What does the IMF really tell us about star formation?

AbstractObtaining accurate measurements of the initial mass function (IMF) is often considered to be the key to understanding star formation, and a universal IMF is often assumed to imply a universal star formation process. Here, we illustrate that different modes of star formation can result in the same IMF, and that, in order to truly understand star formation, a deeper understanding of the primordial binary population is necessary. Detailed knowledge on the binary fraction, mass ratio distribution, and other binary parameters, as a function of mass, is a requirement for recovering the star formation process from stellar population measurements.

Download Full-text

The influence of star clusters on galactic disks: new insights in star-formation in galaxies

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308027610 ◽

2008 ◽

Vol 4 (S254) ◽

pp. 209-220

Author(s):

Pavel Kroupa

Keyword(s):

Star Formation ◽

Galaxy Evolution ◽

Stellar Population ◽

Dwarf Galaxy ◽

Star Clusters ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Disk Galaxies ◽

Galactic Disks

AbstractStars form in embedded star clusters which play a key role in determining the properties of a galaxy's stellar population. A large fraction of newly born massive stars are shot out from dynamically unstable embedded-cluster cores spreading them to large distances before they explode. Embedded clusters blow out their gas once the feedback energy from the new stellar population overcomes its binding energy, leading to cluster expansion and in many cases dissolution into the galaxy. Galactic disks may be thickened by such processes, and some thick disks may be the result of an early epoch of vigorous star-formation. Binary stellar systems are disrupted in clusters leading to a lower fraction of binaries in the field, while long-lived clusters harden degenerate-stellar binaries such that the SNIa rate may increase by orders of magnitude in those galaxies that were able to form long-lived clusters. The stellar initial mass function of the whole galaxy must be computed by adding the IMFs in the individual clusters. The resulting integrated galactic initial mass function (IGIMF) is top-light for SFRs < 10 M⊙/yr, and its slope and, more importantly, its upper stellar mass limit depend on the star-formation rate (SFR), explaining naturally the mass–metallicity relation of galaxies. Based on the IGIMF theory, the re-calibrated Hα-luminosity–SFR relation implies dwarf irregular galaxies to have the same gas-depletion time-scale as major disk galaxies, implying a major change of our concept of dwarf-galaxy evolution. A galaxy transforms about 0.3 per cent of its neutral gas mass every 10 Myr into stars. The IGIMF-theory also naturally leads to the observed radial Hα cutoff in disk galaxies without a radial star-formation cutoff. It emerges that the thorough understanding of the physics and distribution of star clusters may be leading to a major paradigm shift in our understanding of galaxy evolution.

Download Full-text

The Initial Mass Function of Be Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900195270 ◽

2004 ◽

Vol 215 ◽

pp. 83-84

Author(s):

J. Zorec ◽

R. Levenhagen ◽

J. Chauville ◽

Y. Frémat ◽

D. Ballereau ◽

...

Keyword(s):

Star Formation ◽

Main Sequence ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Be Stars ◽

Fast Rotation ◽

Formation Conditions ◽

The Imf

Allowing for systematic differences in the counting of Be Stars due to their overluminosity, changes produced by their fast rotation on spectral types and time spent in the main sequence, a difference between the IMF (Be) and IMF(B) appears, which indicates that the appearance of the Be phenomenon may relay on differences in the initial star formation conditions.

Download Full-text

Star Formation in Cooling Flows

Symposium - International Astronomical Union ◽

10.1017/s0074180900185158 ◽

1987 ◽

Vol 127 ◽

pp. 167-177

Author(s):

R. W. O'Connell

Keyword(s):

Star Formation ◽

Stellar Populations ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Flow Forming ◽

Cooling Flows ◽

Cd Galaxies ◽

Ngc 1275 ◽

The Imf

Star formation, probably with an abnormal initial mass function, represents the most plausible sink for the large amounts of material being accreted by cD galaxies from cooling flows. There are three prominent cases (NGC 1275, PKS 0745-191, and Abell 1795) where cooling flows have apparently induced unusual stellar populations. Recent studies show that about 50% of other accreting cD's have significant ultraviolet excesses. It therefore appears that detectable accretion populations are frequently associated with cooling flows. The questions of the form of the IMF, the fraction of the flow forming stars, and the lifetime of the flow remain open.

Download Full-text

The Initial Mass Function in Young Star Clusters

Highlights of Astronomy ◽

10.1017/s1539299600006821 ◽

1986 ◽

Vol 7 ◽

pp. 489-499

Author(s):

Hans Zinnecker

Keyword(s):

Star Formation ◽

Recent Work ◽

Star Clusters ◽

Mass Function ◽

Young Star ◽

Initial Mass Function ◽

Initial Mass ◽

Stellar Content ◽

Young Star Clusters ◽

The Imf

AbstractThis review discusses both the earlier and the most recent work on the IMF in young star clusters. It is argued that the study of the stellar content of young star clusters offers the best chance of developing a theory of star formation and of the IMF.

