Topic modeling is widely used for analytically evaluating large collections of textual data. One ... more Topic modeling is widely used for analytically evaluating large collections of textual data. One of the most popular topic techniques is Latent Dirichlet Allocation (LDA), which is flexible and adaptive, but not optimal for e.g. short texts from various domains. We explore how the state-of-the-art BERTopic algorithm performs on short multi-domain text and find that it generalizes better than LDA in terms of topic coherence and diversity. We further analyze the performance of the HDBSCAN clustering algorithm utilized by BERTopic and find that it classifies a majority of the documents as outliers. This crucial, yet overseen problem excludes too many documents from further analysis. When we replace HDBSCAN with k-Means, we achieve similar performance, but without outliers.
The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develo... more The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Despite this, the project has always been and remains to this day effectively unfunded. Further, contributors receive little or no formal recognition for creating and supporting what is now critical software. This paper explores the problem in detail, outlines possible solutions to correct this, and presents a few suggestions on how to address the sustainability of general purpose astronomical software.
It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see ... more It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see most stars in the night sky as single stars or binary systems, not in clusters. This is an indication that most star clusters disrupt on short timescales. A favorite candidate mechanism responsible for this destruction of clusters is the removal of the residual gas (after star formation) by the ionizing radiation, strong stellar winds and supernovae of the massive stars in the cluster. Cluster dynamics will be discussed with emphasis on the rst 10 Myr and the role the massive stars can play will be highlighted. With the use of simulations more realistic scenarios can be investigated. It will become clear that massive stars can indeed easily destroy most of the stellar clusters in their early days, creating the huge eld star population we observe in most galaxies. 1
It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see ... more It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see most stars in the night sky as single stars or binary systems, not in clusters. This is an indication that most star clusters disrupt on short timescales. A favorite candidate mechanism responsible for this destruction of clusters is the removal of the residual gas (after star formation) by the ionizing radiation, strong stellar winds and supernovae of the massive stars in the cluster. Cluster dynamics will be discussed with emphasis on the first 10 Myr and the role the massive stars can play will be highlighted. With the use of simulations more realistic scenarios can be investigated. It will become clear that massive stars can indeed easily destroy most of the stellar clusters in their early days, creating the huge field star population we observe in most galaxies.
The Astropy Project (this http URL) is, in its own words, "a community effort to develop a s... more The Astropy Project (this http URL) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Despite this, the project has always been and remains to this day effectively unfunded. Further, contributors receive little or no formal recognition for creating and supporting what is now critical software. This paper explores the problem in detail, outlines possible solutions to correct this, and presents a few suggestions on how to address the sustainability of general purpose astronomical software.
Monthly Notices of the Royal Astronomical Society, 2013
We use hydrodynamical simulations from the OWLS project to investigate the dependence of the phys... more We use hydrodynamical simulations from the OWLS project to investigate the dependence of the physical properties of galaxy populations at redshift 2 on metal-line cooling and feedback from star formation and active galactic nuclei (AGN). We find that if the sub-grid feedback from star formation is implemented kinetically, the feedback is only efficient if the initial wind velocity exceeds a critical value. This critical velocity increases with galaxy mass and also if metal-line cooling is included. This suggests that radiative losses quench the winds if their initial velocity is too low. If the feedback is efficient, then the star formation rate is inversely proportional to the amount of energy injected per unit stellar mass formed (which is proportional to the initial mass loading for a fixed wind velocity). This can be understood if the star formation is self-regulating, i.e. if the star formation rate (and thus the gas fraction) increase until the outflow rate balances the inflow rate. Feedback from AGN is efficient at high masses, while increasing the initial wind velocity with gas pressure or halo mass allows one to generate galaxy-wide outflows at all masses. Matching the observed galaxy mass function requires efficient feedback. In particular, the predicted faint-end slope is too steep unless we resort to highly mass loaded winds for low-mass objects. Such efficient feedback from lowmass galaxies (M * ≪ 10 10 M ⊙) also reduces the discrepancy with the observed specific star formation rates, which are higher than predicted unless the feedback transitions from highly efficient to inefficient just below M * ∼ 5 × 10 9 M ⊙ .
The hypervelocity OB stars in the Milky Way Galaxy were ejected from the central regions some 10-... more The hypervelocity OB stars in the Milky Way Galaxy were ejected from the central regions some 10-100 million years ago. We argue that these stars, as well as many more abundant bound OB stars in the innermost few parsecs, were generated by the interactions of an AGN jet from the central black hole with a dense molecular cloud. Considerations of the associated energy and momentum injection have broader implications for the possible origin of the Fermi bubbles and for the enrichment of the intergalactic medium.
