LINC-NIRVANA. The Stellar Mass Profile and the DM Fraction of a High-Mass Compact Elliptical Galaxy at z 1.3

Transcription

1 LINC-NIRVANA The Stellar Mass Profile and the DM Fraction of a High-Mass Compact Elliptical Galaxy at z 1.3 Doc. No. LN-INAFM-TN-SCI-001 Short Title M and DM at z 1.3 Issue 1.3 Date 27 February, 2015 P. Saracco, P. Ciliegi, F. Ciocca, A. Gargiulo, M. Longhetti, I. Lonoce, S. Tamburri S. Zaggia Prepared Name Date Signature N. Surname Approved Name Date Signature N. Surname

2 Released Name Date Signature

3 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue 1.3 iii Change Record Issue Date Sect. Reason/Initiation/Documents/Remarks all new document all 2 nd Version all 3 rd Version all Final Version

4 iv LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue 1.3 Contents 1 Introduction 1 2 Applicable Documents 1 3 Acronyms and Abbreviations 1 4 Executive Summary 2 5 Scientific Justification Scientific Background Scientific Goals Target List Feasibility Evaluation Observing set-up Description of Simulations Results of Simulations Instrument Requirements Proposed Observations Improvement Over Existing and Near Term Facilities 12 8 References 13 List of Figures 1 Main properties of the galaxy S2F Target Observability Simulated Images SBP for different exposure time and Strehl Ratio values Strehl Ratio across FoV List of Tables 1 Executive Summary Table Target List Required Properties

5 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue Introduction This document provides detailed information on one project from the LINC-NIRVANA Lean- MCAO Early Science Plan (full details on the early science plan can be found in the document LN- MPIA-TN-SCI-010). This project will study the stellar mass profile and the dark matter fraction of a high-mass compact elliptical galaxy at z 1.3 using continuum LINC-NIRVANA Lean-MCAO imaging. 2 Applicable Documents Title LINC-NIRVANA in a nutshell. The LN-Lean-MCAO Early Science Plan Number and Issue LN-MPIA-GEN-SCI-001 LN-MPIA-TN-SCI Acronyms and Abbreviations LBT PSF DM HST SBP SCAO Large Binocular Telescope Point Spread Function Dark Matter Hubble Space Telescope Surface Brightness Profile Single Conjugate AO

6 2 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue Executive Summary Despite the apparent simplicity and homogeneity of the observed properties of elliptical galaxies, the main physical processes involved in their stellar mass growth and in their shaping are still unclear. There are currently two suggested formation scenarios for such galaxies: 1) Ellipticals form most of their stars at high redshift in the central starburst of a major gas-rich merger (Naab et al. 2009; Sommer-Larsen & Toft 2010), with their remaining stars being accreted over longer periods during mostly minor dry mergers (Hilz et al. 2013). 2) Elliptical galaxies form most of their mass from cosmological cold streams and violent disk instability (Dekel et al. 2009). These formation scenarios make different predictions for the behaviour of the dark matter fraction of the galaxies with time, 1) predicts that the effective radius and DM fraction should increase with time, while 2) predicts a constant DM fraction with time. It has proven difficult to thoroughly test which of these cases is correct due to the difficulty of obtaining the necessary observations of galaxies at high redshift. The few measurements of velocity dispersion obtained so far for highredshift (z > 1) massive (M M ) compact elliptical galaxies provide unusually high values. Whether this high velocity dispersion is due to a high stellar mass density toward the center of these galaxies or to a high fraction of dark matter (DM) is still debated, principally because of the need to adequately resolve the light distribution (and hence stellar mass) in the inner regions of distant galaxies. We propose to use LN-Lean-MCAO to accurately trace the stellar mass profile of a massive spheroidal galaxy at z = 1.33 for which we have already established its high stellar mass (M > M ) and compactness through HST-NIC2 observations (Longhetti et al. 2007) and for which we have already accurately measured its stellar velocity dispersion σ V from deep VLT-FORS2 spectroscopic observations (Gargiulo et al. 2015). The proposed LN-Lean-MCAO observations will provide a factor of 3 higher spatial resolution than the HST-NIC2 observations allowing us to determine the stellar mass profile in unprecedented detail. We will use these observations to disentangle the stellar and dark matter components of the galaxy in order to examine whether the dark matter fraction of this galaxy deviates significantly from those of z=0 galaxies as expected under formation scenario 1. As with the other project to study higher redshift galaxies, this project does not require offset skies to be obtained, small (< 1 ) dithers are sufficient to allow adequate background subtraction. However, small offsets to a nearby bright star to act as a PSF reference will be required periodically, as no bright star is located within the LN-Lean-MCAO FoV. Based on the results of our simulations, we propose to observe our target for 3 hours (3.75 hours considering the overheads) with a minimum Strehl Ratio of 0.3. The simulations performed with the software LOST predicted that we will be able to achieve the required Strehl Ratio with the available asterism. Name R.A. Dec. Filters No. Total 1 Min Max Special hh:mm:ss dd:mm:ss Fields Exp (s) Strehl Airmass Mode S2F :06: :02:44.80 K Yes Table 1: Target List. 1 Including overheads

