Greif, Thomas; White, Simon; Klessen, Ralf; Springel, Volker
http://adsabs.harvard.edu/abs/2011arXiv1101.5493G
The authors take into account the streaming velocities between gas and dark matter in their simulations, and present results on how this affects the virialization of minihaloes and hence the star formation rate of Pop III stars.
25 February 2011
24 February 2011
Dissecting the size evolution of elliptical galaxies since z~1: puffing up vs minor merging scenarios
17 February 2011
X-ray mass proxies from hydrodynamic simulations of galaxy clusters (paper I)
Authors: Fabjan, Borgani, Rasia, Bonafede, Dolag, Murante & Tornatore
Paper: http://arxiv.org/abs/1102.2903
Two sets of zoom hydrodynamical simulations of galaxy clusters is used in this work to test the robustness and evolution of scaling relations between the total cluster mass and 3 mass proxies: the gas mass, the temperature of the intra-cluster medium and the product of the two: YX = MgasT. The largest set of about 140 galaxy clusters offers the opportunity to statistically test the intrinsic scatter of the scaling relations between the different proxies. A second smaller set allows to quantify the robustness of those relations against the effects of changing the physical processes included into the simulations. The following mechanisms have thus been compared: (i) thermal conduction, (ii) artificial viscosity, (iii) cooling and star formation, (iv) galactic winds, and (v) AGN feedback.
As found in the previous study of Kravtsov et al. (2006), the authors confirm that the relation with the YX parameter is the least sensitive to the variation of the ICM physics and stay lose to the predictions of the self-similar model along its redshift evolution.
Paper: http://arxiv.org/abs/1102.2903
Two sets of zoom hydrodynamical simulations of galaxy clusters is used in this work to test the robustness and evolution of scaling relations between the total cluster mass and 3 mass proxies: the gas mass, the temperature of the intra-cluster medium and the product of the two: YX = MgasT. The largest set of about 140 galaxy clusters offers the opportunity to statistically test the intrinsic scatter of the scaling relations between the different proxies. A second smaller set allows to quantify the robustness of those relations against the effects of changing the physical processes included into the simulations. The following mechanisms have thus been compared: (i) thermal conduction, (ii) artificial viscosity, (iii) cooling and star formation, (iv) galactic winds, and (v) AGN feedback.
As found in the previous study of Kravtsov et al. (2006), the authors confirm that the relation with the YX parameter is the least sensitive to the variation of the ICM physics and stay lose to the predictions of the self-similar model along its redshift evolution.
Chromospheres in Metal-Poor Stars Evidenced from the He I 10830 Angstrom Line
Authors: Takeda & Takada-Hidai
Link to paper: http://arxiv.org/abs/1102.3210
Observations are presented which reveal the presence of hot chromospheres in extremely metal-poor stars. This raises a challenge to the conventional view that old low-mass stars should have little chromospheric activity as a result of magnetic braking (which slows the rotation of the star, and hence could shut off the dynamo process that produces the magnetic activity which, in turn, powers the chromosphere). Interestingly, this result also strongly suggests that old metal-free (Pop III) stars may also exhibit substantial chromospheric activity. If this is the case, then these stars should launch a weak solar-like wind, which we have found (Johnson & Khochfar 2011) would likely prevent the pollution of their surfaces by ISM material. This provides reason for optimism about the possibility of detecting, unequivocally, Pop III stars in the Galaxy today, if such low-mass primordial stars formed (as suggested by recent high-resolution cosmological simulations; Clark et al. 2011, Greif et al. 2011).
Link to paper: http://arxiv.org/abs/1102.3210
Observations are presented which reveal the presence of hot chromospheres in extremely metal-poor stars. This raises a challenge to the conventional view that old low-mass stars should have little chromospheric activity as a result of magnetic braking (which slows the rotation of the star, and hence could shut off the dynamo process that produces the magnetic activity which, in turn, powers the chromosphere). Interestingly, this result also strongly suggests that old metal-free (Pop III) stars may also exhibit substantial chromospheric activity. If this is the case, then these stars should launch a weak solar-like wind, which we have found (Johnson & Khochfar 2011) would likely prevent the pollution of their surfaces by ISM material. This provides reason for optimism about the possibility of detecting, unequivocally, Pop III stars in the Galaxy today, if such low-mass primordial stars formed (as suggested by recent high-resolution cosmological simulations; Clark et al. 2011, Greif et al. 2011).
