Extremely Inefficient Star Formation in the Outer Disks of Nearby Galaxies

(Abridged) We combine data from The HI Nearby Galaxy Survey and the GALEX Nearby Galaxy Survey to study the relationship between atomic hydrogen (HI) and far-ultraviolet (FUV) emission outside the optical radius (r25) in 17 spiral and 5 dwarf galaxies. In this regime, HI is likely to represent most of the ISM and FUV emission to trace recent star formation with little bias due to extinction, so that the two quantities closely trace the underlying relationship between gas and star formation rate (SFR). The azimuthally averaged HI and FUV intensities both decline with increasing radius in this regime, with the scale length of the FUV profile typically half that of the HI profile. Despite the mismatch in profiles, there is a significant spatial correlation (at 15" resolution) between local FUV and HI intensities; near r25 this correlation is quite strong, in fact stronger than anywhere inside r25, and shows a decline towards larger radii. The star formation efficiency (SFE) - defined as the ratio of FUV/HI and thus the inverse of the gas depletion time - decreases with galactocentric radius across the outer disks, though much shallower than across the optical disks. On average, we find the gas depletion times to be well above a Hubble time (~10^11 yr). We observe a clear relationship between FUV/HI and HI column in the outer disks, with the SFE increasing with increasing HI column. Despite observing systematic variations in FUV/HI, we find no clear evidence for step-function type star formation thresholds. When compared with results from inside r25, we find outer disk star formation to be distinct in several ways: it is extremely inefficient (depletion times of many Hubble times) with column densities and SFRs lower than found anywhere inside the optical disks. It appears that the HI column is one of, perhaps even the key environmental factor in setting the SFR in outer galaxy disks.Comment: Accepted for Publication in The Astronomical Journa

The Gas Consumption History to z ~ 4

Using the observations of the star formation rate and HI densities to z ~ 4, with measurements of the Molecular Gas Depletion Rate (MGDR) and local density of H_2 at z = 0, we derive the history of the gas consumption by star formation to z ~ 4. We find that closed-box models in which H_2 is not replenished by HI require improbably large increases in rho(H_2) and a decrease in the MGDR with lookback time that is inconsistent with observations. Allowing the H_2 used in star formation to be replenished by HI does not alleviate the problem because observations show that there is very little evolution of rho(HI) from z = 0 to z = 4. We show that to be consistent with observational constraints, star formation on cosmic timescales must be fueled by intergalactic ionized gas, which may come from either accretion of gas through cold (but ionized) flows or from ionized gas associated with accretion of dark matter halos. We constrain the rate at which the extraglactic ionized gas must be converted into HI and ultimately into H_2. The ionized gas inflow rate roughly traces the SFRD: about 1 - 2 x 10^8 M_sun Gyr^-1 Mpc^-3 from z ~ 1 - 4, decreasing by about an order of magnitude from z=1 to z=0 with details depending largely on MGDR(t). All models considered require the volume averaged density of rho(H_2) to increase by a factor of 1.5 - 10 to z ~ 1.5 over the currently measured value. Because the molecular gas must reside in galaxies, it implies that galaxies at high z must, on average, be more molecule rich than they are at the present epoch, which is consistent with observations. These quantitative results, derived solely from observations, agree well with cosmological simulations.Comment: 11 pages, 6 figures. Accepted for publication in the Astrophysical Journal

Galaxy-scale Star Formation on the Red Sequence: the Continued Growth of S0s and the Quiescence of Ellipticals

