Monthly Notices of the Royal Astronomical Society

Published by Oxford University Press

Print ISSN: 0035-8711


Narrow polarized components in the OH 1612-MHz maser emission from supergiant OH-IR sources
  • Article
  • Full-text available

February 1987


161 Reads

R J Cohen


G Downs


R Emerson




High-resolution OH 1612-MHz spectra are presented of the supergiant OH–IR sources VY CMa, VX Sgr, IRC 10420 and NML Cyg. The spectra have a resolution of 300 Hz. Narrow components in the spectra have linewidths as small as 550 Hz (0.1 km s–1) but there is no evidence for components narrower than this. These results are in accord with present understanding of maser line-narrowing and of the physical conditions in the OH maser regions. Many of the narrow components have an appreciable degree of circular polarization which is not apparent at the lower frequency resolutions usually employed. The circular polarization indicates the presence of magnetic fields of $\sim{1}\text{mG}$ in the circumstellar envelopes, at distances of $\sim{3}\times {10}^{16}\text{cm}$ from the central stars. These fields are strong enough to influence the outflow from the stars, and may help to explain some of the asymmetries which are seen in their circumstellar envelopes.

Can interstellar H2S be formed via gas-phase reactions? Calculations concerning the rates of the ternary and radiative association reactions between HS+ and H2

February 1989


38 Reads

Based on laboratory work involving the ternary association reaction of HS+ and H2 at 80 K, we have estimated the rate of the analogous radiative association reaction under interstellar conditions. Both the ternary and radiative association reactions appear to occur via a mechanism in which the electronic spin of the H3S+ complex changes before the complex is stabilized. Although this spin change is of low probability, it leads to a radiative association rate coefficient at 80 K of $7\times10^{-16}\enspace \text {cm}^3 \enspace \text s^{-1}$ if radiative stabilization occurs at a rate of 103 s−1. This value of the radiative association rate coefficient at 80 K is large enough to lead to the observed abundance of H2S in the ambient ridge source in Orion.

Automatic morphological classification of galaxy images

November 2009


511 Reads

We describe an image analysis supervised learning algorithm that can automatically classify galaxy images. The algorithm is first trained using a manually classified images of elliptical, spiral, and edge-on galaxies. A large set of image features is extracted from each image, and the most informative features are selected using Fisher scores. Test images can then be classified using a simple Weighted Nearest Neighbor rule such that the Fisher scores are used as the feature weights. Experimental results show that galaxy images from Galaxy Zoo can be classified automatically to spiral, elliptical and edge-on galaxies with accuracy of ~90% compared to classifications carried out by the author. Full compilable source code of the algorithm is available for free download, and its general-purpose nature makes it suitable for other uses that involve automatic image analysis of celestial objects.

Figure 1. Integrated flux density of the OH 1612-(upper panel) and 1667-MHz (lower panel) spectra of OH127 as a function of time. The uncertainties of the measurements are comparable with the symbol sizes. The blue line shows the model light curve. The arrow marks the date of the MERLIN polarimetric observation.
Table 1 . Measured and derived parameters for OH127.8+0.0.
Table 2 . Mean and median values of the polarization parameters for the brightest OH features derived from the multi-epoch data.
Long-term polarimetric observations of OH127.8+0.0

February 2013


83 Reads

OH 1612- and 1667-MHz masers from the well-known object OH127.8+0.0 were monitored in full polarization mode over a period of 6.5 yr and mapped with MERLIN at one epoch. The OH variability pattern of the star is typical of extremely long-period asymptotic giant branch stars. The distance determined from the 1612-MHz light curve and a new measurement of the angular radius is 3.87 ± 0.28 kpc. At both frequencies, the flux of polarized emission tightly follows the total flux variations while, the degrees of circular and linear polarization are constant within measurement accuracy. There is net polarization at both lines. The magnetic field strength estimated from a likely Zeeman pair is −0.6 mG at the distance of 5400 au from the star. At the near and far sides of the envelope, the polarization vectors are well aligned implying a regular structure of the magnetic field. The polarization characteristics of the OH maser emission suggest a radial magnetic field which is frozen in the stellar wind.

Figure 5. Mass-loss rate as a function of time for the 7.5 M ⊙ Z=0.0001 with low-T fixed (red) and low-T variable (black) composition molecular opacities.
Figure 6. Temperature at the base of the convective envelope as a function of total stellar mass for the 7.5 M ⊙ Z=0.0001 models with VW93 mass-loss rate with fixed (red) and variable (black) molecular low temperature opacities.
Super and massive AGB stars - III. Nucleosynthesis in metal-poor and very metal-poor stars - Z=0.001 and 0.0001

March 2014


82 Reads

We present a new grid of stellar models and nucleosynthetic yields for super-AGB stars with metallicities Z = 0.001 and 0.0001, applicable for use within galactic chemical evolution models. Contrary to more metal-rich stars where hot bottom burning is the main driver of the surface composition, in these lower metallicity models the effect of third dredge-up and corrosive second dredge-up also have a strong impact on the yields. These metal-poor and very metal-poor super-AGB stars create large amounts of 4He, 13C, 14N and 27Al as well as the heavy magnesium isotopes 25Mg and 26Mg. There is a transition in yield trends at metallicity Z ≈ 0.001, below which we find positive yields of 12C, 16O, 15N and 28Si, which is not the case for higher metallicities. We explore the large uncertainties derived from wind prescriptions in super-AGB stars, finding ≈2 orders of magnitude difference in yields of 22Ne, 23Na, 24, 25, 26Mg, 27Al and our s-process proxy isotope g. We find inclusion of variable composition low-temperature molecular opacities is only critical for super-AGB stars of metallicities below Z ≈ 0.001. We analyse our results, and those in the literature, to address the question: Are super-AGB stars the polluters responsible for extreme population in the globular cluster NGC 2808? Our results, as well as those from previous studies, seem unable to satisfactorily match the extreme population in this globular cluster.

