arXiv subjects

Pascal Oesch

Publications and source records attributed to Pascal Oesch.

85 records · Page 5Linked to original sources

arXiv2015

Forming Compact Massive Galaxies

In this paper we study a key phase in the formation of massive galaxies: the transition of star forming galaxies into massive (M_stars~10^11 Msun), compact (r_e~1 kpc) quiescent galaxies, which takes place from z~3 to z~1.5. We use HST grism redshifts and extensive photometry in all five 3D-HST/CANDELS fields, more than doubling the area used previously for such studies, and combine these data with Keck MOSFIRE and NIRSPEC spectroscopy. We first confirm that a population of massive, compact, star forming galaxies exists at z~2, using K-band spectroscopy of 25 of these objects at 2.0<z<2.5. They have a median NII/Halpha ratio of 0.6, are highly obscured with SFR(tot)/SFR(Halpha)~10, and have a large range of observed line widths. We infer from the kinematics and spatial distribution of Halpha that the galaxies have rotating disks of ionized gas that are a factor of ~2 more extended than the stellar distribution. By combining measurements of individual galaxies, we find that the kinematics are consistent with a nearly Keplerian fall-off from V_rot~500 km/s at 1 kpc to V_rot~250 km/s at 7 kpc, and that the total mass out to this radius is dominated by the dense stellar component. Next, we study the size and mass evolution of the progenitors of compact massive galaxies. Even though individual galaxies may have had complex histories with periods of compaction and mergers, we show that the population of progenitors likely followed a simple inside-out growth track in the size-mass plane of d(log r_e) ~ 0.3 d(log M_stars). This mode of growth gradually increases the stellar mass within a fixed physical radius, and galaxies quench when they reach a stellar density or velocity dispersion threshold. As shown in other studies, the mode of growth changes after quenching, as dry mergers take the galaxies on a relatively steep track in the size-mass plane.

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arXiv2015

The Sizes of Candidate $z\sim9-10$ Galaxies: confirmation of the bright CANDELS sample and relation with luminosity and mass

Recently, a small sample of six $z\sim9-10$ candidates was discovered in CANDELS that are $\sim10-20\times$ more luminous than any of the previous $z\sim9-10$ galaxies identified over the HUDF/XDF and CLASH fields. We measure the sizes of these candidates to map out the size evolution of galaxies from the earliest observable times. Their sizes are also used to provide a valuable constraint on whether these unusual galaxy candidates are at high redshift. Using galfit to derive sizes from the CANDELS F160W images of these candidates, we find a mean size of 0.13$\pm$0.02" (or 0.5$\pm$0.1 kpc at $z\sim9-10$). This handsomely matches the 0.6 kpc size expected extrapolating lower redshift measurements to $z\sim9-10$, while being much smaller than the 0.59" mean size for lower-redshift interlopers to $z\sim9-10$ photometric selections lacking the blue IRAC color criterion. This suggests that source size may be an effective constraint on contaminants from $z\sim9-10$ selections lacking IRAC data. Assuming on the basis of the strong photometric evidence that the Oesch et al. 2014 sample is entirely at $z\sim9-10$, we can use this sample to extend current constraints on the size-luminosity, size-mass relation, and size evolution of galaxies to $z\sim10$. We find that the $z\sim9-10$ candidate galaxies have broadly similar sizes and luminosities as $z\sim6$-8 counterparts with star-formation-rate surface densities in the range of $\rm Σ_{SFR}=1-20\, M_\odot~ yr^{-1}\, kpc^{-2}$. The stellar mass-size relation is uncertain, but shallower than those inferred for lower-redshift galaxies. In combination with previous size measurements at z=4-7, we find a size evolution of $(1+z)^{-m}$ with $m=1.0\pm0.1$ for $>0.3L^*_{z=3}$ galaxies, consistent with the evolution previously derived from $2 < z < 8$ galaxies.

