arXiv subjects

Bradford P. Holden

Publications and source records attributed to Bradford P. Holden.

At least 19 recordsLinked to original sources

arXiv2026

Radial Velocity Evidence for a Post-Mass-Transfer Massive Binary System NaSt1

We present multi-epoch high-resolution optical spectroscopy ($R \simeq 80{,}000$) of the emission-line object NaSt1 to test its proposed binary nature, along with long-term multiband photometry, mid-infrared spectroscopy, and spatially resolved integral field unit (IFU) spectroscopy to probe the circumstellar kinematics of the system. We detect two groups of 36 emission lines showing radial velocity (RV) variation with a mean period of 311 $\pm$ 5 d, but varying in opposite phase. We associate these two groups with the optically thick wind of the stripped primary star and the wind-wind collision region with the companion star, providing strong evidence for binarity. The RV and light curve (LC) periods are consistent within the uncertainties, ruling out ellipsoidal modulation, which would require an orbital period of about 620 d. The RV-LC phase relationship and high-ionization lines favor binary interaction over pulsations. We model the 1--5~$μ$m spectrum of NaSt1 and find two optically thin dust components: hot $T_{\rm h} \simeq 1230$ K, $M_{\rm h} \simeq 2 \times 10^{-10} M_{\odot}$ and warm $T_{\rm c} \simeq 660$ K, $M_{\rm c} \simeq 3 \times 10^{-8} M_{\odot}$. IFU spectroscopy spatially resolves the circumstellar medium in the [\ion{N}{2}] $\lambda6548$ and $\lambda6584$ emission lines, showing a deprojected expansion velocity of $\sim31$ km~s$^{-1}$, implying a dynamical age of $\sim40$ yr. This short timescale suggests that the nebula was produced by recent mass loss. The system may represent a Galactic analog of a massive binary undergoing a mass-loss process to become a stripped-envelope supernova progenitor.

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arXiv2026

The Discovery of K2-232c: Divergent Formation Histories for Hot and Warm Jupiters Based on Outer Companion Eccentricity

Ever since their discovery, hot Jupiters have been one of the most studied types of exoplanets to exist thanks to their significant size, their proximity to their host star, and their significant departure from anything present in our solar system. Yet, the details of their formation and evolution remain unclear, including their connection, if any, to the wider-orbiting warm Jupiter population. In this work, we present the discovery of K2-232c, an eccentric cold Jupiter ($P = 1950 ^{+140}_{-120}$ days, $e=0.352^{+0.095}_{-0.076}$, $M\sin{i} = 5.31^{+0.48}_{-0.45} {M_{\rm Jup}}$) companion in a known warm Jupiter ($P = 11.1684377 \pm 0.0000010$ days, $e=0.245^{+0.023}_{-0.024}$, $M = 0.427^{+0.039}_{-0.036} {M_{\rm Jup}}$) system. Placing this system in context with the literature, we find that cold Jupiter eccentricities are generally higher in hot Jupiter systems as compared to warm Jupiter systems, suggesting the formation of hot Jupiters is more dynamically violent than warm Jupiters. This adds further evidence to the claim that these two populations are independent from one another with cold Jupiters appearing to play a crucial role in shaping the formation of both.

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arXiv2022

The Mass Scale of High-Redshift Galaxies: Virial Mass Estimates Calibrated with Stellar Dynamical Models from LEGA-C

Dynamical models for $673$ galaxies at $z=0.6-1.0$ with spatially resolved (long-slit) stellar kinematic data from LEGA-C are used to calibrate virial mass estimates defined as $M_{\rm{vir}}=K σ'^2_{\star,\rm{int}} R$, with $K$ a scaling factor, $σ'_{\star,\rm{int}}$ the spatially-integrated stellar velocity second moment from the LEGA-C survey and $R$ the effective radius measured from a Sérsic profile fit to HST imaging. The sample is representative for $M_{\star}>3\times10^{10}~M_{\odot}$ and includes all types of galaxies, irrespective of morphology and color. We demonstrate that using $R=R_{\rm{sma}}$~(the semi-major axis length of the ellipse that encloses 50\% of the light) in combination with an inclination correction on $σ'_{\star,\rm{int}}$~produces an unbiased $M_{\rm{vir}}$. We confirm the importance of projection effects on $σ'_{\star,\rm{int}}$ by showing the existence of a similar residual trend between virial mass estimates and inclination for the nearby early-type galaxies in the ATLAS$^{\rm{3D}}$~survey. Also, as previously shown, when using a Sérsic profile-based $R$ estimate, then a Sérsic index-dependent correction to account for non-homology in the radial profiles is required. With respect to analogous dynamical models for low-redshift galaxies from the ATLAS$^{\rm{3D}}$~survey we find a systematic offset of 0.1 dex in the calibrated virial constant for LEGA-C, which may be due to physical differences between the galaxy samples or an unknown systematic error. Either way, with our work we establish a common mass scale for galaxies across 8 Gyr of cosmic time with a systematic uncertainty of at most 0.1 dex.

