arXiv subjects

R. J. Tuffs

Publications and source records attributed to R. J. Tuffs.

At least 55 records · Page 3Linked to original sources

arXiv2011

Galaxy And Mass Assembly (GAMA): The star formation rate dependence of the stellar initial mass function

The stellar initial mass function (IMF) describes the distribution in stellar masses produced from a burst of star formation. For more than fifty years, the implicit assumption underpinning most areas of research involving the IMF has been that it is universal, regardless of time and environment. We measure the high-mass IMF slope for a sample of low-to-moderate redshift galaxies from the Galaxy And Mass Assembly survey. The large range in luminosities and galaxy masses of the sample permits the exploration of underlying IMF dependencies. A strong IMF-star formation rate dependency is discovered, which shows that highly star forming galaxies form proportionally more massive stars (they have IMFs with flatter power-law slopes) than galaxies with low star formation rates. This has a significant impact on a wide variety of galaxy evolution studies, all of which rely on assumptions about the slope of the IMF. Our result is supported by, and provides an explanation for, the results of numerous recent explorations suggesting a variation of or evolution in the IMF.

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arXiv2010

Galaxy And Mass Assembly (GAMA): Galaxies at the faint end of the Halpha luminosity function

We present an analysis of the properties of the lowest Halpha-luminosity galaxies (L_Halpha<4x10^32 W; SFR<0.02 Msun/yr) in the Galaxy And Mass Assembly (GAMA) survey. These galaxies make up the the rise above a Schechter function in the number density of systems seen at the faint end of the Halpha luminosity function. Above our flux limit we find that these galaxies are principally composed of intrinsically low stellar mass systems (median stellar mass =2.5x10^8 Msun) with only 5/90 having stellar masses M>10^10 Msun. The low SFR systems are found to exist predominantly in the lowest density environments (median density ~0.02 galaxy Mpc^-2 with none in environments more dense than ~1.5 galaxy Mpc^-2). Their current specific star formation rates (SSFR; -8.5 < log(SSFR[yr^-1])<-12.) are consistent with their having had a variety of star formation histories. The low density environments of these galaxies demonstrates that such low-mass, star-forming systems can only remain as low-mass and forming stars if they reside sufficiently far from other galaxies to avoid being accreted, dispersed through tidal effects or having their gas reservoirs rendered ineffective through external processes.

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arXiv2010

The Detection of a Population of Submillimeter-Bright, Strongly-Lensed Galaxies

Gravitational lensing is a powerful astrophysical and cosmological probe and is particularly valuable at submillimeter wavelengths for the study of the statistical and individual properties of dusty starforming galaxies. However the identification of gravitational lenses is often time-intensive, involving the sifting of large volumes of imaging or spectroscopic data to find few candidates. We used early data from the Herschel Astrophysical Terahertz Large Area Survey to demonstrate that wide-area submillimeter surveys can simply and easily detect strong gravitational lensing events, with close to 100% efficiency.

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arXiv2010

Dust emission and star formation in Stephan's Quintet

We analyse a comprehensive set of MIR/FIR observations of Stephan's Quintet (SQ), taken with the Spitzer Space Observatory. Our study reveals the presence of a luminous (L_{IR}\approx 4.6x10^43 erg/s) and extended component of infrared dust emission, not connected with the main bodies of the galaxies, but roughly coincident with the X-ray halo of the group. We fitted the inferred dust emission spectral energy distribution of this extended source and the other main infrared emission components of SQ, including the intergalactic shock, to elucidate the mechanisms powering the dust and PAH emission, taking into account collisional heating by the plasma and heating through UV and optical photons. Combining the inferred direct and dust-processed UV emission to estimate the star formation rate (SFR) for each source we obtain a total SFR for SQ of 7.5 M(sun)/yr, similar to that expected for non-interacting galaxies with stellar mass comparable to the SQ galaxies. Although star formation in SQ is mainly occurring at, or external to the periphery of the galaxies, the relation of SFR per unit physical area to gas column density for the brightest sources is similar to that seen for star-formation regions in galactic disks. We also show that available sources of dust in the group halo can provide enough dust to produce up to L_{IR}\approx 10^42 erg/s powered by collisional heating. Though a minority of the total infrared emission (which we infer to trace distributed star-formation), this is several times higher than the X-ray luminosity of the halo, so could indicate an important cooling mechanism for the hot IGM and account for the overall correspondence between FIR and X-ray emission.