Figure 2. Surface abundances (in logarithm of mole fraction Y) of selected isotopes as a function of time for the 8.5 M ⊙ Z=0.02 model. The time axis has been offset with the zero at the time of first thermal pulse.
Figure 3. Surface abundances of the number ratio C/O for our standard VW93 mass-loss rates cases. The horizontal dashed line represents the divide between M stars (O-rich) and the C stars (C-rich). The time axis has been offset with the zero at the time of first thermal pulse.
Figure 7. Mass-loss rate in M ⊙ yr −1 as a function of time during the thermally pulsing super AGB phase for the set of 8.5 M ⊙ Z=0.02 models.
Figure 12. 26 Al surface abundance in log mole fraction Y as a function of time for the series of 8.5 M ⊙ Z=0.02 stellar model test cases.
Figure 14. Difference between standard yield production factors and those which include the effects of extrapolated pulses for VW93 (top panel) and VW-M (bottom panel) models. Colors and symbols as per Figure 4. The error bars on the 8.5 M ⊙ Z=0.02 model encompass the range of values found for the test cases.
Super and Massive AGB Stars: II - Nucleosynthesis and Yields - Z=0.02, 0.008 and 0.004

October 2013


134 Reads

We have computed detailed evolution and nucleosynthesis models for super and massive asymptotic giant branch (AGB) stars over the mass range 6.5–9.0 M⊙ in divisions of 0.5 M⊙ with metallicities Z = 0.02, 0.008 and 0.004. These calculations, in which we find third dredge-up and hot bottom burning, fill the gap between existing low- and intermediate-mass AGB star models and high-mass star models that become supernovae. For the considered metallicities, the composition of the yields is largely dominated by the thermodynamic conditions at the base of the convective envelope rather than by the pollution arising from third dredge-up. We investigate the effects of various uncertainties, related to the mass-loss rate, mixing length parameter, and the treatment of evolution after the envelope instability that develops near the end of the (super)AGB phase. Varying these parameters alters the yields mainly because of their impact on the amount of third dredge-up enrichment, and to a lesser extent on the hot bottom burning conditions. Our models produce significant amounts of 4He, 7Li (depending on the mass-loss formulation) 13C, 14N, 17O, 23Na, 25Mg, as well the radioactive isotope 26Al in agreement with previous investigation. In addition, our results show enrichment of 22Ne, 26Mg and 60Fe, as well as a substantial increase in our proxy neutron capture species representing all species heavier than iron. These stars may provide important contributions to the Galaxy's inventory of the heavier Mg isotopes, 14N, 7Li and 27Al.

Galaxy groups and clouds in the Local (z~0.01) universe

November 2010


133 Reads

We present an all-sky catalogue of 395 nearby galaxy groups revealed in the Local Supercluster and its surroundings. The groups and their associations are identified among 10 914 galaxies at |b| > 15° with radial velocities VLG < 3500 km s−1. Our group-finding algorithm requires the group members to be located inside their zero-velocity surface. Hereby, we assume that individual galaxy masses are proportional to their total K-band luminosities, M/LK= 6 M⊙/L⊙. The sample of our groups, where each group has n≥ 4 members, is characterized by the following medians: mean projected radius 〈R〉= 268 kpc, radial velocity dispersion σV= 74 km s−1, K-band luminosity LK= 1.2 × 1011 L⊙, virial and projected masses Mvir= 2.4 × 1012 and Mp= 3.3 × 1012 M⊙, respectively. Accounting for measurement error reduces the median masses by 30 per cent. For 97 per cent of identified groups the crossing time does not exceed the cosmic time, 13.7 Gyr, with the median at 3.8 Gyr. We examine different properties of the groups, in particular of the known nearby groups and clusters in Virgo and Fornax. About a quarter of our groups can be classified as fossil groups where the dominant galaxy is at least 10 times brighter than the other group members. In total, our algorithm identifies 54 per cent of galaxies to be members of groups. Together with triple systems and pairs they gather 82 per cent of the K-band light in the local Universe. We have obtained the local value of matter density to be Ωm= 0.08 ± 0.02 within a distance of ∼40 Mpc assuming H0= 73 km s−1 Mpc−1. It is significantly smaller than the cosmic value, 0.28, in the standard ΛCDM model. The discrepancy between the global and local quantities of Ωm may be caused by the existence of a dark matter component unrelated to the virial masses of the galaxy systems.

Dwarf galaxies in four rich clusters with 0.02 < z < 0.14

July 1997


32 Reads

Deep measurements are presented of four rich clusters of galaxies: Abell 1367 (z=0.022), Abell 2199 (z=0.030), Abell 1795 (z=0.063), and Abell 1146 (z=0.141). All clusters have an excess of galaxies at faint magnitudes above blank sky fields. We correct for background contamination and measure the luminosity function of these galaxies in each cluster, and then combine these luminosity functions to get better statistics. The resultant combined luminosity function is rising at faint magnitudes, with a logarithmic slope -1.5 < \alpha < -1.2 for -18 < M_B < -13 and -19 < M_R < -15. This is similar to what has been observed independently in the Coma cluster. The colours of these faint galaxies suggest that they are dwarf spheroidals.

