Stellar halos around galaxies retain fundamental evidence of the processes
which lead to their build up. Sophisticated models of galaxy formation in a
cosmological context yield quantitative predictions about various observable
characteristics, including the amount of substructure, the slope of radial mass
profiles and three dimensional shapes, and the properties of the stellar
populations in the halos. The comparison of such models with the observations
provides constraints on the general picture of galaxy formation in the
hierarchical Universe, as well as on the physical processes taking place in the
halos formation. With the current observing facilities, stellar halos can be
effectively probed only for a limited number of nearby galaxies. In this paper
we illustrate the progress that we expect in this field with the future ground
based large aperture telescopes (E-ELT) and with space based facilities as
JWST.Comment: 6 pages, 4 figures, to appear in Proc. IAU Symposium 317 "The General
  Assembly of Stellar Halos: Structure, Origin and Evolution.

Falomo, Renato

Greggio, Laura

Uslenghi, Michela

English

arXiv

Stellar halos around galaxies retain fundamental evidence of the processes which lead to their build up. Sophisticated models of galaxy formation in a cosmological context yield quantitative predictions about various observable characteristics, including the amount of substructure, the slope of radial mass profiles and three dimensional shapes, and the properties of the stellar populations in the halos. The comparison of such models with the observations provides constraints on the general picture of galaxy formation in the hierarchical Universe, as well as on the physical processes taking place in the halos formation. With the current observing facilities, stellar halos can be effectively probed only for a limited number of nearby galaxies. In this paper we illustrate the progress that we expect in this field with the future ground based large aperture telescopes (E-ELT) and with space based facilities as JWST

