Exploring cloudy gas accretion as a source of interstellar turbulence in the outskirts of disks

High--resolution 2D--MHD numerical simulations have been carried out to investigate the effects of continuing infall of clumpy gas in extended HI galactic disks. Given a certain accretion rate, the response of the disk depends on its surface gas density and temperature. For Galactic conditions at a galactocentric distance of ~20 kpc, and for mass accretion rates consistent with current empirical and theoretical determinations in the Milky Way, the rain of compact high velocity clouds onto the disk can maintain transonic turbulent motions in the warm phase (~2500 K) of HI. Hence, the HI line width is expected to be ~6.5 km/s for a gas layer at 2500 K, if infall were the only mechanism of driving turbulence. Some statistical properties of the resulting forcing flow are shown in this Letter. The radial dependence of the gas velocity dispersion is also discussed.Comment: 13 pages, 3 figures, accepted for publication in ApJ Letter

Galactic-scale macro-engineering: Looking for signs of other intelligent species, as an exercise in hope for our own

If we consider Big History as simply 'our' example of the process of cosmic evolution playing out, then we can seek to broaden our view of our possible fate as a species by asking questions about what paths or trajectories other species' own versions of Big History might take or have taken. This paper explores the broad outlines of possible scenarios for the evolution of long-lived intelligent engineering species---scenarios which might have been part of another species' own Big History story, or which may yet lie ahead in our own distant future. A sufficiently long-lived engineering-oriented species may decide to undertake a program of macro-engineering projects that might eventually lead to a re-engineered galaxy so altered that its artificiality may be detectable from Earth. We consider activities that lead ultimately to a galactic structure consisting of a central inner core surrounded by a more distant ring of stars separated by a relatively sparser 'gap', where star systems and stellar materials may have been removed, 'lifted' or turned into Dyson Spheres. When one looks to the sky, one finds that such galaxies do indeed exist---including the beautiful ringed galaxy known as 'Hoag's Object' (PGC 54559) in the constellation Serpens. This leads us to pose the question: Is Hoag's Object an example of galaxy-scale macro-engineering? And this suggests a program of possible observational activities and theoretical explorations, several of which are presented here, that could be carried out in order to begin to investigate this beguiling question.Comment: 17 pages. Published in: Teaching and Researching Big History: Exploring a New Scholarly Field; L. Grinin, D. Baker, E. Quaedackers and A. Korotayev (eds). Uchitel Publ House, Volgograd, Russia, 2014, Chapter 14, pp. 283-304. ISBN: 978-5-7057-4027-

HI power spectrum of the spiral galaxy NGC628

We have measured the HI power spectrum of the nearly face-on spiral galaxy NGC628 (M74) using a visibility based estimator. The power spectrum is well fitted by a power law $P(U)=AU^{\alpha}$, with $\alpha =- 1.6\pm0.2$ over the length scale $800 {\rm pc} {\rm to} 8 {\rm kpc}$. The slope is found to be independent of the width of the velocity channel. This value of the slope is a little more than one in excess of what has been seen at considerably smaller length scales in the Milky-Way, Small Magellanic Cloud (LMC), Large Magellanic Cloud (SMC) and the dwarf galaxy DDO210. We interpret this difference as indicating a transition from three dimensional turbulence at small scales to two dimensional turbulence in the plane of the galaxy's disk at length scales larger than galaxy's HI scale height. The slope measured here is similar to that found at large scales in the LMC. Our analysis also places an upper limit to the galaxy's scale height at $800\ {\rm pc}$ .Comment: 4 Pages, 2 Figures, 1 Table. Accepted for Publication in MNRAS LETTER

XMM-Newton observations of the spiral galaxy M74 (NGC 628)

The face-on spiral galaxy M74 (NGC 628) was observed by XMM on 2002 February 2. In total, 21 sources are found in the inner 5' from the nucleus (after rejection of a few sources associated to foreground stars). Hardness ratios suggest that about half of them belong to the galaxy. The higher-luminosity end of the luminosity function is fitted by a power-law of slope -0.8. This can be interpreted as evidence of ongoing star formation, in analogy with the distributions found in disks of other late-type galaxies. A comparison with previous Chandra observations reveals a new ultraluminous X-ray transient (L_x \~ 1.5 x 10^39 erg/s in the 0.3--8 keV band) about 4' North of the nucleus. We find another transient black-hole candidate (L_x ~ 5 x 10^38 erg/s) about 5' North-West of the nucleus. The UV and X-ray counterparts of SN 2002ap are also found in this XMM observation.Comment: submitted to ApJL. Based on publicly available data, see http://xmm.vilspa.esa.es/external/xmm_news/items/sn_2002_ap/index.shtm