Discovery of a 0.02 Hz QPO feature in the transient X-ray pulsar KS 1947+300

May 2010


51 Reads

We report the discovery of quasi-periodic oscillations (QPOs) at 0.02 Hz in a transient high-mass X-ray binary pulsar KS 1947+300 using the RXTE Proportional Counter Array (PCA). The QPOs were detected during 2001 May–June, at the end of a long outburst. This is the ninth transient accretion-powered high magnetic field X-ray pulsar in which QPOs have been detected, and the QPO frequency of this source is lowest in this class of sources. The unusual feature of this source is that though the outburst lasted for more than 100 days, the QPOs were detected only during the last few days of the outburst when the X-ray intensity had decayed to 1.6 per cent of the peak intensity. The rms value of the QPO is large, ∼15.4 ± 1.0 per cent with a slight positive correlation with energy. The detection of QPOs and strong pulsations at a low luminosity level suggests that the magnetic field strength of the neutron star is not as high as was predicted earlier on the basis of a correlation between the spin-up torque and the X-ray luminosity.

Realistic Ionizing Fluxes for Young Stellar Populations from 0.05 to twice solar metallicity

July 2002


43 Reads

We present a new grid of ionizing fluxes for O and Wolf—Rayet (W—R) stars for use with evolutionary synthesis codes and single-star H ii region analyses. A total of 230 expanding, non-LTE, line-blanketed model atmospheres have been calculated for five metallicities (0.05, 0.2, 0.4, 1 and 2 Z⊙) using the wm-basic code of Pauldrach, Hoffmann & Lennon for O stars and the cmfgen code of Hillier & Miller for W—R stars. The stellar wind parameters are scaled with metallicity for both O and W—R stars. We compare the ionizing fluxes of the new models with the CoStar models of Schaerer & de Koter and the pure helium W—R models of Schmutz, Leitherer & Gruenwald. We find significant differences, particularly above 54 eV, where the emergent flux is determined by the wind density as a function of metallicity. The new models have lower ionizing fluxes in the He i continuum with important implications for nebular line ratios. We incorporate the new models into the evolutionary synthesis code starburst99 and compare the ionizing outputs for an instantaneous burst and continuous star formation with the work of Schaerer & Vacca (SV98), and Leitherer et al. The changes in the output ionizing fluxes as a function of age are dramatic. We find that, in contrast to previous studies, nebular He iiλ4686 will be at, or just below, the detection limit in low metallicity starbursts during the W—R phase. The new models have lower fluxes in the He i continuum for Z≥ 0.4 Z⊙ and ages ≤7 Myr because of the increased line blanketing. We test the accuracy of the new model atmosphere grid by constructing photoionization models for simple H ii regions, and assessing the impact of the new ionizing fluxes on important nebular diagnostic line ratios. For the case of an H ii region where the ionizing flux is given by the WM-basic dwarf O star grid, we show that He iλ5786/Hβ decreases between 1 and 2 Z⊙ in a similar manner to observations. We find that this decline is caused by the increased effect of line blanketing above solar metallicity. We therefore suggest that a lowering of the upper mass limit at high abundances is not required to explain the diminishing strength of He iλ5786/Hβ, as has been suggested in the past. For an H ii region where the ionizing flux is provided by an instantaneous burst of total mass 10⁶ M⊙, we plot the softness parameter η′ against the abundance indicator R23 for ages of 1–5 Myr. The new models are coincident with the observational data of Bresolin et al., particularly during the W—R phase, unlike the previous models of SV98 which generally over-predict the hardness of the ionizing radiation. The new model grid and updated starburst99 code can be downloaded from

Galaxy And Mass Assembly (GAMA): The galaxy stellar mass function at z < 0.06

November 2011


146 Reads

We determine the low-redshift field galaxy stellar mass function (GSMF) using an area of 143 deg2 from the first three years of the Galaxy And Mass Assembly (GAMA) survey. The magnitude limits of this redshift survey are r < 19.4 mag over two-thirds and 19.8 mag over one-third of the area. The GSMF is determined from a sample of 5210 galaxies using a density-corrected maximum volume method. This efficiently overcomes the issue of fluctuations in the number density versus redshift. With H0= 70 km s−1 Mpc−1, the GSMF is well described between 108 and 1011.5 M⊙ using a double Schechter function with , , α1=−0.35, and α2=−1.47. This result is more robust to uncertainties in the flow-model corrected redshifts than from the shallower Sloan Digital Sky Survey main sample (r < 17.8 mag). The upturn in the GSMF is also seen directly in the i-band and K-band galaxy luminosity functions. Accurately measuring the GSMF below 108 M⊙ is possible within the GAMA survey volume but as expected requires deeper imaging data to address the contribution from low surface-brightness galaxies.