GREGGIO, Laura

FALOMO, Renato

USLENGHI, MICHELA

OA@INAF - Istituto Nazionale di Astrofisica

2016Publication Year2020-07-03T14:05:46ZAcceptance in OA@INAFStudying stellar halos with future facilitiesTitleGREGGIO, Laura; FALOMO, Renato; USLENGHI, MICHELAAuthors10.1017/S1743921315007024DOIhttp://hdl.handle.net/20.500.12386/26332HandlePROCEEDINGS OF THE INTERNATIONAL ASTRONOMICAL UNIONSeriesvol. 11, S317NumberThe General Assembly of Galaxy Halos: Structure,Origin and EvolutionProceedings IAU Symposium No. 317, 2015A. Bragaglia, M. Arnaboldi, M. Rejkuba & D. Romano, eds.c© International Astronomical Union 2016doi:10.1017/S1743921315007024Studying stellar halos with future facilitiesLaura Greggio1, Renato Falomo1 and Michela Uslenghi21 INAF, Osservatorio Astronomico di Padova,Vicolo dell’Osservatorio 5, 35122 Padova, Italyemai:laura.greggio@oapd.inaf.it, renato.falomo@oapd.inaf.it2 INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica,Via Bassini 15, 20133 Milano, ItalyAbstract. Stellar halos around galaxies retain fundamental evidence of the processes whichlead to their build up. Sophisticated models of galaxy formation in a cosmological contextyield quantitative predictions about various observable characteristics, including the amount ofsubstructure, the slope of radial mass profiles and three dimensional shapes, and the propertiesof the stellar populations in the halos. The comparison of such models with the observationsprovides constraints on the general picture of galaxy formation in the hierarchical Universe, aswell as on the physical processes taking place in the halos formation. With the current observingfacilities, stellar halos can be effectively probed only for a limited number of nearby galaxies. Inthis paper we illustrate the progress that we expect in this field with the future ground basedlarge aperture telescopes (E-ELT) and with space based facilities as JWST.Keywords. instrumentation: adaptive optics, galaxies: halos, galaxies: formation.1. IntroductionNowadays there is plenty of evidence for the presence of streams and substructures inthe halo of galaxies, as is expected in the hierarchical models of galaxy formation (e.g.Atkinson, Abraham, & Ferguson 2013). These features, detected down to very low surfacebrightness (μV ∼ 29 mag/arcsec2), are hard to measure against the sky background. Evenmore difficult is characterizing their stellar populations, estimating total magnitude andcolor, which trace respectively the mass and the age/metallicity of their stars. In recentyears galaxy formation models in a cosmological context have become very sophisticated,yielding detailed predictions about the structure of stellar halos, the amount of substruc-ture, the shapes of shells and streams, the characteristics of the stellar populations inthe different components, and more (see Johnston et al. 2008; Font et al. 2011; Cooperet al. 2013; Pillepich et al. 2014). The predictions depend on the techniques adopted toconstruct the models (e.g. whether full hydrodynamical simulations, or n-body modelswith particle tagging), as well as on parameters describing the physical processes whichoccur in the troubled galaxy life (e.g. star formation (SF), the initial mass function, thefeedback). Although the ubiquitous presence of substructures in galaxy halos qualita-tively supports these models, we need to perform a quantitative comparison betweenpredictions and observations, especially to constrain the models’ parameters. This canbe done in different ways, among which (i) analyzing the demographics of substructuresof different types and the properties of their stellar populations; (ii) comparing the stel-lar density profile, checking for the presence and extent of an in situ component; (iii)measuring the metallicity gradient of stellar halos. For example, halos completely builtwith stars shed by accreted companions will hardly show a metallicity gradient, whileaccording to full hydrodynamical simulations, a sizable metallicity difference betweenthe outer and the inner halo should exist, since the latter hosts heated disk stars. These209https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921315007024Downloaded from https://www.cambridge.org/core. IP address: 2.37.249.218, on 02 Jun 2020 at 09:20:15, subject to the Cambridge Core terms of use, available at210 L. Greggio, R. Falomo & M. UslenghiFigure 1. Isochrones computed with the CMD tool (stev.oapd.inaf.it/cgi-bin/cmd) for theindicated ages and metallicity.The right axis is labelled with the distance modulus at whichsources of magnitude given on the left axis are measured with S/N=5 in a 3 hrs exposurewit MICADO. The locations of Red Supegiants (RSGs), Asymptotic Giant Branch (AGBs),Horizontal Branch (HBs) and Red Giant Branch (RGBs) stars are indicated.kind of studies have been done only for nearby