We present the luminosity function (LF) of star clusters in M51 based on HST/ACS observations tak... more We present the luminosity function (LF) of star clusters in M51 based on HST/ACS observations taken as part of the Hubble Heritage project. The clusters are selected based on their size and with the resulting 5 990 clusters we present one of the largest cluster samples of a single galaxy. We find that the LF can be approximated with a double power-law distribution with a break around MV = −8.9. On the bright side the index of the power-law distribution is steeper (α = 2.75) than on the faint-side (α = 1.93), similar to what was found earlier for the "Antennae" galaxies. The location of the bend, however, occurs about 1.6 mag fainter in M51. We confront the observed LF with the model for the evolution of integrated properties of cluster populations of Gieles et al. (2005b), which predicts that a truncated cluster initial mass function would result in a bend in, and a double power-law behaviour of, the integrated LF. The combination of the large field-of view and the high star cluster formation rate of M51 make it possible to detect such a bend in the LF. Hence, we conclude that there exists a fundamental upper limit to the mass of star clusters in M51. Assuming a power-law cluster initial mass function with exponentional cutoff of the form N dM ∝ M −β exp(−M/MC) dM , we find that MC = 10 5 M⊙. A direct comparison with the LF of the "Antennae" suggests that there MC = 4 × 10 5 M⊙.
Proceedings of the International Astronomical Union
If all stars form in clusters and both stars and clusters follow a power-law distribution which f... more If all stars form in clusters and both stars and clusters follow a power-law distribution which favours the creation of low-mass objects, the numerous low-mass clusters will be deficient in high-mass stars. Therefore, the stellar mass function integrated over the entire galaxy (the integrated galactic initial mass function; IGIMF) will be steeper at the high-mass end than the underlying stellar IMF. We show how the steepness of the IGIMF depends on the sampling method and on the assumptions made regarding the star cluster mass function. We also investigate the O-star content, integrated photometry and chemical enrichment of galaxies that result from several IGIMFs compared to more standard IMFs.
Proceedings of the International Astronomical Union
In recent years, a series of papers (Kroupa & Weidner 2003, Weidner & Kroupa 2004, Weidner & Krou... more In recent years, a series of papers (Kroupa & Weidner 2003, Weidner & Kroupa 2004, Weidner & Kroupa 2005 and Weidner & Kroupa 2006, WK06 from now on) have proposed that the stellar content of an entire galaxy may not be well described by the same initial mass function (IMF) that describes the distribution of stellar masses in the star clusters, where these stars form. The reason is that star clusters also form with a cluster mass function (CMF), which is a power law with a power law index of ~−2. If the lowest mass clusters are of masses smaller than the physical upper mass limit for stars they will be deficient in high mass stars. Therefore, if the stellar content of all clusters is added together, making up the Integrated Galactic Initial Mass Function (IGIMF), the distribution of stellar masses may be steeper at the high mass end, depending on the exact shape of the CMF.
We study several aspects of the formation of galaxies, using numerical simulations. We investigat... more We study several aspects of the formation of galaxies, using numerical simulations. We investigate the influence of about thirty different sub-grid physics recipes for cooling, star formation, supernova feedback, AGN feedback etc. on the resulting galaxy populations with large SPH simulations. We investigate several parameters that quantify the environment of galaxies and present the strongest measure of halo mass and
To investigate feedback between relativistic jets emanating from Active Galactic Nuclei (AGN) and... more To investigate feedback between relativistic jets emanating from Active Galactic Nuclei (AGN) and the stellar population of the host galaxy, we analyze the long-term evolution of the galaxy-scale simulations by Gaibler et al. (2012) of jets in massive, gas-rich galaxies at z ∼ 2-3 and of stars formed in the host galaxies. We find strong, jet-induced differences in the resulting stellar populations of galaxies that host relativistic jets and galaxies that do not, including correlations in stellar locations, velocities, and ages. Jets are found to generate distributions of increased radial and vertical velocities that persist long enough to effectively extend the stellar structure of the host. The jets cause the formation of bow shocks that move out through the disk, generating rings of star formation within the disk. The bow shock often accelerates pockets of gas in which stars form, yielding populations of stars with significant radial and vertical velocities, some of which have large enough velocities to escape the galaxy. These stellar population signatures can serve to identify past jet activity as well as jet-induced star formation.