Figure 7. Scatter plot showing the colour-magnitude (CM) distribution of galaxies in the SubS (blue circles) and OptS (black- grey circles). The division between Red Sequence and Blue Cloud defined in the text is shown as the black solid line. 
GAMA/H-ATLAS: Linking the properties of submm detected and undetected early-type galaxies - I. z ≤ 0.06 sample

February 2013


189 Reads

We present two large, nearby (0.013$\le$z$\le$0.06) samples of Early-Type Galaxies (ETGs): a visually classified sample of 220 ETGs, created using source-matched data from the Galaxy and Mass Assembly (GAMA) database with FIR/sub-mm detections from $Herschel$-ATLAS; and a visually classified sample of 551 ETGs which are undetected with $Herschel$-ATLAS. Active galactic nuclei (AGN) are removed from our samples using optical emission line diagnostics. These samples are scrutinised to determine characteristics of sub-mm detected versus undetected ETGs. We find similarities in the stellar mass distributions of the two ETG samples but testing other properties uncovers significant differences. The sub-mm detected sample is shown to have lower concentration and S\'ersic indices than those in the undetected sample - a result which may be linked to the presence of dust in the former. Optical and UV-optical colours are also shown to be much bluer, indicating that the dust is linked with recent star formation. The intrinsic effective radii are on average 1.5 times larger for the sub-mm detected ETGs. Surface densities and groups data from the GAMA database are examined for the two samples, leading to the conclusion that dusty ETGs inhabit sparser environments than non-dusty ETGs in the nearby universe, although environments of the brightest ETGs are shown to differ the least. Modified Planck functions are fit to the H-ATLAS detected PACS and SPIRE fluxes for ETGs with sub-mm flux densities of at least 3$\sigma$ in the 350$\mu$m SPIRE band, giving a resultant mean cold dust temperature of T$_{d}$=22.1K, with a range of 9-30K. The corresponding mean dust mass is 1.8$\times10^{7}$M$_{\odot}$, with a range of (0.08-35.0)$\times10^{7}$M$_{\odot}$. The dust masses calculated from these fits, normalised by stellar mass, are shown to increase with decreasing stellar mass and bluer colours.

The z = 0.0777 C III Absorber Towards PHL 1811 as a Case Study of a Low Redshift Weak Metal Line Absorber

December 2009


27 Reads

We consider the physical conditions and origin of the z= 0.0777 absorption system observed in C iii, C ii, Si iii, C iv, O vi and H i absorption along the line of sight towards the quasar PHL 1811. We analysed the Hubble Space Telescope/Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer spectra of this quasar and compared the results to cloudy photoionization and collisional ionization models in order to derive densities, temperatures and metallicities of the absorbing gas. The absorption can be explained by two C iii clouds, offset by 35 km s−1 in velocity, with metallicities of approximately one-tenth the solar value. One cloud has a density of the order of nH= 1.2+0.9−0.5× 10−3 cm−3 (thickness 0.4+0.3−0.2 kpc) and produces the observed C ii and Si iii absorption, while the other has a density of the order of nH= 1.2+0.9−0.5× 10−5 cm−3 (thickness 80+70−40 kpc) and gives rise to the observed weak C iv absorption. Cloud temperatures are ∼14 000+3000−2000 K and ∼34 000+2000−4000 K for photoionized models. Although collisionally ionized clouds with T∼ 70 000 K are possible, they are less likely because of the short cooling time-scales involved. Previous studies revealed no luminous galaxy at the absorber's redshift, so it is probably related to tidal debris, ejected material, a dwarf galaxy or other halo material in a galaxy group. Our models also indicate that one of the two clouds would produce detectable weak Mg ii absorption if spectral coverage of that transition existed. We predict what the system would look like at z∼ 1 when the ionizing background radiation was more intense. We find that at z∼ 1 the denser component resembles a C iv absorber. The second C iii cloud in this z= 0.0777 absorber may be analogous to a subset of the more diffuse O vi absorbers at higher redshift.

Parsec-scale structures and diffuse bands in a translucent interstellar medium at z 0.079

October 2012


121 Reads

We present a detailed study of the QSO–galaxy pair [SDSS J163956.35+112758.7 (zq = 0.993) and SDSS J163956.38+112802.1 (zg = 0.079)] based on observations carried out using the Giant Metrewave Radio Telescope (GMRT), the Very Large Baseline Array (VLBA), the Sloan Digital Sky Survey and the ESO New Technology Telescope. We show that the interstellar medium of the galaxy probed by the QSO line of sight has near-solar metallicity [12+log(O/H) = 8.47 ± 0.25] and dust extinction [E(B − V) ∼ 0.83 ± 0.11] typical of what is usually seen in translucent clouds. We report the detection of absorption in the λ6284 diffuse interstellar band (DIB) with a rest equivalent width of 1.45 ± 0.20 Å. Our GMRT spectrum shows a strong 21-cm absorption at the redshift of the galaxy with an integrated optical depth of 15.70 ± 0.13 km s−1. Follow-up VLBA observations show that the background radio source is resolved into three components with a maximum projected separation of 89 pc at the redshift of the galaxy. One of these components is too weak to provide useful 21-cm H i information. The integrated H i optical depth towards the other two components are higher than that measured in our GMRT spectrum and differ by a factor 2. By comparing the GMRT and VLBA spectra we show the presence of structures in the 21-cm optical depth on parsec scales. We discuss the implications of such structures for the spin-temperature measurements in high-z damped Lyman α systems. The analysis presented here suggests that this QSO–galaxy pair is an ideal target for studying the DIBs and molecular species using future observations in optical and radio wavebands.

