arXiv subjects

A. Weiss

Publications and source records attributed to A. Weiss.

At least 73 records · Page 4Linked to original sources

arXiv2017

Physical conditions of the molecular gas in metal-poor galaxies

Studying the molecular component of the interstellar medium in metal-poor galaxies has been challenging because of the faintness of carbon monoxide emission, the most common proxy of H2. Here we present new detections of molecular gas at low metallicities, and assess the physical conditions in the gas through various CO transitions for 8 galaxies. For one, NGC 1140 (Z/Zsun ~ 0.3), two detections of 13CO isotopologues and atomic carbon, [CI](1-0), and an upper limit for HCN(1-0) are also reported. After correcting to a common beam size, we compared 12CO(2-1)/12CO(1-0) (R21) and 12CO(3-2)/12CO(1-0) (R31) line ratios of our sample with galaxies from the literature and find that only NGC 1140 shows extreme values (R21 ~ R31 ~ 2). Fitting physical models to the 12CO and 13CO emission in NGC 1140 suggests that the molecular gas is cool (kinetic temperature Tkin<=20 K), dense (H2 volume density nH2 >= $10^6$ cm$^{-3}$), with moderate CO column density (NCO ~ $10^{16}$ cm$^{-2}$) and low filling factor. Surprisingly, the [12CO]/[13CO] abundance ratio in NGC 1140 is very low (~ 8-20), lower even than the value of 24 found in the Galactic Center. The young age of the starburst in NGC 1140 precludes 13C enrichment from evolved intermediate-mass stars; instead we attribute the low ratio to charge-exchange reactions and fractionation, because of the enhanced efficiency of these processes in cool gas at moderate column densities. Fitting physical models to 12CO and [CI](1-0) emission in NGC 1140 gives an unusually low [12CO]/[12C] abundance ratio, suggesting that in this galaxy atomic carbon is at least 10 times more abundant than 12CO.

astro-ph.GA↗

arXiv2017

The most distant, luminous, dusty star-forming galaxies: redshifts from NOEMA and ALMA spectral scans

We present 1.3- and/or 3-mm continuum images and 3-mm spectral scans, obtained using NOEMA and ALMA, of 21 distant, dusty, star-forming galaxies (DSFGs). Our sample is a subset of the galaxies selected by Ivison et al. (2016) on the basis of their extremely red far-infrared (far-IR) colours and low {\it Herschel} flux densities; most are thus expected to be unlensed, extraordinarily luminous starbursts at $z \gtrsim 4$, modulo the considerable cross-section to gravitational lensing implied by their redshift. We observed 17 of these galaxies with NOEMA and four with ALMA, scanning through the 3-mm atmospheric window. We have obtained secure redshifts for seven galaxies via detection of multiple CO lines, one of them a lensed system at $z=6.027$ (two others are also found to be lensed); a single emission line was detected in another four galaxies, one of which has been shown elsewhere to lie at $z=4.002$. Where we find no spectroscopic redshifts, the galaxies are generally less luminous by 0.3-0.4 dex, which goes some way to explaining our failure to detect line emission. We show that this sample contains amongst the most luminous known star-forming galaxies. Due to their extreme star-formation activity, these galaxies will consume their molecular gas in $\lesssim 100$ Myr, despite their high molecular gas masses, and are therefore plausible progenitors of the massive, `red-and-dead' elliptical galaxies at $z \approx 3$.

astro-ph.GA↗

arXiv2017

HIFI Spectroscopy of ${\rm H_2O}$ submm Lines in Nuclei of Actively Star Forming Galaxies