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arXiv2010

Galaxy and Mass Assembly (GAMA): survey diagnostics and core data release

The Galaxy And Mass Assembly (GAMA) survey has been operating since February 2008 on the 3.9-m Anglo-Australian Telescope using the AAOmega fibre-fed spectrograph facility to acquire spectra with a resolution of R~1300 for 120,862 SDSS selected galaxies. The target catalogue constitutes three contiguous equatorial regions centred at 9h (G09), 12h (G12) and 14.5h (G15) each of 12 x 4 sq.deg to limiting fluxes of r < 19.4, r < 19.8, and r < 19.4 mag respectively (and additional limits at other wavelengths). Spectra and reliable redshifts have been acquired for over 98 per cent of the galaxies within these limits. Here we present the survey footprint, progression, data reduction, redshifting, re-redshifting, an assessment of data quality after 3 years, additional image analysis products (including ugrizYJHK photometry, Sersic profiles and photometric redshifts), observing mask, and construction of our core survey catalogue (GamaCore). From this we create three science ready catalogues: GamaCoreDR1 for public release, which includes data acquired during year 1 of operations within specified magnitude limits (February 2008 to April 2008); GamaCoreMainSurvey containing all data above our survey limits for use by the GAMA team and collaborators; and GamaCoreAtlasSv containing year 1, 2 and 3 data matched to Herschel-ATLAS Science Demonstration data. These catalogues along with the associated spectra, stamps and profiles can be accessed via the GAMA website: http://www.gama-survey.org/

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arXiv2010

Galaxy And Mass Assembly (GAMA): The input catalogue and star-galaxy separation

We describe the spectroscopic target selection for the Galaxy And Mass Assembly (GAMA) survey. The input catalogue is drawn from the Sloan Digital Sky Survey (SDSS) and UKIRT Infrared Deep Sky Survey (UKIDSS). The aim is to measure redshifts for galaxies in three 4x12 degree regions at 9h, 12h and 14.5h, on the celestial equator, with magnitude selections r < 19.4, z < 18.2 and K(AB) < 17.6 over all three regions, and r < 19.8 in the 12-h region. The target density is 1080 per sq. deg. in the 12-h region and 720 per sq. deg. in the other regions. The average GAMA target density and area are compared with completed and ongoing galaxy redshift surveys. The GAMA survey implements a highly complete star-galaxy separation that jointly uses an intensity-profile separator (delta(sg) = r-band psf mag - model mag) as per the SDSS) and a colour separator. The colour separator is defined as delta(sg,jk) = J - K - f(g-i), where f(g-i) is a quadratic fit to the J-K colour of the stellar locus over the range 0.3 < g-i < 2.3. All galaxy populations investigated are well separated with delta(sg,jk) > 0.2. From two years out of a three-year AAOmega program on the Anglo-Australian Telescope, we have obtained 79599 unique galaxy redshifts. Previously known redshifts in the GAMA region bring the total up to 98497. The median galaxy redshift is 0.2 with 99% at z < 0.5. We present some of the global statistical properties of the survey, including K-band galaxy counts, colour-redshift relations and preliminary n(z).

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arXiv2009

Galaxy and Mass Assembly (GAMA): Optimal Tiling of Dense Surveys with a Multi-Object Spectrograph

A heuristic greedy algorithm is developed for efficiently tiling spatially dense redshift surveys. In its first application to the Galaxy and Mass Assembly (GAMA) redshift survey we find it rapidly improves the spatial uniformity of our data, and naturally corrects for any spatial bias introduced by the 2dF multi object spectrograph. We make conservative predictions for the final state of the GAMA redshift survey after our final allocation of time, and can be confident that even if worse than typical weather affects our observations, all of our main survey requirements will be met.

