The Gas Phase in a Low Metallicity ISM

Original article can be found at: http://journals.cambridge.org/ Copyright International Astronomical Union. DOI: 10.1017/S1743921308024927We present several results from our analysis of dwarf irregular galaxies culled from The HI Nearby Galaxy Survey (THINGS). We analyse the rotation curves of two galaxies based on “bulk” velocity fields, i.e. velocity maps from which random non–circular motions are removed. We confirm that their dark matter distribution is best fit by an isothermal halo model. We show that the star formation properties of dIrr galaxies resemble those of the outer parts of larger, spiral systems. Lastly, we study the large scale (3–D) distribution of the gas, and argue that the gas disk in dIrrs is thick, both in a relative, as well as in an absolute sense as compared to spirals. Massive star formation through subsequent supernova explosions is able to redistribute the bulk of the ISM, creating large cavities. These cavities are often larger, and longer–lived than in spiral galaxies.Peer reviewe

The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales

(Abridged) We present a comprehensive analysis of the relationship between star formation rate surface density (SFR SD) and gas surface density (gas SD) at sub-kpc resolution in a sample of 18 nearby galaxies. We use high resolution HI data from THINGS, CO data from HERACLES and BIMA SONG, 24 micron data from the Spitzer Space Telescope, and UV data from GALEX. We target 7 spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H2-dominated centers of spiral galaxies, their HI-dominated outskirts and the HI-rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N=1.0+-0.2 relates the SFR SD and the H2 SD across our sample of spiral galaxies, i.e., that H2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ~2*10^9 yrs. We interpret the linear relation and constant depletion time as evidence that stars are forming in GMCs with approximately uniform properties and that the H2 SD may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas SD and SFR SD varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between the HI SD and the SFR SD. As a result, the star formation efficiency (SFE = SFR SD / gas SD) varies strongly across our sample and within individual galaxies. We show that in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. Another general feature of our sample is a sharp saturation of the HI SD at ~9 M_sol/pc^2 in both the spiral and dwarf galaxies.Comment: Accepted for publication in the AJ special THINGS issue. For a high-resolution version visit: http://www.mpia.de/THINGS/Publications.htm

Environment, Ram Pressure, and Shell Formation in HoII

Neutral hydrogen VLA D-array observations of the dwarf irregular galaxy HoII, a prototype galaxy for studies of shell formation, are presented. HI is detected to radii over 16' or 4 R_25, and M_HI=6.44x10^8 M_sun. The total HI map has a comet-like appearance suggesting that HoII is affected by ram pressure from an intragroup medium (IGM). A rotation curve corrected for asymmetric drift was derived and an analysis of the mass distribution yields a total mass 6.3x10^9 M_sun, of which about 80% is dark. HoII lies northeast of the M81 group's core, along with Kar52 (M81dwA) and UGC4483. No signs of interaction are observed and it is argued that HoII is part of the NGC2403 subgroup, infalling towards M81. A case is made for ram pressure stripping and an IGM in the M81 group. Stripping of the disk outer parts would require an IGM density n_IGM>=4.0x10^-6 atoms/cm^3 at the location of HoII. This corresponds to 1% of the virial mass of the group uniformly distributed over a volume just enclosing HoII and is consistent with the X-ray properties of small groups. It is argued that existing observations of HoII do not support self-propagating star formation scenarios, whereby the HI holes and shells are created by supernova explosions and stellar winds. Many HI holes are located in low surface density regions of the disk, where no star formation is expected or observed. Ram pressure has the capacity to enlarge preexisting holes and lower their creation energies, helping to bridge the gap between the observed star formation rate and that required to create the holes. (abridged)Comment: 43 pages, including 7 figures. 4 figures available as JPEG only. Complete manuscript including full resolution figures available at http://www.strw.leidenuniv.nl/~bureau/pub_list.html . Accepted for publication in The Astronomical Journa