Two-dimensional kinematics of SLACS lenses: II. Combined lensing and dynamics analysis of early-type galaxies at z = 0.08 - 0.33

April 2009


43 Reads

We present the first detailed analysis of the mass and dynamical structure of a sample of six early-type lens galaxies, selected from the Sloan Lens ACS Survey, in the redshift range 0.08 < z < 0.33. Both Hubble Space Telescope (HST)/ACS high-resolution imaging and VLT VIMOS integral field spectroscopy are available for these systems. The galaxies are modelled - under the assumptions of axial symmetry and two-integral stellar distribution function - by making use of the CAULDRON code, which self-consistently combines gravitational lensing and stellar dynamics, and is fully embedded within the framework of Bayesian statistics. The principal results of this study are: (i) all galaxies in the sample are well described by a simple axisymmetric power-law profile for the total density, with a logarithmic slope gamma very close to isothermal ( = 1.98 +/- 0.05 and an intrinsic spread close to 5 per cent) showing no evidence of evolution over the probed range of redshift; (ii) the axial ratio of the total density distribution is rounder than 0.65 and in all cases, except for a fast rotator, does not deviate significantly from the flattening of the intrinsic stellar distribution; (iii) the dark matter fraction within the effective radius has a lower limit of about 15 to 30 per cent; (iv) the sample galaxies are only mildly anisotropic, with delta <= 0.16; (v) the physical distinction among slow and fast rotators, quantified by the v/sigma ratio and the intrinsic angular momentum, is already present at z > 0.1. Altogether, early-type galaxies at z = 0.08 - 0.33 are found to be markedly smooth and almost isothermal systems, structurally and dynamically very similar to their nearby counterparts. (Abridged)

Figure 7. The spectral region containing Hα and the [N II] doublet. All three lines are consistent with the being unresolved at the instrumental FWHM. The relative strength of the doublet lines is consistent with that predicted from the electron transition probabilities. 
Figure 9. A comparison of the Balmer series emission lines in the GRB host galaxy, scaled by the appropriate line ratios for Case B H I recombination (T=10 4 K, low density, Osterbrock & Ferland 2006), such that in the absence of dust, all peaks would be expected to match Hβ in intensity. The line intensities are offset from zero for clarity. Even accounting for blending with the [N II] doublet, Hα still shows a relative excess, consistent with the blue-wards lines being attenuated by a dusty line of sight. 
GRB 080517: A local, low luminosity GRB in a dusty galaxy at z=0.09

September 2014


134 Reads

We present an analysis of the photometry and spectroscopy of the host galaxy of Swift-detected GRB 080517. From our optical spectroscopy, we identify a redshift of z = 0.089 ± 0.003, based on strong emission lines, making this a rare example of a very local, low-luminosity, long gamma-ray burst. The galaxy is detected in the radio with a flux density of S4.5 GHz = 0.22 ± 0.04 mJy – one of relatively few known gamma-ray bursts hosts with a securely measured radio flux. Both optical emission lines and a strong detection at 22 μm suggest that the host galaxy is forming stars rapidly, with an inferred star formation rate ∼16 M⊙ yr−1 and a high dust obscuration (E(B − V) > 1, based on sightlines to the nebular emission regions). The presence of a companion galaxy within a projected distance of 25 kpc, and almost identical in redshift, suggests that star formation may have been triggered by galaxy–galaxy interaction. However, fitting of the remarkably flat spectral energy distribution from the ultraviolet through to the infrared suggests that an older, 500 Myr post-starburst stellar population is present along with the ongoing star formation. We conclude that the host galaxy of GRB 080517 is a valuable addition to the still very small sample of well-studied local gamma-ray burst hosts.

Figure 1. Results of single component curve of growth analysis. The best fitted curve for the absorption from J = 2 and J = 3 levels are shown overlayed with data points. The ordinate errors are based on the measured equivalent width errors and the abscissa errors are due to column density errors from our COG analysis.
Figure 4. R-band image covering 1’ × 1’ centered around the QSO. The QSO and 5 galaxies close to the QSO in the angular scale are marked. Only galaxy G1 has the photometric redshift consistent with the z abs of the DLA. 
Molecular hydrogen from z = 0.0963 DLA towards the QSO J1619+3342

June 2014


175 Reads

We report the detection of H2 in a zabs = 0.0963 Damped Lyman α (DLA) system towards zem = 0.4716 QSO J1619+3342. This DLA has log N(H i) = 20.55 ± 0.10, 18.13 ≤ log N(H2) ≤ 18.40, [S/H] = −0.62 ± 0.13, [Fe/S] = −1.00 ± 0.17 and the molecular fraction − 2.11 ≤ log[f(H2)] ≤ −1.85. The inferred gas kinetic temperature using the rotational level population is in the range 95-132 K. We do not detect C i or C ii* absorption from this system. Using R- and V-band deep images, we identify a sub-L* galaxy at an impact parameter of 14 kpc from the line of sight, having consistent photometric redshift, as a possible host for the absorber. We use the photoionization code cloudy to get the physical conditions in the H2 component using the observational constrains from H2, C i, C ii* and Mg i. All the observations can be consistently explained if one or more of the following is true: (i) carbon is underabundant by more than 0.6 dex as seen in halo stars with Z ∼ 0.1 Z⊙, (ii) H i associated with H2 component is less than 50 per cent of the H i measured along the line of sight and (iii) the H2 formation rate on the dust grains is at least a factor of 2 higher than what is typically used in analytic calculations for Milky Way interstellar medium. Even when these are satisfied, the gas kinetic temperature in the models is much lower than what is inferred from the ortho-to-para ratio of the molecular hydrogen. Alternatively, the high kinetic temperature could be a consequence of contribution to the gas heating from non-radiative heating processes seen in hydrodynamical simulations.

