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Halo white dwarfs and the hot intergalactic medium
NUCLEAR PHYSICS A ELSEVIER
Nuclear Physics A621 (1997) 580c-583c
Halo White Dwarfs and the Hot Intergalactic Medium B. D. Fields and G. J. Mathews a and D. N. Schramm b aUniversity of Notre Dame, Department of Physics Notre Dame, IN 46635, USA bUniversity of Chicago Chicago, IL 60637, USA We describe the formation of hot intergalactic gas along with baryonic remnants in the halo. In this scenario, the mass and metallicity of the hot intracluster gas relates directly to the production of baryonic remnants during the collapse of the galactic halos. We construct a schematic but self-consistent model in which early bursts of star formation lead to a large remnant population in the halo, and to the outflow of stellar ejecta into the halo and ultimately the Local Group. This study suggests that the microlensing objects in the halo may predominantly be ~ 0.5Me white dwarfs, assuming the initial mass function for early star formation favored the formation of intermediate mass stars with m ~> 1M o. 1. I N T R O D U C T I O N We describe here a model which relates recent observations of dark matter in the Galactic halo, and hot intergalactic gas in groups and clusters of galaxies [1]. On the one hand, dark halos have long been known to be present in other galaxies and almost certainly our own as well. Recently, the presence of dark matter in our Galactic halo has been directly confirmed by microlensing observations towards the LMC [2,3]. These remarkable experiments have detected massive compact halo objects (MACHOs), and thus are the first direct detection of dark matter in the universe. Present estimates of the lensing object's mass, m ~ 0.5Me, is suggestive of white dwarfs [2]. On the other hand, hot intergalactic gas is found to be ubiquitous in clusters, and has recently been observed in groups [4,5]. This X-ray gas is metal enriched, more so in clusters (Fe ~ Fee/3 ) than in groups (Fe/Fe o ~ 1/10), but in both cases the composition is far from primordial. The presence of these heavy element implies that some of the gas has undergone significant processing through massive stars, and subsequent ejection. Our model is an attempt to connect these observations. To have halo white dwarfs demands that large stellar processing occurred in the past. The presence of metals in the hot gas also requires stellar processing, as well as a mechanism for the ejecta to escape to the intergalactic medium (IGM). Therefore, we posit that there were strong bursts of star formation in the early Galaxy. Most of the stars are now dead: the remnants are MACHOs; the ejecta were lost in galactic wind, become IGM. There are important constraints on such a scenario [6]. We have considered these, and 0375-9474/97/$17.00 © 1997- Elsevier Science B.V. All rights reserved. PII: S0375-9474(97)00307-2
B.D. Fields et al./Nuclear Physics A621 (1997) 580c-583c
58 lc
find that they are obeyed within a specific parameter range. Thus, we find that this model is presently viable; furthermore, it is close to the observational limits on several fronts and so should be confirmed or ruled out soon. 2. O B S E R V A T I O N S
AND CONSTRAINTS
Our model should apply generally to groups and clusters, but here we focus on the Local Group. This consists of two spiral galaxies, each having components both luminous and not. The visible portion of our Galaxy consists in a disk of mass ~ 6 x 101°MG, and a central bulge with about about 101°M®. The halo mass is uncertain as it is not clear how far the halo extends. Nevertheless, the halo seems to contain at least 5 × 101~Mo. The Local Group itself is estimated [7] to have a total mass 3 - 5 x 1012MG. We will assume that the Local Group's X-ray properties are similar to those of similar groups. In groups which clearly evince X-ray gas [4], the temperatures are typically very close to T ~ keV, with metallicity that is poorly determined [5] but of order 10-20% solar. The gas masses inferred for these groups vary more than the temperature does, and moreover the gas mass apparently correlates strongly with the morphology of the group members. Specifically, the gas is directly correlated with the content of early-type galaxies [4]. Recent observations [8] have complicated this issue, suggesting that some spiral-dominated groups might still have a large amount of gas, but it is cool (T ~ 0.3 keV, at the limit of the ROSAT sensitivity). At any rate it appear to be true that if there is X-ray gas in spiral-rich groups, it is not both massive and hot. 3. T H E M O D E L