This paper examines star formation (SF) in relatively massive, primarily early-type galaxies (ETGs) at z~0.1. A sample is drawn from bulge-dominated GALEX/SDSS galaxies on the optical red sequence with strong UV excess and yet quiescent SDSS spectra. High-resolution far-UV imaging of 27 such ETGs using HST ACS/SBC reveals structured UV morphology in 93% of the sample, consistent with low-level ongoing SF (~0.5 Ms/yr). In 3/4 of the sample the SF is extended on galaxy scales (25-75 kpc), while the rest contains smaller (5-15 kpc) SF patches in the vicinity of an ETG - presumably gas-rich satellites being disrupted. Optical imaging reveals that all ETGs with galaxy-scale SF in our sample have old stellar disks (mostly S0 type). None is classified as a true elliptical. In our sample, galaxy-scale SF takes the form of UV rings of varying sizes and morphologies. For the majority of such objects we conclude that the gas needed to fuel current SF has been accreted from the IGM, probably in a prolonged, quasi-static manner, leading in some cases to additional disk buildup. The remaining ETGs with galaxy-scale SF have UV and optical morphologies consistent with minor merger-driven SF or with the final stages of SF in fading spirals. Our analysis excludes that all recent SF on the red sequence resulted from gas-rich mergers. We find further evidence that galaxy-scale SF is almost exclusively an S0 phenomenon (~20% S0s have SF) by examining the overall optically red SDSS ETGs. Conclusion is that significant number of field S0s maintain or resume low-level SF because the preventive feedback is not in place or is intermittent. True ellipticals, on the other hand, stay entirely quiescent even in the field.Comment: Accepted for publication in ApJ. Contains color figures, but compatible with non-color printer

Estimating gas accretion in disc galaxies using the Kennicutt-Schmidt law

We show how the existence of a relation between the star formation rate and the gas density, i.e. the Kennicutt-Schmidt law, implies a continuous accretion of fresh gas from the environment into the discs of spiral galaxies. We present a method to derive the gas infall rate in a galaxy disc as a function of time and radius, and we apply it to the disc of the Milky Way and 21 galaxies from the THINGS sample. For the Milky Way, we found that the ratio between the past and current star formation rates is about 2-3, averaged over the disc, but it varies substantially with radius. In the other disc galaxies there is a clear dependency of this ratio with galaxy stellar mass and Hubble type, with more constant star formation histories for small galaxies of later type. The gas accretion rate follows very closely the SFR for every galaxy and it dominates the evolution of these systems. The Milky Way has formed two thirds of its stars after z=1, whilst the mass of cold gas in the disc has remained fairly constant with time. In general, all discs have accreted a significant fraction of their gas after z=1. Accretion moves from the inner regions of the disc to the outer parts, and as a consequence star formation moves inside-out as well. At z=0 the peak of gas accretion in the Galaxy is at about 6-7 kpc from the centre.Comment: 14 pages, 10 figures, accepted for publication in MNRA

Spectro-photometric close pairs in GOODS-S: major and minor companions of intermediate-mass galaxies

(Abriged) Our goal here is to provide merger frequencies that encompass both major and minor mergers, derived from close pair statistics. We use B-band luminosity- and mass-limited samples from an Spitzer/IRAC-selected catalogue of GOODS-S. We present a new methodology for computing the number of close companions, Nc, when spectroscopic redshift information is partial. We select as close companions those galaxies separated by 6h^-1 kpc < rp < 21h^-1 kpc in the sky plane and with a difference Delta_v <= 500 km s^-1 in redshift space. We provide Nc for four different B-band-selected samples. Nc increases with luminosity, and its evolution with redshift is faster in more luminous samples. We provide Nc of M_star >= 10^10 M_Sun galaxies, finding that the number including minor companions (mass ratio >= 1/10) is roughly two times the number of major companions alone (mass ratio >= 1/3) in the range 0.2 <= z < 1.1. We compare the major merger rate derived by close pairs with the one computed by morphological criteria, finding that both approaches provide similar merger rates for field galaxies when the progenitor bias is taken into account. Finally, we estimate that the total (major+minor) merger rate is ~1.7 times the major merger rate. Only 30% to 50% of the M_star >= 10^10 M_Sun early-type (E/S0/Sa) galaxies that appear z=1 and z=0 may have undergone a major or a minor merger. Half of the red sequence growth since z=1 is therefore unrelated to mergers.Comment: Accepted in A&A. 14 pages, 6 figures, 8 tables. We have tested the method with a local, volume-limited spectroscopic sample

