being ejected in the same direction at velocities +/- 900 km/sec and not rotating about a massive black hole. Thus it does not provide evidence for a black hole in the center. Subject headings: galaxies: nuclei: individual (NGC 4258) -- black holes -- masersComment: 7 pages, 1 Figure, LaTex using epsf.tex, submitted to Ap.J.Letter

Chandra Observations and the Nature of the Anomalous Arms of NGC 4258 (M 106)

This paper presents high resolution X-ray observations with Chandra of NGC 4258 and infers the nature of the so called ``anomalous arms'' in this galaxy. The anomalous arms dominate the X-ray image; diffuse X-ray emission from the ``plateaux'' regions, seen in radio and H$\alpha$ imaging, is also found. X-ray spectra have been obtained at various locations along the anomalous arms and are well described by thermal (mekal) models with kT in the range 0.37 - 0.6 keV. The previously known kpc-scale radio jets are surrounded by cocoons of hot X-ray emitting gas for the first 350 pc of their length. The radio jets, seen in previous VLBA and VLA observations, propagate perpendicular to the compact nuclear gas disk (imaged in water vapor maser emission). The angle between the jets and the rotation axis of the galactic disk is 60$^{\circ}$. The jets shock the normal interstellar gas along the first 350 pc of their length, causing the hot, X-ray emitting cocoons noted above. At a height of z = 175 pc from the disk plane, the jets exit the normal gas disk and then propagate though the low density halo until they reach ``hot spots'' (at 870 pc and 1.7 kpc from the nucleus), which are seen in radio, optical line and X-ray emission. These jets must drive mass motions into the low density halo gas. This high velocity halo gas impacts on the dense galactic gas disk and shock heats it along and around a ``line of damage'', which is the projection of the jets onto the galactic gas disk as viewed down the galaxy disk rotation axis. However, because NGC 4258 is highly inclined ($i$ = 64$^{\circ}$), the ``line of damage'' projects on the sky in a different direction to the jets themselves. We calculate the expected p.a. of the ``line of damage'' on the sky and find that it coincides with the anomalous arms to within 2$^{\circ}$. (Abstract truncated).Comment: 12 pages plus 9 figures, to be published in the Astrophysical Journal, v560, nr 1, pt 1 (Oct 10, 2001 issue

Clusters of Extragalactic Ultra Compact HII Regions

We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young, embedded star birth regions, which are likely to be clusters of ultracompact HII regions. All 35 of the sources presented in this article have positive radio spectral indices alpha>0 suggesting an optically thick thermal bremsstrahlung emission arising in the HII region surrounding hot stars. Energy requirements indicate a range of a several to >500 O7V star equivalents powering each HII region. Assuming a Salpeter IMF, this corresponds to integrated stellar masses of 0.1--60,000 Msun. For roughly half of the sources in our sample, there is no obvious optical counterpart, giving further support for their deeply embedded nature. Their luminosities and radio spectral energy distributions are consistent with HII regions having electron densities from 1500 cm^-3 to 15000 cm^-3 and radii of 1 - 7 pc. We suggest that the less luminous of these sources are extragalactic ultracompact HII region complexes, those of intermediate luminosity are similar to W49 in the Galaxy, while the brightest will be counterparts to 30 Doradus. These objects constitute the lower mass range of extragalactic ``ultradense HII regions'' which we argue are the youngest stages of massive star cluster formation yet observed. This sample is beginning to fill in the continuum of objects between small associations of ultracompact HII regions and the massive extragalactic clusters that may evolve into globular clusters.Comment: 37 pages, uses AASTeX; scheduled to appear in ApJ v. 559 October 2001. Full postscript version available from http://www.astro.wisc.edu/~chip/Papers/Johnson_Kobulnicky_etal_ApJ559.ps.g

Radio Continuum Jet in NGC 7479

The barred galaxy NGC 7479 hosts a remarkable jet-like radio continuum feature: bright, 12-kpc long in projection, and hosting an aligned magnetic field. The degree of polarization is 6%-8% along the jet, and remarkably constant, which is consistent with helical field models. The radio brightness of the jet suggests strong interaction with the ISM and hence a location near the disk plane. We observed NGC 7479 at four wavelengths with the VLA and Effelsberg radio telescopes. The equipartition strength is 35-40 micro-G for the total and >10 micro-G for the ordered magnetic field in the jet. The jet acts as a bright, polarized background. Faraday rotation between 3.5 and 6 cm and depolarization between 6 and 22 cm can be explained by magneto-ionic gas in front of the jet, with thermal electron densities of ~0.06 cm**(-3) in the bar and ~0.03 cm**(-3) outside the bar. The regular magnetic field along the bar points toward the nucleus on both sides. The regular field in the disk reveals multiple reversals, probably consisting of field loops stretched by a shearing gas flow in the bar. The projection of the jet bending in the sky plane is in the sense opposite to that of the underlying stellar and gaseous spiral structure. The bending in 3-D is most easily explained as a precessing jet, with an age less than 10**6 years. Our observations are consistent with very recent triggering, possibly by a minor merger. NGC 7479 provides a unique opportunity to study interaction-triggered 15-kpc scale radio jets within a spiral galaxy.Comment: 18 pages, 21 figures, accepted for publication in the Astrophysical Journa