11 February 2011
The Halo Occupation Distribution of Black Holes
Authors: Colin DeGraf, Matthew Oborski, Tiziana Di Matteo, Suchetana Chatterjee, Daisuke Nagai, Zheng Zheng, Jonathan Richardson
http://arxiv.org/abs/1102.1437
This paper investigates the halo occupation distribution (HOD) of black holes within a hydrodynamic cosmological simulation that directly follows black hole growth. Functional fits are derived for the number of BH per halo mass, the conditional mass function of BHs, and the radial distribution of BHs.
http://arxiv.org/abs/1102.1437
This paper investigates the halo occupation distribution (HOD) of black holes within a hydrodynamic cosmological simulation that directly follows black hole growth. Functional fits are derived for the number of BH per halo mass, the conditional mass function of BHs, and the radial distribution of BHs.
Synthetic Observations of Simulated AGN Jets: X-ray Cavities
Authors: Peter J. Mendygral, Sean M. O'Neill, Thomas W. Jones
Link to paper: http://arxiv.org/abs/1102.1481
X-ray cavities in galaxy clusters are now commonly used to determine the mechanical energy input from radio sources. The authors provide a detailed assessment of these observational methods by computing realistic X-ray emission maps for the simulated sources, determining the cavity enthalpy as done for real observations and comparing the results to the known and directly measureable properties in the simulation. Determined cavity enthalpies are typically within a factor of 2 of the simulation values; cavity ages seem more vulnerable to projection effects and problems of the methods, in particular the buoyant rise time.
Link to paper: http://arxiv.org/abs/1102.1481
X-ray cavities in galaxy clusters are now commonly used to determine the mechanical energy input from radio sources. The authors provide a detailed assessment of these observational methods by computing realistic X-ray emission maps for the simulated sources, determining the cavity enthalpy as done for real observations and comparing the results to the known and directly measureable properties in the simulation. Determined cavity enthalpies are typically within a factor of 2 of the simulation values; cavity ages seem more vulnerable to projection effects and problems of the methods, in particular the buoyant rise time.
4 February 2011
Clustering of HI galaxies
Clustering of HI galaxies in HIPASS and ALFALFA
Authors: S. S. Passmoor, C. M. Cress, A. Faltenbacher
http://arxiv.org/abs/1101.5007
This paper studies the clustering of HI galaxies. It presents
results for the correlation functions, and compare them to dark-matter.
This gives constraints on galaxy formation and its relation to dark-matter.
Authors: S. S. Passmoor, C. M. Cress, A. Faltenbacher
http://arxiv.org/abs/1101.5007
This paper studies the clustering of HI galaxies. It presents
results for the correlation functions, and compare them to dark-matter.
This gives constraints on galaxy formation and its relation to dark-matter.
27 January 2011
Title: Dynamical Delays Between Starburst and AGN Activity in Galaxy Nuclei
Author: Philip F. Hopkins
Link to paper: http://arxiv.org/abs/1101.4230
The author investigates the observed (possible) delay between the onset of AGN activity and the peak of star formation in star-burst galaxies. He shows that the delay can be reproduced in simulations of merging galaxies, and give a physical explanation of it. He argues of implications for the role of the AGN feedback in suppressing star formation.
Link to paper: http://arxiv.org/abs/1101.4230
The author investigates the observed (possible) delay between the onset of AGN activity and the peak of star formation in star-burst galaxies. He shows that the delay can be reproduced in simulations of merging galaxies, and give a physical explanation of it. He argues of implications for the role of the AGN feedback in suppressing star formation.
