Niv Drory

The structure of classical bulges and pseudobulges: the link between pseudobulges and Sérsic index

In this paper, we study the properties of pseudobulges (bulges that appear similar to disk galaxies) and classical bulges (bulges which appear similar to E-type galaxies) in bulge-disk decompositions. We show that the distribution of bulge Sersic indices, nb , is bimodal, and this bimodality correlates with the morphology of the bulge. Pseudobulges have nb 2 and classical bulges have nb 2 with little to no overlap. Also, pseudobulges do not follow the correlations of Sersic index with structural parameters or the photometric projections of the fundamental plane in the same way that classical bulges and elliptical galaxies do. We find that pseudobulges are systematically flatter than classical bulges and thus more disk-like in both their morphology and shape. We do not find significant differences between different bulge morphologies which we are collectively calling pseudobulges (nuclear spirals, nuclear rings, nuclear bars, and nuclear patchiness); they appear to behave similarly in all parameter correlations. In the Sersic index, flattening, and bulge-to-total ratio, the distinction appears to be between classical bulges and pseudobulges, not between different pseudobulge morphologies. The Sersic index of the pseudobulges does not correlate with B/T, in contrast to classical bulges. Also, the half-light radius of the pseudobulge correlates with the scale length of the disk; this is not the case for classical bulges. The correlation of Sersic index and scale lengths with bulge morphology suggests that secular evolution is creating pseudobulges with low-Sersic index and that other processes (e.g., major mergers) are responsible for the higher Sersic index in classical bulges and elliptical galaxies.

Bulgeless giant galaxies challenge our picture of galaxy formation by hierarchical clustering

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 ? ?/FWHM 15, 000) to measure stellar velocity dispersions in the nuclear star clusters and (pseudo)bulges of the pure-disk galaxies M 33, M 101, NGC?3338, NGC?3810, NGC?6503, and NGC?6946. The dispersions range from 20 ? 1?km?s?1 in the nucleus of M 33 to 78 ? 2?km?s?1 in the pseudobulge of NGC?3338. We use HST archive images to measure the brightness profiles of the nuclei and (pseudo)bulges in M 101, 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. (1) Upper limits on the masses of any supermassive black holes are M ? (2.6 ? 0.5) ? 106 M ? in M 101 and M ? (2.0 ? 0.6) ? 106 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 galaxies are common or rare. We find that at least 11 of 19 galaxies with V circ > 150?km?s?1, including M 101, 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.

The HETDEX pilot survey - II. The evolution of the Lyα escape fraction from the ultraviolet slope and luminosity function of 1.9 < z < 3.8 LAEs

We study the escape of Lyα photons from Lyα emitting galaxies (LAEs) and the overall galaxy population using a sample of 99 LAEs at 1.9 (3-6) × 1042 erg s–1 (0.25-0.5 L*), have a mean E(B – V) = 0.13 ± 0.01, implying an attenuation of ~70% in the UV. They show a median UV uncorrected SFR = 11 M ☉ yr–1, dust-corrected SFR = 34 M ☉ yr–1, and Lyα equivalent widths (EWs) which are consistent with normal stellar populations. We measure a median Lyα escape fraction of 29%, with a large scatter and values ranging from a few percent to 100%. The Lyα escape fraction in LAEs correlates with E(B – V) in a way that is expected if Lyα photons suffer from similar amounts of dust extinction as UV continuum photons. This result implies that a strong enhancement of the Lyα EW with dust, due to a clumpy multi-phase interstellar medium (ISM), is not a common process in LAEs at these redshifts. It also suggests that while in other galaxies Lyα can be preferentially quenched by dust due to its scattering nature, this is not the case in LAEs. We find no evolution in the average dust content and Lyα escape fraction of LAEs from z ~ 4 to 2. We see hints of a drop in the number density of LAEs from z ~ 4 to 2 in the redshift distribution and the Lyα luminosity function, although larger samples are required to confirm this. The mean Lyα escape fraction of the overall galaxy population decreases significantly from z ~ 6 to z ~ 2, in agreement with recent results. Our results point toward a scenario in which star-forming galaxies build up significant amounts of dust in their ISM between z ~ 6 and 2, reducing their Lyα escape fraction, with LAE selection preferentially detecting galaxies which have the highest escape fractions given their dust content. The fact that a large escape of Lyα photons is reached by z ~ 6 implies that better constraints on this quantity at higher redshifts might detect re-ionization in a way that is uncoupled from the effects of dust.