Galaxy And Mass Assembly (GAMA): mass-size relations of z$<$0.1 galaxies subdivided by S\'ersic index, colour and morphology

November 2014


93 Reads

We use data from the Galaxy And Mass Assembly (GAMA) survey in the redshift range 0.01 < z < 0.1 (8399 galaxies in g to Ks bands) to derive the stellar mass–half-light radius relations for various divisions of ‘early’- and ‘late’-type samples. We find that the choice of division between early and late (i.e. colour, shape, morphology) is not particularly critical; however, the adopted mass limits and sample selections (i.e. the careful rejection of outliers and use of robust fitting methods) are important. In particular, we note that for samples extending to low stellar mass limits (${<}10^{10}\mathcal {M_{\odot }}$) the Sérsic index bimodality, evident for high-mass systems, becomes less distinct and no-longer acts as a reliable separator of early- and late-type systems. The final set of stellar mass–half-light radius relations are reported for a variety of galaxy population subsets in 10 bands (ugrizZY JHKs) and are intended to provide a comprehensive low-z benchmark for the many ongoing high-z studies. Exploring the variation of the stellar mass–half-light radius relations with wavelength, we confirm earlier findings that galaxies appear more compact at longer wavelengths albeit at a smaller level than previously noted: at $10^{10}\mathcal {M_{\odot }}$ both spiral systems and ellipticals show a decrease in size of 13 per cent from g to Ks (which is near linear in log wavelength). Finally, we note that the sizes used in this work are derived from 2D Sérsic light profile fitting (using galfit3), i.e. elliptical semimajor half-light radii, improving on earlier low-z benchmarks based on circular apertures.

Figure 3. The pointing centre and the primary beam size of the WSRT are overlaid on a SDSS image of CNOC2 0920+37. The solid line and the broken line indicate the HPBW of the primary beam ( ∼ 36 ′ ) and 10 per cent level of the primary beam ( ∼ 1.0 ◦ in diameter), respectively. The blue and red circles indicate 
Figure 4. The radio continuum image of the CNOC2 0920+37 field at z ∼ 0.1 without applying primary beam correction. This image shows ∼ 1.4 deg 2 of 
Neutral atomic hydrogen (HI) gas evolution in field galaxies at z ~ 0.1 and 0.2

August 2013


139 Reads

We measure the neutral atomic hydrogen (H i) gas content of field galaxies at intermediate redshifts of z ∼ 0.1 and ∼0.2 using hydrogen 21-cm emission lines observed with the Westerbork Synthesis Radio Telescope. In order to make high signal-to-noise ratio detections, an H i signal stacking technique is applied: H i emission spectra from multiple galaxies, optically selected by the second Canadian Network for Observational Cosmology redshift survey project, are co-added to measure the average H i mass of galaxies in the two redshift bins. We calculate the cosmic H i gas densities (ΩHi) at the two redshift regimes and compare those with measurements at other redshifts to investigate the global evolution of the H i gas density over cosmic time. From a total of 59 galaxies at z ∼ 0.1 we find ΩHi = (0.33 ± 0.05) × 10−3, and at z ∼ 0.2 we find ΩHi = (0.34 ± 0.09) × 10−3, based on 96 galaxies. These measurements help bridge the gap between high-z damped Lyman α observations and blind 21-cm surveys at z = 0. We find that our measurements of ΩHi at z ∼ 0.1 and ∼0.2 are consistent with the H i gas density at z ∼ 0 and that all measurements of ΩHi from 21-cm emission observations at z ≲ 0.2 are in agreement with no evolution of the H i gas content in galaxies during the last 2.4 Gyr.

Figure 1: Suzaku XIS1 image of CTB 37A in the 0.3–10 keV energy band. The spectral integration regions for the source (annular apertures) and the background are indicated by the solid blue and dashed black circles, respectively. The solid red circle represents the X-ray source (CXOU J171419.8–383023). The corner of the CCD chip illuminated by the 55Fe calibration source is excluded from the image as shown by the dashed red circle. The coordinates (RA and Dec.) refer to epoch J2000.
Figure 2: CTB 37A is displayed in the 0.3–10 keV energy band, with X-ray data obtained from the Suzaku XIS0 detector. The image is overlaid by 843-MHz radio isocontours logarithmically scaled (0.23, 0.36, 0.57, 0.90, 1.42, 2.23 and 3.51 mJy beam−1). Radio emission clearly shows two radio SNRs, CTB 37A and G348.5–0.0. The X-ray data are smoothed with a three-pixel Gaussian in order to highlight the structure. The colour coding shows the brightness levels from 1.3 counts pix−1 to 13.36 counts pix−1 in logarithmic scale from yellow to black. The masers from the field are indicated with blue and white crosses at velocities ∼−65 and ∼−22 km s−1, respectively. The diamond shows the extended non-thermal X-ray source CXOU J171419.8–383023. The location of HESS J1714–385 is represented by the circle, in which the positional error is specified with the radius of 1.2 arcmin. The coordinates (RA and Dec.) refer to epoch J2000.
Figure 3: Background-subtracted XIS spectra of CTB 37A in the 0.3–10 keV energy band fitted with an absorbed VMEKAL and PL model. XIS1, XIS0 and XIS3 are shown in red, black and green, respectively. The bottom panel shows the residuals from the best-fitting model.
Figure 4: The radial variation of the electron temperature of CTB 37A.
{\it Suzaku} observation of Galactic supernova remnant CTB 37A (G348.5+0.1)

July 2011


119 Reads

We present here the results of the observation of CTB 37A obtained with the X-ray Imaging Spectrometer onboard the {\it Suzaku} satellite. The X-ray spectrum of CTB 37A is well fitted by two components, a single-temperature ionization equilibrium component (VMEKAL) with solar abundances, an electron temperature of $kT_{\rm e}\sim0.6$ keV, absorbing column density of $N_{\rm H}\sim3\times10^{22}$ ${\rm cm^{-2}}$ and a power-law component with photon index of $\Gamma$ $\sim 1.6$. The X-ray spectrum of CTB 37A is characterized by clearly detected K-shell emission lines of Mg, Si, S, and Ar. The plasma with solar abundances supports the idea that the X-ray emission originates from the shocked interstellar material. The ambient gas density, and age of the remnant are estimated to be $\sim1f^{-1/2}$${\rm cm^{-3}}$ and $\sim3\times10^{4}f^{1/2}$ yr, respectively. The center-filling X-ray emission surrounded by a shell-like radio structure and other X-ray properties indicate that this remnant would be a new member of mixed-morphology supernova remnant class.