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arXiv2018

Simulating the M-R Relation from APF follow up of TESS targets: Survey design and strategies for overcoming mass biases

We present simulations of multi-year RV follow up campaigns of the {\it TESS} small exoplanet yield on the Automated Planet Finder telescope, using four different schemes to sample the transiting planets' RV phase curves. For planets below roughly 10 M$_\oplus$ we see a systematic bias of measured masses that are higher than the true planet mass, regardless of the observing scheme used. This produces a statistically significant difference in the mass-radius relation we recover, where planet masses are predicted to be too high and too similar across the entire super-Earth to Neptune radius range. This bias is due in part to only reporting masses that are measured with high statistical significance. Incorporating all mass measurements, even those that are essentially only upper limits, significantly mitigates this bias. We also find statistically significant differences between the mean number of planets measured at the 1-, 3-, and 5$σ_{\rm K}$ level by the different prioritization schemes. Our results show that prioritization schemes that more evenly sample the RV phase curves produce a larger number of significant mass detections. The scheme that aims to most uniformly sample the phase curve performs best, followed closely by the scheme which randomly samples, and then an in quadrature sampling approach. The fourth scheme, out of quadrature, performs noticeably worse. These results have important implications for determining accurate planet compositions and for designing effective RV follow up campaigns in the era of large planet detection surveys such as $K2$, $TESS$, and $PLATO$.

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arXiv2018

The Detection of Intergalactic Halpha Emission from the Slug Nebula at z~2.3

The Slug Nebula is one of the largest and most luminous Lyman-alpha (LyA) nebulae discovered to date, extending over 450 kiloparsecs (kpc) around the bright quasar UM287 at z=2.283. Characterized by high surface brightnesses over intergalactic scales, its LyA emission may either trace high-density ionized gas ("clumps") or large column densities of neutral material. To distinguish between these two possibilities, information from a non-resonant line such as Halpha is crucial. Therefore, we analyzed a deep MOSFIRE observation of one of the brightest LyA emitting regions in the Slug Nebula with the goal of detecting associated Halpha emission. We also obtained a deep, moderate resolution LyA spectrum of the nearby brightest region of the Slug. We detected an Halpha flux of F_(Halpha)= 2.62 +/- 0.47 x 10^-17 erg/cm^2/s (SB_(Halpha)=2.70 +/- 0.48 x 10^-18 erg/cm^2/s/sq") at the expected spatial and spectral location. Combining the Halpha detection with its corresponding LyA flux (determined from the narrow-band imaging) we calculate a flux ratio of F_(LyA_/F_(Halpha)= 5.5 +/- 1.1. The presence of a skyline at the location of the Halpha emission decreases the signal to noise ratio of the detection and our ability to put stringent constraints on the Halpha kinematics. Our measurements argue for the origin of the LyA emission being recombination radiation, suggesting the presence of high-density ionized gas. Finally, our high-resolution spectroscopic study of the LyA emission does not show evidence of a rotating disk pattern and suggest a more complex origin for at least some parts of the Slug Nebula.

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arXiv2018

A Low Lyman Continuum Escape Fraction of $<10\%$ for Extreme [OIII] Emitters in an Overdensity at z$\sim3.5$