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arXiv2006

Modelling the spectral energy distribution of galaxies. IV Correcting apparent disk scalelengths and central surface brightnesses for the effect of dust at optical and near-infrared wavelengths

We present corrections for the change in the apparent scalelengths, central surface brightnesses and axis ratios due to the presence of dust in pure disk galaxies, as a function of inclination, central face-on opacity in the B-band (tau^f_B) and wavelength. The correction factors were derived from simulated images of disk galaxies created using geometries for stars and dust which can reproduce the entire spectral energy distribution from the ultraviolet (UV) to the Far-infrared (FIR)/submillimeter (submm) and can also account for the observed surface-brightness distributions in both the optical/Near-infrared and FIR/submm. We found that dust can significantly affect both the scalelength and central surface brightness, inducing variations in the apparent to intrinsic quantities of up to 50 percent in scalelength and up to 1.5 magnitudes in central surface brightness. We also identified some astrophysical effects for which, although the absolute effect of dust is non-negligible, the predicted variation over a likely range in opacity is relatively small, such that an exact knowledge of opacity is not needed. Thus, for a galaxy at a typical inclination of 37 degrees and having any tau^f_B>2, the effect of dust is to increase the scalelength in B relative to that in I by a factor of 1.12 +- 0.02 and to change the B-I central colour by 0.36 +- 0.05 magnitudes. Finally we use the model to analyse the observed scalelength ratios between B and I for a sample of disk-dominated spiral galaxies, finding that the tendency for apparent scalelength to increase with decreasing wavelength is primarily due to the effects of dust.

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arXiv2006

Powerful High Velocity-Dispersion Molecular Hydrogen Associated with an Intergalactic Shock Wave in Stephan's Quintet

We present the discovery of strong mid-infrared emission lines of molecular hydrogen of apparently high velocity dispersion (~870 km/s) originating from a group-wide shock wave in Stephan's Quintet. These Spitzer Space Telescope observations reveal emission lines of molecular hydrogen and little else. this is the first time an almost pure H_2 line spectrum has been seen in an extragalactic object. Along with the absence of PAH features and very low excitation ionized gas tracers, the spectra resemble shocked gas seen in Galactic supernova remnants, but on a vast scale. The molecular emission extends over 24 kpc along the X-ray emitting shock-front, but has ten times the surface luminosity as the soft X-rays, and about one-third the surface luminosity of the IR continuum. We suggest that the powerful H_2 emission is generated by the shock wave caused when a high-velocity intruder galaxy collides with filaments of gas in the galaxy group. Our observations suggest a close connection between galaxy-scale shock-waves and strong broad H_2 emission lines, like those seen in the spectra of Ultraluminous Infrared Galaxies where high-speed collisions between galaxy disks are common.

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arXiv2005

Discovery of an extended ultraviolet disk in the nearby galaxy NGC4625

Recent far-UV (FUV) and near-UV (NUV) observations of the nearby galaxy NGC4625 made by the Galaxy Evolution Explorer (GALEX) show the presence of an extended UV disk reaching 4 times the optical radius of the galaxy. The UV-to-optical colors suggest that the bulk of the stars in the disk of NGC4625 are currently being formed, providing a unique opportunity to study today the physics of star formation under conditions similar to those when the normal disks of spiral galaxies like the Milky Way first formed. In the case of NGC4625, the star formation in the extended disk is likely to be triggered by interaction with NGC4618 and possibly also with the newly discovered galaxy NGC4625A. The positions of the FUV complexes in the extended disk coincide with peaks in the H I distribution. The masses of these complexes are in the range 10^3-10^4 Msun with their Halpha emission (when present) being dominated by ionization from single stars.