galaxies (Deason, Belorukov & Evans2011; Greggio et al. 2014; Ibata et al. 2014; Crnojevic, this volume); with future largeaperture telescopes we expect to pursue these issues on more distant objects, enrichingthe samples with a greater number and more types of galaxies. In this paper we illustratesome examples of these future possibilities, concentrating on photometry of individualsources, stars and Globular Clusters (GC), which trace the halo formation history.2. Instrumental set up and stellar probesOur results are based on simulations of images computed with the AETC † (v3.0)tool, adopting two instrumental set ups: the NIRcam camera on board of JWST, andthe MICADO ‡ camera as baseline for the ELTCAM for the E-ELT telescope. The latterconfiguration exemplifies the more general case of a ∼ 30m-class ground based telescope,working close to the diffraction limit thanks to an efficient adaptive optics module.The most important features of the MICADO and the NIRcam set ups include the fieldof view (FoV), respectively of  0.78 and  9.33 arcmin2, the pixel size, of ∼ 3 and ∼ 32mas , and the magnitude limits for an isolated point source, which, in 3 hrs of integrationand at S/N=5, are respectively I = 30.3 and 28.8, J = 29.5 and 28.3. Thus, the NIRcamFoV is  10 times wider than the MICADO FoV, but the latter yields a factor of ∼ 10better resolution. This is important for the photometry in crowded fields, but becomesof little relevance in the low density, halo environment. On the other hand, the muchlarger collecting area of E-ELT allows deeper photometry with MICADO compared toNIRcam, by more than 1 mag. With these constraints how far can we detect the stellarprobes of the halo formation history? Fig.1 shows theoretical isochrones for a wide rangeof ages. The right axis, labelled with M(I) + 30.3, indicates the distance modulus upto which the various stellar probes can be measured with good accuracy on a MICADOimage. It appears that bright RSGs will be detected up to very large distances (hundredsof Mpc), but they trace only the most recent SF. Using AGB stars we will probe the SF† http://aetc.oapd.inaf.it‡ http://www.mpe.mpg.de/ir/micadohttps://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921315007024Downloaded from https://www.cambridge.org/core. IP address: 2.37.249.218, on 02 Jun 2020 at 09:20:15, subject to the Cambridge Core terms of use, available atStudying halos with future facilities 211Figure 2. MICADO (left) and NIRcam (right) 3 × 3 arcsec2 I-band simulated images of astellar population typical for a stream in a galaxy halo at 8 Mpc distance (see text). A totalintegration time of 3 hrs has been adopted. Circles, with a diameter of 10 (left) and 6 (right)pixels, show the position of red clump stars (28.5  I  29.5).of the last few Gyr, but to access the whole SF history we need to sample the brightRGB stars, which can be measured in the galaxy halos up ∼ 40 Mpc away. The RGBcolors are almost insensitive to age, so these stars yield information on the integratedmass of the stellar population, but not on the detailed SF history. On the other hand,they can can be used to evaluate a photometric metallicity. Finally, red HB stars can bemeasured up to ∼ 10 Mpc, but blue HB stars can be detected well only within  5 Mpc.In summary, we will be able to catch currently forming stars up to distances of hundredsof Mpcs, which will be very interesting for interacting galaxies. However, in order to derivethe fundamental properties of galaxy halos we need to probe ages of several Gyrs, andthe brightest objects we can use to achieve this goal are RGB stars.3. Resolved stars in streams and smooth component in galaxy halosThe characterization of the SF history in the halo substructures and streams yieldsfundamental information on their progenitors. To illustrate this case we produced simu-lated images of a synthetic stellar population obtained with a constant SF rate over 10Gyr, and located at a distance of 8 Mpc, with an average surface brightness of μV = 24.5mag/arcsec2 (Fig. 2). The luminosity sampled in the field is only of 8 × 104 LV ,, andthere are 450 stars brighter than I = 30, but the total MICADO FoV is more than 300times wider, providing large statistics. The impressive resolution of the MICADO imageis readily appreciated, with faint stars clearly visible through the halo of the bright ob-jects. The core helium burning stars (marked with red circles) are too faint and crowdedto be measured well on the NIRcam image, while they are very well detected on theMICADO’s. On the other hand, NIRcam will sample well the bright RGB stars, allowingus to measure the density profile, the shape, and the metallicity of the stellar halo. Tobetter explore this matter we have simulated a 3 hrs integration full NIRcam I-bandimage containing one stellar population with an