Diffuse HI Disks in Isolated Galaxies

In order to investigate the contribution of diffuse components to their total HI emission, we have obtained high precision HI line flux densities with the 100m Green Bank Telescope for a sample of 100 isolated spiral and irregular galaxies which we have previously observed with the 43m telescope. A comparison of the observed HI line fluxes obtained with the two different telescopes, characterized by half-power beam widths of 9 arcmin and 21 arcmin respectively, exploits a ``beam matching'' technique to yield a statistical determination of the occurrence of diffuse HI components in their disks. A simple model of the HI distribution within a galaxy well describes ~75 % of the sample and accounts for all of the HI line flux density. The remaining galaxies are approximately evenly divided into two categories: ones which appear to possess a significantly more extensive HI distribution than the model predicts, and ones for which the HI distribution is more centrally concentrated than predicted. Examples of both extremes can be found in the literature but little attention has been paid to the centrally concentrated HI systems. Our sample has demonstrated that galaxies do not commonly possess extended regions of low surface brightness HI gas which is not accounted for by our current understanding of the structure of HI disks. Eight HI-rich companions to the target objects are identified, and a set of extragalactic HI line flux density calibrators is presented.Comment: 26 page

The Extreme Outer Regions of Disk Galaxies: I. Chemical Abundances of HII Regions

We present the first results of an ongoing project to investigate the present-day chemical abundances of the extreme outer parts of galactic disks, as probed by the emission line spectra of a new sample of HII regions. The galaxies studied here, NGC628, NGC1058 and NGC6946, are all late-type spiral galaxies, characterized by larger than average HI-to-optical sizes. Our deep Halpha images have revealed the existence of recent massive star formation, traced by HII regions, out to, and beyond, two optical radii in these galaxies (defined by the B-band 25th magnitude isophote). Optical spectra of these newly-discovered HII regions are used to investigate their densities, ionization parameters, extinctions and in particular their oxygen and nitrogen abundances. Our measurements reveal gas-phase abundances of O/H~10-15% of the solar value, and N/O~20-25% of the solar value, at radii of 1.5-2 R25. Clear evidence also exists for diminished dust extinction (Av~0-0.2) at large radii. The combination of our measurements of outer disk HII region abundances with those for inner disk HII regions published in the literature is a powerful probe of the shape of abundance gradients over unprecedented radial baselines. Within the limits of the current dataset, the radial abundance variations are consistent with single log-linear relationships, although the derived slopes can often differ considerably from those found if only inner disk HII regions are used to define the fit. Interestingly, both the mean level of enrichment and the ratio of N/O measured in extreme outer galactic disks are similar to those values measured in some high redshift damped Lyman-alpha absorbers, suggesting that outer disks at the present epoch are relatively unevolved. (abridged)Comment: 36 pages, 10 embedded postscript files, 3 jpeg files, 7 postscript tables; accepted for publication in the Astronomical Journal (August issue

The Cool ISM in S0 Galaxies. I. A Survey of Molecular Gas

Lenticular galaxies remain remarkably mysterious as a class. Observations to date have not led to any broad consensus about their origins, properties and evolution, though they are often thought to have formed in one big burst of star formation early in the history of the Universe, and to have evolved relatively passively since then. In that picture, current theory predicts that stellar evolution returns substantial quantities of gas to the interstellar medium; most is ejected from the galaxy, but significant amounts of cool gas might be retained. Past searches for that material, though, have provided unclear results. We present results from a survey of molecular gas in a volume-limited sample of field S0 galaxies, selected from the Nearby Galaxies Catalog. CO emission is detected from 78 percent of the sample galaxies. We find that the molecular gas is almost always located inside the central few kiloparses of a lenticular galaxy, meaning that in general it is more centrally concentrated than in spirals. We combine our data with HI observations from the literature to determine the total masses of cool and cold gas. Curiously, we find that, across a wide range of luminosity, the most gas rich galaxies have about 10 percent of the total amount of gas ever returned by their stars. That result is difficult to understand within the context of either monolithic or hierarchical models of evolution of the interstellar medium.Comment: 26 pages of text, 15 pages of tables, 10 figures. Accepted for publication in the Astrophysical Journa

Discovery of Recent Star Formation in the Extreme Outer Regions of Disk Galaxies

We present deep Halpha images of three nearby late-type spiral galaxies (NGC628, NGC1058 and NGC6946), which reveal the presence of HII regions out to, and beyond, two optical radii (defined by the 25th B-band isophote). The outermost HII regions appear small, faint and isolated, compared to their inner disk counterparts, and are distributed in organized spiral arm structures, likely associated with underlying HI arms and faint stellar arms. The relationship between the azimuthally--averaged Halpha surface brightness (proportional to star formation rate per unit area) and the total gas surface density is observed to steepen considerably at low gas surface densities. We find that this effect is largely driven by a sharp decrease in the covering factor of star formation at large radii, and not by changes in the rate at which stars form locally. An azimuthally--averaged analysis of the gravitational stability of the disk of NGC6946 reveals that while the existence of star formation in the extreme outer disk is consistent with the Toomre-Q instability model, the low rates observed are only compatible with the model when a constant gaseous velocity dispersion is assumed. We suggest that observed behaviour could also be explained by a model in which the star formation rate has an intrinsic dependence on the azimuthally-averaged gas volume density, which decreases rapidly in the outer disk due to the vertical flaring of the gas layer.Comment: 10 pages, 2 embedded postscript files, 3 jpeg images; accepted for publication in ApJ Letter