7 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue Scientific Justification 5.1 Scientific Background Despite the apparent simplicity and homogeneity of the observed properties of elliptical galaxies, the main physical processes involved in their stellar mass growth and in their shaping are still unclear. The old stellar populations observed in most of the elliptical galaxies, the homogeneity of their properties and the tightness of the observed color-magnitude relation and of other scaling relations such as the fundamental plane (FP), suggest that most of the stars were formed and assembled at very high redshifts, z > 4 5, through an intense episode of star formation (e.g. Naab et al. 2007; Khochfar & Silk 2006) or through dry mergers, the coalescence of pre-existing old stellar systems (e.g. De Lucia et al. 2006). A current view involves a combination of these two processes acting at different times: ellipticals are thought to form at high redshifts from a main major gas-rich merger during which almost 80% of their final stellar mass should be formed in the dissipative central starburst (Naab et al. 2009; Sommer-Larsen & Toft 2010). The resulting spheroid should further grow through dry minor mergers of smaller satellites re-arranging stars and accreting dark matter (DM) mostly in the outskirt (Hilz et al. 2013), while keeping the central part and the total stellar mass nearly unchanged, the so called inside-out accretion (e.g. Oser et al. 2010, Naab et al. 2009; Hopkins et al. 2010; Bezanson et al. 2009). According to this scheme, the effective radius is expected to increase as well as the DM fraction. The cold-accretion scenario (Dekel et al. 2009) is actually the only alternative scenario to the above one. In this scenario, the formation of elliptical galaxies is driven by cosmological cold streams and violent disk instability rather than by merging. The incoming primordial gas starts falling into the DM halo forming a disk with its angular momentum. When the external gas stream is particularly clumpy, the disk breaks up (van den Bergh 1996; Elmegreen et al. 2005) due to gravitational instability into massive clumps. These clumps dissipatively merge toward the center forming a spheroid over short time scales ( 0.5 Gyr) that should evolve passively in time. The resulting population of stars would be nearly coeval, the central stellar mass density should be high and dominate the velocity dispersion while the DM fraction should not change in time. The DM fraction is a key ingredient in both the scenarios and may be a useful tool to discriminate between them. However, this quantity has no strong constraints from observations especially at high redshift, where the comparison would be particularly stringent given the little time available to assemble a massive (M> M ) spheroid. The fraction and the properties of DM can in principle be tightly constrained by combining galaxy dynamical measures, sensible to the total matter (DM + stars), with those related to the baryonic component only, if the latter is very well known. The few measurements of velocity dispersion obtained so far for high-redshift (z> 1), massive (M M ) compact elliptical galaxies provide usually high values. Whether this high velocity dispersion is due to a high stellar mass density toward the center of these galaxies or due to a high fraction of dark matter (DM) is still debated. The apparent compactness (effective radius Re kpc) from HST H-band observations reaching a resolution of about FWHM 0.2 arcsec ( 1.7 kpc at z = 1.2), suggests that the stellar mass of these ellipticals is centrally concentrated. This would justify the observed high velocity dispersion and relegate the DM component to a negligible contribution (< 10%), and is in tension with what is expected from simulations. Theory and simulations show indeed that the gas cooling and the dissipative mechanisms needed to produce dense