THE BIMODAL GALAXY STELLAR MASS FUNCTION IN THE COSMOS SURVEY TO z ∼ 1: A STEEP FAINT END AND A NEW GALAXY DICHOTOMY

We present a new analysis of stellar mass functions in the COSMOS field to fainter limits than has been previously probed at z ≤ 1. The increase in dynamic range reveals features in the shape of the stellar mass function that deviate from a single Schechter function. Neither the total nor the red (passive) or blue (star-forming) galaxy stellar mass functions can be well fitted with a single Schechter function once the mass completeness limit of the sample probes below ~3 × 10^9 M_⊙. We observe a dip or plateau at masses ~10^(10) M_⊙, just below the traditional M^*, and an upturn toward a steep faint-end slope of α ~ –1.7 at lower mass at all redshifts ≤ 1. This bimodal nature of the mass function is not solely a result of the blue/red dichotomy. Indeed, the blue mass function is by itself bimodal at z ~ 1. This suggests a new dichotomy in galaxy formation that predates the appearance of the red sequence. We propose two interpretations for this bimodal distribution. If the gas fraction increases toward lower mass, galaxies with M_ (baryon) ~ 10^(10) M_⊙ would shift to lower stellar masses, creating the observed dip. This would indicate a change in star formation efficiency, perhaps linked to supernovae feedback becoming much more efficient below ~10^(10) M_⊙. Therefore, we investigate whether the dip is present in the baryonic (stars+gas) mass function. Alternatively, the dip could be created by an enhancement of the galaxy assembly rate at ~10^(11) M_⊙, a phenomenon that naturally arises if the baryon fraction peaks at M_(halo) ~ 10^(12) M_⊙. In this scenario, galaxies occupying the bump around M_* would be identified with central galaxies and the second fainter component of the mass function having a steep faint-end slope with satellite galaxies. The low-mass end of the blue and total mass functions exhibit a steeper slope than has been detected in previous work that may increasingly approach the halo mass function value of –2. While the dip feature is apparent in the total mass function at all redshifts, it appears to shift from the blue to the red population, likely as a result of transforming high-mass blue galaxies into red ones. At the same time, we detect a drastic upturn in the number of low-mass red galaxies. Their increase with time seems to reflect a decrease in the number of blue systems and so we tentatively associate them with satellite dwarf (spheroidal) galaxies that have undergone quenching due to environmental processes.

Bulges of Nearby Galaxies with Spitzer: Scaling Relations in Pseudobulges and Classical Bulges

We investigate scaling relations of bulges using bulge-disk decompositions at 3.6 ?m and present bulge classifications for 173 E-Sd galaxies within 20?Mpc. Pseudobulges and classical bulges are identified using S?rsic index, Hubble Space Telescope morphology, and star formation activity (traced by 8 ?m emission). In the near-IR pseudobulges have nb 2, as found in the optical. S?rsic index and morphology are essentially equivalent properties for bulge classification purposes. We confirm, using a much more robust sample, that the S?rsic index of pseudobulges is uncorrelated with other bulge structural properties, unlike for classical bulges and elliptical galaxies. Also, the half-light radius of pseudobulges is not correlated with any other bulge property. We also find a new correlation between surface brightness and pseudobulge luminosity; pseudobulges become more luminous as they become more dense. Classical bulges follow the well-known scaling relations between surface brightness, luminosity, and half-light radius that are established by elliptical galaxies. We show that those pseudobulges (as indicated by S?rsic index and nuclear morphology) that have low specific star formation rates are very similar to models of galaxies in which both a pseudobulge and classical bulge exist. Therefore, pseudobulge identification that relies only on structural indicators is incomplete. Our results, especially those on scaling relations, imply that pseudobulges are very different types of objects than elliptical galaxies.

The HETDEX pilot survey. III. The low metallicities of high-redshift lyα galaxies

We present the results of Keck/NIRSPEC spectroscopic observations of three Lyα emitting galaxies (LAEs) at z~ 2.3 discovered with the HETDEX pilot survey. We detect Hα, [O III], and Hβ emission from two galaxies at z= 2.29 and 2.49, designated HPS194 and HPS256, respectively, representing the first detection of multiple rest-frame optical emission lines in galaxies at high redshift selected on the basis of their Lyα emission. We find that the redshifts of the Lyα emission from these galaxies are offset redward of the systemic redshifts (derived from the Hα and [O III] emission) by Δv = 162 ± 37 (photometric) ± 42 (systematic) km s–1 for HPS194 and Δv = 36 ± 35 ± 18 km s–1 for HPS256. An interpretation for HPS194 is that a large-scale outflow may be occurring in its interstellar medium. This outflow is likely powered by star-formation activity, as examining emission line ratios implies that neither LAE hosts an active galactic nucleus. Using the upper limits on the [N II] emission, we place meaningful constraints on the gas-phase metallicities in these two LAEs of Z< 0.17 and < 0.28 Z ☉ (1σ). Measuring the stellar masses of these objects via spectral energy distribution (SED) fitting (~1010 and 6 × 108 M ☉, respectively), we study the nature of LAEs in a mass-metallicity plane. At least one of these two LAEs appears to be more metal poor than continuum-selected star-forming galaxies at the same redshift and stellar mass, implying that objects exhibiting Lyα emission may be systematically less chemically enriched than the general galaxy population. We use the SEDs of these two galaxies to show that neglecting the contribution of the measured emission line fluxes when fitting stellar population models to the observed photometry can result in overestimates of the population age by orders of magnitude and the stellar mass by a factor of ~2. This effect is particularly important at z 7, where similarly strong emission lines may masquerade in the photometry as a 4000 A break.