We present a systematic survey of multiple velocity-resolved H$_2$O spectra using Herschel/HIFI towards nine nearby actively star forming galaxies. The ground-state and low-excitation lines (E$_{\rm up}\,\le 130\,{\rm K}$) show profiles with emission and absorption blended together, while absorption-free medium-excitation lines ($130\,{\rm K}\, \le\, E_{\rm up}\,\le\,350\,{\rm K}$) typically display line shapes similar to CO. We analyze the HIFI observation together with archival SPIRE/PACS H$_2$O data using a state-of-the-art 3D radiative transfer code which includes the interaction between continuum and line emission. The water excitation models are combined with information on the dust- and CO spectral line energy distribution to determine the physical structure of the interstellar medium (ISM). We identify two ISM components that are common to all galaxies: A warm ($T_{\rm dust}\,\sim\,40-70\,{\rm K}$), dense ($n({\rm H})\,\sim\,10^5-10^6\,{\rm cm^{-3}}$) phase which dominates the emission of medium-excitation H$_2$O lines. This gas phase also dominates the FIR emission and the CO intensities for $J_{\rm up} > 8$. In addition a cold ($T_{\rm dust}\,\sim\,20-30\,{\rm K}$), dense ($n({\rm H})\sim\,10^4- 10^5\,{\rm cm^{-3}}$) more extended phase is present. It outputs the emission in the low-excitation H$_2$O lines and typically also produces the prominent line absorption features. For the two ULIRGs in our sample (Arp 220 and Mrk 231) an even hotter and more compact (R$_s\,\le\,100$ pc) region is present which is possibly linked to AGN activity. We find that collisions dominate the water excitation in the cold gas and for lines with $E_{\rm up}\le300\,{\rm K}$ and $E_{\rm up}\le800\,{\rm K}$ in the warm and hot component, respectively. Higher energy levels are mainly excited by IR pumping.

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arXiv2016

ALMA observations of atomic carbon in z~4 dusty star-forming galaxies

We present ALMA [CI]($1-0$) (rest frequency 492 GHz) observations for a sample of 13 strongly-lensed dusty star-forming galaxies originally discovered at 1.4mm in a blank-field survey by the South Pole Telescope. We compare these new data with available [CI] observations from the literature, allowing a study of the ISM properties of $\sim 30$ extreme dusty star-forming galaxies spanning a redshift range $2 < z < 5$. Using the [CI] line as a tracer of the molecular ISM, we find a mean molecular gas mass for SPT-DSFGs of $6.6 \times 10^{10}$ M$_{\odot}$. This is in tension with gas masses derived via low-$J$ $^{12}$CO and dust masses; bringing the estimates into accordance requires either (a) an elevated CO-to-H$_2$ conversion factor for our sample of $α_{\rm CO} \sim 2.5$ and a gas-to-dust ratio $\sim200$, or (b) an high carbon abundance $X_{\rm CI} \sim 7\times10^{-5}$. Using observations of a range of additional atomic and molecular lines (including [CI], [CII], and multiple transitions of CO), we use a modern Photodissociation Region code (3D-PDR) to assess the physical conditions (including the density, UV radiation field strength, and gas temperature) within the ISM of the DSFGs in our sample. We find that the ISM within our DSFGs is characterised by dense gas permeated by strong UV fields. We note that previous efforts to characterise PDR regions in DSFGs may have significantly underestimated the density of the ISM. Combined, our analysis suggests that the ISM of extreme dusty starbursts at high redshift consists of dense, carbon-rich gas not directly comparable to the ISM of starbursts in the local Universe.

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arXiv2016

The space density of luminous dusty star-forming galaxies at $z>4$: SCUBA-2 and LABOCA imaging of ultrared galaxies from $Herschel$-ATLAS

Until recently, only a handful of dusty, star-forming galaxies (DSFGs) were known at $z>4$, most of them significantly amplified by gravitational lensing. Here, we have increased the number of such DSFGs substantially, selecting galaxies from the uniquely wide 250-, 350- and 500-$μ$m Herschel-ATLAS imaging survey on the basis of their extremely red far-infrared colors and faint 350- and 500-$μ$m flux densities - ergo they are expected to be largely unlensed, luminous, rare and very distant. The addition of ground-based continuum photometry at longer wavelengths from the JCMT and APEX allows us to identify the dust peak in their SEDs, better constraining their redshifts. We select the SED templates best able to determine photometric redshifts using a sample of 69 high-redshift, lensed DSFGs, then perform checks to assess the impact of the CMB on our technique, and to quantify the systematic uncertainty associated with our photometric redshifts, $σ=0.14\,(1+z)$, using a sample of 25 galaxies with spectroscopic redshifts, each consistent with our color selection. For Herschel-selected ultrared galaxies with typical colors of $S_{500}/S_{250}\sim 2.2$ and $S_{500}/S_{350}\sim 1.3$ and flux densities, $S_{500}\sim 50\,$mJy, we determine a median redshift, $\hat{z}_{\rm phot}=3.66$, an interquartile redshift range, 3.30$-$4.27, with a median rest-frame 8$-$1000-$μ$m luminosity, $\hat{L}_{\rm IR}$, of $1.3\times 10^{13}\,$L$_\odot$. A third lie at $z>4$, suggesting a space density, $ρ_{z>4}$, of $\approx 6 \times 10^{-7}\,$Mpc$^{-3}$. Our sample contains the most luminous known star-forming galaxies, and the most over-dense cluster of starbursting proto-ellipticals yet found.