8 4 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue 1.3 stellar systems induce a strong contraction of the DM halo (e.g. Gnedin et al. 2011; Blumenthal et al. 1986), contrary to the feedback processes and to dry mergers that induce a DM halo expansion (e.g. Johansson et al. 2009; Governato et al. 2010; Maccio et al. 2012). Thus, quantifing the DM fraction in massive compact elliptical galaxies can discriminate among the different mechanisms of mass accretion. 5.2 Scientific Goals An accurate reconstruction of the luminosity profile and hence of the stellar mass profile in the core of these massive elliptical galaxies coupled with an accurate measure of their velocity dispersion sensible to stellar and DM component, would allow one to estimate the real DM fraction and to constrain the mechanism responsible of their stellar mass growth. We therefore propose to use LN-Lean-MCAO to obtain deep K band observations of the galaxy SF1-527, in order to allow us to accurately trace the stellar mass profile of a massive spheroidal galaxy at z = 1.33 for which we have already established its high stellar mass (M> M ) and compactness through HST-NIC2 observations (Longhetti et al. 2007) and for which we have already accurately measured its stellar velocity dispersion σ v from deep VLT-FORS2 spectroscopic observations (Gargiulo et al. 2015). The observations we propose will allow us to resolve the profile at an angular scale smaller than 0.1 arcsec (see Fig. 5) hence sampling the very central region of the galaxy, fundamental to derive an accurate estimate of the stellar mass. This region is critical, indeed, to estimate the stellar mass of the galaxy since a small variation of the profile can produce a large variation in the total stellar mass. From the accurate sampling of the PSF (see below) we will reconstruct the intrinsic luminosity profile of the galaxy using the software Galfit (Peng et al. 2002). We will use Galfit to fit a Sersic profile convolved with the PSF of the instrument to the observed light profile thus fixing the slope of the profile at angular resolution lower than < 0.1 arcsec (< 0.8 kpc at z 1.3), i.e. in the very central regions. This will also provide us with an accurate measure of the effective radius R e, the radius encompassing half of the luminosity of the galaxy. We will derive the stellar mass profile taking into account small radial variations of the mass-to-light ratio (M/L) according to the observed color gradients (Gargiulo et al. 2012). In this derivation, we will take also into account the possibile different mass normalization of the stellar initial mass function (IMF) on the basis of the recent constraints imposed by observations. We will use the relation between the mass normalization and the galaxy velocity dispersion recently found for dense and high-mass elliptical galaxies (Gargiulo et al. 2015), like the one we will observe. Then, by solving Jeans equation for the observed stellar mass profile (no dark matter component) we will derive the expected velocity dispersion due to the stellar mass component at different radii and integrated at the radius sampled by the VLT spectroscopic observations (0.8 arcsec wide slit). We will compare this predicted velocity dispersion (baryon dominated) with the one derived from integrated velocity dispersion (baryon+dm) measured on the VLT spectra. The resulting possible mismatch between the two values, will provide the DM fraction with which models will have to be confronted. Our choice to model the total mass profile of the galaxy with a spherical and isotropic mass model is supported by recent observations. Actually, high-z dense ETGs are structurally and kinematically similar to dense local ellipticals (Trujillo et al. 2014; van der Wel et al. 2012; Buitrago et al. 2013; Gargiulo et al. 2015) which are found to preferentially be slow rotator and bulge dominated