Recent work has suggested extreme [OIII] emitting star-forming galaxies are important to reionization. Relatedly, [OIII]/[OII] has been put forward as an indirect estimator of the Lyman Continuum (LyC) escape fraction ($f_{esc}$) at $z\gtrsim4.5$ when the opaque IGM renders LyC photons unobservable. Using deep archival U-band (VLT/VIMOS) imaging of a recently confirmed overdensity at $z\sim3.5$ we calculate tight constraints on $f_{esc}$ for a sample (N=73) dominated by extreme [OIII] emitters. We find no Lyman Continuum signal ($f_{esc}^{rel} < 6.3^{+0.7}_{-0.7} \%$ at $1σ$) in a deep U-band stack of our sample (31.98 mag at 1$σ$). This constraint is in agreement with recent studies of star-forming galaxies spanning $z\sim1-4$ that have found very low average $f_{esc}$. Despite the galaxies in our study having an estimated average rest-frame EW([OIII]$\lambda5007$)$\sim400Å$ and [OIII]/[OII]$\sim 4$ from composite SED-fitting, we find no LyC detection, which brings into question the potential of [OIII]/[OII] as an effective probe of the LyC--a majority of LyC emitters have [OIII]/[OII]$>3$, but we establish here that [OIII]/[OII]$>3$ does not guarantee significant LyC leakage for a population. Since even extreme star-forming galaxies are unable to produce the $f_{esc}\sim10-15\%$ required by most theoretical calculations for star-forming galaxies to drive reionization, there must either be a rapid evolution of $f_{esc}$ between $z\sim3.5$ and the Epoch of Reionization, or hitherto observationally unstudied sources (e.g. ultra-faint low-mass galaxies with $\log(M/M_\odot)\sim7-8.5$) must make an outsized contribution to reionization.

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arXiv2017

Three's Company: An additional non-transiting super-Earth in the bright HD 3167 system, and masses for all three planets

HD 3167 is a bright (V = 8.9), nearby K0 star observed by the NASA K2 mission (EPIC 220383386), hosting two small, short-period transiting planets. Here we present the results of a multi-site, multi-instrument radial velocity campaign to characterize the HD 3167 system. The masses of the transiting planets are 5.02+/-0.38 MEarth for HD 3167 b, a hot super-Earth with a likely rocky composition (rho_b = 5.60+2.15-1.43 g/cm^3), and 9.80+1.30-1.24 MEarth for HD 3167 c, a warm sub-Neptune with a likely substantial volatile complement (rho_c = 1.97+0.94-0.59 g/cm^3). We explore the possibility of atmospheric composition analysis and determine that planet c is amenable to transmission spectroscopy measurements, and planet b is a potential thermal emission target. We detect a third, non-transiting planet, HD 3167 d, with a period of 8.509+/-0.045 d (between planets b and c) and a minimum mass of 6.90+/-0.71 MEarth. We are able to constrain the mutual inclination of planet d with planets b and c: we rule out mutual inclinations below 1.3 degrees as we do not observe transits of planet d. From 1.3-40 degrees, there are viewing geometries invoking special nodal configurations which result in planet d not transiting some fraction of the time. From 40-60 degrees, Kozai-Lidov oscillations increase the system's instability, but it can remain stable for up to 100Myr. Above 60 degrees, the system is unstable. HD 3167 promises to be a fruitful system for further study and a preview of the many exciting systems expected from the upcoming NASA TESS mission.

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arXiv2017

A comparison of the most massive quiescent galaxies from $z \sim 3$ to the present: slow evolution in size, and spheroid-dominated

We use Hubble Space Telescope imaging to study the structural properties of ten of the most massive ($M \geq 10^{11.25}$ Msun) quiescent galaxies (QGs) in the UKIDSS UDS at $2.5<z<3.0$. The low spatial density of these galaxies required targeted WFC3 $H_{160}$ imaging, as such systems are rare in existing surveys like CANDELS. We fit Sersic models to the 2D light profiles and find that the median half-light radius is $R_e \sim 3$ kpc, a factor of $\sim 3$ smaller than QGs with similar masses at $z \sim 0$. Complementing our sample with similarly massive QGs at lower redshifts, we find that the median size evolves as $R_e \propto H(z)^{-0.85 \pm 0.12}$ (or alternatively, $R_e \propto (1+z)^{-0.90 \pm 0.12}$). This rate of evolution is slower than that for lower mass QGs. When compared to low redshift QGs, the axis ratio distribution for our high redshift massive QG sample is most consistent with those in which spheroids are dominant. These observations point to earlier size growth among massive QGs that also resulted in spheroidal systems. Finally, we measured residual-corrected surface brightness profiles for our sample. These show that the Sersic parameterization is generally representative out to several effective radii and does not miss excess low surface brightness light. The sizes inferred from the light profiles therefore confirm the compactness of these most massive high redshift QGs.