Star Formation from DLA Gas in the Outskirts of Lyman Break Galaxies at z~3

We present evidence for spatially extended low surface brightness emission around Lyman break galaxies (LBGs) in the V band image of the Hubble Ultra Deep Field, corresponding to the z~3 rest-frame FUV light, which is a sensitive measure of star formation rates (SFRs). We find that the covering fraction of molecular gas at z~3 is not adequate to explain the emission in the outskirts of LBGs, while the covering fraction of neutral atomic-dominated hydrogen gas at high redshift is sufficient. We develop a theoretical framework to connect this emission around LBGs to the expected emission from neutral H I gas i.e., damped Lyman alpha systems (DLAs), using the Kennicutt-Schmidt (KS) relation. Working under the hypothesis that the observed FUV emission in the outskirts of LBGs is from in situ star formation in atomic-dominated hydrogen gas, the results suggest that the SFR efficiency in such gas at z~3 is between factors of 10 and 50 lower than predictions based on the local KS relation. The total SFR density in atomic-dominated gas at z~3 is constrained to be ~10% of that observed from the inner regions of LBGs. In addition, the metals produced by in situ star formation in the outskirts of LBGs yield metallicities comparable to those of DLAs, which is a possible solution to the "Missing Metals" problem for DLAs. Finally, the atomic-dominated gas in the outskirts of galaxies at both high and low redshifts has similar reduced SFR efficiencies and is consistent with the same power law.Comment: 26 pages, 12 figures, 2 tables, appendix, accepted by ApJ, proof corrections include

The Baryonic Assembly of Dark Matter Halos

We use a suite of cosmological hydrodynamic simulations to quantify the accretion rates of baryons into dark matter halos and the resulting baryon mass fractions, as a function of halo mass, redshift, and baryon type (including cold and hot gas). We find that the net baryonic accretion rates through the virial radius are sensitive to galactic outflows and explore a range of outflow parameters to illustrate the effects. We show that the cold gas accretion rate is in general not a simple universal factor of the dark matter accretion rate, and that galactic winds can cause star formation rates to deviate significantly from the external gas accretion rates, both via gas ejection and re-accretion. Furthermore, galactic winds can inject enough energy and momentum in the surrounding medium to slow down accretion altogether, especially in low-mass halos and at low redshift. By resolving the accretion rates versus radius from the halo centers, we show how cold streams penetrate the hot atmospheres of massive halos at z>2, but gradually disappear at lower redshift. The total baryon mass fraction is also strongly suppressed by outflows in low-mass halos, but is nearly universal in the absence of feedback in halos above the UV background suppression scale. The transition halo mass, at which the gas mass in halos is equal for the cold and hot components, is roughly constant at ~10^11.5 Msun and does not depend sensitively on the wind prescription. We provide simple fitting formulae for the cold gas accretion rate into halos in the no-wind case. Finally, we show that cold accretion is broadly consistent with driving the bulk of the highly star-forming galaxies observed at z~2, but that the more intense star formers likely sample the high end of the accretion rate distribution, and may be additionally fueled by a combination of gas recycling, gas re-accretion, hot mode cooling, and mergers.Comment: 20 pages, 10 figures. MNRAS, in pres

The drop in the cosmic star formation rate below redshift 2 is caused by a change in the mode of gas accretion and by AGN feedback