It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see ... more It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see most stars in the night sky as single stars or binary systems, not in clusters. This is an indication that most star clusters disrupt on short timescales. A favorite candidate mechanism responsible for this destruction of clusters is the removal of the residual gas (after star formation) by the ionizing radiation, strong stellar winds and supernovae of the massive stars in the cluster. Cluster dynamics will be discussed with emphasis on the rst 10 Myr and the role the massive stars can play will be highlighted. With the use of simulations more realistic scenarios can be investigated. It will become clear that massive stars can indeed easily destroy most of the stellar clusters in their early days, creating the huge eld star population we observe in most galaxies.
Monthly Notices of the Royal Astronomical Society: Letters, 2009
Galaxy formation models typically assume that the size and rotation speed of galaxy disks are lar... more Galaxy formation models typically assume that the size and rotation speed of galaxy disks are largely dictated by the mass, concentration, and spin of their surrounding dark matter haloes. Equally important, however, are the fraction of baryons in the halo that collect into the central galaxy, as well as the net angular momentum that they are able to retain during its assembly process. We explore the latter using a set of four large cosmological Nbody/gasdynamical simulations drawn from the OWLS (OverWhelmingly Large Simulations) project. These runs differ only in their implementation of feedback from supernovae. We find that, when expressed as fractions of their virial values, galaxy mass and net angular momentum are tightly correlated. Galaxy mass fractions, m d = M gal /M vir , depend strongly on feedback, but only weakly on halo mass or spin over the halo mass range explored here (M vir > 10 11 h −1 M ⊙). The angular momentum of a galaxy, expressed in units of that of its surrounding halo, j d = J gal /J vir , correlates with m d in a manner that is insensitive to feedback and that deviates strongly from the simple j d = m d assumption often adopted in semi-analytic models of galaxy formation. The m d-j d correlation implies that, in a given halo, galaxy disk size is maximal when the central galaxy makes up a substantial fraction (∼ 20-30%) of all baryons within the virial radius (i.e., m d ∼ 0.03-0.05). At z = 2, such systems may host gaseous disks with radial scale lengths as large as those reported for star-forming disks by the SINS survey, even in moderately massive haloes of average spin. Extended disks at z = 2 may thus signal the presence of systems where galaxy formation has been particularly efficient, rather than the existence of haloes with unusually high spin parameter.
Monthly Notices of the Royal Astronomical Society: Letters, 2011
We investigate the correlation between nine different dark matter halo properties using a rank co... more We investigate the correlation between nine different dark matter halo properties using a rank correlation analysis and a Principal Component Analysis for a sample of haloes spanning five orders of magnitude in mass. We consider mass and dimensionless measures of concentration, age, relaxedness, sphericity, triaxiality, substructure, spin, and environment, where the latter is defined in a way that makes it insensitive to mass. We find that concentration is the most fundamental property. Except for environment, all parameters are strongly correlated with concentration. Concentration, age, substructure, mass, sphericity and relaxedness can be considered a single family of parameters, albeit with substantial scatter. In contrast, spin, environment, and triaxiality are more independent, although spin does correlate strongly with substructure and both spin and triaxiality correlate substantially with concentration. Although mass sets the scale of a halo, all other properties are more sensitive to concentration.
Monthly Notices of the Royal Astronomical Society, 2014
We investigate the properties of damped Lyα absorption systems (DLAs) in semianalytic models of g... more We investigate the properties of damped Lyα absorption systems (DLAs) in semianalytic models of galaxy formation, including new modeling of the partitioning of cold gas into atomic, molecular, and ionized phases, and a star formation recipe based on the density of molecular gas. We use three approaches for partitioning gas into atomic and molecular constituents: a pressure-based recipe and metallicity-based recipes with fixed and varying UV radiation fields. We identify DLAs by adopting an assumed gas density profile for galactic discs and passing lines of sight through our simulations to compute H I column densities. We find that models with "standard" gas radial profiles-computed assuming that the average specific angular momentum of the gas disc is equal to that of the host dark matter halo-fail to reproduce the observed column density distribution of DLAs, regardless of the assumed gas partitioning. These models also fail to reproduce the distribution of velocity widths ∆v of low-ionization state metal systems, overproducing low ∆v relative to high ∆v systems. Models with "extended" radial gas profiles-corresponding to gas discs with higher specific angular momentum, or gas in an alternate extended configuration-are able to reproduce quite well the column density distribution of absorbers over the column density range 19 < log N HI < 22.5 in the redshift range 2 < z < 3.5. The model with pressure-based gas partitioning and the metallicity-based recipe with a varying UV radiation field also reproduce the observed line density of DLAs, H I gas density, and ∆v distribution at z < 3 well. However all of the models investigated here underproduce DLAs and the H I gas density at z > 3. This may indicate that DLAs at high redshift arise from a different physical phenomenon, such as outflows or filaments. If this is the case, the flatness in the number of DLAs and H I gas density over the redshift interval 0 < z < 5 may be due to a cosmic coincidence where the majority of DLAs at z > 3 arise from intergalactic gas in filaments or streams while those at z < 3 arise predominantly in galactic discs. We further investigate the dependence of DLA metallicity on redshift and ∆v in our favored models, and find good agreement with the observations, particularly when we include the effects of metallicity gradients.
