Bulgeless Giant Galaxies Challenge Our Picture of Galaxy Formation By

RECEIVED 2010 APRIL 12; ACCEPTED 2010 AUGUST 18 Preprint typeset using LATEX style emulateapj v. 04/03/99 BULGELESS GIANT GALAXIES CHALLENGE OUR PICTURE OF GALAXY FORMATION BY HIERARCHICAL CLUSTERING1,2 JOHN KORMENDY3,4,5,NIV DRORY5,RALF BENDER4,5, AND MARK E. CORNELL3 Received 2010 April 12; Accepted 2010 August 18 ABSTRACT To better understand the prevalence of bulgeless galaxies in the nearby field, we dissect giant Sc – Scd galaxies with Hubble Space Telescope (HST) photometry and Hobby-Eberly Telescope (HET) spectroscopy. We use the HET High Resolution Spectrograph (resolution R λ/FWHM 15,000) to measure stellar velocity dispersions in the nuclear star clusters and (pseudo)bulges of≡ the pure-disk≃ galaxies M33, M101, NGC 3338, NGC 3810, NGC 6503, and NGC 6946. The dispersions range from 20 1kms−1 in the nucleus of M33 to 78 2kms−1 in the pseudobulge of NGC 3338. We use HST archive images± to measure the brightness profiles of the± nuclei and (pseudo)bulges in M101, NGC 6503, and NGC 6946 and hence to estimate their masses. The results imply small mass-to-light ratios consistent with young stellar populations. These observations lead to two conclusions: 6 (1) Upper limits on the masses of any supermassive black holes (BHs) are M• < (2.6 0.5) 10 M⊙ in M101 6 ± × and M• < (2.0 0.6) 10 M⊙ in NGC 6503. ∼ (2) We∼ show± that the× above galaxies contain only tiny pseudobulges that make up < 3 % of the stellar mass. This provides the strongest constraints to date on the lack of classical bulges in the biggest∼ pure-disk galaxies. We inventory the galaxies in a sphere of radius 8 Mpc centered on our Galaxy to see whether giant, pure-disk −1 galaxies are common or rare. We find that at least 11 of 19 galaxies with Vcirc > 150 km s , including M101, NGC 6946, IC 342, and our Galaxy, show no evidence for a classical bulge. Four may contain small classical bulges that contribute 5–12% of the light of the galaxy. Only four of the 19 giant galaxies are ellipticals or have classical bulges that contribute 1/3 of the galaxy light. We conclude that pure-disk galaxies are far from rare. It is hard to understand how bulgeless∼ galaxies could form as the quiescent tail of a distribution of merger histories. Recognition of pseudobulges makes the biggest problem with cold dark matter galaxy formation more acute: How can hierarchical clustering make so many giant, pure-disk galaxies with no evidence for merger-built bulges? Finally, we emphasize that this problem is a strong function of environment: the Virgo cluster is not a puzzle, because more than 2/3 of its stellar mass is in merger remnants. Subject headings: galaxies: evolution — galaxies: formation — galaxies: individual (M33, NGC 3338, NGC 3810, NGC 5457, NGC 6503, NGC 6946) — galaxies: nuclei — galaxies: photometry — galaxies: structure 1. INTRODUCTION and 3 are long. Readers who need σ and M• results can find them in Figure 1 and Table 1. Readers who are interested in the This paper has two aims. First, we derive upper limits on smallest pseudobulges can concentrate on §3. Readers whose the masses M• of any supermassive BHs in two giant, pure- disk galaxies. This provides data for a study (Kormendy et al. interest is the challenge that pure-disk galaxies present for our picture of galaxy formation should skip directly to §4. 2010) of the lack of correlation (Kormendy & Gebhardt 2001) between BHs and galaxy disks. Second, we inventory disks, pseudobulges, and classical bulges in the nearby universe and 1.2. Introduction to the Velocity Dispersion Measurements show that giant, pure-disk galaxies are not rare. This highlights Nuclei are expected to have velocity dispersions that range the biggest problem with our mostly well supported picture of from those of globular clusters, σ 10 km s−1, to values hierarchical clustering: How can so many pure-disk galaxies similar to those in the smallest classical∼ bulges and ellipticals form, given so much merger violence? Both studies need (e.g., M32: σ 60 km s−1; Tonry 1984; 1987; Dressler & arXiv:1009.3015v1 [astro-ph.GA] 15 Sep 2010 the same observations: photometry to measure structure and Richstone 1988;≃ van der Marel et al. 1994a, b; Bender et al. spectroscopy to measure velocity dispersions and masses. 1996). But nuclei are faint and embedded in bright disks. The σ constraint implies that we need high dispersion, and the 1.1. A Practical Guide to Readers faintness implies that we need a large telescope. As a result, In §2, we measure stellar velocity dispersions in high-mass, few σ measurements of nuclei are available. The best object– Sc–Scd galaxies that contain only nuclei or extremely small now very well measured–is M33, whose exceptionally well pseudobulges. In §3, we derive HST- and ground-basedsurface defined nucleus has a velocity dispersion of σ 21 3kms−1 photometry of the most useful subset of our galaxies to see (Kormendy & McClure 1993; Gebhardt et al.