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arXiv2014

Dense cores in galaxies out to z=2.5 in SDSS, UltraVISTA, and the five 3D-HST/CANDELS fields

The dense interiors of massive galaxies are among the most intriguing environments in the Universe. In this paper we ask when these dense cores were formed and determine how galaxies gradually assembled around them. We select galaxies that have a stellar mass >3x10^10 Msun inside r=1 kpc out to z=2.5, using the 3D-HST survey and data at low redshift. Remarkably, the number density of galaxies with dense cores appears to have decreased from z=2.5 to the present. This decrease is probably mostly due to stellar mass loss and the resulting adiabatic expansion, with some contribution from merging. We infer that dense cores were mostly formed at z>2.5, consistent with their largely quiescent stellar populations. While the cores appear to form early, the galaxies in which they reside show strong evolution: their total masses increase by a factor of 2-3 from z=2.5 to z=0 and their effective radii increase by a factor of 5-6. As a result, the contribution of dense cores to the total mass of the galaxies in which they reside decreases from ~50% at z=2.5 to ~15% at z=0. Because of their early formation, the contribution of dense cores to the total stellar mass budget of the Universe is a strong function of redshift. The stars in cores with M_1kpc>3x10^10 Msun make up ~0.1% of the stellar mass density of the Universe today but 10%-20% at z~2, depending on their IMF. The formation of these cores required the conversion of ~10^11 Msun of gas into stars within ~1 kpc, while preventing significant star formation at larger radii.

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arXiv2014

3D-HST WFC3-selected Photometric Catalogs in the Five CANDELS/3D-HST Fields: Photometry, Photometric Redshifts and Stellar Masses

The 3D-HST and CANDELS programs have provided WFC3 and ACS spectroscopy and photometry over ~900 square arcminutes in five fields: AEGIS, COSMOS, GOODS-North, GOODS-South, and the UKIDSS UDS field. All these fields have a wealth of publicly available imaging datasets in addition to the HST data, which makes it possible to construct the spectral energy distributions (SEDs) of objects over a wide wavelength range. In this paper we describe a photometric analysis of the CANDELS and 3D-HST HST imaging and the ancillary imaging data at wavelengths 0.3um to 8um. Objects were selected in the WFC3 near-IR bands, and their SEDs were determined by carefully taking the effects of the point spread function in each observation into account. A total of 147 distinct imaging datasets were used in the analysis. The photometry is made available in the form of six catalogs: one for each field, as well as a master catalog containing all objects in the entire survey. We also provide derived data products: photometric redshifts, determined with the EAZY code, and stellar population parameters determined with the FAST code. We make all the imaging data that were used in the analysis available, including our reductions of the WFC3 imaging in all five fields. 3D-HST is a spectroscopic survey with the WFC3 and ACS grisms, and the photometric catalogs presented here constitute a necessary first step in the analysis of these grism data. All the data presented in this paper are available through the 3D-HST website.

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arXiv2014

Observations of environmental quenching in groups in the 11 Gyr since z=2.5: different quenching for central and satellite galaxies

We present direct observational evidence for star formation quenching in galaxy groups in the redshift range 0<z<2.5. We utilize a large sample of nearly 6000 groups, selected by fixed cumulative number density from three photometric catalogs, to follow the evolving quiescent fractions of central and satellite galaxies over roughly 11 Gyr. At z~0, central galaxies in our sample range in stellar mass from Milky Way/M31 analogs (M=6.5x10^10 M\solar) to nearby massive ellipticals (M=1.5x10^11 M\solar). Satellite galaxies in the same groups reach masses as low as twice that of the Large Magellanic Cloud (M=6.5x10^9 M\solar). Using statistical background subtraction, we measure the average rest-frame colors of galaxies in our groups and calculate the evolving quiescent fractions of centrals and satellites over seven redshift bins. Our analysis shows clear evidence for star formation quenching in group halos, with a different quenching onset for centrals and their satellite galaxies. Using halo mass estimates for our central galaxies, we find that star formation shuts off in centrals when typical halo masses reach between 10^12 and 10^13 M\solar, consistent with predictions from the halo quenching model. In contrast, satellite galaxies in the same groups most likely undergo quenching by environmental processes, whose onset is delayed with respect to their central galaxy. Although star formation is suppressed in all galaxies over time, the processes that govern quenching are different for centrals and satellites. While mass plays an important role in determining the star formation activity of central galaxies, quenching in satellite galaxies is dominated by the environment in which they reside.