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arXiv2015

Three Super-Earths Orbiting HD 7924

We report the discovery of two super-Earth mass planets orbiting the nearby K0.5 dwarf HD 7924 which was previously known to host one small planet. The new companions have masses of 7.9 and 6.4 M$_\oplus$, and orbital periods of 15.3 and 24.5 days. We perform a joint analysis of high-precision radial velocity data from Keck/HIRES and the new Automated Planet Finder Telescope (APF) to robustly detect three total planets in the system. We refine the ephemeris of the previously known planet using five years of new Keck data and high-cadence observations over the last 1.3 years with the APF. With this new ephemeris, we show that a previous transit search for the inner-most planet would have covered 70% of the predicted ingress or egress times. Photometric data collected over the last eight years using the Automated Photometric Telescope shows no evidence for transits of any of the planets, which would be detectable if the planets transit and their compositions are hydrogen-dominated. We detect a long-period signal that we interpret as the stellar magnetic activity cycle since it is strongly correlated with the Ca II H and K activity index. We also detect two additional short-period signals that we attribute to rotationally-modulated starspots and a one month alias. The high-cadence APF data help to distinguish between the true orbital periods and aliases caused by the window function of the Keck data. The planets orbiting HD 7924 are a local example of the compact, multi-planet systems that the Kepler Mission found in great abundance.

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arXiv2014

The Assembly Histories of Quiescent Galaxies Since z=0.7 from Absorption Line Spectroscopy

We present results from modeling the optical spectra of a large sample of quiescent galaxies between 0.1 < z < 0.7 from the Sloan Digital Sky Survey (SDSS) and the AGN and Galaxy Evolution Survey (AGES). We examine how the stellar ages and abundance patterns of galaxies evolve over time as a function of stellar mass from 10^{9.6}-10^{11.8} Msun. Galaxy spectra are stacked in bins of mass and redshift, and modeled over a wavelength range from 4000 A to 5500 A. Full spectrum stellar population synthesis modeling provides estimates of the age and the abundances of the elements Fe, Mg, C, N, and Ca. We find negligible evolution in elemental abundances at fixed stellar mass over roughly 7 Gyr of cosmic time. In addition, the increase in stellar ages with time for massive galaxies is consistent with passive evolution since z = 0.7. Taken together, these results favor a scenario in which the inner ~ 0.3-3 R_e of massive quiescent galaxies have been passively evolving over the last half of cosmic time. Interestingly, the derived stellar ages are considerably younger than the age of the universe at all epochs, consistent with an equivalent single-burst star formation epoch of z < 1.5. These young stellar population ages coupled with the existence of massive quiescent galaxies at z > 1 indicate the inhomogeneous nature of the z < 0.7 quiescent population. The data also permit the addition of newly-quenched galaxies at masses below ~10^{10.5} Msun at z < 0.7. Additionally, we analyze very deep Keck DEIMOS spectra of the two brightest quiescent galaxies in a cluster at z = 0.83. There is tentative evidence that these galaxies are older than their counterparts in low-density environments. In the Appendix, we demonstrate that our full spectrum modeling technique allows for accurate and reliable modeling of galaxy spectra to low S/N (~20 A^{-1}) and/or low spectral resolution (R ~ 500).

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arXiv2013

HST/WFC3 Confirmation of the Inside-Out Growth of Massive Galaxies at 0<z<2 and Identification of their Star Forming Progenitors at z~3

We study the structural evolution of massive galaxies by linking progenitors and descendants at a constant cumulative number density of n_c=1.4x10^{-4} Mpc^{-3} to z~3. Structural parameters were measured by fitting Sersic profiles to high resolution CANDELS HST WFC3 J_{125} and H_{160} imaging in the UKIDSS-UDS at 1<z<3 and ACS I_{814} imaging in COSMOS at 0.25<z<1. At a given redshift, we selected the HST band that most closely samples a common rest-frame wavelength so as to minimize systematics from color gradients in galaxies. At fixed n_c, galaxies grow in stellar mass by a factor of ~3 from z~3 to z~0. The size evolution is complex: galaxies appear roughly constant in size from z~3 to z~2 and then grow rapidly to lower redshifts. The evolution in the surface mass density profiles indicates that most of the mass at r<2 kpc was in place by z~2, and that most of the new mass growth occurred at larger radii. This inside-out mass growth is therefore responsible for the larger sizes and higher Sersic indices of the descendants toward low redshift. At z<2, the effective radius evolves with the stellar mass as r_e M^{2.0}, consistent with scenarios that find dissipationless minor mergers to be a key driver of size evolution. The progenitors at z~3 were likely star-forming disks with r_e~2 kpc, based on their low Sersic index of n~1, low median axis ratio of b/a~0.52, and typical location in the star-forming region of the U-V versus V-J diagram. By z~1.5, many of these star-forming disks disappeared, giving rise to compact quiescent galaxies. Toward lower redshifts, these galaxies continued to assemble mass at larger radii and became the local ellipticals that dominate the high mass end of the mass function at the present epoch.