Monthly Notices of the Royal Astronomical Society, 2010
We investigate the physics driving the cosmic star formation (SF) history using the more than fif... more We investigate the physics driving the cosmic star formation (SF) history using the more than fifty large, cosmological, hydrodynamical simulations that together comprise the OverWhelmingly Large Simulations (OWLS) project. We systematically vary the parameters of the model to determine which physical processes are dominant and which aspects of the model are robust. Generically, we find that SF is limited by the build-up of dark matter haloes at high redshift, reaches a broad maximum at intermediate redshift, then decreases as it is quenched by lower cooling rates in hotter and lower density gas, gas exhaustion, and self-regulated feedback from stars and black holes. The higher redshift SF is therefore mostly determined by the cosmological parameters and to a lesser extent by photo-heating from reionization. The location and height of the peak in the SF history, and the steepness of the decline towards the present, depend on the physics and implementation of stellar and black hole feedback. Mass loss from intermediate-mass stars and metal-line cooling both boost the SF rate at late times. Galaxies form stars in a self-regulated fashion at a rate controlled by the balance between, on the one hand, feedback from massive stars and black holes and, on the other hand, gas cooling and accretion. Paradoxically, the SF rate is highly insensitive to the assumed SF law. This can be understood in terms of self-regulation: if the SF efficiency is changed, then galaxies adjust their gas fractions so as to achieve the same rate of production of massive stars. Self-regulated feedback from accreting black holes is required to match the steep decline in the observed SF rate below redshift two, although more extreme feedback from SF, for example in the form of a top-heavy initial stellar mass function at high gas pressures, can help.
Monthly Notices of the Royal Astronomical Society, 2011
We study the rate at which gas accretes on to galaxies and haloes and investigate whether the acc... more We study the rate at which gas accretes on to galaxies and haloes and investigate whether the accreted gas was shocked to high temperatures before reaching a galaxy. For this purpose we use a suite of large cosmological, hydrodynamical simulations from the OWLS project, which uses a modified version of the smoothed particle hydrodynamics code gadget-3. We improve on previous work by considering a wider range of halo masses and redshifts, by distinguishing accretion on to haloes and galaxies, by including important feedback processes, and by comparing simulations with different physics. Gas accretion is mostly smooth, with mergers only becoming important for groups and clusters. The specific rate of gas accretion on to haloes is, like that for dark matter, only weakly dependent on halo mass. For halo masses M halo ≫ 10 11 M ⊙ it is relatively insensitive to feedback processes. In contrast, accretion rates on to galaxies are determined by radiative cooling and by outflows driven by supernovae and active galactic nuclei. Galactic winds increase the halo mass at which the central galaxies grow the fastest by about two orders of magnitude to M halo ∼ 10 12 M ⊙. Gas accretion is bimodal, with maximum past temperatures either of order the virial temperature or 10 5 K. The fraction of gas accreted on to haloes in the hot mode is insensitive to feedback and metal-line cooling. It increases with decreasing redshift, but is mostly determined by halo mass, increasing gradually from less than 10% for ∼ 10 11 M ⊙ to greater than 90% at ∼ 10 13 M ⊙. In contrast, for accretion on to galaxies the cold mode is always significant and the relative contributions of the two accretion modes are more sensitive to feedback and metal-line cooling. On average, the majority of stars present in any mass halo at any redshift were formed from gas accreted in the cold mode, although the hot mode contributes typically over 10% for M halo 10 11 M ⊙. Thus, while gas accretion on to haloes can be robustly predicted, the rate of accretion on to galaxies is sensitive to uncertain feedback processes. Nevertheless, it is clear that galaxies, but not necessarily their gaseous haloes, are predominantly fed by gas that did not experience an accretion shock when it entered the host halo.