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arXiv2004

A comparative study of the spatial distribution of ultraviolet and far-infrared fluxes from M 101

The total ultraviolet (UV) flux (from 1412 to 2718 AA) of M 101 is compared on a pixel-to-pixel basis with the total far-infrared (FIR) flux (from 60 to 170 micron), using the maps of the galaxy taken by GALEX in the near-UV and far-UV and by ISOPHOT at 60, 100 and 170 micron. The main result of this investigation is the discovery of a tight dependence of the FIR/UV ratio on radius, with values monotonically decreasing from around 4 in the nuclear region to nearly zero towards the edge of the optical disk. Although the tightness of this dependence is in part attributable to resolution effects, the result is consistent with the presence of a large-scale distribution of diffuse dust having a face-on optical depth which decreases with radius and which dominates over the more localized variations in opacity between the arm and interarm regions. We also find a trend for the FIR/UV ratio to take on higher values in the regions of diffuse interarm emission than in the spiral-arm regions, at a given radius. This is interpreted quantitatively in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.

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arXiv2004

Far-infrared and sub-mm observations of the Crab nebula

We present FIR and sub-mm images of the Crab nebula, taken with ISOPHOT on board ISO and with the SCUBA on the JCMT. The ISOPHOT observations were made at 60, 100, 170 micron, with FWHM of equivalent area Gaussian beams of 44, 47 and 93 arcsec, respectively. The 850 micron SCUBA image has a Gaussian FWHM of 17 arcsec. The 60 and 100 micron images show clear excess of emission above that expected from an extrapolation of the synchrotron spectrum of the Crab nebula from lower frequencies. The ISOPHOT images reveal that about half this excess is attributable to two peaks, separated by approximately 80 arcsec. We also present spectra taken using the LWS on board ISO, which show that the FIR excess is not due to line emission. The lower resolution 170 micron image does not show any excess emission. These findings are consistent with a picture in which the FIR excess is due to emission from a small amount of warm dust which radiates predominantly at 60 and 100 micron micron but not at 170 micron, and which is superimposed on a synchrotron spectrum which gradually steepens towards shorter wavelengths throughout the FIR and MIR spectral range. The dust geometry is consistent with a torus of diameter approximately 0.8 pc created by the supernova progenitor, superimposed upon a broadly distributed component which may be supernova condensates in the filaments. The dust-to-gas ratio in the filaments is comparable to the interstellar value. To study spectral index variations in the radio synchrotron emission we compared the 850-micron image with a 20-cm VLA image. These images are very similar, implying that there is little variation in spectral index across the face of the remnant between these wavelengths.

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arXiv2003

Infrared emission towards 11 years after outburst: Properties of the circumstellar dust

Detailed models are presented for the late epoch mid infrared (MIR) emission from collisionally heated grains in the shocked circumstellar gas around SN 1987A. Thermal dust emission from a region of moderate density interior to the thick inner ring seen with the Hubble Space Telescope (HST) is found to be a natural explanation for the MIR spectral energy distribution measured by ISOCAM. The MIR-spectrum can be reproduced by a mixture of silicate-iron or silicate-graphite grains or by a composition of pure graphite grains. A composition of pure iron grains on the other hand can be excluded and a pure silicate composition does not seem to be very likely. The dust-to-gas ratio in the interaction zone is ~0.01%, an order of magnitude lower than estimates for dust abundances in the winds of red supergiant (RSG) stars in the LMC. This low dust abundance can be accounted for by a combination of evaporation through the UV-flash from the supernova outburst and subsequent sputtering in the shocked gas. For this explanation to hold, dust in the pre-supernova circumstellar medium (CSM) would have to have been predominantly composed of grains other than graphite, with a maximum size smaller than ~0.1 microns.

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arXiv2003

The far-IR/radio correlation in the ISO era

We present the correlation between the far-infrared (FIR) and radio emissions from a composite sample of 72 nearby normal galaxies observed with the ISOPHOT instrument on board the Infrared Space Observatory. The galaxies in the sample have measurements at three FIR wavelengths (60, 100 and 170 micron), which allowed a direct determination of the warm and cold FIR emission components. This is the first time that the correlation has been established for the total FIR luminosity, of which most is carried by the cold dust component predominantly emitting longwards of the spectral coverage of IRAS. The slope of this correlation is slightly non-linear (1.10+/-0.03). Separate correlations between the warm and cold FIR emission components and the radio emission have also been derived. The slope of the warm FIR/radio correlation was found to be linear (1.03+/-0.03). For the cold FIR/radio correlation we found a slightly non-linear (1.13+/-0.04) slope. We qualitatively interpret the correlations in terms of star formation rate and find that both the FIR and radio emissions may be consistent with a non-linear dependence on star formation rate for galaxies not undergoing starburst activity.