Multicolor photometry of 145 of the HII regions in M33

This paper is the first in a series presenting CCD multicolor photometry for 145 HII regions, selected from 369 candidate regions from Boulesteix et al. (1974), in the nearby spiral galaxy M33. The observations, which covered the whole area of M33, were carried out by the Beijing Astronomical Observatory 60/90 cm Schmidt Telescope, in 13 intermediate-band filters, covering a range of wavelength from 3800 to 10000 A. This provides a series of maps which can be converted to a multicolor map of M33, in pixels of 1.7''*1.7''. Using aperture photometry we obtain the spectral energy distributions (SEDs) for these HII regions. We also give their identification charts. Using the relationship between the BATC intermediate-band system used for the observations and the UBVRI broad-band system, the magnitudes in the B and V bands are then derived. Histograms of the magnitudes in V and in B-V are plotted, and the color-magnitude diagram is also given. The distribution of magnitudes in the V band shows that the apparent magnitude of almost all the regions is brighter than 18, corresponding to an absolute magnitude of -6.62 for an assumed distance modulus of 24.62, which corresponds to a single main sequence O5 star, while the distribution of color shows that the sample is blue, with a mode close to -0.05 as would be expected from a range of typical young clusters.Comment: 34 pages, 19 figures including 16 of jpg form, will appear in the December 2002 issue of A

The M33 Metallicity Project: Resolving the Abundance Gradient Discrepancies in M33

We present a new determination of the metallicity gradient in M33, based on Keck/LRIS measurements of oxygen abundances using the temperature-sensitive emission line [OIII] 4363 A in 61 HII regions. These data approximately triple the sample of direct oxygen abundances in M33. We find a central abundance of 12 + log(O/H) = 8.36+/-0.04 and a slope of -0.027+/-0.012 dex/kpc, in agreement with infrared measurements of the neon abundance gradient but much shallower than most previous oxygen gradient measurements. There is substantial intrinsic scatter of 0.11 dex in the metallicity at any given radius in M33, which imposes a fundamental limit on the accuracy of gradient measurements that rely on small samples of objects. We also show that the ionization state of neon does not follow the ionization state of oxygen as is commonly assumed, suggesting that neon abundance measurements from optical emission lines require careful treatment of the ionization corrections.Comment: 10 pages, Accepted to Ap

Detection of Cold Atomic Clouds in the Magellanic Bridge

We report a detection of cold atomic hydrogen in the Magellanic Bridge using 21-cm absorption spectroscopy toward the radio source B0312-770. With a column density of N_HI=1.2E20 cm^-2, a maximum absorption optical depth of tau=0.10 and a maximum 21-cm emission brightness temperature of 1.4 K, this line of sight yields a spin temperature, T_s, between 20 K and 40 K. H I 21-cm absorption and emission spectroscopy toward 7 other low column density sightlines on the periphery of the LMC and SMC reveal absorption toward one additional background radio source behind the SMC with tau=0.03. The data have typical sensitivities of sigma_tau=0.005 to 0.070 in absorption and sigma_{T_B}=0.03 K in emission. These data demonstrate the presence of a cold atomic phase which is probably accompanied by molecular condensations in the tenuous interstellar medium of the Bridge region. Young OB stars observed in the Magellanic Bridge could form "in situ" from these cold condensations rather than migrate from regions of active star formation in the main body of the SMC. The existence of cold condensations and star formation in the Magellanic Bridge might be understood as a small scale version of the mechanism that produces star formation in the tidal tails of interacting galaxies.Comment: 25 pages, uses AASTeX and psfig; Accepted for Publication in the Astronomical Journa

On the Dynamical and Physical State of the `Diffuse Ionized Medium' in Nearby Spiral Galaxies

We have analyzed deep narrow-band H$\alpha$ images and high-resolution long-slit spectra for a sample of the nearest and brightest late-type galaxies to study the morphology, physical state, and kinematics of the `Diffuse Ionized Medium' (`DIM'). We find that the DIM covers most of the star-forming disk, and is morphologically related to the presence of the giant HII regions. In addition, the DIM and the giant HII regions differ systematically in their physical and dynamical state. The DIM is characterized by enhanced emission in the low-ionization forbidden lines ([OI], [NII], and [SII]), and even the high-ionization [OIII]$\lambda$5007 line is moderately strong in the DIM. We verify the inference made by Lehnert & Heckman that the DIM contributes significantly to the global emission-line ratios measured in late-type galaxies. We also find that the DIM is more disturbed kinematically than the gas in the giant HII regions. The intrinsic FWHMs of the H$\alpha$ and [NII]$\lambda$6584 lines range from 30 to 100 km s$^{-1}$ in the DIM compared to 20-50 km s$^{-1}$ in HII regions. The high-ionization gas in the DIM is even more kinematically disturbed than the low-ionization gas: the [OIII]$\lambda$5007 lines have intrinsic FWHMs of 70-150 km s$^{-1}$. The differing kinematics implies that `the DIM' is not a single monolithic phase of the ISM. Instead, it may consist of a `quiescent DIM' with a low ionization-state and small scale-height (few hundred pc) and a `disturbed DIM' with a high ionization state and moderate scale-height (0.5 to 1 kpc). We argue that the quiescent DIM is most likely photoionized by radiation leaking out of giant HII regions, while the disturbed DIM is most likely heated by the mechanical energy supplied by supernovae and stellar winds.Comment: 37 pages(including 7 tables) and 12 figures. To appear in the Dec 10, 1997 issue of The Astrophysical Journa