9 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue Figure 1: Left - K-band ground-based image (FWHM 1.2 arcsec) of the 1 1 arcmin field encompassing the target galaxy S2F1-527 (K(Vega) 18.2) and the bright (R(Vega) 13 mag) star. Center - HST-NIC2 image (6 6 arcsec, FWHM 0.2 arcsec) in the F160W band of the high-mass (M M ) target spheroidal galaxy having an effective radius of R e = 1.6 kpc (Longhetti et al. 2007; Saracco et al. 2009). Right - VLT-FORS2 rest-frame (unsmoothed and not-rebinned) spectrum (black line) obtained with 8hrs of integration for the galaxy S2F1-527 with the best-fit ppxf model superimposed (red line). The estimated velocity dispersion is σ v = 240 ± 26 km/s (Gargiulo et al. 2015). (Cappellari et al. 2013). We propose to carry out LN-Lean-MCAO K-band observations of the spheroidal galaxy S2F1-527 at z = to accurately trace its stellar mass profile. To this end, dedicated observations for an accurate PSF sampling are needed. This target galaxy is very bright (K(Vega) 18.2) and has a stellar mass > M, hence populating the high-mass end of the mass function of galaxies. According to the recent estimate of the merger rate by Man et al. (2014) who found less than one merger at redshift for galaxies with masses logm>10.8 M, our target galaxy has most likely completed its growth at its redshift. The HST-NIC2 images show that it has a small radius when compared to the mean size of local ellipticals with similar stellar mass. VLT observations gave an accurate measure of the integrated velocity dispersion. Hence, this is one of the densest elliptical galaxies at high redshift with known velocity dispersion. For these reasons, this galaxy is the perfect target to carry out this kind analysis. The main properties of the galaxy are reported in Table 1. In Fig. 1, a K-band 1 1 arcmin image of the field surrounding the target (left panel), the HST-NIC2 image (central panel) and the VLT-FORS2 spectrum (right panel) of the target are shown. At 18 arcsec from the target there is a bright (R 13 mag) star (see left panel of Fig. 1). We ask for K -band imaging for two main reasons. The first one is that the K band at the redshift of the galaxy samples the rest-frame light at λ 9000 Å where the emission is dominated by the low-mass ( 1 M ) stars constituting the bulk of the stellar mass of the galaxy that we are interested to accurately trace. The second reason is that the best performances of the instrument can be reached in K band. 5.3 Target List We selected the galaxy S2F It is the ideal candidate to reach our goal due to its high stellar mass (M > M ) and compactness obtained through HST-NIC2 observations (Longhetti et

10 LN-INAFM-TN-SCI-001 M? and DM at z 1.3 Issue al. 2007) and due to the availability of its stellar velocity dispersion σv obtained from deep VLTFORS2 spectroscopic observations (Gargiulo et al. 2015). The main properties of S2F1-527 are summarised in the following table. Name R.A. hh:mm:ss 03:06:43.34 S2F1-527 Dec. dd:mm:ss 00:02:44.80 K(Vega) z M /M Re [kpc] Table 2: Target properties. The left panel of Figure 2 shows the available observing time below airmass 1.25, 1.5, 2 and 2.5 for our target S2F Hours of Darkness GalaxyS2F GWS Total Mag = MHWS Total Mag = Dec Days 300 Dec Nov Oct Sep Aug Jul Jun May Apr Mar Feb 2 Jan Hours Above Airmass 1.25, 1.5 or RA Figure 2: Left Panel: Available observing time below Airmass 1.25, 1.5, 2 and 2.5 for the galaxy S2F Right Panel: The available AO asterism for S2F1 527.