A deep X-ray observation of supernova remnant G304.6+0.1 (Kes 17) with Suzaku

March 2012


66 Reads

In this paper, we present the analysis of a deep (99.6 ks) observation of G304.6+0.1 with the X-ray Imaging Spectrometer on board the Suzaku satellite. The X-ray spectral data are well fitted with a plasma model consisting of a thermal component, in collisional ionization equilibrium, and a non-thermal component. The thermal emission is well fitted with the vmekal model with an electron temperature of kTe∼ 0.75 keV, a high absorbing column density of NH∼ 3.9 × 1022 cm−2 and near/lower solar abundances, which indicate that the X-ray emitting plasma of G304.6+0.1 is dominated by a swept-up ambient medium. The non-thermal component is well fitted with a power-law model with a photon index of Γ∼ 1.4. We have found a relatively high electron density ne∼ 2.3f−1/2 cm−3, an age t∼ 1.4 × 104f1/2 yr and an X-ray emitting mass Mx∼ 380f1/2 M⊙ at an adopted distance of d= 10 kpc. Using the morphological and spectral X-ray data, we confirm that the remnant is a new member of the mixed-morphology supernova remnants.

Figure 1. The three BPT diagrams showing the selection criteria defined by Kewley et al. (2006) used to classify the emission-line galaxies as: Seyfert (black dots) and LINER (grey crosses) AGN. The solid lines separate the star-forming galaxies from the AGN and the dashed lines represent the Seyfert-LINER demarcation. 
Figure 3. Normalized distributions of matched system properties for: Seyfert-HR (black solid lines), Seyfert-LR (grey solid lines) and their respective control samples, Control-HR galaxies (black dashed lines) and Control-LR galaxies (grey dotted lines). 
Figure 6. Top: Total stellar mass (per bin) of the blue neighbouring galaxies around the target samples as a function of galaxy density within 1h −1 Mpc. Bottom: Total stellar mass (per bin) of the red neighbouring galaxies.
Figure 7. Local number density of galaxies, within rp = 1h −1 Mpc, as a function of stellar mass, M * , of the targets.
On the relation between Seyfert 2 accretion rate and environment at z < 0.1

October 2013


321 Reads

We analyse different properties of the small scale environment of Seyfert 2 for two samples selected according to the accretion rate parameter, R, from the DR7-SDSS survey. We compare the results with two control samples of non-active galaxies that cover the same redshift range, luminosity, colours, morphology, age and stellar mass content. Our study shows that both high and low accretion rate subsamples reside in bluer and lower density environments than the control samples. However, we find that this difference is at least two times stronger for the low accretion rate Seyferts. In the vicinity of Seyfert 2, red galaxies have systematically lower values of stellar-mass as compared with corresponding control samples. The lower values of stellar mass for red neighbours is more significant at higher density environments and it is more evident for low accretion rate Seyfert. We also find that this effect is independent of the host's stellar mass. Our results are consistent with a scenario where AGN occurrence is higher in lower/medium density environments with a higher merger rate and a lack of a dense intergalactic medium (that can strip gas from these systems) that provide suitable conditions for the central black hole feeding. We find this particularly evident for the low accretion rate Seyferts that could compensate through the intergalactic medium the lack of gas of their hosts.

An estimate of the electron density in filaments of galaxies at z~0.1

April 2011


29 Reads

Most of the baryons in the Universe are thought to be contained within filaments of galaxies, but as yet, no single study has published the observed properties of a large sample of known filaments to determine typical physical characteristics such as temperature and electron density. This paper presents a comprehensive large-scale search conducted for X-ray emission from a population of 41 bona fide filaments of galaxies to determine their X-ray flux and electron density. The sample is generated from the filament catalogue of Pimbblet et al., which is in turn sourced from the two-degree Field Galaxy Redshift Survey (2dFGRS). Since the filaments are expected to be very faint and of very low density, we used stacked ROSAT All-Sky Survey data. We detect a net surface brightness from our sample of filaments of (1.6 ± 0.1) × 10−14 erg cm−2 s−1 arcmin−2 in the 0.9–1.3 keV energy band for 1-keV plasma, which implies an electron density of ne= (4.7 ± 0.2) × 10−4 h1/2100 cm−3. Finally, we examine if a filament’s membership to a supercluster leads to an enhanced electron density as reported by Kull & Böhringer. We suggest it remains unclear if supercluster membership causes such an enhancement.

