arXiv subjects

Hassen M. Yesuf

Publications and source records attributed to Hassen M. Yesuf.

49 records · Page 3Linked to original sources

arXiv2020

Quenching as a Contest between Galaxy Halos and their Central Black Holes

Existing models of galaxy formation have not yet explained striking correlations between structure and star-formation activity in galaxies, notably the sloped and moving boundaries that divide star-forming from quenched galaxies in key structural diagrams. This paper uses these and other relations to ``reverse-engineer'' the quenching process for central galaxies. The basic idea is that star-forming galaxies with larger radii (at a given stellar mass) have lower black-hole masses due to lower central densities. Galaxies cross into the green valley when the cumulative effective energy radiated by their black hole equals $\sim4\times$ their halo-gas binding energy. Since larger-radii galaxies have smaller black holes, one finds they must evolve to higher stellar masses in order to meet this halo-energy criterion, which explains the sloping boundaries. A possible cause of radii differences among star-forming galaxies is halo concentration. The evolutionary tracks of star-forming galaxies are nearly parallel to the green-valley boundaries, and it is mainly the sideways motions of these boundaries with cosmic time that cause galaxies to quench. BH-scaling laws for star-forming, quenched, and green-valley galaxies are different, and most BH mass growth takes place in the green valley. Implications include: the radii of star-forming galaxies are an important second parameter in shaping their black holes; black holes are connected to their halos but in different ways for star-forming, quenched, and green-valley galaxies; and the same BH-halo quenching mechanism has been in place since $z \sim 3$. We conclude with a discussion of black hole-galaxy co-evolution, the origin and interpretation of BH scaling laws.

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arXiv2020

Structural and Stellar Population Properties vs. Bulge Types in Sloan Digital Sky Survey Central Galaxies

This paper studies pseudo-bulges (P-bulges) and classical bulges (C-bulges) in Sloan Digital Sky Survey central galaxies using the new bulge indicator $ΔΣ_1$, which measures relative central stellar-mass surface density within 1 kpc. We compare $ΔΣ_1$ to the established bulge-type indicator $Δ\langleμ_e\rangle$ from Gadotti (2009) and show that classifying by $ΔΣ_1$ agrees well with $Δ\langleμ_e\rangle$. $ΔΣ_1$ requires no bulge-disk decomposition and can be measured on SDSS images out to $z = 0.07$. Bulge types using it are mapped onto twenty different structural and stellar-population properties for 12,000 SDSS central galaxies with masses 10.0 < log $M_*$/$M_{\odot}$ < 10.4. New trends emerge from this large sample. Structural parameters show fairly linear log-log relations vs. $ΔΣ_1$ and $Δ\langleμ_e\rangle$ with only moderate scatter, while stellar-population parameters show a highly non-linear "elbow" in which specific star-formation rate remains roughly flat with increasing central density and then falls rapidly at the elbow, where galaxies begin to quench. P-bulges occupy the low-density end of the horizontal arm of the elbow and are universally star-forming, while C-bulges occupy the elbow and the vertical branch and exhibit a wide range of star-formation rates at fixed density. The non-linear relation between central density and star-formation rate has been seen before, but this mapping onto bulge class is new. The wide range of star-formation rates in C-bulges helps to explain why bulge classifications using different parameters have sometimes disagreed in the past. The elbow-shaped relation between density and stellar indices suggests that central structure and stellar-populations evolve at different rates as galaxies begin to quench.

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arXiv2019

The Activation of Galactic Nuclei and Their Accretion Rates are Linked to the Star Formation Rates and Bulge-types of Their Host Galaxies