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arXiv2016

Kiloparsec-scale dust disks in high-redshift luminous submillimeter galaxies

We present high-resolution (0.16$"$) 870um Atacama Large Millimeter/submillimeter Array (ALMA) imaging of 16 luminous (L_IR ~ 4 x 10^12 L_sun) submillimeter galaxies (SMGs) from the ALESS survey of the Extended Chandra Deep Field South. This dust imaging traces the dust-obscured star formation in these z~2.5 galaxies on ~1.3 kpc scales. The emission has a median effective radius of $R_e=0.24" \pm 0.02"$, corresponding to a typical physical size of $R_{e}=1.8\pm$0.2 kpc. We derive a median Sérsic index of $n=0.9\pm0.2$, implying that the dust emission is remarkably disk-like at the current resolution and sensitivity. We use different weighting schemes with the visibilities to search for clumps on 0.12$"$ (~1.0 kpc) scales, but we find no significant evidence for clumping in the majority of cases. Indeed, we demonstrate using simulations that the observed morphologies are generally consistent with smooth exponential disks, suggesting that caution should be exercised when identifying candidate clumps in even moderate S/N interferometric data. We compare our maps to comparable-resolution HST H$_{160}$-band images, finding that the stellar morphologies appear significantly more extended and disturbed, and suggesting that major mergers may be responsible for driving the formation of the compact dust disks we observe. The stark contrast between the obscured and unobscured morphologies may also have implications for SED fitting routines that assume the dust is co-located with the optical/near-IR continuum emission. Finally, we discuss the potential of the current bursts of star formation to transform the observed galaxy sizes and light profiles, showing that the $z\sim0$ descendants of these SMGs are expected to have stellar masses, effective radii, and gas surface densities consistent with the most compact massive (M_* ~ 1-2 x 10^11 M_sun) early-type galaxies observed locally.

astro-ph.GA↗

arXiv2016

ALMA spectroscopic survey in the Hubble Ultra Deep Field: Continuum number counts, resolved 1.2-mm extragalactic background, and properties of the faintest dusty star forming galaxies

We present an analysis of a deep (1$σ$=13 $μ$Jy) cosmological 1.2-mm continuum map based on ASPECS, the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field. In the 1 arcmin$^2$ covered by ASPECS we detect nine sources at $>3.5σ$ significance at 1.2-mm. Our ALMA--selected sample has a median redshift of $z=1.6\pm0.4$, with only one galaxy detected at z$>$2 within the survey area. This value is significantly lower than that found in millimeter samples selected at a higher flux density cut-off and similar frequencies. Most galaxies have specific star formation rates similar to that of main sequence galaxies at the same epoch, and we find median values of stellar mass and star formation rates of $4.0\times10^{10}\ M_\odot$ and $\sim40~M_\odot$ yr$^{-1}$, respectively. Using the dust emission as a tracer for the ISM mass, we derive depletion times that are typically longer than 300 Myr, and we find molecular gas fractions ranging from $\sim$0.1 to 1.0. As noted by previous studies, these values are lower than using CO--based ISM estimates by a factor $\sim$2. The 1\,mm number counts (corrected for fidelity and completeness) are in agreement with previous studies that were typically restricted to brighter sources. With our individual detections only, we recover $55\pm4\%$ of the extragalactic background light (EBL) at 1.2 mm measured by the Planck satellite, and we recover $80\pm7\%$ of this EBL if we include the bright end of the number counts and additional detections from stacking. The stacked contribution is dominated by galaxies at $z\sim1-2$, with stellar masses of (1-3)$\times$10$^{10}$ M$_\odot$. For the first time, we are able to characterize the population of galaxies that dominate the EBL at 1.2 mm.