On Lyman-limit Systems
On Lyman-limit Systems and the Evolution of the Intergalactic Ionizing Background
Authors: Matthew McQuinn, S. Peng Oh, Claude-Andre Faucher-Giguere
http://arxiv.org/abs/1101.1964v1
The authors use cosmological simulations to study the properties of Lyman-limit systems at 3 < z < 6 and the relation between the ionising emissivity of galaxies and the amplitude of the intergalactic ionising background. This could explain the rapid decrease in the Ly-alpha forest transmission observed around z=6 without invoking reionisation.
Authors: Matthew McQuinn, S. Peng Oh, Claude-Andre Faucher-Giguere
http://arxiv.org/abs/1101.1964v1
The authors use cosmological simulations to study the properties of Lyman-limit systems at 3 < z < 6 and the relation between the ionising emissivity of galaxies and the amplitude of the intergalactic ionising background. This could explain the rapid decrease in the Ly-alpha forest transmission observed around z=6 without invoking reionisation.
19 January 2011
Conformal Cyclic Cosmology in WMAP data?
Authors: V.G.Gurzadyan, R.Penrose
Link to Paper: arxiv.org/abs/1011.3706
The Conformal Cyclic Cosmology model. A dynamic and controversial model (R. Penrose) that argues for various aeons preceding and following our current universe. The authors also explain that the validity of this model can be tested via observations (CMB anisotropies) and even claim to have found proof for the same!
Link to Paper: arxiv.org/abs/1011.3706
The Conformal Cyclic Cosmology model. A dynamic and controversial model (R. Penrose) that argues for various aeons preceding and following our current universe. The authors also explain that the validity of this model can be tested via observations (CMB anisotropies) and even claim to have found proof for the same!
An Actively Accreting Massive Black Hole in the Dwarf Starburst Galaxy Henize 2-10
Authors: Reines et al.
Link to the paper: arXiv:1101.1309
The authors report observations strongly suggesting the presence of a central black hole in a blue compact dwarf galaxy. The mass of the black hole is estimated to be ~10^6 MSun, which is intriguing given that the galaxy itself has no bulge, nucleus, or nuclear stellar cluster. In turn, the authors suggest that in the early universe black holes may have formed before the stellar components of the first dwarf galaxies.
Link to the paper: arXiv:1101.1309
The authors report observations strongly suggesting the presence of a central black hole in a blue compact dwarf galaxy. The mass of the black hole is estimated to be ~10^6 MSun, which is intriguing given that the galaxy itself has no bulge, nucleus, or nuclear stellar cluster. In turn, the authors suggest that in the early universe black holes may have formed before the stellar components of the first dwarf galaxies.
8 November 2010
The first galaxies: assembly of disks and prospects for direct detection
Authors: A. Pawlik, M. Milosavljevic and V. Bromm
Link to the paper: arXiv:1011.0438
The authors study the formation of a galaxy embedded in a 109 solar masses halo at redshift z=10. Galaxies of this mass should be possible to observe with theJames Webb Space Telescope (JWST).
The authors do not include star formation and feedback, but concentrate on the formation history of the galaxy in the presence of primordial and molecular cooling. The first result is that the galaxy for a gas disk with and without the inclusion of molecular cooling. Indeed, molecular cooling seems to act only on the thickness and fragmentation of the disk, but not on its assembly and gas content. This is one of the first simulations showing that such a halo can host a disk.
The second result is that a simple modeling of the star formation history of the galaxy gives estimates of its detectability with JWST. The authors predict that JWST could distinguish between normal and top-heavy IMF in the starburts case, and between starburst and continuous star formation. Their model can be rescaled to different values of parameters like, e.g., the escape fraction of photons.
Link to the paper: arXiv:1011.0438
The authors study the formation of a galaxy embedded in a 109 solar masses halo at redshift z=10. Galaxies of this mass should be possible to observe with the
The authors do not include star formation and feedback, but concentrate on the formation history of the galaxy in the presence of primordial and molecular cooling. The first result is that the galaxy for a gas disk with and without the inclusion of molecular cooling. Indeed, molecular cooling seems to act only on the thickness and fragmentation of the disk, but not on its assembly and gas content. This is one of the first simulations showing that such a halo can host a disk.
