The transients following GRB970228 and GRB970508 showed that these (and
probably all) GRBs are cosmological. However, the host galaxies expected to be
associated with these and other bursts are largely absent, indicating that
either bursts are further than expected or the host galaxies are underluminous.
This apparent discrepancy does not invalidate the cosmological hypothesis, but
instead host galaxy observations can test more sophisticated models.Comment: 5 pages, AIPPROC LaTeX, to appear in "Gamma-Ray Bursts, 4th
  Huntsville Symposium," eds. C. Meegan, R. Preece and T. Koshu

Band, David L.

Hartmann, Dieter H.

English

arXiv

Dieter H. Hartmann

David L. Band

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Properties of GRB host galaxies

The transients following GRB970228 and GRB970508 showed that these (and probably all) GRBs are cosmological. However, the host galaxies expected to be associated with these and other bursts are largely absent, indicating that either bursts are further than expected or the host galaxies are underluminous. This apparent discrepancy does not invalidate the cosmological hypothesis, but instead host galaxy observations can test more sophisticated models

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Clemson UniversityTigerPrintsPublications Physics and AstronomyWinter 12-9-1997Properties of GRB Host GalaxiesDieter H. HartmannDepartment of Physics and Astronomy, Clemson University, hdieter@clemson.eduDavid L. BandCASS, UC San DiegoFollow this and additional works at: https://tigerprints.clemson.edu/physastro_pubsThis Article is brought to you for free and open access by the Physics and Astronomy at TigerPrints. It has been accepted for inclusion in Publicationsby an authorized administrator of TigerPrints. For more information, please contact kokeefe@clemson.edu.Recommended CitationPlease use publisher's recommended citation.arXiv:astro-ph/9712125v1  9 Dec 1997Properties of GRB Host GalaxiesDieter H. Hartmann1 and David L. Band21Department of Physics and Astronomy, Clemson University, Clemson, SC 296342CASS, UC San Diego, La Jolla, CA 92093Abstract. The transients following GRB970228 and GRB970508 showed that these(and probably all) GRBs are cosmological. However, the host galaxies expected to beassociated with these and other bursts are largely absent, indicating that either burstsare further than expected or the host galaxies are underluminous. This apparentdiscrepancy does not invalidate the cosmological hypothesis, but instead host galaxyobservations can test more sophisticated models.THE ABSENCE OF THE EXPECTED HOSTGALAXIESObservations of the optical transients (OTs) from GRB970228 [19] andGRB970508 [2] have finally provided the smoking gun that bursts are cosmologi-cal. In most cosmological models bursts occur in host galaxies: are these galaxiespresent, and conversely, what can we learn from them? Underlying any confronta-tion of theory and data must be a well defined model. Here we show that the hostgalaxy observations are not consistent with the expectations of the simplest cosmo-logical model, and that these observations can be used to test more sophisticatedmodels.In the simplest (“minimal”) cosmological model the distance scale is derivedfrom the intensity distribution logN–logP assuming bursts are standard candleswhich do not evolve in rate or intensity. Bursts occur in normal galaxies at arate proportional to a galaxy’s luminosity. This model predicts the host galaxydistribution for a given burst. Are the expected host galaxies present?For GRB970228 an underlying extended object was found [19], but its redshiftand nature have not been established. If the observed “fuzz” is indeed a galaxy atz ∼ 1/4, it is ∼5 magnitudes fainter than expected for a galaxy at this redshift.For GRB970508 no obvious underlying galaxy was observed [14] and the nearestextended objects have separations of several arcseconds, but spectroscopy with theKeck telescopes [10] led to the discovery of absorption and emission lines giving aGRB redshift of z ≥ 0.835. The HST magnitude limit Rlim ∼ 25.5 [14] for a galaxycoincident with the transient again suggests a host galaxy fainter than expected.Similar conclusions follow from the inspection of IPN error boxes [1,16,17], butsee also [6,20]. This absence of sufficiently bright host galaxies is often called the“no-host” problem, which is a misnomer. The point simply is that if galaxies suchas the Milky Way provide the hosts to most bursters, and if