skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Hierarchical Star Formation in Turbulent Media: Evidence from Young Star Clusters

Abstract

We present an analysis of the positions and ages of young star clusters in eight local galaxies to investigate the connection between the age difference and separation of cluster pairs. We find that star clusters do not form uniformly but instead are distributed so that the age difference increases with the cluster pair separation to the 0.25–0.6 power, and that the maximum size over which star formation is physically correlated ranges from ∼200 pc to ∼1 kpc. The observed trends between age difference and separation suggest that cluster formation is hierarchical both in space and time: clusters that are close to each other are more similar in age than clusters born further apart. The temporal correlations between stellar aggregates have slopes that are consistent with predictions of turbulence acting as the primary driver of star formation. The velocity associated with the maximum size is proportional to the galaxy’s shear, suggesting that the galactic environment influences the maximum size of the star-forming structures.

Authors:
;  [1];  [2]; ;  [3]; ; ; ; ;  [4];  [5];  [6];  [7];  [8];  [9];  [10];  [11];  [12];  [13]
  1. Astronomy Department, University of Massachusetts, Amherst, MA 01003 (United States)
  2. IBM Research Division, T.J. Watson Research Center, Yorktown Heights, NY (United States)
  3. Department of Astronomy, The Oskar Klein Centre, Stockholm University, Stockholm (Sweden)
  4. Space Telescope Science Institute, Baltimore, MD (United States)
  5. California Institute of Technology, 1200 East California Boulevard, Pasadena, CA (United States)
  6. Department of Physics and Astronomy, University of Wyoming, Laramie, WY (United States)
  7. Institute for Computational Cosmology and Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham (United Kingdom)
  8. Department of Astronomy, University of Wisconsin–Madison, Madison, WI (United States)
  9. Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, D-69120 Heidelberg (Germany)
  10. Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, D-69120, Heidelberg (Germany)
  11. Department of Astronomy, New Mexico State University, Las Cruces, NM (United States)
  12. Gemini Observatory, La Serena (Chile)
  13. Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611 (Australia)
Publication Date:
OSTI Identifier:
22663527
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 842; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CORRELATIONS; FORECASTING; MILKY WAY; SPACE; STAR CLUSTERS; STARS; TURBULENCE; ULTRAVIOLET RADIATION; VELOCITY