Discovery of 90 Type Ia supernovae among 700 000 Sloan spectra: The Type Ia supernova rate versus galaxy mass and star formation rate at redshift ~0.1

August 2012


213 Reads

Using a method to discover and classify supernovae (SNe) in galaxy spectra, we find 90 Type Ia SNe (SNe Ia) and 10 Type II SNe among the ∼700 000 galaxy spectra in the Sloan Digital Sky Survey Data Release 7 that have star-formation histories (SFHs) derived with the VErsatile SPectral Analysis code (vespa). We use the SN Ia sample to measure SN Ia rates per unit stellar mass. We confirm, at the median redshift of the sample, z = 0.1, the inverse dependence on galaxy mass of the SN Ia rate per unit mass, previously reported by Li et al. for a local sample. We further confirm, following Kistler et al., that this relation can be explained by the combination of galaxy ‘downsizing’ and a power-law delay-time distribution (DTD; the distribution of times that elapse between a hypothetical burst of star formation and the subsequent SN Ia explosions) with an index of −1, inherent to the double-degenerate progenitor scenario. We use the method of Maoz et al. to recover the DTD by comparing the number of SNe Ia hosted by each galaxy in our sample with the vespa-derived SFH of the stellar population within the spectral aperture. In this galaxy sample, which is dominated by old and massive galaxies, we recover a ‘delayed’ component to the DTD of 4.5 ± 0.6 (statistical) ${_{-0.5}\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!^{\rm +0.3}}$ (systematic) × 10− 14 SNe M⊙− 1 yr− 1 for delays in the range >2.4 Gyr. The mass-normalized SN Ia rate, averaged over all masses and redshifts in our galaxy sample, is $R_{{\rm Ia,M}}(z=0.1) = 0.10 \pm 0.01\ (\rm statistical) \pm 0.01\ (\rm systematic)\ {\rm SNuM}$, and the volumetric rate is RIa, V(z = 0.1) = 0.247 ${_{-0.026}\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!^{\rm +0.029}}$ (statistical) ${_{-0.031}\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!^{\rm +0.016}}$ (systematic) × 10− 4 SNe yr− 1 Mpc− 3. This rate is consistent with the rates and rate evolution from other recent SN Ia surveys, which together also indicate a ∼t−1 DTD.

Figure 2. Rest-frame g − r colour-stellar mass diagrams for eagle galaxies using photometry and dust reddening from model GD+O at z = 0 . 1 for simulation Ref-100 ( left panel ), and a composite set of eagle galaxies obtained by combining the higher-resolution model Recal-25 for galaxies with M 10 9 M and Ref-100 for M > 10 9 M ( right panel ), cross-fading the sampling probability of the two galaxy populations linearly in log 10 ( M / M ) (see text for details). Colours represent the point density of eagle galaxies of given M and g − r colour (see text for details), with black points representing individual outlying eagle galaxies. Contours represent the colour- M distribution for a volume-limited set of gama galaxies from Taylor et al. (2014), with grey points representing individual outlying galaxies. The masses of observed galaxies are obtained through SED fitting, see Taylor et al. (2014). The colours are mapped to their equivalent contours in the colour bar. The colour bar covers 2 dex in point density with a contour spacing of 0.28 dex. A lack of numerical resolution makes lower-mass eagle galaxies too red in the left panel. The transition of colours of observed galaxies, from the red-sequence at stellar masses above M ∼ 10 10 . 5 M to the blue cloud at lower stellar masses, is reproduced in the simulation, although the blue cloud extends to slightly higher M . 
Figure 4. Rest-frame g − r colour as a function of r -band absolute magnitude for eagle galaxies (colours) compared to a volume- limited sample of gama galaxies (Taylor et al. 2014, contour lines). contours and point shading is the same as in Fig 2. eagle photometry is obtained using the GD+O model. A composite eagle galaxy population is used, consisting of galaxies from Ref-100 at M > 10 9 M and Recal-25 at M < 10 9 M as in 
Figure C1. g − r colour distributions for eagle galaxies for the dust-free model. The 4 panels show colour distributions for 4 bins of stellar mass, as indicated by the legend. The solid, dotted and dashed lines show the eagle SSP values and the eagle SSP values with Gallazzi et al. (2005) metallicities and ages respectively. Gallazzi metallicities and ages are assigned to each galaxy, based on the median parameter values at the galactic stellar mass. Z values are simply taken as the observed median value. The LWA values are sampled from a normal distribution with standard deviation taken from Gallazzi et. al. (2005), assuming that the scatter in age and metallicity is uncorrelated. We see that the complex star formation histories of eagle provide a better match to the observed colour distributions than a single SSP model using empirical values for age and metallicity.
Colours and luminosities of z = 0.1 galaxies in the EAGLE simulation

April 2015


490 Reads

We calculate the colours and luminosities of redshift z = 0.1 galaxies from the EAGLE simulation suite using the GALAXEV population synthesis models. We take into account obscuration by dust in birth clouds and diffuse ISM using a two-component screen model, following the prescription of Charlot and Fall. We compare models in which the dust optical depth is constant to models where it depends on gas metallicity, gas fraction and orientation. The colours of EAGLE galaxies for the more sophisticated models are in broad agreement with those of observed galaxies. In particular, EAGLE produces a red sequence of passive galaxies and a blue cloud of star forming galaxies, with approximately the correct fraction of galaxies in each population and with g-r colours within 0.1 magnitudes of those observed. Luminosity functions from UV to NIR wavelengths differ from observations at a level comparable to systematic shifts resulting from a choice between Petrosian and Kron photometric apertures. Despite the generally good agreement there are clear discrepancies with observations. The blue cloud of EAGLE galaxies extends to somewhat higher luminosities than in the data, consistent with the modest underestimate of the passive fraction in massive EAGLE galaxies. There is also a moderate excess of bright blue galaxies compared to observations. The overall level of agreement with the observed colour distribution suggests that EAGLE galaxies at z = 0.1 have ages, metallicities and levels of obscuration that are comparable to those of observed galaxies.

Top-cited authors