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arXiv2013

Tracing the Mass Growth and Star Formation Rate Evolution of Massive Galaxies from z~6 to z~1 in the Hubble Ultra-Deep Field

We present an analysis of $\sim$1500 H160-selected photometric galaxies detected to a limiting magnitude of 27.8 in the HUDF, using imaging from the HST WFC3/IR camera in combination with archival UV, optical, and NIR imaging. We fit photometric redshifts and stellar population estimates for all galaxies with well-determined Spitzer IRAC fluxes, allowing for the determination of the cumulative mass function within the range $1 4$.

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arXiv2013

Slow Evolution of the Specific Star Formation Rate at z>2: The Impact of Dust, Emission Lines, and A Rising Star Formation History

We measure the evolution of the specific star formation rate (sSFR = SFR / Mstellar) between redshift 4 and 6 to investigate the previous reports of "constant" sSFR at z>2. We obtain photometry on a large sample of galaxies at z~4-6 located in the GOODS-S field that have high quality imaging from HST and Spitzer. We have derived stellar masses and star formation rates (SFRs) through stellar population modeling of their spectral energy distributions (SEDs). We estimate the dust extinction from the observed UV colors. In the SED fitting process we have studied the effects of assuming a star formation history (SFH) both with constant SFR and one where the SFR rises exponentially with time. The latter SFH is chosen to match the observed evolution of the UV luminosity function. We find that neither the mean SFRs nor the mean stellar masses change significantly when the rising SFR (RSF) model is assumed instead of the constant SFR model. When focusing on galaxies with Mstar ~ 5x10^9 Msun, we find that the sSFR evolves weakly with redshift (sSFR(z) \propto (1+z)^(0.6+/-0.1) Gyr^-1), consistent with previous results and with recent estimates of the sSFR at z~2-3 using similar assumptions. We have also investigated the impact of optical emission lines on our results. We estimate that the contribution of emission lines to the rest-frame optical fluxes is only modest at z~4 and 5 but it could reach ~50% at z~6. When emission lines of this strength are taken into account, the sSFR shows somewhat higher values at high redshifts, according to the relation sSFR(z) \propto (1+z)^(1.0+/-0.1) Gyr^-1, i.e., ~2.3x higher at z~6 than at z~2. However, the observed evolution is substantially weaker than that found at z<2 or that expected from current models (which corresponds to sSFR(z) \propto (1+z)^(2.5) Gyr^-1). -abridged-

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arXiv2013

A Predicted New Population of UV-faint Galaxies at z>4

We show that a bursty model of high redshift star formation explains several puzzling observations of the high redshift galaxy population. We begin by pointing out that the observed specific star formation rate requires a duty-cycle of ~10%, which is much lower than found in many hydro-dynamical simulations. This value follows directly from the fact that the observed star formation rate in galaxies integrated over a Hubble time exceeds the observed stellar mass by an order of magnitude. We use the large observed specific star formation rate to calibrate the efficiency of feedback in a model for the high redshift star formation rate which includes merger driven star formation regulated by SNe feedback. This model reproduces the star formation rate density function and the stellar mass function of galaxies at 4<z<7. A prediction of the model is that the specific star formation rate does not evolve very rapidly with either mass or redshift, in agreement with observation. This is in contrast to results from hydrodynamical simulations where the star formation closely follows the accretion rate, and so increases strongly towards high redshift. The bursty star formation model naturally explains the observation that there is not enough stellar mass at z~2-4 to account for all of the star-formation observed, without invoking properties like an evolving initial mass function of stars. The finding of a duty cycle that is ~10% implies that there should be ten times the number of known galaxies at fixed stellar mass that have not yet been detected through standard UV selection at high redshift. We therefore predict the existence of a large undetected population of UV-faint galaxies that accounts for most of the stellar mass density at z~4-8.