astro-ph.GA↗

arXiv2016

ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Search for [CII] line and dust emission in 6<z<8 galaxies

We present a search for [CII] line and dust continuum emission from optical dropout galaxies at $z>6$ using ASPECS, our ALMA Spectroscopic Survey in the Hubble Ultra-Deep Field (UDF). Our observations, which cover the frequency range $212-272$ GHz, encompass approximately the range $6 $4.5 $σ$, two of which correspond to blind detections with no optical counterparts. At this significance level, our statistical analysis shows that about 60\% of our candidates are expected to be spurious. For one of our blindly selected [CII] line candidates, we tentatively detect the CO(6-5) line in our parallel 3-mm line scan. None of the line candidates are individually detected in the 1.2 mm continuum. A stack of all [CII] candidates results in a tentative detection with $S_{1.2mm}=14\pm5μ$Jy. This implies a dust-obscured star formation rate (SFR) of $(3\pm1)$ M$_\odot$ yr$^{-1}$. We find that the two highest--SFR objects have candidate [CII] lines with luminosities that are consistent with the low-redshift $L_{\rm [CII]}$ vs. SFR relation. The other candidates have significantly higher [CII] luminosities than expected from their UV--based SFR. At the current sensitivity it is unclear whether the majority of these sources are intrinsically bright [CII] emitters, or spurious sources. If only one of our line candidates was real (a scenario greatly favored by our statistical analysis), we find a source density for [CII] emitters at $6<z<8$ that is significantly higher than predicted by current models and some extrapolations from galaxies in the local universe.

astro-ph.GA↗

arXiv2016

ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: implications for spectral line intensity mapping at millimeter wavelengths and CMB spectral distortions

We present direct estimates of the mean sky brightness temperature in observing bands around 99GHz and 242GHz due to line emission from distant galaxies. These values are calculated from the summed line emission observed in a blind, deep survey for specrtal line emission from high redshift galaxies using ALMA (the 'ASPECS' survey). In the 99 GHz band, the mean brightness will be dominated by rotational transitions of CO from intermediate and high redshift galaxies. In the 242GHz band, the emission could be a combination of higher order CO lines, and possibly [CII] 158$μ$m line emission from very high redshift galaxies ($z \sim 6$ to 7). The mean line surface brightness is a quantity that is relevant to measurements of spectral distortions of the cosmic microwave background, and as a potential tool for studying large-scale structures in the early Universe using intensity mapping. While the cosmic volume and the number of detections are admittedly small, this pilot survey provides a direct measure of the mean line surface brightness, independent of conversion factors, excitation, or other galaxy formation model assumptions. The mean surface brightness in the 99GHZ band is: $T_B = 0.94\pm 0.09$ $μ$K. In the 242GHz band, the mean brightness is: $T_B = 0.55\pm 0.033$ $μ$K. These should be interpreted as lower limits on the average sky signal, since we only include lines detected individually in the blind survey, while in a low resolution intensity mapping experiment, there will also be the summed contribution from lower luminosity galaxies that cannot be detected individually in the current blind survey.

astro-ph.GA↗

arXiv2016

Microphysics and dynamics of the Gamma-Ray Burst 121024A

Aims. The aim of the study is to constrain the physics of gamma-ray bursts (GRBs) by analysing the multi-wavelength afterglow data set of GRB 121024A that covers the full range from radio to X-rays. Methods. Using multi-epoch broad-band observations of the GRB 121024A afterglow, we measured the three characteristic break frequencies of the synchrotron spectrum. We used six epochs of combined XRT and GROND data to constrain the temporal slopes, the dust extinction, the X-ray absorption, and the spectral slope with high accuracy. Two more epochs of combined data from XRT, GROND, APEX, CARMA, and EVLA were used to set constraints on the break frequencies and therefore on the micro-physical and dynamical parameters. Results. The XRT and GROND light curves show a simultaneous and achromatic break at around 49 ks. As a result, the crossing of the synchrotron cooling break is no suitable explanation for the break in the light curve. The multi wavelength data allow us to test two plausible scenarios explaining the break: a jet break, and the end of energy injection. The jet-break scenario requires a hard electron spectrum, a very low cooling break frequency, and a non-spreading jet. The energy injection avoids these problems, but requires $ε_e > 1 (k = 2)$, spherical outflow, and $ε_B < 10^{-9}$. Conclusions. In light of the extreme microphysical parameters required by the energy-injection model, we favour a jet-break scenario where $ν_m < ν_{sa}$ to explain the observations. This scenario gives physically meaningful microphysical parameters, and it also naturally explains the reported detection of linear and circular polarisation.