We use bulge-type classifications of 809 representative SDSS galaxies by Gadotti (2009) to classify a large sample of galaxies into real bulges (classical or elliptical) and pseudobulges using Random Forest. We use structural and stellar population predictors that can easily be measured without image decomposition. Multiple parameters such as the central mass density with 1 kpc, concentration index, Sérsic index and velocity dispersion do result in accurate bulge classifications when combined together. We classify $\sim 44,500$ face-on galaxies above stellar mass of 10$^{10}$ M$_\odot$ and redshift $ 0.02 < z < 0.07$ into real bulges or pseudobulges with $93 \pm 2$\% accuracy. We show that $\sim 75 - 90\%$ of AGNs identified by the optical line ratio diagnostic are hosted by real bulges. The pseudobulge fraction significantly decreases with AGN signature as the line ratios change from indicating pure star formation ($\sim 54 \pm 4$ \%), to composite of star formation and AGN ($\sim 18 \pm 3$\%), and to AGN-dominated galaxies ($\sim 5 \pm 3$\%). Using the dust-corrected [\ion{O}{3}] luminosity as an AGN accretion indicator, and the stellar mass and radius as proxies for a black hole mass, we find that AGNs in real bulges have lower Eddington ratios than AGNs in pseudobulges. Real bulges have a wide range of AGN and star formation activities, although most of them are weak AGNs. For both bulge-types, their Eddington ratios are correlated with specific star formation rates (SSFR). Real bulges have lower specific accretion rate but higher AGN fraction than pseudobulges do at similar SSFRs.

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arXiv2019

Dirt-cheap Gas Scaling Relations: Using Dust Absorption, Metallicity and Galaxy Size to Predict Gas Masses for Large Samples of Galaxies

We apply novel survival analysis techniques to investigate the relationship between a number of the properties of galaxies and their atomic ($M_\mathrm{HI}$) and molecular ($M_{\mathrm{H_2}}$) gas mass, with the aim of devising efficient, effective empirical estimators of the cold gas content in galaxies that can be applied to large optical galaxy surveys. We find that dust attenuation, {\AV}, of both the continuum and nebular emission, shows significant partial correlations with $M_{\mathrm{H_2}}$, after controlling for the effect of star formation rate (SFR). The partial correlation between {\AV} and $M_\mathrm{HI}$, however, is weak. This is expected because in poorly dust-shielded regions molecular hydrogen is dissociated by far-ultraviolet photons. We also find that the stellar half-light radius, $R_{50}$, shows significant partial correlations with both $M_{\mathrm{H_2}}$ and $M_\mathrm{HI}$. This hints at the importance of environment (e.g., galactocentric distance) on the gas content of galaxies and the interplay between gas and SFR. We fit multiple regression to summarize the median, mean, and the $0.15/0.85$ quantile multivariate relationships among $M_{\mathrm{H_2}}$, {\AV}, metallicity, and/or $R_{50}$. A linear combination of {\AV} and metallicity (inferred from stellar mass) or {\AV} and $R_{50}$, can estimate molecular gas masses within $\sim 2.5-3$ times the observed masses. If SFR is used in addition, $M_\mathrm{H_2}$ can be predicted to within a factor $\lesssim 2$. In this case, {\AV} and $R_{50}$ are the two best secondary parameters that improve the primary relation between $M_\mathrm{H_2}$ and SFR. Likewise, $M_\mathrm{HI}$ can be predicted to within a factor $\lesssim 3$ using $R_{50}$ and SFR.

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arXiv2018

On the Transition of the Galaxy Quenching Mode at 0.5<z<1 in CANDELS

We investigate the galaxy quenching process at intermediate redshift using a sample of $\sim4400$ galaxies with $M_{\ast} > 10^{9}M_{\odot}$ between redshift 0.5 and 1.0 in all five CANDELS fields. We divide this sample, using the integrated specific star formation rate (sSFR), into four sub-groups: star-forming galaxies (SFGs) above and below the ridge of the star-forming main sequence (SFMS), transition galaxies and quiescent galaxies. We study their $UVI$ ($U-V$ versus $V-I$) color gradients to infer their sSFR gradients out to twice effective radii. We show that on average both star-forming and transition galaxies at all masses are not fully quenched at any radii, whereas quiescent galaxies are fully quenched at all radii. We find that at low masses ($M_{\ast} = 10^{9}-10^{10}M_{\odot}$) SFGs both above and below the SFMS ridge generally have flat sSFR profiles, whereas the transition galaxies at the same masses generally have sSFRs that are more suppressed in their outskirts. In contrast, at high masses ($M_{\ast} > 10^{10.5}M_{\odot}$), SFGs above and below the SFMS ridge and transition galaxies generally have varying degrees of more centrally-suppressed sSFRs relative to their outskirts. These findings indicate that at $z\sim~0.5-1.0$ the main galaxy quenching mode depends on its already formed stellar mass, exhibiting a transition from "the outside-in" at $M_{\ast} \leq 10^{10}M_{\odot}$ to "the inside-out" at $M_{\ast} > 10^{10.5}M_{\odot}$. In other words, our findings support that internal processes dominate the quenching of massive galaxies, whereas external processes dominate the quenching of low-mass galaxies.