their redshifts are lessthan unity, as predicted by the minimal model, we expect to find bright galaxiesinside a large fraction of the smallest IPN error boxes.To demonstrate this quantitatively, consider the apparent magnitude of a typicalhost galaxy, which we assume has M∗(B) = −20 (approximately the absolutemagnitude of an L∗ galaxy—see discussion below). Using Peebles’ notation [13],the apparent magnitude ism = 42.38 +M + 5 log [y(z)(1 + z)] +K(z) + E(z) + A(Ω, z) + χ(z) , (1)where K(z) is the usual K-correction, E(z) corrects for the possible evolution of thehost galaxy’s spectrum, A is the sum of Galactic foreground (position dependent)and intergalactic extinction, and χ(z) represents any corrections that apply inhierarchical galaxy formation scenarios, where galaxies are assembled through themerger of star forming subunits. The commonly found term 5 log(h) is alreadyabsorbed in eq. (1). Neglecting potentially large corrections from the K, E, A,and χ(z) terms, a host like the Milky Way with M ∼ −20 would have an apparentmagnitude m ∼ 22 for redshifts of order unity. Several small IPN error boxeshave no galaxy of this magnitude or brighter. Our simplified treatment agrees withSchaefer’s conclusion [16,17] that typical galaxies at the calculated burst distanceare absent from burst error boxes.Thus bursts are further than predicted from the logN–logP distribution withoutevolution, or they occur in underluminous galaxies; an extreme limit of the latteralternative is that bursts do not occur in galaxies.HOST GALAXIES AS A PROBE OF COSMOLOGICALMODELSThe search for host galaxies is a powerful test of cosmological burst models.From the two above mentioned OTs we conclude that GRBs are cosmological,but the observations have not fixed the distance scale quantitatively, nor havethey determined the energy source. While the x-ray, optical, and radio lightcurves(for GRB970508 only) are consistent with the predictions of the basic “fireballafterglow” picture, the fireball’s central engine could be the merger of a neutronstar binary, the collapse of a massive, rotating star, or the jet produced by accretiononto a massive black hole residing at the center of an otherwise normal galaxy.The host galaxies found within burst error boxes are a powerful discriminantbetween different models for the burst energy source. Note that the region withinwhich a host would be acceptable surrounding the sub-arcsecond localizations of anOT is effectively the error box for the host galaxy. Almost all models assume thatbursts are associated with galaxies; the issue is the relationship between the burstand the host. In models such as the momentary activation of a dormant massiveblack hole the burst rate per galaxy is constant. On the other hand bursts are anendpoint of stellar evolution in most models, and therefore to first order we expectthe burst rate per galaxy in these models to be proportional to the galaxy’s massand thus luminosity. These two model classes have different host galaxy luminosityfunctions ψ(M) with different average values of M (the absolute magnitude). Inthe first case, ψ(M) is proportional to the normal galaxy luminosity function, whilein the second case ψ(M) is proportional to the normal galaxy luminosity functionweighted by the luminosity L ∝ 10−0.4M . We approximate the normal galaxyluminosity function with the Schechter function:Φ(M) = κ 100.4(M∗−M)(α+1)[exp(−100.4(M∗−M))](2)where κ is the normalization, α is the slope of the faint end, andM∗ is the absolutemagnitude of an L∗ galaxy. Here we use α = −1. In the B band M∗(B) = −19.53,which corresponds to L∗(B) = 1.8×1010L⊙ h−275 ∼ 3×1011 L⊙(B) h−275 . In Figure 1we show the cumulative distributions for the host galaxy magnitudes for the twomodel classes. As can be seen, the average host galaxy magnitude (i.e., at 0.5)differs by ∼ 1.75 magnitudes.However, we can make better predictions about the host galaxies in cosmologicalmodels where bursts are a stellar endpoint. In such models, the burst rate shouldbe a function of the star formation rate (SFR). If there is a substantial delay (e.g.,of order a billion years or more) between the GRB event and the star formingactivity that created the progenitor, then the burst rate integrates over a galaxy’sSFR, and we would not expect the host galaxy to display the signatures of recentstar formation. Furthermore, if the progenitor is given a large velocity, then it maytravel a large distance from the host galaxy before bursting, and it may becomeimpossible