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arXiv2012

The stellar mass structure of massive galaxies from z=0 to z=2.5; surface density profiles and half-mass radii

We present stellar mass surface density profiles of a mass-selected sample of 177 galaxies at 0.5 < z < 2.5, obtained using very deep HST optical and near-infrared data over the GOODS-South field, including recent CANDELS data. Accurate stellar mass surface density profiles have been measured for the first time for a complete sample of high-redshift galaxies more massive than 10^10.7 M_sun. The key advantage of this study compared to previous work is that the surface brightness profiles are deconvolved for PSF smoothing, allowing accurate measurements of the structure of the galaxies. The surface brightness profiles account for contributions from complex galaxy structures such as rings and faint outer disks. Mass profiles are derived using radial rest-frame u-g color profiles and a well-established empirical relation between these colors and the stellar mass-to-light ratio. We derive stellar half-mass radii from the mass profiles, and find that these are on average ~25% smaller than rest-frame g band half-light radii. This average size difference of 25% is the same at all redshifts, and does not correlate with stellar mass, specific star formation rate, effective surface density, Sersic index, or galaxy size. Although on average the difference between half-mass size and half-light size is modest, for approximately 10% of massive galaxies this difference is more than a factor two. These extreme galaxies are mostly extended, disk-like systems with large central bulges. These results are robust, but could be impacted if the central dust extinction becomes high. ALMA observations can be used to explore this possibility. These results provide added support for galaxy growth scenarios wherein massive galaxies at these epochs grow by accretion onto their outer regions.

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arXiv2012

The Evolution of Mass-size Relation for Lyman Break Galaxies From z=1 to z=7

For the first time, we study the evolution of the stellar mass-size relation for star-forming galaxies from z ~ 4 to z ~ 7 from Hubble-WFC3/IR camera observations of the HUDF and Early Release Science (ERS) field. The sizes are measured by determining the best fit model to galaxy images in the rest-frame 2100 Å\ with the stellar masses estimated from SED fitting to rest-frame optical (from Spitzer/IRAC) and UV fluxes. We show that the stellar mass-size relation of Lyman-break galaxies (LBGs) persists, at least to z ~ 5, and the median size of LBGs at a given stellar mass increases towards lower redshifts. For galaxies with stellar masses of 9.5<Log(M*/Msun)<10.4 sizes evolve as $(1+z)^{-1.20\pm0.11}$. This evolution is very similar for galaxies with lower stellar masses of 8.6<Log(M*/Msun)<9.5 which is $r_{e} \propto (1+z)^{-1.18\pm0.10}$, in agreement with simple theoretical galaxy formation models at high z. Our results are consistent with previous measurements of the LBGs mass-size relation at lower redshifts (z ~ 1-3).