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arXiv2018

The Isophotal Structure of Star-forming Galaxies at $0.5< z <1.8$ in CANDELS: Implications for the Evolution of Galaxy Structure

We have measured the radial profiles of isophotal ellipticity ($\varepsilon$) and disky/boxy parameter A$_4$ out to radii of about three times the semi-major axes for $\sim4,600$ star-forming galaxies (SFGs) at intermediate redshifts $0.5 1$. Moreover, the median A$_4$ values at all radii are almost zero. In contrast, the highly-inclined, low-mass LSFGs in the same mass-redshift bins generally have monotonically increasing $\varepsilon$ with radius and are dominated by disky values at intermediate radii. These findings at intermediate redshifts imply that low-mass SSFGs are not disk-like, while low-mass LSFGs appear to harbour disk-like components flattened by significant rotation. At high masses ($M_{\ast} > 10^{10}M_{\odot}$), highly-inclined SSFGs and LSFGs both exhibit a general, distinct trend for both $\varepsilon$ and A$_4$ profiles: increasing values with radius at lower radii, reaching maxima at intermediate radii, and then decreasing values at larger radii. Such a trend is more prevalent for more massive ($M_{\ast} > 10^{10.5}M_{\odot}$) galaxies or those at lower redshifts ($z<1.4$). The distinct trend in $\varepsilon$ and A$_4$ can be simply explained if galaxies possess all three components: central bulges, disks in the intermediate regions, and halo-like stellar components in the outskirts.

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arXiv2017

The Nature of Massive Transition Galaxies in CANDELS, GAMA, and Cosmological Simulations

We explore observational and theoretical constraints on how galaxies might transition between the "star-forming main sequence" (SFMS) and varying "degrees of quiescence" out to $z=3$. Our analysis is focused on galaxies with stellar mass $M_*>10^{10}M_{\odot}$, and is enabled by GAMA and CANDELS observations, a semi-analytic model (SAM) of galaxy formation, and a cosmological hydrodynamical "zoom in" simulation with momentum-driven AGN feedback. In both the observations and the SAM, transition galaxies tend to have intermediate Sérsic indices, half-light radii, and surface stellar mass densities compared to star-forming and quiescent galaxies out to $z=3$. We place an observational upper limit on the average population transition timescale as a function of redshift, finding that the average high-redshift galaxy is on a "fast track" for quenching whereas the average low-redshift galaxy is on a "slow track" for quenching. We qualitatively identify four physical origin scenarios for transition galaxies in the SAM: oscillations on the SFMS, slow quenching, fast quenching, and rejuvenation. Quenching timescales in both the SAM and the hydrodynamical simulation are not fast enough to reproduce the quiescent population that we observe at $z\sim3$. In the SAM, we do not find a clear-cut morphological dependence of quenching timescales, but we do predict that the mean stellar ages, cold gas fractions, SMBH masses, and halo masses of transition galaxies tend to be intermediate relative to those of star-forming and quiescent galaxies at $z<3$.