Topic modeling is widely used for analytically evaluating large collections of textual data. One ... more Topic modeling is widely used for analytically evaluating large collections of textual data. One of the most popular topic techniques is Latent Dirichlet Allocation (LDA), which is flexible and adaptive, but not optimal for e.g. short texts from various domains. We explore how the state-of-the-art BERTopic algorithm performs on short multi-domain text and find that it generalizes better than LDA in terms of topic coherence and diversity. We further analyze the performance of the HDBSCAN clustering algorithm utilized by BERTopic and find that it classifies a majority of the documents as outliers. This crucial, yet overseen problem excludes too many documents from further analysis. When we replace HDBSCAN with k-Means, we achieve similar performance, but without outliers.
The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develo... more The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Despite this, the project has always been and remains to this day effectively unfunded. Further, contributors receive little or no formal recognition for creating and supporting what is now critical software. This paper explores the problem in detail, outlines possible solutions to correct this, and presents a few suggestions on how to address the sustainability of general purpose astronomical software.
It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see ... more It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see most stars in the night sky as single stars or binary systems, not in clusters. This is an indication that most star clusters disrupt on short timescales. A favorite candidate mechanism responsible for this destruction of clusters is the removal of the residual gas (after star formation) by the ionizing radiation, strong stellar winds and supernovae of the massive stars in the cluster. Cluster dynamics will be discussed with emphasis on the rst 10 Myr and the role the massive stars can play will be highlighted. With the use of simulations more realistic scenarios can be investigated. It will become clear that massive stars can indeed easily destroy most of the stellar clusters in their early days, creating the huge eld star population we observe in most galaxies. 1
It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see ... more It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see most stars in the night sky as single stars or binary systems, not in clusters. This is an indication that most star clusters disrupt on short timescales. A favorite candidate mechanism responsible for this destruction of clusters is the removal of the residual gas (after star formation) by the ionizing radiation, strong stellar winds and supernovae of the massive stars in the cluster. Cluster dynamics will be discussed with emphasis on the first 10 Myr and the role the massive stars can play will be highlighted. With the use of simulations more realistic scenarios can be investigated. It will become clear that massive stars can indeed easily destroy most of the stellar clusters in their early days, creating the huge field star population we observe in most galaxies.
The Astropy Project (this http URL) is, in its own words, "a community effort to develop a s... more The Astropy Project (this http URL) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Despite this, the project has always been and remains to this day effectively unfunded. Further, contributors receive little or no formal recognition for creating and supporting what is now critical software. This paper explores the problem in detail, outlines possible solutions to correct this, and presents a few suggestions on how to address the sustainability of general purpose astronomical software.
Monthly Notices of the Royal Astronomical Society, 2013
We use hydrodynamical simulations from the OWLS project to investigate the dependence of the phys... more We use hydrodynamical simulations from the OWLS project to investigate the dependence of the physical properties of galaxy populations at redshift 2 on metal-line cooling and feedback from star formation and active galactic nuclei (AGN). We find that if the sub-grid feedback from star formation is implemented kinetically, the feedback is only efficient if the initial wind velocity exceeds a critical value. This critical velocity increases with galaxy mass and also if metal-line cooling is included. This suggests that radiative losses quench the winds if their initial velocity is too low. If the feedback is efficient, then the star formation rate is inversely proportional to the amount of energy injected per unit stellar mass formed (which is proportional to the initial mass loading for a fixed wind velocity). This can be understood if the star formation is self-regulating, i.e. if the star formation rate (and thus the gas fraction) increase until the outflow rate balances the inflow rate. Feedback from AGN is efficient at high masses, while increasing the initial wind velocity with gas pressure or halo mass allows one to generate galaxy-wide outflows at all masses. Matching the observed galaxy mass function requires efficient feedback. In particular, the predicted faint-end slope is too steep unless we resort to highly mass loaded winds for low-mass objects. Such efficient feedback from lowmass galaxies (M * ≪ 10 10 M ⊙) also reduces the discrepancy with the observed specific star formation rates, which are higher than predicted unless the feedback transitions from highly efficient to inefficient just below M * ∼ 5 × 10 9 M ⊙ .
The hypervelocity OB stars in the Milky Way Galaxy were ejected from the central regions some 10-... more The hypervelocity OB stars in the Milky Way Galaxy were ejected from the central regions some 10-100 million years ago. We argue that these stars, as well as many more abundant bound OB stars in the innermost few parsecs, were generated by the interactions of an AGN jet from the central black hole with a dense molecular cloud. Considerations of the associated energy and momentum injection have broader implications for the possible origin of the Fermi bubbles and for the enrichment of the intergalactic medium.