≃ 2001).± We use whether they contain small classical bulges, pseudobulges, or it as a test case for our observations. The “gold standard” of nuclei and to measure nuclear masses and M• limits. Sections 2 nuclear dispersion measurements is Walcher et al. (2005); they 1Based on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen. 2Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at STScI, which is operated by AURA, Inc., under NASA contract NAS 5-26555. These observations are associated with program numbers 7330, 7919, 8591, 8597, 8599, 9293, 9360, 9490, 9788, and 11080. 3Department of Astronomy, The University of Texas at Austin, 1 University Station C1400, Austin, Texas 78712-0259, USA; [email protected]; [email protected] 4Universitäts-Sternwarte, Scheinerstrasse 1, München D-81679, Germany 5Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching-bei-München, Germany; [email protected]; [email protected] 1 2 Kormendy et al. used the Ultraviolet and Visual Echelle Spectrograph on the the K0 – K3 stars in many previous papers; they reliably fit Very Large Telescope to measure 9 nuclei of generally modest- the spectra of low- to moderate-dispersion galaxies very well. sized galaxies at a resolution of R = 35,000. In any case, (1) the Ca infrared triplet region is relatively This paper reports R = 15,000 measurements of σ in M33, insensitive to template mismatch (Dressler 1984), and (2) NGC3338, NGC3810, NGC5457, NGC6503, and NGC6946. the FCQ program that provides our final dispersion values is In choosing targets, we favored the largest pure-disk galaxies specifically engineered to minimize problems with template that have the smallest possible pseudobulges (Kormendy & mismatch (Bender 1990). Kennicutt 2004) and the smallest possible distances. The most 2.2. Preprocessing of Spectra important galaxies for our purposes are NGC 5457 = M101 and NGC 6946. A closely similar object is IC 342, for which Spectral reductions were carried out using the interactive Böker et al. (1999) measured a nuclear dispersion of σ =33 3 image processing system IRAF6 (Tody 1993). The echelle km s−1 at a spectral resolution of R = 21,500. All three± are software package was used to remove instrumental signatures Scd galaxies with extremely small pseudobulges or nuclear star from the data. For each night, we created a bias frame, a clusters but essentially the largest possible asymptotic rotation continuum flat, and a Thorium-Argon arc-lamp spectrum from velocities 200 km s−1 consistent with our requirement that calibration data taken before and after the observations. Science they contain∼ no classical bulges. frames were first bias and overscan corrected. Bad pixels Late in our data reduction, we were scooped by Ho et al. were flagged. Next, we removed cosmic ray hits using the (2009), who measured σ and collected published σ data for spectroscopy-optimizedversion of L.A.COSMIC (van Dokkum 428 galaxies. All of our objects are included in their paper. 2001), and we coadded the three spectra obtained for each However, their instrumental velocity dispersion is σinstr = 42 galaxy. We traced and fitted the spectral orders in the km s−1, whereas ours is 8 km s−1. Our measurements therefore continuum flats using 3rd-order Legendre polynomials. We provide important confirmation. Most of their measurements removed the spectral signature of the continuum lamp from th prove to be remarkably accurate, even when σ < σinstr. We the flat fields by fitting the continuum using 7 -order Legendre disagree on two values. Also, our measurementshave estimated polynomials along the dispersion direction and divided the flat errors that are a factor of 4 smaller than theirs. Confidence in field frames by the fits. Next, we extracted the science spectra our understanding of the∼ smallest central velocity dispersions (galaxies and velocity standard stars) using these normalized in the biggest pure-disk galaxies is correspondingly increased. flat fields to obtain one-dimensional spectra for each order. This provided one, multi-order spectrum of the galaxy nucleus 2. OBSERVATIONS AND DATA REDUCTION from the object fiber plus two “sky” spectra at galactocentric distances of 10′′ bracketing the nucleus. Finally, all spectra 2.1. Observations were wavelength calibrated using the Th-Ar lamp spectra to an The spectra were obtained with the High Resolution accuracy of 0.02 pixel 0.003 Å 0.1 km s−1. Spectrograph (HRS: Tull 1988) and the 9 m Hobby-Eberly The remaining∼ tasks are≃ sky subtraction≃ and the combination Telescope (HET: Ramsey et al.

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