astro-ph.HE↗

arXiv2016

A survey of the cold molecular gas in gravitationally lensed star-forming galaxies at z>2

Using the Australia Telescope Compact Array (ATCA), we conducted a survey of CO J=1-0 and J=2-1 line emission towards strongly lensed high-redshift dusty star forming galaxies (DSFGs) previously discovered with the South Pole Telescope (SPT). Our sample comprises 17 sources that had CO-based spectroscopic redshifts obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX). We detect all sources with known redshifts in either CO J=1-0 or J=2-1. Twelve sources are detected in the 7-mm continuum. The derived CO luminosities imply gas masses in the range (0.5-11)x10^{10} M_sun and gas depletion timescales <200 Myr, using a CO to gas mass conversion factor alpha_CO=0.8 M_sun (K km/s pc^2)^{-1}. Combining the CO luminosities and dust masses, along with a fixed gas-to-dust ratio, we derive alpha_CO factors in the range 0.4-1.8, similar to what is found in other starbursting systems. We find small scatter in alpha_CO values within the sample, even though inherent variations in the spatial distribution of dust and gas in individual cases could bias the dust-based alpha_CO estimates. We find that lensing magnification factors based on the CO linewidth to luminosity relation (mu_CO) are highly unreliable, but particularly when mu<5. Finally, comparison of the gas and dynamical masses suggest that the average molecular gas fraction stays relatively constant at z=2-5 in the SPT DSFG sample.

astro-ph.GA↗

arXiv2015

Theory of stellar convection II: first stellar models

We present here the first stellar models on the Hertzsprung-Russell diagram (HRD), in which convection is treated according to the novel scale-free convection theory (SFC theory) by Pasetto et al. (2014). The aim is to compare the results of the new theory with those from the classical, calibrated mixing-length (ML) theory to examine differences and similarities. We integrate the equations describing the structure of the atmosphere from the stellar surface down to a few percent of the stellar mass using both ML theory and SFC theory. The key temperature over pressure gradients, the energy fluxes, and the extension of the convective zones are compared in both theories. The analysis is first made for the Sun and then extended to other stars of different mass and evolutionary stage. The results are adequate: the SFC theory yields convective zones, temperature gradients of the ambient and of the convective element, and energy fluxes that are very similar to those derived from the "calibrated" MT theory for main sequence stars. We conclude that the old scale dependent ML theory can now be replaced with a self-consistent scale-free theory able to predict correct results, one which is simpler and more physically grounded than the ML theory. Fundamentally, the SFC theory offers a deeper insight of the underlying physics than numerical simulations.

astro-ph.SR↗

arXiv2015

The ATLASGAL survey: distribution of cold dust in the Galactic plane. Combination with Planck data

Sensitive ground-based submillimeter surveys, such as ATLASGAL, provide a global view on the distribution of cold dense gas in the Galactic plane. Here we use the 353 GHz maps from the Planck/HFI instrument to complement the ground-based APEX/LABOCA observations with information on larger angular scales. The resulting maps reveal the distribution of cold dust in the inner Galaxy with a larger spatial dynamic range. We find examples of elongated structures extending over angular scales of 0.5 degree. Corresponding to >30 pc structures in projection at a distance of 3 kpc, these dust lanes are very extended and show large aspect ratios. Furthermore, we assess the fraction of dense gas ($f_{\rm DG}$), and estimate 2-5% (above A$_{\rm{v}}>$7 mag) on average in the Galactic plane. PDFs of the column density reveal the typically observed log-normal distribution for low- and exhibit an excess at high column densities. As a reference for extragalactic studies, we show the line-of-sight integrated N-PDF of the inner Galaxy, and derive a contribution of this excess to the total column density of $\sim2.2$%, above $N_{\rm H_2} = 2.92\times10^{22}$ cm$^{-2}$. Taking the total flux density, we provide an independent estimate of the mass of molecular gas in the inner Galaxy of $\sim1\times10^9\,M_{\odot}$, which is consistent with previous estimates using CO emission. From the mass and $f_{\rm DG}$ we estimate a Galactic SFR of $\dot M = 1.3\,M_{\odot}$ yr$^{-1}$. While the distribution of diffuse gas is homogenous in the inner Galaxy, the CMZ stands out with a higher dense gas fraction. The low star formation efficiency of the Milky Way is well explained by the low $f_{\rm DG}$ in the Galactic ISM, while the high $f_{\rm DG}$ towards the CMZ, despite its low star formation activity, suggests that, in that particular region of our Galaxy, high-density gas is not the bottleneck for star formation.