We present the luminosity function (LF) of star clusters in M51 based on HST/ACS observations tak... more We present the luminosity function (LF) of star clusters in M51 based on HST/ACS observations taken as part of the Hubble Heritage project. The clusters are selected based on their size and with the resulting 5 990 clusters we present one of the largest cluster samples of a single galaxy. We find that the LF can be approximated with a double power-law distribution with a break around MV = −8.9. On the bright side the index of the power-law distribution is steeper (α = 2.75) than on the faint-side (α = 1.93), similar to what was found earlier for the "Antennae" galaxies. The location of the bend, however, occurs about 1.6 mag fainter in M51. We confront the observed LF with the model for the evolution of integrated properties of cluster populations of Gieles et al. (2005b), which predicts that a truncated cluster initial mass function would result in a bend in, and a double power-law behaviour of, the integrated LF. The combination of the large field-of view and the high star cluster formation rate of M51 make it possible to detect such a bend in the LF. Hence, we conclude that there exists a fundamental upper limit to the mass of star clusters in M51. Assuming a power-law cluster initial mass function with exponentional cutoff of the form N dM ∝ M −β exp(−M/MC) dM , we find that MC = 10 5 M⊙. A direct comparison with the LF of the "Antennae" suggests that there MC = 4 × 10 5 M⊙.
Proceedings of the International Astronomical Union
If all stars form in clusters and both stars and clusters follow a power-law distribution which f... more If all stars form in clusters and both stars and clusters follow a power-law distribution which favours the creation of low-mass objects, the numerous low-mass clusters will be deficient in high-mass stars. Therefore, the stellar mass function integrated over the entire galaxy (the integrated galactic initial mass function; IGIMF) will be steeper at the high-mass end than the underlying stellar IMF. We show how the steepness of the IGIMF depends on the sampling method and on the assumptions made regarding the star cluster mass function. We also investigate the O-star content, integrated photometry and chemical enrichment of galaxies that result from several IGIMFs compared to more standard IMFs.
Proceedings of the International Astronomical Union
In recent years, a series of papers (Kroupa & Weidner 2003, Weidner & Kroupa 2004, Weidner & Krou... more In recent years, a series of papers (Kroupa & Weidner 2003, Weidner & Kroupa 2004, Weidner & Kroupa 2005 and Weidner & Kroupa 2006, WK06 from now on) have proposed that the stellar content of an entire galaxy may not be well described by the same initial mass function (IMF) that describes the distribution of stellar masses in the star clusters, where these stars form. The reason is that star clusters also form with a cluster mass function (CMF), which is a power law with a power law index of ~−2. If the lowest mass clusters are of masses smaller than the physical upper mass limit for stars they will be deficient in high mass stars. Therefore, if the stellar content of all clusters is added together, making up the Integrated Galactic Initial Mass Function (IGIMF), the distribution of stellar masses may be steeper at the high mass end, depending on the exact shape of the CMF.
We study several aspects of the formation of galaxies, using numerical simulations. We investigat... more We study several aspects of the formation of galaxies, using numerical simulations. We investigate the influence of about thirty different sub-grid physics recipes for cooling, star formation, supernova feedback, AGN feedback etc. on the resulting galaxy populations with large SPH simulations. We investigate several parameters that quantify the environment of galaxies and present the strongest measure of halo mass and
To investigate feedback between relativistic jets emanating from Active Galactic Nuclei (AGN) and... more To investigate feedback between relativistic jets emanating from Active Galactic Nuclei (AGN) and the stellar population of the host galaxy, we analyze the long-term evolution of the galaxy-scale simulations by Gaibler et al. (2012) of jets in massive, gas-rich galaxies at z ∼ 2-3 and of stars formed in the host galaxies. We find strong, jet-induced differences in the resulting stellar populations of galaxies that host relativistic jets and galaxies that do not, including correlations in stellar locations, velocities, and ages. Jets are found to generate distributions of increased radial and vertical velocities that persist long enough to effectively extend the stellar structure of the host. The jets cause the formation of bow shocks that move out through the disk, generating rings of star formation within the disk. The bow shock often accelerates pockets of gas in which stars form, yielding populations of stars with significant radial and vertical velocities, some of which have large enough velocities to escape the galaxy. These stellar population signatures can serve to identify past jet activity as well as jet-induced star formation.