Citation Formats

Grasha, K., Calzetti, D., Elmegreen, B. G., Adamo, A., Messa, M., Aloisi, A., Bright, S. N., Lee, J. C., Ryon, J. E., Ubeda, L., Cook, D. O., Dale, D. A., Fumagalli, M., Gallagher III, J. S., Gouliermis, D. A., Grebel, E. K., Kahre, L., Kim, H., and Krumholz, M. R., E-mail: kgrasha@astro.umass.edu. Hierarchical Star Formation in Turbulent Media: Evidence from Young Star Clusters. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA740B.
Grasha, K., Calzetti, D., Elmegreen, B. G., Adamo, A., Messa, M., Aloisi, A., Bright, S. N., Lee, J. C., Ryon, J. E., Ubeda, L., Cook, D. O., Dale, D. A., Fumagalli, M., Gallagher III, J. S., Gouliermis, D. A., Grebel, E. K., Kahre, L., Kim, H., & Krumholz, M. R., E-mail: kgrasha@astro.umass.edu. Hierarchical Star Formation in Turbulent Media: Evidence from Young Star Clusters. United States. doi:10.3847/1538-4357/AA740B.
Grasha, K., Calzetti, D., Elmegreen, B. G., Adamo, A., Messa, M., Aloisi, A., Bright, S. N., Lee, J. C., Ryon, J. E., Ubeda, L., Cook, D. O., Dale, D. A., Fumagalli, M., Gallagher III, J. S., Gouliermis, D. A., Grebel, E. K., Kahre, L., Kim, H., and Krumholz, M. R., E-mail: kgrasha@astro.umass.edu. Sat . "Hierarchical Star Formation in Turbulent Media: Evidence from Young Star Clusters". United States. doi:10.3847/1538-4357/AA740B.
@article{osti_22663527,
title = {Hierarchical Star Formation in Turbulent Media: Evidence from Young Star Clusters},
author = {Grasha, K. and Calzetti, D. and Elmegreen, B. G. and Adamo, A. and Messa, M. and Aloisi, A. and Bright, S. N. and Lee, J. C. and Ryon, J. E. and Ubeda, L. and Cook, D. O. and Dale, D. A. and Fumagalli, M. and Gallagher III, J. S. and Gouliermis, D. A. and Grebel, E. K. and Kahre, L. and Kim, H. and Krumholz, M. R., E-mail: kgrasha@astro.umass.edu},
abstractNote = {We present an analysis of the positions and ages of young star clusters in eight local galaxies to investigate the connection between the age difference and separation of cluster pairs. We find that star clusters do not form uniformly but instead are distributed so that the age difference increases with the cluster pair separation to the 0.25–0.6 power, and that the maximum size over which star formation is physically correlated ranges from ∼200 pc to ∼1 kpc. The observed trends between age difference and separation suggest that cluster formation is hierarchical both in space and time: clusters that are close to each other are more similar in age than clusters born further apart. The temporal correlations between stellar aggregates have slopes that are consistent with predictions of turbulence acting as the primary driver of star formation. The velocity associated with the maximum size is proportional to the galaxy’s shear, suggesting that the galactic environment influences the maximum size of the star-forming structures.},
doi = {10.3847/1538-4357/AA740B},
journal = {Astrophysical Journal},
number = 1,
volume = 842,
place = {United States},
year = {Sat Jun 10 00:00:00 EDT 2017},
month = {Sat Jun 10 00:00:00 EDT 2017}
}
  • We study a model of rapidly cooling shocked stellar winds in young massive clusters and estimate the circumstances under which secondary star formation, out of the reinserted winds from a first stellar generation (1G), is possible. We have used two implementations of the model: a highly idealized, computationally inexpensive, spherically symmetric semi-analytic model, and a complex, three-dimensional radiation-hydrodynamic, simulation; they are in a good mutual agreement. The results confirm our previous findings that, in a cluster with 1G mass 10{sup 7} M {sub ⊙} and half-mass–radius 2.38 pc, the shocked stellar winds become thermally unstable, collapse into dense gaseous structuresmore » that partially accumulate inside the cluster, self-shield against ionizing stellar radiation, and form the second generation (2G) of stars. We have used the semi-analytic model to explore a subset of the parameter space covering a wide range of the observationally poorly constrained parameters: the heating efficiency, η {sub he}, and the mass loading, η {sub ml}. The results show that the fraction of the 1G stellar winds accumulating inside the cluster can be larger than 50% if η {sub he} ≲ 10%, which is suggested by the observations. Furthermore, for low η {sub he}, the model provides a self-consistent mechanism predicting 2G stars forming only in the central zones of the cluster. Finally, we have calculated the accumulated warm gas emission in the H30 α recombination line, analyzed its velocity profile, and estimated its intensity for super star clusters in interacting galaxies NGC4038/9 (Antennae) showing that the warm gas should be detectable with ALMA.« less