The Fine-Scale Structure of the neutral Interstellar Medium in nearby Galaxies

We present an analysis of the properties of HI holes detected in 20 galaxies that are part of "The HI Nearby Galaxy Survey" (THINGS). We detected more than 1000 holes in total in the sampled galaxies. Where they can be measured, their sizes range from about 100 pc (our resolution limit) to about 2 kpc, their expansion velocities range from 4 to 36 km/s, and their ages are estimated to range between 3 and 150 Myr. The holes are found throughout the disks of the galaxies, out to the edge of the HI; 23% of the holes fall outside R25. We find that shear limits the age of holes in spirals (shear is less important in dwarf galaxies) which explains why HI holes in dwarfs are rounder, on average than in spirals. Shear, which is particularly strong in the inner part of spiral galaxies, also explains why we find that holes outside R25 are larger and older. We derive the scale height of the HI disk as a function of galactocentric radius and find that the disk flares up in all galaxies. We proceed to derive the surface and volume porosity (Q2D and Q3D) and find that this correlates with the type of the host galaxy: later Hubble types tend to be more porous. The size distribution of the holes in our sample follows a power law with a slope of a ~ -2.9. Assuming that the holes are the result of massive star formation, we derive values for the supernova rate (SNR) and star formation rate (SFR) which scales with the SFR derived based on other tracers. If we extrapolate the observed number of holes to include those that fall below our resolution limit, down to holes created by a single supernova, we find that our results are compatible with the hypothesis that HI holes result from star formation.Comment: 142 pages, 55 figures, accepted for publication in the Astronomical Journa

Capillary electrophoresis-mass spectrometry using noncovalently coated capillaries for the analysis of biopharmaceuticals

In this work, the usefulness of capillary electrophoresis–electrospray ionization time-of-flight–mass spectrometry for the analysis of biopharmaceuticals was studied. Noncovalently bound capillary coatings consisting of Polybrene-poly(vinyl sulfonic acid) or Polybrene-dextran sulfate-Polybrene were used to minimize protein and peptide adsorption, and achieve good separation efficiencies. The potential of the capillary electrophoresis-mass spectrometry (CE-MS) system to characterize degradation products was investigated by analyzing samples of the drugs, recombinant human growth hormone (rhGH) and oxytocin, which had been subjected to prolonged storage, heat exposure, and/or different pH values. Modifications could be assigned based on accurate masses as obtained with time-of-flight–mass spectrometry (TOF-MS) and migration times with respect to the parent compound. For heat-exposed rhGH, oxidations, sulfonate formation, and deamidations were observed. Oxytocin showed strong deamidation (up to 40%) upon heat exposure at low pH, whereas at medium and high pH, mainly dimer (>10%) and trisulfide formation (6–7%) occurred. Recombinant human interferon-β-1a (rhIFN-β) was used to evaluate the capability of the CE-MS method to assess glycan heterogeneity of pharmaceutical proteins. Analysis of this N-glycosylated protein revealed a cluster of resolved peaks which appeared to be caused by at least ten glycoforms differing merely in sialic acid and hexose N-acetylhexosamine composition. Based on the relative peak area (assuming an equimolar response per glycoform), a quantitative profile could be derived with the disialytated biantennary glycoform as most abundant (52%). Such a profile may be useful for in-process and quality control of rhIFN-β batches. It is concluded that the separation power provided by combined capillary electrophoresis and TOF-MS allows discrimination of highly related protein species

Planetary Nebulae Kinematics in M31

We present kinematics of 135 planetary nebulae in M31 from a survey covering 3.9 square degrees and extending out to 15 kpc from the southwest major axis and more than 20 kpc along the minor axis. The majority of our sample, even well outside the disk, shows significant rotational support (mean line-of-sight velocity 116 km/s). We argue that these PN belong to the outer part of M31's large de Vaucouleurs bulge. Only five PN have velocities clearly inconsistent with this fast rotating bulge. All five may belong to tidal streams in M31's outer halo. One is projected on the Northern Spur, and is counter-rotating with respect to the disk there. Two are projected along the major axis at X=-10 kpc and have M32-like velocities; they could be debris from that galaxy. The remaining two halo PN are located near the center of the galaxy and their velocities follow the gradient found by Ibata et al. (2004), implying that these PN could belong to the Southern Stream. If M31 has a non-rotating, pressure-supported halo, we have yet to find it, and it must be a very minor component of the galaxy.Comment: accepted to ApJ; main body of paper is 36 pages, including 14 figure

On the origin of the neutral hydrogen supershells: the ionized progenitors and the limitations of the multiple supernovae hypothesis