We model the Local Group evolution schematically, in the spirit of a hierarchical clustering picture. Namely, we establish a hierarchy of three mass scales: (1) protogalactic clouds, (2) galactic halos, and the group itself. Each mass scale corresponds to a primordial density fluctuation which must overcome the cosmological expansion. Thus the dynamics of each component comes from a collapse model, namely the behavior of a spherical overdensity [9]. The different mass scales are self-similar. At the smallest scale, the clouds contain stars, as well as both hot and cold gas. The halos include the clouds as sub-components, as well as their own component of hot gas and stars ejected from clouds. The group includes the two galaxies as a sub-component, as well as hot gas ejected from the halos. While the clouds contain only baryons, we introduce a component of non-baryonic dark matter at the halo and group levels. Star formation is a key ingredient of the model; stars provide significant heating, as well as nucleosynthesis products. Star formation occurs only in the cold gas components of the clouds, and has both a quiescent and merger-induced term, with the former dominant [9]. We include the production of metals and helium using the yields of Maeder [10], and we follow the luminosity evolution of the stars in the halo. The different mass scales interact via several mechanisms. One of these is merging, which reduces the number of clouds while increasing their average mass. After starting with about 106 clouds each with a mass (initially, 106Mo), these finally coalesce to become one massive object, the progenitor of the disk and bulge. We do not distinguish these
582c
B.D. Fields et al./Nuclear Physics A621 (1997) 580c-~583c
components nor follow the disk evolution, as we are only interested in the halo. We also allow for stellar enrichment of the gas (predominantly via supernovae), which contributes heat and metals. The gas may also be heated or cooled via p d V work, and radiative cooling is important as well. At each mass scale, the heating leads to evaporation of the hottest gas particles with thermal velocities above the local escape velocity. For the hot gas this process is very efficient: there is a significant wind at work at all levels, with mass loss even from the group itself (this leads to the presence of hot intergroup gas as a significant component of the dark baryons). The wind also functions to remove material after a single stellar processing and so prevents recycling which would otherwise lead to overproduction of metals and helium. The evolution depends sensitively on the choice of the initial mass function (IMF) for the halo. A remnant-rich halo scenario such as ours requires that the halo IMF was different from that of the disk. Whereas in the disk, stars with m < 1M e are by far the most numerous, they must be rare in the halo. as they are very long lived. Such stars would still be shining and lead to an unacceptably bright halo. Thus we must choose an IMF biased away from low mass (< 1Me) stars; arguments for such an IMF in the early Galaxy are presented in [11]. We parameterize the IMF as a log-normal form [12], characterized by a centroid mc and and dimensionless width a. In fact, we are able to obtain good results for a whole spectrum of IMF parameters; here we present results with mc = 2.3M® (the value suggested in [12]) and a = 1.6 (the present-day value; [13]). 