Download Full-text

Initial mass function variations cannot explain the ionizing spectrum of low metallicity starbursts

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834359 ◽

2019 ◽

Vol 621 ◽

pp. A105 ◽

Cited By ~ 22

Author(s):

E. R. Stanway ◽

J. J. Eldridge

Keyword(s):

Stellar Population ◽

Spectral Synthesis ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Photon Flux ◽

Population Synthesis ◽

Stellar Population Synthesis ◽

Galaxy Populations ◽

The Imf

Aims. Observations of both galaxies in the distant Universe and local starbursts are showing increasing evidence for very hard ionizing spectra that stellar population synthesis models struggle to reproduce. Here we explore the effects of the assumed stellar initial mass function (IMF) on the ionizing photon output of young populations at wavelengths below key ionization energy thresholds. Methods. We use a custom set of binary population and spectral synthesis (BPASS) models to explore the effects of IMF assumptions as a function of metallicity, IMF slope, upper mass limit, IMF power law break mass and sampling. Results. We find that while the flux capable of ionizing hydrogen is only weakly dependent on IMF parameters, the photon flux responsible for the He II and O VI lines is far more sensitive to assumptions. In our current models this flux arises primarily from helium and Wolf-Rayet stars which have partially or fully lost their hydrogen envelopes. The timescales for formation and evolution of both Wolf Rayet stars and helium dwarfs, and hence inferred population age, are affected by choice of model IMF. Even the most extreme IMFs cannot reproduce the He II ionizing flux observed in some high redshift galaxies, suggesting a source other than stellar photospheres. Conclusions. We caution that detailed interpretation of features in an individual galaxy spectrum is inevitably going to be subject to uncertainties in the IMF of its contributing starbursts. We remind the community that the IMF is fundamentally a statistical construct, and that stellar population synthesis models are most effective when considering entire galaxy populations rather than individual objects.

Download Full-text

Fornax 3D project: a two-dimensional view of the stellar initial mass function in the massive lenticular galaxy FCC 167

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935360 ◽

2019 ◽

Vol 626 ◽

pp. A124 ◽

Cited By ~ 9

Author(s):

I. Martín-Navarro ◽

M. Lyubenova ◽

G. van de Ven ◽

J. Falcón-Barroso ◽

L. Coccato ◽

...

Keyword(s):

Stellar Population ◽

Population Analysis ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Dynamical Structure ◽

Two Dimensional ◽

Early Type ◽

Stellar Initial Mass Function ◽

The Imf

The stellar initial mass function (IMF) regulates the baryonic cycle within galaxies, and is a key ingredient for translating observations into physical quantities. Although it was assumed to be universal for decades, there is now growing observational evidence showing that the center of massive early-type galaxies hosts a larger population of low-mass stars than is expected based on observations from the Milky Way. Moreover, these variations in the IMF have been found to be related to radial metallicity variations in massive galaxies. We present here a two-dimensional stellar population analysis of the massive lenticular galaxy FCC 167 (NGC 1380) as part of the Fornax3D project. Using a newly developed stellar population fitting scheme, we derive a full two-dimensional IMF map of an early-type galaxy. This two-dimensional analysis allows us go further than a radial analysis, showing how the metallicity changes along a disk-like structure while the IMF follows a distinct, less disky distribution. Thus, our findings indicate that metallicity cannot be the sole driver of the observed radial IMF variations. In addition, a comparison with the orbital decomposition shows suggestive evidence of a coupling between stellar population properties and the internal dynamical structure of FCC 167, where metallicity and IMF maps seem to track the distribution of cold and warm orbits, respectively.

Download Full-text

Fragmentation and the Initial Mass Function

International Astronomical Union Colloquium ◽

10.1017/s0252921100023472 ◽

1989 ◽

Vol 120 ◽

pp. 44-55

Author(s):

Richard B. Larson

Keyword(s):

Star Formation ◽

Power Law ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Solar Mass ◽

Universal Feature ◽

Basic Features ◽

The Imf

A central problem in the theory of star formation is to understand the spectrum of masses, or Initial Mass Function, with which stars are formed. The fundamental role of the IMF in galactic evolution has been described by Tinsley (1980), and an extensive review of evidence concerning the IMF and its possible variability has been presented by Scalo (1986). Although the IMF derived from the observations is subject to many uncertainties, two basic features seem reasonably well established. One is that the typical stellar mass, defined such that equal amounts of matter condense into stars above and below this mass, is within a factor of 3 of one solar mass. A theory of star formation should therefore be able to explain why most stars are formed with masses of order one solar mass. The second apparently universal feature is that the IMF for relatively massive stars can be approximated by a power law with a slope not greatly different from that originally proposed by Salpeter (1955). Thus we also need to understand why the IMF always has a similar power-law tail toward higher masses.

Download Full-text