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arXiv2012

Shape Evolution of Massive Early-Type Galaxies: Confirmation of Increased Disk Prevalence at z>1

We use high-resolution K-band VLT/HAWK-I imaging over 0.25 square degrees to study the structural evolution of massive early-type galaxies since z~1. Mass-selected samples, complete down to log(M/M_sun)~10.7 such that `typical' L* galaxies are included at all redshifts, are drawn from pre-existing photometric redshift surveys. We then separated the samples into different redshift slices and classify them as late- or early-type galaxies on the basis of their specific star-formation rate. Axis-ratio measurements for the ~400 early-type galaxies in the redshift range 0.6 1 are, on average, flatter than at z<1 and the median projected axis ratio at a fixed mass decreases with redshift. However, we also find that at all epochs z<~2 the very most massive early-type galaxies (log(M/M_sun)>11.3) are the roundest, with a pronounced lack among them of galaxies that are flat in projection. Merging is a plausible mechanism that can explain both results: at all epochs merging is required for early-type galaxies to grow beyond log(M/M_sun)~11.3, and all early types over time gradually and partially loose their disk-like characteristics.

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arXiv2011

The UVJ Selection of Quiescent and Star Forming Galaxies: Separating Early and Late-Type Galaxies and Isolating Edge-on Spirals

We utilize for the first time HST ACS imaging to examine the structural properties of galaxies in the rest-frame U-V versus V-J diagram (i.e., the UVJ diagram) using a sample at 0.6 10.25). The use of the UVJ diagram as a tool to distinguish quiescent galaxies from star forming galaxies (SFGs) is becoming more common due to its ability to separate red quiescent galaxies from reddened SFGs. Quiescent galaxies occupy a small and distinct region of UVJ color space and we find most of them to have concentrated profiles with high Sersic indices (n>2.5) and smooth structure characteristic of early-type systems. SFGs populate a broad, but well-defined sequence of UVJ colors and are comprised of objects with a mix of Sersic indices. Interestingly, most UVJ-selected SFGs with high Sersic indices also display structure due to dust and star formation typical of the n<2.5 SFGs and late-type systems. Finally, we find that the position of a SFG on the sequence of UVJ colors is determined to a large degree by the mass of the galaxy and its inclination. Systems that are closer to edge-on generally display redder colors and lower [OII]3727 luminosity per unit mass as a consequence of the reddening due to dust within the disks. We conclude that the two main features seen in UVJ color space correspond closely to the traditional morphological classes of early and late-type galaxies.

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arXiv2011

The Star Formation Rate-Density Relation at 0.6<z<0.9 and the Role of Star Forming Galaxies

We study the star formation rates (SFRs) of galaxies as a function of local galaxy density at 0.6 1.8x10^{10} Msun to conduct our main analysis. With three different SFR indicators, (1) Spitzer MIPS 24-micron imaging, (2) SED fitting, and (3) [OII]3727 emission, we find the median specific SFR (SSFR) and SFR to decline from the low-density field to the cores of groups and a rich cluster. For the SED and [OII] based SFRs, the decline in SSFR is roughly an order of magnitude while for the MIPS based SFRs, the decline is a factor of ~4. We find approximately the same magnitude of decline in SSFR even after removing the sample of galaxies near the cluster. Galaxies in groups and a cluster at these redshifts therefore have lower star formation (SF) activity than galaxies in the field, as is the case at z~0. We investigated whether the decline in SFR with increasing density is caused by a change in the proportion of quiescent and star forming galaxies (SFGs) or by a decline in the SFRs of SFGs. Using the rest-frame U-V and V-J colors to distinguish quiescent galaxies from SFGs we find the fraction of quiescent galaxies increases from ~32% to 79% from low to high density. In addition, we find the SSFRs of SFGs, selected based on U-V and V-J colors, to decline with increasing density by factors of ~5-6 for the SED and [OII] based SFRs. The MIPS based SSFRs for SFGs decline with a shallower slope. The order of magnitude decline in the SSFR-density relation at 0.6<z<0.9 is therefore driven by both a combination of declining SFRs of SFGs as well as a changing mix of SFGs and quiescent galaxies [ABRIDGED].