4. R E S U L T S To sketch the basic results, we summarize the mass and metal budgets. The clouds merge to form the proto-disk and bulge; starting with a mass 106M@ and ending with 6.8 x 101°Me . The halos begin with a mass 1.3 x 1012M®, of which % is baryonic. The final mass is 4.5 x 1011M®, of which 33% is baryonic. The remnants are mostly white dwarfs, with 12% neutron stars. Thus there is a net loss of 7.8 x 1011Me of gas from the galaxies into the IGM. The group itself begins with a mass of 5.6 x 1012Me, and ends with a mass 4.3 x 1012Me, 16% of which is baryonic. Of the baryonic group mass, 2.7 x 1011Me resides in hot gas, a level below but close to the ROSAT limits. The group as a whole loses 1.3 x 1012Me of gas to the intergalactic medium medium; thus about 65% of the initial baryonic mass in the group is ejected later into intergalactic space. This amount of hot (ionized) material is consistent with Gunn-Peterson limits on the intergalactic medium. A similar anaylsis shows that the gas and star metallicities are reasonable. Constraints are considered in detail as discussed in [1]. Aside from nucleosynthesis issues, there are various luminosities to consider, arising from: long-lived stars in halo [6]; the now-cooled halo white dwarfs; and the diffuse radiation from the (presumably universal) occurrence of the starbursts themselves [14]. We find all of these luminosities to be acceptably low, but the white dwarf luminosity, and perhaps the diffuse background are near current limits and thus are potentially detectable. 5. D I S C U S S I O N
AND CONCLUSIONS
We have shown that one may a plausible model of galaxy evolution that relates the hot gas seen in galaxy aggregates to a halo population of remnants. Since we link these
B.D, Fields et al./Nuclear Physics A621 (1997) 580c-583c
583c
phenomena, confirmation of one in the Local Groups would imply, in our model, the existence of the other. Further, signatures of the model are within reach. Unless the Local Group is anomalous, then the expected hot gas mass is too cool to provide a source of the diffuse X-ray background, as suggested by [15]. However, such gas may still be observable (and perhaps already observed [8]), and so further work on such systems is crucial. Also promising are the signatures of white dwarfs. Aside from the microlensing experiments, the luminosity of these objects should also be detectable with further pencil beam and wide angle observations. Also, it is intriguing that several edge-on galaxies have an observed IR halo, as one would expect from cooled white dwarfs [16-19]. Finally, it is worth noting that, given the reality of the microlensing objects, it a very conservative guess that they are remnant (particularly since brown dwarfs are disallowed [20,21]). Thus, if white dwarf scenarios such as this one can be ruled out, we would be led to demand that the galaxy be dominated by something stranger still. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
B.D. Fields, G.J. Mathews, and D.N. Schramm (1996) submitted to Ap.J. D. Bennett, these proceedings. E. Aubourg et al. Nature365 (1993) 623. J.S. Mulchaey, D.S. Davis, R.F. Mushotzky, and D. Burstein, Ap.J. 456 (1996) 80. D.S. Davis, J.S. Mulchaey, R.F. Mushotzky, and D. Burstein, Ap.J. 460 (1996) 601. D. Ryu, K.A. Olive, and J. Silk, Ap.J. 353 (1990) 81. M. Fich, and S. Tremaine, ARAA (1991) 409. Ponman, T.J., Bourner, P.D.J., Ebeling, H., & BShringer, H. 1996, MNRAS, in press (astro-ph/9607114) G.J. Mathews, and D.N. Schramm, Ap.J. 404 (1993) 468. A. Maeder, Astron. and Astrophys. 264 (1992) 105. J. Silk, Phys. Reports 227 (1993) 143. F.C. Adams and G. Laughlin (1996) submitted to Ap.J. (astro-ph/9502006). Miller, G.E., & Scalo, J.M. 1979, ApJS, 41,513 S. Charlot, and J. Silk, Ap.J. 445 (1995) 124. Y. Suto, K. Makishima, Y. Ishisaki, and Y. Ogasaka, Ap.J. (1996) in press (astroph/9602061). Barnaby D., & Thronson, H.A. 1994, AJ, 107, 1717 Sackett, P.D., Morrison, H.L., Harding, P., & Boroson, T.A. 1994, Nature, 370, 441 Lequeux, J., Fort, B., Dantel-Fort, M., Cuillandre, J.C., & Mellier, Y. 1996, A~A, in press (astro-ph/9606172) Lehnert, M.D., &: Heckman, T.M. 1996, ApJ, 462, 651 C. Flynn, A. Gould, and J.N. Baheall, (1996), submitted to Ap.J. (astro-ph/9503035). S. Kwaler, Ap.J. (1996) in press.