  • We present a study of the hierarchical clustering of the young stellar clusters in six local (3–15 Mpc) star-forming galaxies using Hubble Space Telescope broadband WFC3/UVIS UV and optical images from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey). We identified 3685 likely clusters and associations, each visually classified by their morphology, and we use the angular two-point correlation function to study the clustering of these stellar systems. We find that the spatial distribution of the young clusters and associations are clustered with respect to each other, forming large, unbound hierarchical star-forming complexes that are in general very young. Themore » strength of the clustering decreases with increasing age of the star clusters and stellar associations, becoming more homogeneously distributed after ∼40–60 Myr and on scales larger than a few hundred parsecs. In all galaxies, the associations exhibit a global behavior that is distinct and more strongly correlated from compact clusters. Thus, populations of clusters are more evolved than associations in terms of their spatial distribution, traveling significantly from their birth site within a few tens of Myr, whereas associations show evidence of disruption occurring very quickly after their formation. The clustering of the stellar systems resembles that of a turbulent interstellar medium that drives the star formation process, correlating the components in unbound star-forming complexes in a hierarchical manner, dispersing shortly after formation, suggestive of a single, continuous mode of star formation across all galaxies.« less
  • We analyze the relationship between maximum cluster mass and surface densities of total gas ({Sigma}{sub gas}), molecular gas ({Sigma}{sub H{sub 2}}), neutral gas ({Sigma}{sub H{sub I}}), and star formation rate ({Sigma}{sub SFR}) in the grand-design galaxy M51, using published gas data and a catalog of masses, ages, and reddenings of more than 1800 star clusters in its disk, of which 223 are above the cluster mass distribution function completeness limit. By comparing the two-dimensional distribution of cluster masses and gas surface densities, we find for clusters older than 25 Myr that M{sub 3rd}{proportional_to}{Sigma}{sub H{sub I}{sup 0.4{+-}0.2}}, whereM{sub 3rd} is themore » median of the five most massive clusters. There is no correlation with{Sigma}{sub gas},{Sigma}{sub H2}, or{Sigma}{sub SFR}. For clusters younger than 10 Myr, M{sub 3rd}{proportional_to}{Sigma}{sub H{sub I}{sup 0.6{+-}0.1}} and M{sub 3rd}{proportional_to}{Sigma}{sub gas}{sup 0.5{+-}0.2}; there is no correlation with either {Sigma}{sub H{sub 2}} or{Sigma}{sub SFR}. The results could hardly be more different from those found for clusters younger than 25 Myr in M33. For the flocculent galaxy M33, there is no correlation between maximum cluster mass and neutral gas, but we have determined M{sub 3rd}{proportional_to}{Sigma}{sub gas}{sup 3.8{+-}0.3}, M{sub 3rd}{proportional_to}{Sigma}{sub H{sub 2}{sup 1.2{+-}0.1}}, and M{sub 3rd}{proportional_to}{Sigma}{sub SFR}{sup 0.9{+-}0.1}. For the older sample in M51, the lack of tight correlations is probably due to the combination of strong azimuthal variations in the surface densities of gas and star formation rate, and the cluster ages. These two facts mean that neither the azimuthal average of the surface densities at a given radius nor the surface densities at the present-day location of a stellar cluster represent the true surface densities at the place and time of cluster formation. In the case of the younger sample, even if the clusters have not yet traveled too far from their birth sites, the poor resolution of the radio data compared to the physical sizes of the clusters results in measured{Sigma} that are likely quite diluted compared to the actual densities relevant for the formation of the clusters.« less
  • We analyze the relationship between maximum cluster mass M{sub max} and surface densities of total gas ({Sigma}{sub gas}), molecular gas ({Sigma}{sub H{sub 2}}), and star formation rate ({Sigma}{sub SFR}) in the flocculent galaxy M 33, using published gas data and a catalog of more than 600 young star clusters in its disk. By comparing the radial distributions of gas and most massive cluster masses, we find that M{sub max}{proportional_to}{Sigma}{sup 4.7{+-}0.4}{sub gas}, M{sub max}{proportional_to}{Sigma}{sup 1.3{+-}0.1}{sub H{sub 2}}, and M{sub max}{proportional_to}{Sigma}{sup 1.0{+-}0.1}{sub SFR}. We rule out that these correlations result from the size of the sample; hence, the change of the maximummore » cluster mass must be due to physical causes.« less
  • The popular idea that star formation has proceeded sequentially from lowest to highest mass members in open clusters is examined critically. For extremely young clusters, such as NGC 2264 and NGC 6530, this sequential hypothesis is a consequence of the assignment of pre-main-sequence contraction ages to all member stars. However, such ages yield a formation history which is implausible from a physical point of view, since the critical time for the onset of formation at any stellar mass is equal to the pre-main-sequence contraction time for that mass. Moreover, these ages are in conflict with the strong observational evidence thatmore » a substantial fraction of cluster members have already reached the main sequence. After reconsideration of the probable main-sequence members, the stellar ages in NGC 2264 and NGC 6530 are consistent with a variety of formation histories, and, in particular, with the view that all stellar masses form in approximately the same interval of time within a given cluster, i.e., that there is no mass-age correlation. A notion closely related to the sequential hypothesis, that the total star-formation rate increases exponentially with time, is subject to the same criticism.« less