astro-ph.SR↗

arXiv2015

Probing star formation in the dense environments of z~1 lensing halos aligned with dusty star-forming galaxies detected with the South Pole Telescope

We probe star formation in the environments of massive $\sim10^{13}\,M_{\odot}$ dark matter halos at redshifts of $z$$\sim$$1$. This star formation is linked to a sub-millimetre clustering signal which we detect in maps of the Planck High Frequency Instrument that are stacked at the positions of a sample of high-redshift ($z$$>$$2$) strongly-lensed dusty star-forming galaxies (DSFGs) selected from the South Pole Telescope (SPT) 2500 deg$^2$ survey. The clustering signal has sub-millimetre colours which are consistent with the mean redshift of the foreground lensing halos ($z$$\sim$$1$). We report a mean excess of star formation rate (SFR) compared to the field, of $(2700\pm700)\,M_{\odot}\,{yr}^{-1}$ from all galaxies contributing to this clustering signal within a radius of 3.5' from the SPT DSFGs. The magnitude of the Planck excess is in broad agreement with predictions of a current model of the cosmic infrared background. The model predicts that 80$\%$ of the excess emission measured by Planck originates from galaxies lying in the neighbouring halos of the lensing halo. Using Herschel maps of the same fields, we find a clear excess, relative to the field, of individual sources which contribute to the Planck excess. The mean excess SFR compared to the field is measured to be ($370\pm40)$$\,M_{\odot}\,{yr}^{-1}$ per resolved, clustered source. Our findings suggest that the environments around these massive $z$$\sim$$1$ lensing halos host intense star formation out to about $2\,$Mpc. The flux enhancement due to clustering should also be considered when measuring flux densities of galaxies in Planck data.

astro-ph.GA↗

arXiv2015

Molecular depletion times and the CO-to-H2 conversion factor in metal-poor galaxies

Tracing molecular hydrogen content with carbon monoxide in low-metallicity galaxies has been exceedingly difficult. Here we present a new effort, with IRAM 30-m observations of 12CO(1-0) of a sample of 8 dwarf galaxies having oxygen abundances ranging from 12+logO/H=7.7 to 8.4. CO emission is detected in all galaxies, including the most metal-poor galaxy of our sample (0.1 Zsun); to our knowledge this is the largest number of 12CO(1-0) detections ever reported for galaxies with 12+logO/H<=8 (0.2 Zsun) outside the Local Group. We calculate stellar masses (Mstar) and star-formation rates (SFRs), and analyze our results by combining our observations with galaxy samples from the literature. Extending previous results for a correlation of the molecular gas depletion time, tau(dep), with Mstar and specific SFR (sSFR), we find a variation in tau(dep) of a factor of 200 or more (from <50 Myr to 10 Gyr) over a spread of 1000 in sSFR and Mstar. We exploit the variation of tau(dep) to constrain the CO-to-H2 mass conversion factor alpha(CO) at low metallicity, and assuming a power-law variation find alpha(CO) \propto (Z/Zsun)^1.9, similar to results based on dust continuum measurements compared with gas mass. By including HI measurements, we show that the fraction of total gas mass relative to the baryonic mass is higher in galaxies that are metal poor, of low mass, and of high sSFR. Finally, comparisons of the data with star-formation models of the molecular gas phases suggest that, at metallicities Z/Zsun<=0.2, there are some discrepancies with model predictions.

astro-ph.GA↗

arXiv2015

Sub-kiloparsec Imaging of Cool Molecular Gas in Two Strongly Lensed Dusty, Star-Forming Galaxies