age of 6 Gyr, metallicity [Fe/H] = -1,and total luminosity of 107 LV ,, located at a distance of 8 Mpc, for an average surfacebrightnes of μV  28, typical of a galaxy halo. Photometry of the  3500 stars brighterhttps://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921315007024Downloaded from https://www.cambridge.org/core. IP address: 2.37.249.218, on 02 Jun 2020 at 09:20:15, subject to the Cambridge Core terms of use, available at212 L. Greggio, R. Falomo & M. UslenghiFigure 3. The lenticular galaxy NGC4452 member of the Virgo Cluster, with superimposedthe FoV of NIRcam (cyan) and MICADO (yellow).than I = 27.5 (MI  -2) shows that they are measured with a mean error of  0.04 mag.Font et al. (2011) models for Milky Way type galaxies feature a metallicity gradient with[Fe/H] ∼ -1.2 and -0.5 respectively in the outer and inner halo regions, which implies acolour difference of Δ(I − J)  0.15 mag between the inner and outer halo bright RGBstars. Comparing this color difference to the photometric accuracy mentioned above itappears that it will be possible to test these models, and in general derive fundamentalinformation on the metallicity of stellar halos of galaxies at this distance.The VIRGO cluster of galaxies is the closest rich aggregate of all morphological types.It is then interesting to examine to which extent we will be able to study the halos of itsmembers. Fig. 3 shows a SDSS image of a VIRGO member galaxy, with superimposedthe MICADO and the NIRcam FoV. With 3 hrs integration, the two set ups will allowsampling the upper 3 and 1.5 mag on the RGB: both limits are quite adequate to derivethe photometric metallicity, though MICADO will yield more accurate results. On theother hand, due to its wide FoV, NIRcam will be more efficient to map the widespreadstellar halo, while MICADO will allow exquisite photometry in the inner halo regions,at the interface with the disk, at high surface brightness.Finally, we can ask how far we can detect substructures in galaxy halos using starcounts, arguably the most robust way to trace overdensities at very low surface brightness.Due to the combination of RGB stars color and limiting magnitude, the J band is moreefficient than the I band to this end. At half solar metallicity, the tip of the RGB islocated at M(J)  -5.5; at the magnitude limits mentioned above for MICADO, we willhttps://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921315007024Downloaded from https://www.cambridge.org/core. IP address: 2.37.249.218, on 02 Jun 2020 at 09:20:15, subject to the Cambridge Core terms of use, available atStudying halos with future facilities 213Figure 4. MICADO simulated image in the J -band of a 12 Gyr old stellar population, with[Fe/H]=-1.3, and M(V)=-9.7, adopting a 3 hrs exposure and a distance of 1 Mpc. Stars aredistributed following a King profile with core and tidal radii of 1 and 30 pc, respectively. Onthe right panel we show a section of the image, extending from 1.5 pc to 8.7 pc from the clustercenter (lower left and upper right corners), with turn-off stars (J  28) circled in red.sample the upper magnitude on the RGB up to m−M ∼ 34. At such distance, MICADOmaps a field of ∼ 15 kpc on a side, so we will still need some mosaicking to sample the 100 kpc extension of the halo, but we will be able to detect structures at very low surfacebrightness. A stellar population of 104 LV , has ∼ 1.4 stars in this magnitude range,if its age is older than a few Gyr. Thus the surface brightness scales with the surfacedensity σ of such stars according to μV = −4.8 − 2.5log σ + m − M . At (m − M) = 34,μV = 33 corresponds to σ=0.03 stars/arcsec2 which is almost 100 times higher thanthe expected density of foregorund stars in this magnitude range, estimated from theTRILEGAL † simulator. Therefore, we expect to be able to trace structures to very faintsurface brightness; this is particularly important, since galaxy formation models predictthat most substructures are found at μV  30 (Johnston et al. 2008).4. Globular Clusters and Globular Cluster systems in GalaxiesOne important application for a large aperture telescope working at the diffractionlimit deals with photometry of resolved stars in distant GCs for, e.g., their age dating, thedetection of multiple populations and their characterization. To assess the capabilities inthis field a large grid of simulations should be computed and analyzed with sophisticatedphotometric tools which allow a full exploitation of the data. Here we just remark thatit will be possible to construct Color-Magnitude diagrams down to the turn offs of oldclusters in the whole Local Group. Fig. 4 shows an example of a simulated image for aGC at 1 Mpc from us. The stars visible on the left panel are mostly bright giants, butthe circled sources on the right panel are turn off stars, with 27.8  J  