to associate a galaxy with the burst.FIGURE 1. The cumulative distribution of host galaxy magnitudes if their luminosity functionis weighted by number or luminosity. If GRBs trace light, the 50% point is near M∗. If it isproportional to galaxy number a typical host would be ∼ 2 magnitudes fainter.In many models the burst occurs shortly after its progenitor star forms (e.g.,within a hundred million years or less). We would then expect that on averagebursts would occur in galaxies showing evidence of recent star formation. Theburst rate should be proportional to the SFR, both for individual galaxies and fora given cosmological epoch.In particular, the burst rate and the SFR should have the same history, as wasrecently considered by several groups [12,15,18,21]. Extensive redshift surveys anddata from the Hubble Deep Field have reliably determined the cosmic star formationhistory to z ∼ 5 [3,4,7–9]. The data clearly suggest a rapid increase in the comovingSFR density with increasing redshift, SFR ∝ (1 + z)4, reaching a peak rate (atz ∼ 1.5) about 10–20 times higher than the present-day rate, and decreasing slowlyto the present value by z ∼ 5. This evolution function, η(z), enters the differentialrate vs. (bolometric) peak flux∂PR ∝ P−5/2 E(z)−1 η(z) (1 + z)−3 [(1 + z)∂zy(z) + y(z)]−1 , (3)where E(z) and y(z) are defined in [13]. For small redshifts the logarithmic slopeof this function is Euclidean, i.e. −5/2. The solid curve of Figure 2 shows theeffects of geometry (bending of logN–logP) and the dashed curve demonstrates howη(z) compensates for the geometry out to the redshift at which the cosmic SFRpeaks. At larger redshifts the effects of geometry and decreasing SFR then combineand the slope flattens quickly. Comparison with BATSE data suggests that thisSFR model deviates from the pseudo-Euclidean slope too abruptly. While severalstudies [15,18,21] report that the observed SFR generates a brightness distributionconsistent with BATSE data, our findings support the different result of Petrosian &Lloyd [12], who suggest that other evolutionary effects must be present in additionto the density evolution described by η(z). While a good fit to the data requiresFIGURE 2. The slope of the differential GRB brightness distribution. Non-evolving sources(solid curve) quickly show significant deviation from the Euclidean value −5/2 with increasingredshift. If the GRB rate is proportional to the SFR (see text) the apparent Euclidean slopeextends to z ∼ 1 (dashed curve). For greater z the geometry of the universe together with a nowdecreasing burst rate cause the slope to deviate rapidly from −5/2.a more sophisticated model of source evolution, the basic message is likely to bethe same: the logN–logP distribution does not exclude GRB redshifts much greaterthan unity.Therefore, the absence of the host galaxies predicted by the “minimal” cosmo-logical model does not call the cosmological origin of bursts into question. Instead,host galaxy observations will teach us where bursts occur.REFERENCES1. Band, D., and Hartmann, D. H., Ap. J. 493, in press (1998).2. Bond, H. E., IAU Circ. 6654 (1997).3. Connolly, A. J., et al., astro-ph/9706255 (1997).4. Ellis, R. S., ARAA 35, 389 (1997).5. Fuller, G. M., and Shi, X., astro-ph/9711020 (1997).6. Larson, S. B., Ap. J. 491, in press (1997).7. Lilly, S., in Critical Dialogues in Cosmology, ed. N. Turok, Singapore: World Scien-tific, 1997, p. 465.8. Madau, P., et al., M.N.R.A.S 283, 1388 (1996)9. Madau, P., et al., astro-ph/9709147 (1997).10. Metzger, M. R., et al., IAU Circ. 6631, 6655, and 6676 (1997).11. Paczynski, B., astro-ph/9710086 (1997).12. Petrosian, V., and Lloyd, N. M., these proceedings (1998).13. Peebles, P. J. E., Principles of Physical Cosmology, Princeton: Princeton UniversityPress, 1993.14. Pian, E., et al., Ap. J., submitted (1997).15. Sahu, K. C., et al., Ap. J. Lett. 489, L127 (1997).16. Schaefer, B. E., in Gamma-Ray Bursts: Observations, Analyses and Theories, eds.C. Ho, R. I. Epstein, and E. E. Fenimore, Cambridge: Cambridge University Press,1992, p. 107.17. Schaefer, B. E., et al., Ap. J. 489, 693 (1997).18. Totani, T., Ap. J. Lett. 486, L71 (1997).19. Van Paradijs, J., et al., Nature 386, 686 (1997).20. Vrba, F. J., et al., these proceedings (1998).21. Wijers, R.A.M.J., et al., M.N.R.A.S., in press (1997).

Properties of GRB Host Galaxies

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Properties of GRB Host Galaxies

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