We present spatially-resolved imaging obtained with the Australia Telescope Compact Array (ATCA) of three CO lines in two high-redshift gravitationally lensed dusty star-forming galaxies, discovered by the South Pole Telescope. Strong lensing allows us to probe the structure and dynamics of the molecular gas in these two objects, at z=2.78 and z=5.66, with effective source-plane resolution of less than 1kpc. We model the lensed emission from multiple CO transitions and the dust continuum in a consistent manner, finding that the cold molecular gas as traced by low-J CO always has a larger half-light radius than the 870um dust continuum emission. This size difference leads to up to 50% differences in the magnification factor for the cold gas compared to dust. In the z=2.78 galaxy, these CO observations confirm that the background source is undergoing a major merger, while the velocity field of the other source is more complex. We use the ATCA CO observations and comparable resolution Atacama Large Millimeter/submillimeter Array dust continuum imaging of the same objects to constrain the CO-H_2 conversion factor with three different procedures, finding good agreement between the methods and values consistent with those found for rapidly star-forming systems. We discuss these galaxies in the context of the star formation - gas mass surface density relation, noting that the change in emitting area with observed CO transition must be accounted for when comparing high-redshift galaxies to their lower redshift counterparts.

astro-ph.GA↗

arXiv2015

What Powers Lyman alpha Blobs?

Lyman alpha blobs (LABs) are spatially extended lyman alpha nebulae seen at high redshift. The origin of Lyman alpha emission in the LABs is still unclear and under debate. To study their heating mechanism(s), we present Australia Telescope Compact Array (ATCA) observations of the 20 cm radio emission and Herschel PACS and SPIRE measurements of the far-infrared (FIR) emission towards the four LABs in the protocluster J2143-4423 at z=2.38. Among the four LABs, B6 and B7 are detected in the radio with fluxes of 67+/-17 microJy and 77+/-16 microJy, respectively, and B5 is marginally detected at 3 sigma (51+/-16 microJy). For all detected sources, their radio positions are consistent with the central positions of the LABs. B6 and B7 are obviously also detected in the FIR. By fitting the data with different templates, we obtained redshifts of 2.20$^{+0.30}_{-0.35}$ for B6 and 2.20$^{+0.45}_{-0.30}$ for B7 which are consistent with the redshift of the lyman alpha emission within uncertainties, indicating that both FIR sources are likely associated with the LABs. The associated FIR emission in B6 and B7 and high star formation rates strongly favor star formation in galaxies as an important powering source for the lyman alpha emission in both LABs. However, the other two, B1 and B5, are predominantly driven by the active galactic nuclei or other sources of energy still to be specified, but not mainly by star formation. In general, the LABs are powered by quite diverse sources of energy.

astro-ph.GA↗

arXiv2015

High Resolution Imaging of PHIBSS z~2 Main Sequence Galaxies in CO J=1-0

We present Karl G. Jansky Very Large Array observations of the CO J=1-0 transition in a sample of four $z\sim2$ main sequence galaxies. These galaxies are in the blue sequence of star-forming galaxies at their redshift, and are part of the IRAM Plateau de Bure HIgh-$z$ Blue Sequence Survey (PHIBSS) which imaged them in CO J=3-2. Two galaxies are imaged here at high signal-to-noise, allowing determinations of their disk sizes, line profiles, molecular surface densities, and excitation. Using these and published measurements, we show that the CO and optical disks have similar sizes in main-sequence galaxies, and in the galaxy where we can compare CO J=1-0 and J=3-2 sizes we find these are also very similar. Assuming a Galactic CO-to-H$_2$ conversion, we measure surface densities of $Σ_{mol}\sim1200$ M$_\odot$pc$^{-2}$ in projection and estimate $Σ_{mol}\sim500-900$ M$_\odot$pc$^{-2}$ deprojected. Finally, our data yields velocity-integrated Rayleigh-Jeans brightness temperature line ratios $r_{31}$ that are approximately unity. In addition to the similar disk sizes, the very similar line profiles in J=1-0 and J=3-2 indicate that both transitions sample the same kinematics, implying that their emission is coextensive. We conclude that in these two main sequence galaxies there is no evidence for significant excitation gradients or a large molecular reservoir that is diffuse or cold and not involved in active star-formation. We suggest that $r_{31}$ in very actively star-forming galaxies is likely an indicator of how well mixed the star formation activity and the molecular reservoir are.

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