Here we address the question whether the ionized shells associated with giant HII regions can be progenitors of the larger HI shell-like objects found in the Milky Way and other spiral and dwarf irregular galaxies. We use for our analysis a sample of 12 HII shells presented recently by Rela\~no et al. (2005, 2007). We calculate the evolutionary tracks that these shells would have if their expansion is driven by multiple supernovae explosions from the parental stellar clusters. We find, contrary to Rela\~no et al. (2007), that the evolutionary tracks of their sample HII shells are inconsistent with the observed parameters of the largest and most massive neutral hydrogen supershells. We conclude that HII shells found inside giant HII regions may represent the progenitors of small or intermediate HI shells, however they cannot evolve into the largest HI objects unless, aside from the multiple supernovae explosions, an additional energy source contributes to their expansion.Comment: Accepted for publication in ApJ, tentatively scheduled for the ApJ July 1, 2008, v681n1 issue. 19 pages, 4 figure

UV Observations of the Powering Source of the Supergiant Shell in IC2574

A multi-band analysis of the region containing the supergiant HI shell in the nearby dwarf irregular galaxy IC2574 presents evidence of a causal relationship between a central star cluster, the surrounding expanding HI shell, and secondary star formation sites on the rim of the HI shell. Comparisons of the far-UV (FUV, 1521 A), optical broad-band, H-alpha, X-ray, and HI morphologies suggest that the region is in an auspicious moment of star formation triggered by the central stellar cluster. The derived properties of the HI shell, the central stellar cluster, and the star forming regions on the rim support this scenario: The kinematic age of the HI shell is <14 Myr and in agreement with the age of the central stellar cluster derived from the FUV observations (sim 11 Myr). An estimate for the mechanical energy input from SN and stellar winds of the central stellar cluster made from FUV photometry and the derived cluster age is 4.1 x 10^52 erg, roughly a few times higher than the kinetic energy of the HI shell. The requisite energy input needed to create the HI shell, derived in the `standard' fashion from the HI observations (using the numerical models of Chevalier), is 2.6 x 10^53 erg which is almost an order of magnitude higher than the estimated energy input as derived from the FUV data. Given the overwhelming observational evidence that the central cluster is responsible for the expanding HI shell, this discrepancy suggests that the required energy input is overestimated using the `standard' method. This may explain why some other searches for remnant stellar clusters in giant HI holes have been unsuccessful so far. Our observations also show that stellar clusters are indeed able to create supergiant HI shells, even at large galactocentric radii, a scenario which has recently been questioned by a number of authors.Comment: AJ, accepted, 16 pages, 6 figure

DDO 88: A Galaxy-Sized Hole in the Interstellar Medium

We present an HI and optical study of the gas-rich dwarf irregular galaxy DDO 88. Although DDO 88's global optical and HI parameters are normal for its morphological type, it hosts a large (3 kpc diameter) and unusually complete ring of enhanced HI emission. The gas ring is located at approximately one-third of the total HI radius and one-half the optically-defined Holmberg radius, and contains 30% of the total HI of the galaxy. The ring surrounds a central depression in the HI distribution, so it may be a shell formed by a starburst episode. However, the UBV colors in the HI 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, but 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 HI. An age estimate for the ring, however, is uncertain 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 indicate 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. If this ring and the depression in the gas which it surrounds were not formed by stellar winds and supernovae, this would indicate that some other, currently unidentified, mechanism is operating.Comment: 44 pages; 16 figures. To appear in AJ, January 2005. Available from ftp.lowell.edu, cd pub/dah/papers/d88 and http://www.fiu.edu/~simpsonc/d8

Extended HI Rotation Curve and Mass Distribution of M31

New HI observations of Messier 31 (M31) obtained with the Effelsberg and Green Bank 100-m telescopes make it possible to measure the rotation curve of that galaxy out to ~35 kpc. Between 20 and 35 kpc, the rotation curve is nearly flat at a velocity of ~226 km/s. A model of the mass distribution shows that at the last observed velocity point, the minimum dark-to-luminous mass ratio is \~0.5 for a total mass of 3.4 10^11 Msol at R < 35 kpc. This can be compared to the estimated MW mass of 4.9 10^11 Msol for R < 50 kpc.Comment: 4 pages, 2 figures, accepted for publication in ApJ Letter