11 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue Feasibility Evaluation This section presents the results of simulations of LN-Lean-MCAO observations used to determine the feasibility of the proposed project. 6.1 Observing set-up In the right panel of Figure 2 we show a suitable asterism for S2F A total mag of has been obtained for the GWS field while a total mag of has been obtained for the MHWS field using the USNO-B1 catalog. The feasibility of the available asterism (only one bright star with R 13 mag for the MHWS at a distance of 18 arcsec from the target) has been tested by LN team member M. Horrobin (Cologne) using the software LOST. The results of this simulation are reported in Section Description of Simulations Using the latest version of the LN-Lean-MCAO simulation software Make LN MCAO Image.pro (written by M. Norris) we simulated the observation of a spheroidal galaxy with Sersic profile with index 4 and an effective radius of 0.2 arcsec, these values were chosen to match those determined for the target galaxy. The simulation procedure involves producing an image of the simulated galaxy, with flux and noise characteristics matching those of a real observation (i.e. scaled by the exposure and number of reads) which is then convolved with the appropriate LN-Lean-MCAO PSF to produce a simulated LN-Lean-MCAO observation. We assumed a K band filter, three different levels of AO performance (Strehl Ratio of 0.6, 0.3 and 0.1) and four different total exposure time (0.5, 1, 2, and 3 hours), using 3 minutes sub-exposure (20/hrs). 6.3 Results of Simulations In Figure 3 we show the simulated images resulting from the simulation procedure outlined in Section 6.2. These images were obtained assuming a Strehl ratio of 0.6, 0.3 and 0.1 with a total exposure time of 30 minutes (upper row) and 3 hours (bottom row). For each simulated image, we compared the theoretical surface brightness profile (SBP) obtained from the Sersic profile (n=4, no Poisson noise added, no convolution with a PSF), with the observed SBP. The results of our analysis are reported in Figure 4 where, for each exposure time, we report the observed SBP obtained with different Strehl ratio values. As clearly shown in the figure, with an exposure time of 1 hour (or less) we have a poor reconstruction of the real SBP (solid green line) even with a very good Strehl Ratio value. With 2 hours of exposure time we have a good reconstruction of the real SBP only with a Strehl Ratio of 0.6. Finally with an exposure time of 3 hours we have a good reconstruction of the real SBP even with a Strehl Ratio of 0.3. On the basis of these results we propose to observe our target for 3 hours with a minimum Strehl Ratio of 0.3. Finally, the simulations performed by LN team member M. Horrobin with the software LOST (Arcidiacono et al. 2004) predicted that we will be able to achieve the required 0.3 Strehl Ratio with the available asterism. The results of the simulation are reported in Figure 5 where we show the Strehl Ratio variation across the target field and the guide star field both for the single conjugate AO (SCAO) and MCAO mode. As shown in the Figure, the absolute best Strehl would

12 8 LN-INAFM-TN-SCI-001 M? and DM at z 1.3 Issue 1.3 Figure 3: The simulated K band image (2 2 arcsec) of the galaxy S2F1-527 obtained assuming a Strehl ratio of 0.6, 0.3 and 0.1 ( from left to right) and an exposure time of 30 minutes (top row) and 3 hours (bottom row).

13 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue Figure 4: The surface brightness profile of the simulated galaxy with different exposure time (from 0.5 to 3 hours from top left to bottom right) and different Strehl Ratio values : 0.6 (solid black line), 0.3 (dashed blue line), and 0.1 (dot dashed red line). The solid green line shows the theoretical SBP of a galaxy with a Sersic profile with n=4, no Poisson noise added and without convolution for a PSF. be obtained in SCAO mode, but this then has a relatively steep drop off across the science field. When we switch to using MCAO we reduce the maximum predicted Strehl, but we benefit from a substantial flattening of the variation across the science field reaching a Strehl Ratio value of 0.3 on the target (center of the science field). 6.4 Instrument Requirements This project does not require offset skies to be obtained. Small (few arcsec) dithers are sufficient to allow for adequate background subtraction. However, since no bright stars are located within the LN-Lean-MCAO FoV, we have to dither every 5 minutes to a nearby bright (R 13 mag) star at a distance of 18 to act as a PSF reference while holding the AO loop. In this case this should be doable, as the asterism should allow the move without losing any of the AO reference stars. We expect a total of 0.75 hours overheads.

14 10 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue 1.3 Strehl across the S2F field, 0.6 seeing SCAO MCAO high strehl tuned MCAO standard setup Strehl across the guide star field, 14 offset from science field Figure 5: Strehl Ratio variation across the science and guide star fields using the available asterism for the SCAO and MCAO mode as predicted by the LOST software. Although the MCAO mode delivers a slightly lower Strehl ratio than the SCAO mode at the location of S2F1-527, the predicted gradient in the Strehl ratio is significantly lowered in the MCAO case.