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arXiv2011

The Rest Frame UV to Optical Colors and SEDs of z~4-7 Galaxies

We use the ultra-deep HUDF09 and the deep ERS data from the HST WFC3/IR camera, along with the wide area Spitzer/IRAC data from GOODS-S to derive SEDs of star-forming galaxies from the rest-frame UV to the optical over a wide luminosity range (M_1500 ~ -21 to M_1500 ~ -18) from z ~ 7 to z ~ 4. The sample contains ~ 400 z ~ 4, ~ 120 z ~ 5, ~ 60 z ~ 6, and 36 prior z ~ 7 galaxies. Median stacking enables the first comprehensive study of very faint high-z galaxies at multiple redshifts (e.g., [3.6] = 27.4 +/- 0.1 AB mag for the M_1500 ~ -18 sources at z ~ 4). At z ~ 4 our faint median-stacked SEDs reach to ~ 0.06 L*(z=4) and are combined with recently published results at high luminosity L > L* that extend to M_1500 ~ -23. We use the observed SEDs and template fits to derive rest frame UV-to-optical colors (U - V) at all redshifts and luminosities. We find that this color does not vary significantly with redshift at a fixed luminosity. The UV-to-optical color does show a weak trend with luminosity, becoming redder at higher luminosities. This is most likely due to dust. At z >~ 5 we find blue colors [3.6]-[4.5] ~ -0.3 mag that are most likely due to rest-frame optical emission lines contributing to the flux in the IRAC filter bandpasses. The scatter across our derived SEDs remains substantial, but the results are most consistent with a lack of any evolution in the SEDs with redshift at a given luminosity. The similarity of the SEDs suggests a self-similar mode of evolution over a timespan from 0.7 Gyr to 1.5 Gyr that encompasses very substantial growth in the stellar mass density in the universe (from ~ 4x10^6 to ~ 2x10^7 Msun Mpc^-3).

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arXiv2011

The zCOSMOS-SINFONI Project I: Sample Selection and Natural-Seeing Observations

The zCOSMOS SINFONI project is aimed at studying the physical and kinematical properties of a sample of massive z~1.4-2.5 star-forming galaxies, through SINFONI near-IR integral field spectroscopy (IFS), combined with the multi-wavelength information from the zCOSMOS (COSMOS) survey. The project is based on 1 hour of natural-seeing observations per target, and Adaptive Optics (AO) follow-up for a major part of the sample, which includes 30 galaxies selected from the zCOSMOS/VIMOS spectroscopic survey. This first paper presents the sample selection, and the global physical characterization of the target galaxies from multicolor photometry, i.e., star formation rate (SFR), stellar mass, age, etc. The Halpha integrated properties such as, flux, velocity dispersion, and size, are derived from the natural-seeing observations, while the follow up AO observations will be presented in the next paper of this series. Our sample appears to be well representative of star-forming galaxies at z~2, covering a wide range in mass and SFR. The Halpha integrated properties of the 25 Halpha detected galaxies are similar to those of other IFS samples at the same redshifts. Good agreement is found among the SFRs derived from Halpha luminosity and other diagnostic methods, provided the extinction affecting the Halpha luminosity is about twice that affecting the continuum. A preliminary kinematic analysis, based on the maximum observed velocity difference across the source, and on the integrated velocity dispersion, indicates that the sample splits nearly 50-50 into rotation-dominated and velocity dispersion-dominated galaxies, in good agreement with previous surveys.

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arXiv2011

The UDF05 Follow-up of the Hubble Ultra Deep Field. III. The Luminosity Function at z~6

In this paper, we present a derivation of the rest-frame 1400A luminosity function (LF) at redshift six from a new application of the maximum likelihood method by exploring the five deepest HST/ACS fields, i.e., the HUDF, two UDF05 fields, and two GOODS fields. We work on the latest improved data products, which makes our results more robust than those of previous studies. We use un-binned data and thereby make optimal use of the information contained in the dataset. We focus on the analysis to a magnitude limit where the completeness is larger than 50% to avoid possibly large errors in the faint end slope that are difficult to quantify. We also take into account scattering in and out of the dropout sample due to photometric errors by defining for each object a probability that it belongs to the dropout sample. We find the best fit Schechter parameters to the z~6 LF are: alpha = 1.87 +/- 0.14, M* = -20.25 +/- 0.23, and phi*=1.77^{+0.62}_{-0.49} * 10^{-3} Mpc^{-3}. Such a steep slope suggests that galaxies, especially the faint ones, are possibly the main sources of ionizing photons in the universe at redshift six. We also combine results from all studies at z~6 to reach an agreement in 95% confidence level that -20.45<M*<-20.05 and -1.90<alpha<-1.55. The luminosity density has been found not to evolve significantly between z~6 and z~5, but considerable evolution is detected from z~6 to z~3.

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