The second result is that a simple modeling of the star formation history of the galaxy gives estimates of its detectability with JWST. The authors predict that JWST could distinguish between normal and top-heavy IMF in the starburts case, and between starburst and continuous star formation. Their model can be rescaled to different values of parameters like, e.g., the escape fraction of photons.
5 November 2010
Jet-powered molecular hydrogen emission from radio galaxies
Authors: P. Ogle et al.
Link to the paper: arXiv:1009.4533
The authors examine a sample of 55 radio galaxies with z < 0.22 and find 31% percent of those to exhibit particularly strong mid-IR emission from warm H2 (T = 100 - 1500 K, with masses in this phase typically around some 108, up to 2 x 1010 solar masses), putting them in a newly (re)defined class of "molecular hydrogen emission galaxies" (MOHEGs). They argue that the jets in these sources may interact with the cold H2 in these galaxies and shock-heat it, resulting in the observed emission. They find that X-rays originating from the AGN are incapable of powering the H2 emission, but that cosmic rays may still be an alternative explanation if a cosmic ray pressure 25 times higher than thermal pressure is deemed to be reasonable.
The radio MOHEGs show only low to moderate star formation rates (< 3 solar masses per year) and much less 7.7 µm PAH emission then normal star forming galaxies with respect to the mid-IR continuum (a factor of 10 - 100 lower). The H2 luminosity does not show much correlation with radio power, but several sources in clusters with X-ray cavities indicate that the ratio between H2 luminosity and kinetic jet power may lie around 10-4 to some 10-3. Most (14 of 17) radio MOHEGs belong to close galaxy pairs, groups or cluster and the authors conjecture that this environment or past gas-rich mergers may deliver large quantities of gas to the galaxies.
The authors conclude that jet-driven outflows may be responsible for the emission, although the details are yet very unclear. The jet powers, however, would more than sufficient for this.
Link to the paper: arXiv:1009.4533
The authors examine a sample of 55 radio galaxies with z < 0.22 and find 31% percent of those to exhibit particularly strong mid-IR emission from warm H2 (T = 100 - 1500 K, with masses in this phase typically around some 108, up to 2 x 1010 solar masses), putting them in a newly (re)defined class of "molecular hydrogen emission galaxies" (MOHEGs). They argue that the jets in these sources may interact with the cold H2 in these galaxies and shock-heat it, resulting in the observed emission. They find that X-rays originating from the AGN are incapable of powering the H2 emission, but that cosmic rays may still be an alternative explanation if a cosmic ray pressure 25 times higher than thermal pressure is deemed to be reasonable.
The radio MOHEGs show only low to moderate star formation rates (< 3 solar masses per year) and much less 7.7 µm PAH emission then normal star forming galaxies with respect to the mid-IR continuum (a factor of 10 - 100 lower). The H2 luminosity does not show much correlation with radio power, but several sources in clusters with X-ray cavities indicate that the ratio between H2 luminosity and kinetic jet power may lie around 10-4 to some 10-3. Most (14 of 17) radio MOHEGs belong to close galaxy pairs, groups or cluster and the authors conjecture that this environment or past gas-rich mergers may deliver large quantities of gas to the galaxies.
The authors conclude that jet-driven outflows may be responsible for the emission, although the details are yet very unclear. The jet powers, however, would more than sufficient for this.
28 October 2010
SMBH formation by direct collapse: keeping protogalactic gas H2 free in dark matter haloes with Tvir>10^4 K
Authors: Shang et. al.
Link to the Paper: http://arxiv.org/abs/0906.4773
They then determine the J(crit) required to suppress molecular (hydrogen) cooling in each of the three halos simulated above and find that;[i] for a hard spectrum (metal free stars): J(crit) is between 10e4 to 10e5, [ii] for a softer spectrum (normal stellar population) J(crit) lies between 30 to 300.