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arXiv2003

The spiral galaxy M33 mapped in the FIR by ISOPHOT: A spatially resolved study of the warm and cold dust

The Sc galaxy M33 has been mapped with ISOPHOT in the far-infrared, at 60, 100, and 170mue. The spatial resolution of these FIR maps allows the separation of spiral arms and interarm regions and the isolation of a large number of star-forming regions. The spectral energy distribution in the FIR indicates a superposition of two components, a warm one originating from dust at ~45K, and a cold one, at ~16K. The warm component is concentrated towards the spiral arms and the star-forming regions, and is likely heated by the UV radiation from OB stars. The cold component is more smoothly distributed over the disk, and heated by the diffuse interstellar radiation. For the about 60 star-forming regions detected the H-alpha/FIR flux ratio increases significantly with the distance from the galaxy center, probably due to decreasing extinction. An anti-correlation of F_Ha/F_60 with F_170 suggests the intrinsic extinction to be related to the cold dust surface brightness according to A_V/S_170~0.03mag/MJy*sr. For the total galaxy the star formation rate (SFR) derived from the FIR is in agreement with that derived from the de-extincted H-alpha emission. For individual star-forming regions, a consistency between SFRs derived from the optical and from the FIR requires only a fraction of the UV radiation to be absorbed locally. The individual star-forming regions also show a local radio-FIR correlation. This local correlation is, however, due to quite different components than to those that lead to the well-known global radio-FIR correlation for entire galaxies.

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arXiv2003

Physical Conditions and Star Formation Activity in the Intragroup Medium of Stephan's Quintet

New multi-band observations of the famous compact group of galaxies Stephan's Quintet (SQ) are presented and analyzed. These include far infrared (FIR) images at 60$μm$ and 100$μm$ (ISOPHOT C-100 camera), radio continuum images at 1.4 GHz (VLA B-array) and 4.86 GHz (VLA C-array), and long-slit optical spectrographs (Palomar $200"$ telescope). With these new data, we aim to learn more about the X-ray/radio ridge in the middle of the intragroup medium (IGM) and the IGM starburst SQ-A, both are likely to be caused by the high speed collision ($\sim 900$ km s$^{-1}$) between the intruder galaxy NGC 7318b ($v = 5700$ km s$^{-1}$) and the IGM ($v = 6600$ km s$^{-1}$).

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arXiv2002

The percentage of stellar light re-radiated by dust in late-type Virgo Cluster galaxies

We show that the mean percentage of stellar light re-radiated by dust is ~30 percent for the Virgo Cluster late-spirals measured with ISOPHOT by Tuffs et al. (2002). A strong dependence of this ratio with morphological type was found, ranging from typical values of ~15 percent for early spirals to up to ~50 percent for some late spirals. The extreme BCDs can have even higher percentages of their bolometric output re-radiated in the thermal infrared. Luminosity correction factors for the cold dust component are given for general use in converting far-infrared (FIR) luminosities derived from IRAS.

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arXiv2002

Photometric Mapping with ISOPHOT using the "P32" Astronomical Observation Template

The ``P32'' Astronomical Observation Template (AOT) provided a means to map large areas of sky (up to 45x45 arcmin) in the FIR at high redundancy and with sampling close to the Nyquist limit using the ISOPHOT C100 (3x3) and C200 (2x2) detector arrays on board ISO. However, the transient response behaviour of the Ga:Ge detectors, if uncorrected, can lead to severe systematic photometric errors and distortions of source morphology on maps. Here we describe the basic concepts of an algorithm which can successfully correct for transient response artifacts in P32 observations. Examples are given demonstrating the photometric and imaging performance of ISOPHOT P32 observations corrected using the algorithm.

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