Nuclear Physics A621 (1997) 580c-583c
Halo White Dwarfs and the Hot Intergalactic Medium B. D. Fields and G. J. Mathews a and D. N. Schramm b aUniversity of Notre Dame, Department of Physics Notre Dame, IN 46635, USA bUniversity of Chicago Chicago, IL 60637, USA We describe the formation of hot intergalactic gas along with baryonic remnants in the halo. In this scenario, the mass and metallicity of the hot intracluster gas relates directly to the production of baryonic remnants during the collapse of the galactic halos. We construct a schematic but self-consistent model in which early bursts of star formation lead to a large remnant population in the halo, and to the outflow of stellar ejecta into the halo and ultimately the Local Group. This study suggests that the microlensing objects in the halo may predominantly be ~ 0.5Me white dwarfs, assuming the initial mass function for early star formation favored the formation of intermediate mass stars with m ~> 1M o. 1. I N T R O D U C T I O N We describe here a model which relates recent observations of dark matter in the Galactic halo, and hot intergalactic gas in groups and clusters of galaxies [1]. On the one hand, dark halos have long been known to be present in other galaxies and almost certainly our own as well. Recently, the presence of dark matter in our Galactic halo has been directly confirmed by microlensing observations towards the LMC [2,3]. These remarkable experiments have detected massive compact halo objects (MACHOs), and thus are the first direct detection of dark matter in the universe. Present estimates of the lensing object's mass, m ~ 0.5Me, is suggestive of white dwarfs [2]. On the other hand, hot intergalactic gas is found to be ubiquitous in clusters, and has recently been observed in groups [4,5]. This X-ray gas is metal enriched, more so in clusters (Fe ~ Fee/3 ) than in groups (Fe/Fe o ~ 1/10), but in both cases the composition is far from primordial. The presence of these heavy element implies that some of the gas has undergone significant processing through massive stars, and subsequent ejection. Our model is an attempt to connect these observations. To have halo white dwarfs demands that large stellar processing occurred in the past. The presence of metals in the hot gas also requires stellar processing, as well as a mechanism for the ejecta to escape to the intergalactic medium (IGM). Therefore, we posit that there were strong bursts of star formation in the early Galaxy. Most of the stars are now dead: the remnants are MACHOs; the ejecta were lost in galactic wind, become IGM. There are important constraints on such a scenario [6]. We have considered these, and 0375-9474/97/$17.00 © 1997- Elsevier Science B.V. All rights reserved. PII: S0375-9474(97)00307-2
B.D. Fields et al./Nuclear Physics A621 (1997) 580c-583c
58 lc
find that they are obeyed within a specific parameter range. Thus, we find that this model is presently viable; furthermore, it is close to the observational limits on several fronts and so should be confirmed or ruled out soon. 2. O B S E R V A T I O N S
AND CONSTRAINTS
Our model should apply generally to groups and clusters, but here we focus on the Local Group. This consists of two spiral galaxies, each having components both luminous and not. The visible portion of our Galaxy consists in a disk of mass ~ 6 x 101°MG, and a central bulge with about about 101°M®. The halo mass is uncertain as it is not clear how far the halo extends. Nevertheless, the halo seems to contain at least 5 × 101~Mo. The Local Group itself is estimated [7] to have a total mass 3 - 5 x 1012MG. We will assume that the Local Group's X-ray properties are similar to those of similar groups. In groups which clearly evince X-ray gas [4], the temperatures are typically very close to T ~ keV, with metallicity that is poorly determined [5] but of order 10-20% solar. The gas masses inferred for these groups vary more than the temperature does, and moreover the gas mass apparently correlates strongly with the morphology of the group members. Specifically, the gas is directly correlated with the content of early-type galaxies [4]. Recent observations [8] have complicated this issue, suggesting that some spiral-dominated groups might still have a large amount of gas, but it is cool (T ~ 0.3 keV, at the limit of the ROSAT sensitivity). At any rate it appear to be true that if there is X-ray gas in spiral-rich groups, it is not both massive and hot. 3. T H E M O D E L