The values are ~ 3 to 10 percent lower than previous estimates. They argue that this improved estimate resulted from a better hydrogen molecule-dissociational rate that they adopted. As seen in the Dijkstra paper (see previous post), the reduction in J(crit) exponentially increases the number of rare halos exposed critical radiation; there by preventing fragmentation and ensuing direct collapse. This might give rise to 10e5 solar mass objects at the centre of these haloes- progenitors for SMBH.
Fluctuations in the high-z LW background: close halo pairs as the origins of SMBH
Author: Dijkstra et. al.
Link to the paper: http://arxiv.org/abs/0810.0014
They take into account the (i) the clustering of DM halos, (ii) Poisson fluctuations in the number of corresponding star forming galaxies, and (iii) scatter in the LW luminosity produced by halos. Although most of the haloes would be exposed to global mean value of the LW background, some haloes could be exposed to the high critical LW flux (Jcrit) that can prevent fragmentation.
This fraction, although low ~ 10e-8 to 10e-6 of all the haloes with Tvir>10e4 K, has an exponential dependance on the value of the flux; i.e. any small change in the value of J(crit) will exponentially affect the number of haloes that are exposed to this critical flux.
22 October 2010
Lyman 'bump' galaxies - II. A possible signature of massive extremely metal-poor or metal-free stars in z = 3.1 Lya emitters
Authors: Inoue, et al.
Link to article: arXiv:1010.2582
Observations of both Lya emission and Lyman continuum emission (the Lyman 'bump') are presented for a sample of galaxies at z = 3.1. Besides exhibiting the Lyman 'bump' (blueward of Lya), which could be a sign of a population of metal-poor stars which have hard spectra, some of the galaxies also show a spatial offset between the Lya emission and the Lyman continuum emission. The authors argue that the observed properties of the galaxies may be best explained by the presence of massive, metal-free stars. Interestingly, it is the same galaxies which show the spatial offset which also appear to require the largest fractions of Pop III stars. Indeed, this would be consistent with a scenario in which Pop III star formation may take place at relatively low redshift, albeit only at the outskirts of metal-enriched regions.
Link to article: arXiv:1010.2582
Observations of both Lya emission and Lyman continuum emission (the Lyman 'bump') are presented for a sample of galaxies at z = 3.1. Besides exhibiting the Lyman 'bump' (blueward of Lya), which could be a sign of a population of metal-poor stars which have hard spectra, some of the galaxies also show a spatial offset between the Lya emission and the Lyman continuum emission. The authors argue that the observed properties of the galaxies may be best explained by the presence of massive, metal-free stars. Interestingly, it is the same galaxies which show the spatial offset which also appear to require the largest fractions of Pop III stars. Indeed, this would be consistent with a scenario in which Pop III star formation may take place at relatively low redshift, albeit only at the outskirts of metal-enriched regions.
19 October 2010
Dwarf archaeology
This paper examines two scenarios for the formation of ultra-faint dwarf galaxies (UFDs), representing the formation processes of the first galaxies. It is claimed that the first galaxies are chemical "one-shot" events, where only one (long-lived) stellar generation forms after the first, Population III, SN explosions. This conclusion is established using a comparison between the stellar abundance signatures as observed in the present-day UDFs, and the results of high-resolution hydro simulations.
Reference:
Dwarf archaeology: Probing the first enrichment events with low-luminosity galaxies. Authors: Anna Frebel & Volker Bromm
http://arxiv.org/abs/1010.1261
Reference:
Dwarf archaeology: Probing the first enrichment events with low-luminosity galaxies. Authors: Anna Frebel & Volker Bromm
http://arxiv.org/abs/1010.1261
27 September 2010
Transformations in the Fall: The Birth of the Early-Type Galaxies
The authors discuss in their paper (Feldmann et al. 2010; arXiv:1008.3386) a recent high resolution cosmological hydro simulation of the formation of a galaxy group environment. The remarkable feature of their simulation is to be able to resolve the detailed formation history of individual group members with high mass resolution (SPH particle mass = 1E6 M_sun). Their main conclusion is that elliptical galaxies form by major mergers before the assembly of the group, and that gas cooling and associated star formation is strongly suppressed via ram-pressure stripping once galaxies enter the group environment.