15 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue Proposed Observations Based on the results of our simulation, we propose to observe our target for 3 hrs with a minimum Strehl of 0.3. The observations of the target should consist of short dithered exposures each of them offset by few arcsec in order to obtain a good sky subtraction for each image. Considering a seeing of about 0.6 arcsec in K, we expect to collect almost 70% of the flux of the target within 0.1 arcsec. In order to obtain a good reconstruction of the intrinsic profile of the galaxy we need to sample the PSF. Since no stars are present within the field of view of LN-Lean- MCAO, dedicated observations are needed. The bright star close to the target galaxy could be used as a reference PSF. However, given its brightness particular attention needs to be paid to saturation ( 1 sec exposure). Considering to observe the reference PSF stars for every 5 minutes while holding the AO loop, we expect total overheads (dithering for background subtraction and PSF reference) of about 0.75 hours, so the total requested time for our target is 3.75 hrs. The small pixel scale (5 mas) will allow us to obtain an excellent sampling of the PSF and consequently an excellent modelling of the intrinsic light profile of the galaxy up to more than 3 effective Radius. Name Required Filters Total Exp. Min Strehl Min Airmass with overheads (s) S2F1-527 K Table 3: Observations and conditions required to meet the scientific goals.

16 12 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue Improvement Over Existing and Near Term Facilities Our goal is to model the intrinsic light profile of the galaxy reaching the innermost regions. With its extreme high resolution and small pixel scale (5mas), LN-Lean-MCAO is the perfect instrument to model the light profile of our target. Considering a seeing of 0.6, we expect to collect almost 70% of the flux within 0.1 arcsec, doubling the spatial resolution achievable with HST (HST/WFC3 pixel scale = 130 mas in the IR channel). It is impossible to obtain these spatial resolutions with other instruments like VLT or Keck. Moreover, the presence of a MCAO system will ensure a small variation of the Strehl Ratio value over the entire field of view, ensuring a good Strehl Ratio on our target even at a distance of 18 arcsec from the bright star (R 13 mag) (see left panel of Fig. 1). This is impossible with a single conjugate AO (SCAO) system like LBT-FLAO, as demonstrated by our simulation with the software LOST. In the long term (i.e. after 2018) JWST will be able to repeat this kind of observation for different targets. However this project has the potential to examine whether the dark matter fraction of z 1.5 galaxies deviates significantly from those of z=0 well before JWST launches.

17 LN-INAFM-TN-SCI-001 M and DM at z 1.3 Issue References Arcidiacono, C., et al. 2004, ApOpt,43,4288 Bezanson R., et al. 2009, ApJ 697, 1290 Blumenthal G. R., Faber S. M., Flores R., Primack J. R. 1986, ApJ 301, 27 Buitrago et al. 2013, MNRAS 428, 1460 Cappellari et al. 2013, MNRAS 432, 1862 Dekel A., et al. 2009, ApJ 703, 785 De Lucia G., et al. 2006, MNRAS 366, 499 Elmegreen D. M. 2005, ApJ 623, L71 Gargiulo A., Saracco P., Longhetti M., et al A&A, 573, 110 Gargiulo A., Saracco P., Longhetti M., et al. 2012, MNRAS 425, 2698 Gnedin N. Y., Kravstov A. V. 2011, ApJ 728, 88 Governato F., et al. 2009, Nature 463, 203 Hilz M., et al. 2013, MNRAS 429, 2924 Hopkins P., et al. 2010, ApJ 715, 202 Johansson P. H., Naab T., Ostriker J. P. 2009, ApJ 697, L38 Khochfar S. & Silk J., 2006, MNRAS 370 Longhetti M., Saracco P., Severgnini P., et al. 2007, MNRAS 374, 614 Macció et al. 2012, ApJ 744, L9 Man et al. 2014, arxiv: Naab T., et al. 2007, ApJ, 658 Naab T., et al. 2009, ApJ, 699, L178 Oser L. et al 2010, ApJ 725, 2312 Peng C. Y., et al. 2002, AJ 124, 266 Saracco P., Longhetti M., Andreon S. 2009, MNRAS 392, 718 Sommer-Larsen J., Toft S., 2010, ApJ 721, 1755 Trujillo et al. 2014, ApJ 780, L20 van den Bergh 1996, PASP 108 van der Wel et al. 2012, ApJS 203, 24