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arXiv2011

The Majority of Compact Massive Galaxies at z~2 are Disk Dominated

We investigate the stellar structure of massive, quiescent galaxies at z~2, based on Hubble Space Telescope/WFC3 imaging from the Early Release Science program. Our sample of 14 galaxies has stellar masses of M* > 10^{10.8} Msol and photometric redshifts of 1.5 < z < 2.5. In agreement with previous work, their half-light radii are <2 kpc, much smaller than equally massive galaxies in the present-day universe. A significant subset of the sample appears highly flattened in projection, which implies, considering viewing angle statistics, that a significant fraction of the galaxies in our sample have pronounced disks. This is corroborated by two-dimensional surface brightness profile fits. We estimate that 65% +/- 15% of the population of massive, quiescent z~2 galaxies are disk-dominated. The median disk scale length is 1.5 kpc, substantially smaller than the disks of equally massive galaxies in the present-day universe. Our results provide strong observational evidence that the much-discussed ultra-dense high-redshift galaxies should generally be thought of as disk-like stellar systems with the majority of stars formed from gas that had time to settle into a disk.

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arXiv2010

The Physical Origins of The Morphology-Density Relation: Evidence for Gas Stripping from the SDSS

We provide a physical interpretation and explanation of the morphology-density relation for galaxies, drawing on stellar masses, star formation rates, axis ratios and group halo masses from the Sloan Digital Sky Survey. We first re-cast the classical morphology-density relation in more quantitative terms, using low star formation rate (quiescence) as a proxy for early-type morphology and dark matter halo mass from a group catalog as a proxy for environmental density: for galaxies of a given stellar mass the quiescent fraction is found to increase with increasing dark matter halo mass. Our novel result is that - at a given stellar mass - quiescent galaxies are significantly flatter in dense environments, implying a higher fraction of disk galaxies. Supposing that the denser environments differ simply by a higher incidence of quiescent disk galaxies that are structurally similar to star-forming disk galaxies of similar mass, explains simultaneously and quantitatively these quiescence -nvironment and shape-environment relations. Our findings add considerable weight to the slow removal of gas as the main physical driver of the morphology-density relation, at the expense of other explanations.

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arXiv2010

The Mid-Infrared Luminosities of Normal Galaxies over Cosmic Time

Modern population synthesis models estimate that 50% of the restframe K-band light is produced by TP-AGB stars during the first Gyr of a stellar population, with a substantial fraction continuing to be produced by the TP-AGB over a Hubble time. Between 0.2 and 1.5 Gyr, intermediate mass stars evolve into TP-AGB C stars which, due to significant amounts of circumstellar dust, emit half their energy in the mid-IR. We combine these results using published mid-IR colors of Galactic TP-AGB M and C stars to construct simple models for exploring the contribution of the TP-AGB to 24micron data as a function of stellar population age. We compare these empirical models with an ensemble of galaxies in the CDFS from z=0 to z=2, and with high quality imaging in M81. Within the uncertainties, the TP-AGB appears responsible for a substantial fraction of the mid-IR luminosities of galaxies from z=0 to z=2, the maximum redshift to which we can test our hypothesis, while, at the same time, our models reproduce much of the detailed structure observed in mid-IR imaging of M81. The mid-IR is a good diagnostic of star formation over timescales of ~1.5 Gyr, but this implies that on-going star formation rates at z=1 may be overestimated by factors of ~1.5-6, depending on the nature of star formation events. Our results, if confirmed through subsequent work, have strong implications for the star formation rate density of the universe and the growth of stellar mass over time.

astro-ph.CO↗

arXiv2009

Major Merging: The Way to Make a Massive, Passive Galaxy

We analyze the projected axial ratio distribution, p(b/a), of galaxies that were spectroscopically selected from the Sloan Digital Sky Survey (DR6) to have low star-formation rates. For these quiescent galaxies we find a rather abrupt change in p(b/a) at a stellar mass of ~10^{11} M_sol: at higher masses there are hardly any galaxies with b/a<0.6, implying that essentially none of them have disk-like intrinsic shapes and must be spheroidal. This transition mass is ~3-4 times higher than the threshold mass above which quiescent galaxies dominate in number over star-forming galaxies, which suggests these mass scales are unrelated. At masses lower than ~10^{11} M_sol, quiescent galaxies show a large range in axial ratios, implying a mix of bulge- and disk-dominated galaxies. Our result strongly suggests that major merging is the most important, and perhaps only relevant, evolutionary channel to produce massive (>10^{11} M_sol), quiescent galaxies, as it inevitably results in spheroids.

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