THE HETDEX PILOT SURVEY. I. SURVEY DESIGN, PERFORMANCE, AND CATALOG OF EMISSION-LINE GALAXIES

We present a catalog of emission-line galaxies selected sol ly by their emission-line fluxes using a wide-field integral field spectrograph. This work is partially motivat ed as a pilot survey for the upcoming Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). We describe the observations, reductions, detections, redshift classifications, line fluxes, and counterpart information f r 397 emission-line galaxies detected over 169 ⊓⊔ with a 3500-5800̊A bandpass under 5̊ A full-width-half-maximum (FWHM) spectral resolution. Th e survey’s best sensitivity for unresolved objects under photometric conditions is between 4− 20× 10 erg s cm depending on the wavelength, and Ly α luminosities between3− 6× 10 erg s are detectable. This survey method complements narrowband and color-selection techni ques in the search for high redshift galaxies with its different selection properties and large volume probed. Th e four survey fields within the COSMOS, GOODS-N, MUNICS, and XMM-LSS areas are rich with existing, complemen tary data. We find 104 galaxies via their high redshift Lyα emission at1.9 < z < 3.8, and the majority of the remainder objects are low redshift [ OII]3727 emitters atz < 0.56. The classification between low and high redshift objects de pends on rest frame equivalent width, as well as other indicators, where available. Based o n matches to X-ray catalogs, the active galactic nuclei (AGN) fraction amongst the Ly α emitters (LAEs) is 6%. We also analyze the survey’s complete ness and contamination properties through simulations. We find fi ve high-z, highly-significant, resolved objects with full-width-half-maximum sizes> 44 ⊓⊔ which appear to be extended Ly α nebulae. We also find three high-z objects with rest frame Ly α equivalent widths above the level believed to be achievable with normal star formation, EW0 > 240Å. Future papers will investigate the physical properties o f this sample. Subject headings: galaxies: formation — galaxies: evolution —galaxies: high -redshift — cosmology: observations

The Contribution of Star Formation and Merging to Stellar Mass Buildup in Galaxies

We present a formalism to reveal merging by subtracting the change in the galaxy stellar mass function (MF) due to mass-dependent star formation (SF) from the observed time derivative of the MF. We present the SF rate (SFR) in the FORS Deep Field as a function of stellar mass and time spanning 9 z > 0. At 10 < log M* < 11, galaxies are being preferentially destroyed at early times, while at later times the change in their numbers turns positive. This is an indication of the top-down buildup of the red sequence suggested by other recent observations.

THE EVOLUTION OF EARLY- AND LATE-TYPE GALAXIES IN THE COSMIC EVOLUTION SURVEY UP TO z ≈ 1.2*

The Cosmic Evolution Survey (COSMOS) allows for the first time a highly significant census of environments and structures up to redshift 1, as well as a full morphological description of the galaxy population. In this paper we present a study aimed to constrain the evolution, in the redshift range 0.2 7 × 1010 M ☉) early-type galaxies have similar characteristic ages, colors, and SSFRs independently of the environment they belong to, with those hosting the oldest stars in the universe preferentially belonging to the highest density regions. The whole catalog including morphological information and stellar mass estimates analyzed in this work is made publicly available.

The Evolution of the Tully-Fisher Relation of Spiral Galaxies*

We present the B-band Tully-Fisher relation (TFR) of 60 late-type galaxies with redshifts 0.1–1. The galaxies were selected from the FORS Deep Field with a limiting magnitude of . Spatially resolved rotation curves R p 23 were derived from spectra obtained with FORS2 at the Very Large Telescope. High-mass galaxies with vmax 150 km s 1 show little evolution, whereas the least massive systems in our sample are brighter by ∼1–2 mag compared with their local counterparts. For the entire distant sample, the TFR slope is flatter than for local field galaxies ( vs. ). Thus, we find evidence for the evolution of the slope of the TFR 5.77 0.45 7.92 0.18 with redshift on the 3 j level. This is still true when we subdivide the sample into three redshift bins. We speculate that the flatter tilt of our sample is caused by the evolution of luminosities and an additional population of blue galaxies at . The mass dependence of the TFR evolution also leads to variations for different z 0.2 galaxy types in magnitude-limited samples, suggesting that selection effects can account for the discrepant results of previous TFR studies on the luminosity evolution of late-type galaxies. Subject headings: galaxies: evolution — galaxies: kinematics and dynamics — galaxies: spiral