It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see ... more It is generally believed that most (if not all) stars are born in clusters. Nevertheless, we see most stars in the night sky as single stars or binary systems, not in clusters. This is an indication that most star clusters disrupt on short timescales. A favorite candidate mechanism responsible for this destruction of clusters is the removal of the residual gas (after star formation) by the ionizing radiation, strong stellar winds and supernovae of the massive stars in the cluster. Cluster dynamics will be discussed with emphasis on the rst 10 Myr and the role the massive stars can play will be highlighted. With the use of simulations more realistic scenarios can be investigated. It will become clear that massive stars can indeed easily destroy most of the stellar clusters in their early days, creating the huge eld star population we observe in most galaxies.
Monthly Notices of the Royal Astronomical Society: Letters, 2009
Galaxy formation models typically assume that the size and rotation speed of galaxy disks are lar... more Galaxy formation models typically assume that the size and rotation speed of galaxy disks are largely dictated by the mass, concentration, and spin of their surrounding dark matter haloes. Equally important, however, are the fraction of baryons in the halo that collect into the central galaxy, as well as the net angular momentum that they are able to retain during its assembly process. We explore the latter using a set of four large cosmological Nbody/gasdynamical simulations drawn from the OWLS (OverWhelmingly Large Simulations) project. These runs differ only in their implementation of feedback from supernovae. We find that, when expressed as fractions of their virial values, galaxy mass and net angular momentum are tightly correlated. Galaxy mass fractions, m d = M gal /M vir , depend strongly on feedback, but only weakly on halo mass or spin over the halo mass range explored here (M vir > 10 11 h −1 M ⊙). The angular momentum of a galaxy, expressed in units of that of its surrounding halo, j d = J gal /J vir , correlates with m d in a manner that is insensitive to feedback and that deviates strongly from the simple j d = m d assumption often adopted in semi-analytic models of galaxy formation. The m d-j d correlation implies that, in a given halo, galaxy disk size is maximal when the central galaxy makes up a substantial fraction (∼ 20-30%) of all baryons within the virial radius (i.e., m d ∼ 0.03-0.05). At z = 2, such systems may host gaseous disks with radial scale lengths as large as those reported for star-forming disks by the SINS survey, even in moderately massive haloes of average spin. Extended disks at z = 2 may thus signal the presence of systems where galaxy formation has been particularly efficient, rather than the existence of haloes with unusually high spin parameter.
Monthly Notices of the Royal Astronomical Society: Letters, 2011
We investigate the correlation between nine different dark matter halo properties using a rank co... more We investigate the correlation between nine different dark matter halo properties using a rank correlation analysis and a Principal Component Analysis for a sample of haloes spanning five orders of magnitude in mass. We consider mass and dimensionless measures of concentration, age, relaxedness, sphericity, triaxiality, substructure, spin, and environment, where the latter is defined in a way that makes it insensitive to mass. We find that concentration is the most fundamental property. Except for environment, all parameters are strongly correlated with concentration. Concentration, age, substructure, mass, sphericity and relaxedness can be considered a single family of parameters, albeit with substantial scatter. In contrast, spin, environment, and triaxiality are more independent, although spin does correlate strongly with substructure and both spin and triaxiality correlate substantially with concentration. Although mass sets the scale of a halo, all other properties are more sensitive to concentration.
Monthly Notices of the Royal Astronomical Society, 2014
We investigate the properties of damped Lyα absorption systems (DLAs) in semianalytic models of g... more We investigate the properties of damped Lyα absorption systems (DLAs) in semianalytic models of galaxy formation, including new modeling of the partitioning of cold gas into atomic, molecular, and ionized phases, and a star formation recipe based on the density of molecular gas. We use three approaches for partitioning gas into atomic and molecular constituents: a pressure-based recipe and metallicity-based recipes with fixed and varying UV radiation fields. We identify DLAs by adopting an assumed gas density profile for galactic discs and passing lines of sight through our simulations to compute H I column densities. We find that models with "standard" gas radial profiles-computed assuming that the average specific angular momentum of the gas disc is equal to that of the host dark matter halo-fail to reproduce the observed column density distribution of DLAs, regardless of the assumed gas partitioning. These models also fail to reproduce the distribution of velocity widths ∆v of low-ionization state metal systems, overproducing low ∆v relative to high ∆v systems. Models with "extended" radial gas profiles-corresponding to gas discs with higher specific angular momentum, or gas in an alternate extended configuration-are able to reproduce quite well the column density distribution of absorbers over the column density range 19 < log N HI < 22.5 in the redshift range 2 < z < 3.5. The model with pressure-based gas partitioning and the metallicity-based recipe with a varying UV radiation field also reproduce the observed line density of DLAs, H I gas density, and ∆v distribution at z < 3 well. However all of the models investigated here underproduce DLAs and the H I gas density at z > 3. This may indicate that DLAs at high redshift arise from a different physical phenomenon, such as outflows or filaments. If this is the case, the flatness in the number of DLAs and H I gas density over the redshift interval 0 < z < 5 may be due to a cosmic coincidence where the majority of DLAs at z > 3 arise from intergalactic gas in filaments or streams while those at z < 3 arise predominantly in galactic discs. We further investigate the dependence of DLA metallicity on redshift and ∆v in our favored models, and find good agreement with the observations, particularly when we include the effects of metallicity gradients.