We model the Local Group evolution schematically, in the spirit of a hierarchical clustering picture. Namely, we establish a hierarchy of three mass scales: (1) protogalactic clouds, (2) galactic halos, and the group itself. Each mass scale corresponds to a primordial density fluctuation which must overcome the cosmological expansion. Thus the dynamics of each component comes from a collapse model, namely the behavior of a spherical overdensity [9]. The different mass scales are self-similar. At the smallest scale, the clouds contain stars, as well as both hot and cold gas. The halos include the clouds as sub-components, as well as their own component of hot gas and stars ejected from clouds. The group includes the two galaxies as a sub-component, as well as hot gas ejected from the halos. While the clouds contain only baryons, we introduce a component of non-baryonic dark matter at the halo and group levels. Star formation is a key ingredient of the model; stars provide significant heating, as well as nucleosynthesis products. Star formation occurs only in the cold gas components of the clouds, and has both a quiescent and merger-induced term, with the former dominant [9]. We include the production of metals and helium using the yields of Maeder [10], and we follow the luminosity evolution of the stars in the halo. The different mass scales interact via several mechanisms. One of these is merging, which reduces the number of clouds while increasing their average mass. After starting with about 106 clouds each with a mass (initially, 106Mo), these finally coalesce to become one massive object, the progenitor of the disk and bulge. We do not distinguish these
582c
B.D. Fields et al./Nuclear Physics A621 (1997) 580c-~583c
components nor follow the disk evolution, as we are only interested in the halo. We also allow for stellar enrichment of the gas (predominantly via supernovae), which contributes heat and metals. The gas may also be heated or cooled via p d V work, and radiative cooling is important as well. At each mass scale, the heating leads to evaporation of the hottest gas particles with thermal velocities above the local escape velocity. For the hot gas this process is very efficient: there is a significant wind at work at all levels, with mass loss even from the group itself (this leads to the presence of hot intergroup gas as a significant component of the dark baryons). The wind also functions to remove material after a single stellar processing and so prevents recycling which would otherwise lead to overproduction of metals and helium. The evolution depends sensitively on the choice of the initial mass function (IMF) for the halo. A remnant-rich halo scenario such as ours requires that the halo IMF was different from that of the disk. Whereas in the disk, stars with m < 1M e are by far the most numerous, they must be rare in the halo. as they are very long lived. Such stars would still be shining and lead to an unacceptably bright halo. Thus we must choose an IMF biased away from low mass (< 1Me) stars; arguments for such an IMF in the early Galaxy are presented in [11]. We parameterize the IMF as a log-normal form [12], characterized by a centroid mc and and dimensionless width a. In fact, we are able to obtain good results for a whole spectrum of IMF parameters; here we present results with mc = 2.3M® (the value suggested in [12]) and a = 1.6 (the present-day value; [13]). 