28.2. Thesestars, very crowded inside ∼ 2 core radii, become well separated in the outer region.Viewed as point sources, GCs hold fundamental information on the formation historyof the parent galaxy, as implied by, e.g., the relation between the total halo mass and totalmass of the GC system (Hudson, Harris & Harris 2014), the bimodal color distribution,† stev.oapd.inaf.it/cgi-bin/trilegalhttps://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921315007024Downloaded from https://www.cambridge.org/core. IP address: 2.37.249.218, on 02 Jun 2020 at 09:20:15, subject to the Cambridge Core terms of use, available at214 L. Greggio, R. Falomo & M. Uslenghithe spatial coincidence of clusters and streams (e.g. Blom et al. 2014). Assuming astypical the magnitude distribution of GCs in M31, which peaks at M(J) = -9.2 (Nantaiset al. 2006), and using the above mentioned limiting magnitudes for MICADO, we willderive accurate photometry of the GC system in galaxies up to a distance of ∼ 450 Mpc,and, thanks to the exquisite resolution, we will detect them even on top of the highsurface brightness, central regions. In addition, the large volume accessible with futureinstrumentation will allow us to characterize the GC systems in hosts of Active GalacticNuclei (AGNs) of various types and luminosity, and to study the possible connectionbetween the GCs in the host galaxies and the formation of AGNs in their centers, usinga sample of hundreds of objects.5. Summary and ConclusionsBased on the results of simulations, we conclude that future telescopes will allow ampleand detailed studies of stellar halos with sufficent accuracy to constrain models of galaxyformation. In particular it will be possible to:• catch young stars up to distances of hundreds of Mpcs and witness current starsformation in large samples of interacting galaxies;• costruct the color-magnitude diagram of stars in tidal features down to the red clumpfor galaxies up to ∼ 10 Mpc away, and derive the metallicity distribution and its gradientin the halo of galaxies within 20 Mpc;• map the stellar density in galaxy halos up to a distance of 50 Mpc; overdensities willbe traceable at very faint limits of surface brightness (μV ∼ 33 mag/arcsec2);• analyze the color-magnitude diagram of resolved stars in globular clusters down tothe old turn-off region in the whole Local Group;• measure the luminosity, mass and color distribution of the globular cluster membersof galaxies up to a redshift z  0.1.AcknowledgementsWe acknowledge support from INAF and MIUR through the Progetto premiale T-REX.ReferencesAtkinson, A. M., Abraham, R. G., & Ferguson, A. M. N. 2013, ApJ, 765, 28Blom, C., Forber, D. A., Romanowsky, A. J., et al. 2014, MNRAS, 439, 2420Cooper, A. P., D’Souza, R. Kauffmann, G., et al. 2013, MNRAS, 434, 3348Deason, A. J., Belorukov, W., & Evans, N. W. 2011, MNRAS, 416, 2903Font, A. S., McCarthy, I. G., Crain, R. A., et al. 2013, MNRAS, 416, 2802Greggio, L., Rejkuba, M., Gonzales, O. A., et al. 2014, AA, 562, A73Hudson, M. J., Harris, G. L., & Harris, W. E. 2014, ApJ (Letters), 787, L5Ibata, R. A., Lewis, G. F., McConnachie, A. W., et al. 2014, ApJ, 780, 128Johnston, K. V., Bullock, J. S., Sharma, S., et al. 2008, ApJ, 689, 936Nantais, J. B., Huchra, J. P., Barmby, P., et al. 2006, AJ, 131, 1416Pillepich, A., Vogelsberger, M., Deason, A. J., et al. 2014, MNRAS, 444, 237https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921315007024Downloaded from https://www.cambridge.org/core. IP address: 2.37.249.218, on 02 Jun 2020 at 09:20:15, subject to the Cambridge Core terms of use, available at

Proceedings of the International Astronomical Union

Studying stellar halos with future facilities

AbstractStellar halos around galaxies retain fundamental evidence of the processes which lead to their build up. Sophisticated models of galaxy formation in a cosmological context yield quantitative predictions about various observable characteristics, including the amount of substructure, the slope of radial mass profiles and three dimensional shapes, and the properties of the stellar populations in the halos. The comparison of such models with the observations provides constraints on the general picture of galaxy formation in the hierarchical Universe, as well as on the physical processes taking place in the halos formation. With the current observing facilities, stellar halos can be effectively probed only for a limited number of nearby galaxies. In this paper we illustrate the progress that we expect in this field with the future ground based large aperture telescopes (E-ELT) and with space based facilities as JWST.</jats:p

Laura Greggio

Renato Falomo

Michela Uslenghi

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Studying stellar halos with future facilities

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