Monthly Notices of the Royal Astronomical Society, 2010
We investigate the physics driving the cosmic star formation (SF) history using the more than fif... more We investigate the physics driving the cosmic star formation (SF) history using the more than fifty large, cosmological, hydrodynamical simulations that together comprise the OverWhelmingly Large Simulations (OWLS) project. We systematically vary the parameters of the model to determine which physical processes are dominant and which aspects of the model are robust. Generically, we find that SF is limited by the build-up of dark matter haloes at high redshift, reaches a broad maximum at intermediate redshift, then decreases as it is quenched by lower cooling rates in hotter and lower density gas, gas exhaustion, and self-regulated feedback from stars and black holes. The higher redshift SF is therefore mostly determined by the cosmological parameters and to a lesser extent by photo-heating from reionization. The location and height of the peak in the SF history, and the steepness of the decline towards the present, depend on the physics and implementation of stellar and black hole feedback. Mass loss from intermediate-mass stars and metal-line cooling both boost the SF rate at late times. Galaxies form stars in a self-regulated fashion at a rate controlled by the balance between, on the one hand, feedback from massive stars and black holes and, on the other hand, gas cooling and accretion. Paradoxically, the SF rate is highly insensitive to the assumed SF law. This can be understood in terms of self-regulation: if the SF efficiency is changed, then galaxies adjust their gas fractions so as to achieve the same rate of production of massive stars. Self-regulated feedback from accreting black holes is required to match the steep decline in the observed SF rate below redshift two, although more extreme feedback from SF, for example in the form of a top-heavy initial stellar mass function at high gas pressures, can help.
Monthly Notices of the Royal Astronomical Society, 2011
We study the rate at which gas accretes on to galaxies and haloes and investigate whether the acc... more We study the rate at which gas accretes on to galaxies and haloes and investigate whether the accreted gas was shocked to high temperatures before reaching a galaxy. For this purpose we use a suite of large cosmological, hydrodynamical simulations from the OWLS project, which uses a modified version of the smoothed particle hydrodynamics code gadget-3. We improve on previous work by considering a wider range of halo masses and redshifts, by distinguishing accretion on to haloes and galaxies, by including important feedback processes, and by comparing simulations with different physics. Gas accretion is mostly smooth, with mergers only becoming important for groups and clusters. The specific rate of gas accretion on to haloes is, like that for dark matter, only weakly dependent on halo mass. For halo masses M halo ≫ 10 11 M ⊙ it is relatively insensitive to feedback processes. In contrast, accretion rates on to galaxies are determined by radiative cooling and by outflows driven by supernovae and active galactic nuclei. Galactic winds increase the halo mass at which the central galaxies grow the fastest by about two orders of magnitude to M halo ∼ 10 12 M ⊙. Gas accretion is bimodal, with maximum past temperatures either of order the virial temperature or 10 5 K. The fraction of gas accreted on to haloes in the hot mode is insensitive to feedback and metal-line cooling. It increases with decreasing redshift, but is mostly determined by halo mass, increasing gradually from less than 10% for ∼ 10 11 M ⊙ to greater than 90% at ∼ 10 13 M ⊙. In contrast, for accretion on to galaxies the cold mode is always significant and the relative contributions of the two accretion modes are more sensitive to feedback and metal-line cooling. On average, the majority of stars present in any mass halo at any redshift were formed from gas accreted in the cold mode, although the hot mode contributes typically over 10% for M halo 10 11 M ⊙. Thus, while gas accretion on to haloes can be robustly predicted, the rate of accretion on to galaxies is sensitive to uncertain feedback processes. Nevertheless, it is clear that galaxies, but not necessarily their gaseous haloes, are predominantly fed by gas that did not experience an accretion shock when it entered the host halo.
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Papers by Marcel Haas