4. R E S U L T S To sketch the basic results, we summarize the mass and metal budgets. The clouds merge to form the proto-disk and bulge; starting with a mass 106M@ and ending with 6.8 x 101°Me . The halos begin with a mass 1.3 x 1012M®, of which % is baryonic. The final mass is 4.5 x 1011M®, of which 33% is baryonic. The remnants are mostly white dwarfs, with 12% neutron stars. Thus there is a net loss of 7.8 x 1011Me of gas from the galaxies into the IGM. The group itself begins with a mass of 5.6 x 1012Me, and ends with a mass 4.3 x 1012Me, 16% of which is baryonic. Of the baryonic group mass, 2.7 x 1011Me resides in hot gas, a level below but close to the ROSAT limits. The group as a whole loses 1.3 x 1012Me of gas to the intergalactic medium medium; thus about 65% of the initial baryonic mass in the group is ejected later into intergalactic space. This amount of hot (ionized) material is consistent with Gunn-Peterson limits on the intergalactic medium. A similar anaylsis shows that the gas and star metallicities are reasonable. Constraints are considered in detail as discussed in [1]. Aside from nucleosynthesis issues, there are various luminosities to consider, arising from: long-lived stars in halo [6]; the now-cooled halo white dwarfs; and the diffuse radiation from the (presumably universal) occurrence of the starbursts themselves [14]. We find all of these luminosities to be acceptably low, but the white dwarf luminosity, and perhaps the diffuse background are near current limits and thus are potentially detectable. 5. D I S C U S S I O N
AND CONCLUSIONS
We have shown that one may a plausible model of galaxy evolution that relates the hot gas seen in galaxy aggregates to a halo population of remnants. Since we link these
B.D, Fields et al./Nuclear Physics A621 (1997) 580c-583c
583c
phenomena, confirmation of one in the Local Groups would imply, in our model, the existence of the other. Further, signatures of the model are within reach. Unless the Local Group is anomalous, then the expected hot gas mass is too cool to provide a source of the diffuse X-ray background, as suggested by [15]. However, such gas may still be observable (and perhaps already observed [8]), and so further work on such systems is crucial. Also promising are the signatures of white dwarfs. Aside from the microlensing experiments, the luminosity of these objects should also be detectable with further pencil beam and wide angle observations. Also, it is intriguing that several edge-on galaxies have an observed IR halo, as one would expect from cooled white dwarfs [16-19]. Finally, it is worth noting that, given the reality of the microlensing objects, it a very conservative guess that they are remnant (particularly since brown dwarfs are disallowed [20,21]). Thus, if white dwarf scenarios such as this one can be ruled out, we would be led to demand that the galaxy be dominated by something stranger still. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
B.D. Fields, G.J. Mathews, and D.N. Schramm (1996) submitted to Ap.J. D. Bennett, these proceedings. E. Aubourg et al. Nature365 (1993) 623. J.S. Mulchaey, D.S. Davis, R.F. Mushotzky, and D. Burstein, Ap.J. 456 (1996) 80. D.S. Davis, J.S. Mulchaey, R.F. Mushotzky, and D. Burstein, Ap.J. 460 (1996) 601. D. Ryu, K.A. Olive, and J. Silk, Ap.J. 353 (1990) 81. M. Fich, and S. Tremaine, ARAA (1991) 409. Ponman, T.J., Bourner, P.D.J., Ebeling, H., & BShringer, H. 1996, MNRAS, in press (astro-ph/9607114) G.J. Mathews, and D.N. Schramm, Ap.J. 404 (1993) 468. A. Maeder, Astron. and Astrophys. 264 (1992) 105. J. Silk, Phys. Reports 227 (1993) 143. F.C. Adams and G. Laughlin (1996) submitted to Ap.J. (astro-ph/9502006). Miller, G.E., & Scalo, J.M. 1979, ApJS, 41,513 S. Charlot, and J. Silk, Ap.J. 445 (1995) 124. Y. Suto, K. Makishima, Y. Ishisaki, and Y. Ogasaka, Ap.J. (1996) in press (astroph/9602061). Barnaby D., & Thronson, H.A. 1994, AJ, 107, 1717 Sackett, P.D., Morrison, H.L., Harding, P., & Boroson, T.A. 1994, Nature, 370, 441 Lequeux, J., Fort, B., Dantel-Fort, M., Cuillandre, J.C., & Mellier, Y. 1996, A~A, in press (astro-ph/9606172) Lehnert, M.D., &: Heckman, T.M. 1996, ApJ, 462, 651 C. Flynn, A. Gould, and J.N. Baheall, (1996), submitted to Ap.J. (astro-ph/9503035). S. Kwaler, Ap.J. (1996) in press.