The cosmic star formation rate is observed to drop sharply after redshift z=2. We use a large, cosmological, smoothed particle hydrodynamics simulation to investigate how this decline is related to the evolution of gas accretion and to outflows driven by active galactic nuclei (AGN). We find that the drop in the star formation rate follows a corresponding decline in the global cold-mode accretion rate density onto haloes, but with a delay of order the gas consumption time scale in the interstellar medium. Here we define cold-mode (hot-mode) accretion as gas that is accreted and whose temperature has never exceeded (did exceed) 10^5.5 K. In contrast to cold-mode accretion, which peaks at z~3, the hot mode continues to increase to z~1 and remains roughly constant thereafter. By the present time, the hot mode strongly dominates the global accretion rate onto haloes. Star formation does not track hot-mode halo accretion because most of the hot halo gas never accretes onto galaxies. AGN feedback plays a crucial role by preferentially preventing gas that entered haloes in the hot mode from accreting onto their central galaxies. Consequently, in the absence of AGN feedback, gas accreted in the hot mode would become the dominant source of fuel for star formation and the drop off in the cosmic star formation rate would be much less steep.Comment: Accepted for publication in MNRAS, 9 pages and 3 figures. Revised version: minor change

The Allen Telescope Array Pi GHz Sky Survey I. Survey Description and Static Catalog Results for the Bootes Field

The Pi GHz Sky Survey (PiGSS) is a key project of the Allen Telescope Array. PiGSS is a 3.1 GHz survey of radio continuum emission in the extragalactic sky with an emphasis on synoptic observations that measure the static and time-variable properties of the sky. During the 2.5-year campaign, PiGSS will twice observe ~250,000 radio sources in the 10,000 deg^2 region of the sky with b > 30 deg to an rms sensitivity of ~1 mJy. Additionally, sub-regions of the sky will be observed multiple times to characterize variability on time scales of days to years. We present here observations of a 10 deg^2 region in the Bootes constellation overlapping the NOAO Deep Wide Field Survey field. The PiGSS image was constructed from 75 daily observations distributed over a 4-month period and has an rms flux density between 200 and 250 microJy. This represents a deeper image by a factor of 4 to 8 than we will achieve over the entire 10,000 deg^2. We provide flux densities, source sizes, and spectral indices for the 425 sources detected in the image. We identify ~100$ new flat spectrum radio sources; we project that when completed PiGSS will identify 10^4 flat spectrum sources. We identify one source that is a possible transient radio source. This survey provides new limits on faint radio transients and variables with characteristic durations of months.Comment: Accepted for publication in ApJ; revision submitted with extraneous figure remove

The accuracy of frozen section analysis in ultrasound- guided core needle biopsy of breast lesions

<p>Abstract</p> <p>Background</p> <p>Limited data are available to evaluate the accuracy of frozen section analysis and ultrasound- guided core needle biopsy of the breast.</p> <p>Methods</p> <p>In a retrospective analysis data of 120 consecutive handheldultrasound- guided 14- gauge automated core needle biopsies (CNB) in 109 consecutive patients with breast lesions between 2006 and 2007 were evaluated.</p> <p>Results</p> <p>In our outpatient clinic120 CNB were performed. In 59/120 (49.2%) cases we compared histological diagnosis on frozen sections with those on paraffin sections of CNB and finally with the result of open biopsy. Of the cases 42/59 (71.2%) were proved to be malignant and 17/59 (28.8%) to be benign in the definitive histology. 2/59 (3.3%) biopsies had a false negative frozen section result. No false positive results of the intraoperative frozen section analysis were obtained, resulting in a sensitivity, specificity and positive predicting value (PPV) and negative predicting value (NPV) of 95%, 100%, 100% and 90%, respectively. Histological and morphobiological parameters did not show up relevance for correct frozen section analysis. In cases of malignancy time between diagnosis and definitive treatment could not be reduced due to frozen section analysis.</p> <p>Conclusion</p> <p>The frozen section analysis of suspect breast lesions performed by CNB displays good sensitivity/specificity characteristics. Immediate investigations of CNB is an accurate diagnostic tool and an important step in reducing psychological strain by minimizing the period of uncertainty in patients with breast tumor.</p