17 September 2010
On the star formation rates in molecular clouds
Authors: C. Lada, M. Lombardo, J. Alveshttp; http://arxiv.org/abs/1009.2985
The authors study the correlations between SFR and cloud mass in a sample of 8 molecular cloud complexes (11 clouds). They find that only the mass in the cores of the cloud is tightly linked to the SFR.
Total cloud mass and SFR do not correlate well, with variations which are considerable (more than one order of magnitude in scatter). Instead, the correlation between high-density mass (which is roughly 10 per cent the total one for almost all the objects in the sample) and SFR is linear and quite tight (with a scatter of about 2-3).
The only concern is the following: if the correlation between high-density mass and SFR is so tight, and if the high-density mass is roughly 10 per cent the total one, why is the correlation between total mass and SFR so bad?...
The authors study the correlations between SFR and cloud mass in a sample of 8 molecular cloud complexes (11 clouds). They find that only the mass in the cores of the cloud is tightly linked to the SFR.
Total cloud mass and SFR do not correlate well, with variations which are considerable (more than one order of magnitude in scatter). Instead, the correlation between high-density mass (which is roughly 10 per cent the total one for almost all the objects in the sample) and SFR is linear and quite tight (with a scatter of about 2-3).
The only concern is the following: if the correlation between high-density mass and SFR is so tight, and if the high-density mass is roughly 10 per cent the total one, why is the correlation between total mass and SFR so bad?...
10 September 2010
Galaxy and Mass Assembly (GAMA): Dust obscuration in galaxies and their recent star formation histories
Authors: D. B. Wijesinghe, A. M. Hopkins et al.
Link to article: arXiv:1009.0616v1
Star formation rates derived through pan-spectral analysis of a sample of ~ 30.000 galaxies drawn from the Galaxy and Mass Assembly (GAMA) survey. In order to get the SFR in different bands (by means of linear scaling factors derived from population synthesis models), dust obscuration corrections are applied to the Hα, [OII] and UV luminosities by using a range of extinction laws drawn from the literature.
The results of this study clearly show that the fine-tuned Fischera & Dopita (2005) obscuration curve (Rv = 4.5) give the best agreement when comparing the different corrected SFR indicators. One interesting finding of the paper is that the 2200 Å feature present in the obscuration curves of the Milky way, has to be removed in order to obtain complete consistency between all SFR indicators suggesting that this feature may not be common in the average integrated attenuation of galaxy emission.
The last part of this paper attempt to give constraints on the star formation history of local Universe galaxies by comparing the corrected data with the predictions of evolutionary synthesis models. The evolutionary paths are computed by assuming an exponentially decaying SFR and give the best fit to the data by using a Baldry & Glazebrook (2003) IMF with a characteristic SFR decay of 90 Myrs, which correspond to stellar population ages of the galaxies ranging from 200 to 500 Myrs.
The results of this study clearly show that the fine-tuned Fischera & Dopita (2005) obscuration curve (Rv = 4.5) give the best agreement when comparing the different corrected SFR indicators. One interesting finding of the paper is that the 2200 Å feature present in the obscuration curves of the Milky way, has to be removed in order to obtain complete consistency between all SFR indicators suggesting that this feature may not be common in the average integrated attenuation of galaxy emission.
The last part of this paper attempt to give constraints on the star formation history of local Universe galaxies by comparing the corrected data with the predictions of evolutionary synthesis models. The evolutionary paths are computed by assuming an exponentially decaying SFR and give the best fit to the data by using a Baldry & Glazebrook (2003) IMF with a characteristic SFR decay of 90 Myrs, which correspond to stellar population ages of the galaxies ranging from 200 to 500 Myrs.
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