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arXiv2017

The Origins of UV-optical Color Gradients in Star-forming Galaxies at z ~ 2: Predominant Dust Gradients But Negligible sSFR Gradients

The rest-frame UV-optical (i.e., $NUV-B$) color is sensitive to both low-level recent star formation (specific star formation rate - sSFR) and dust. In this Letter, we extend our previous work on the origins of $NUV-B$ color gradients in star-forming galaxies (SFGs) at $z\sim1$ to those at $z\sim2$. We use a sample of 1335 large (semi-major axis radius $R_{\rm SMA}>0.''18$) SFGs with extended UV emission out to $2R_{\rm SMA}$ in the mass range $M_{\ast} = 10^{9}-10^{11}M_{\odot}$ at $1.5<z<2.8$ in the CANDELS/GOODS-S and UDS fields. We show that these SFGs generally have negative $NUV-B$ color gradients (redder centres), and their color gradients strongly increase with galaxy mass. We also show that the global rest-frame $FUV-NUV$ color is approximately linear with $A_{\rm V}$, which is derived by modeling the observed integrated FUV to NIR spectral energy distributions of the galaxies. Applying this integrated calibration to our spatially-resolved data, we find a negative dust gradient (more dust extinguished in the centers), which steadily becomes steeper with galaxy mass. We further find that the $NUV-B$ color gradients become nearly zero after correcting for dust gradients regardless of galaxy mass. This indicates that the sSFR gradients are negligible and dust reddening is likely the principal cause of negative UV-optical color gradients in these SFGs. Our findings support that the buildup of the stellar mass in SFGs at the Cosmic Noon is self-similar inside $2R_{\rm SMA}$.

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arXiv2017

Weak Galactic Winds in Active Galactic Nuclei Post-starburst Host Galaxies at z ~ 0.1

Post-starburst (PSB) galaxies may be in rapid transition from star-forming to quiescence and are excellent candidates to constrain active galactic nuclei (AGN) feedback models. We study galactic winds in the stacked spectrum of 560 AGN PSBs and that of a control sample of star-forming galaxies in the Sloan Digital Sky Survey (SDSS). Using a two component (inter-stellar +wind) absorption-line model of the \ion{Na}{i} doublet, and after accounting for the stellar photospheric absorption, we find that the AGN PSBs have a centroid wind velocity shift of $-252^{+64}_{-57}$ km s$^{-1}$ and a maximum blueshift velocity of $-678^{+54}_{-53}$ km s$^{-1}$. In comparison, the control sample, which is matched with the AGN PSBs in redshift, stellar mass, axis-ratio, the 4000 Å break index, Balmer decrement, and WISE 12 $μ$m to 4.6 $μ$m flux ratio, has a centroid wind velocity shift of $-119^{+33}_{-41}$ km s$^{-1}$ and a maximum velocity of $-406^{+51}_{-61}$ km s$^{-1}$. The equivalent widths due to the winds in both samples are similar: $0.36^{+0.10}_{-0.07}$ Å for the AGN PSBs and $0.24^{+0.07}_{-0.06} $ Å for the control sample. Despite having a higher velocity, the observed winds in the AGN PSBs are still not powerful enough to sweep significant amounts of gas out of the halos of the host galaxies. We also detect winds of similar velocities in the stacked spectra of shocked and quenched PSBs.

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arXiv2017

Molecular Gas during the Post-Starburst Phase: Low Gas Fractions in Green Valley Seyfert Post-Starburst Galaxies

Post-starbursts (PSBs) are candidate for rapidly transitioning from star-bursting to quiescent galaxies. We study the molecular gas evolution of PSBs at z ~ 0.03 - 0.2. We undertook new CO (2-1) observations of 22 Seyfert PSBs candidates using the ARO Submillimeter Telescope. This sample complements previous samples of PSBs by including green valley PSBs with Seyfert-like emission, allowing us to analyze for the first time the molecular gas properties of 116 PSBs with a variety of AGN properties. The distribution of molecular gas to stellar mass fractions in PSBs is significantly different than normal star-forming galaxies in the COLD GASS survey. The combined samples of PSBs with Seyfert-like emission line ratios have a gas fraction distribution which is even more significantly different and is broader (~ 0.03-0.3). Most of them have lower gas fractions than normal star-forming galaxies. We find a highly significant correlation between the WISE 12 micron to 4.6 micron flux ratios and molecular gas fractions in both PSBs and normal galaxies. We detect molecular gas in 27% of our Seyfert PSBs. Taking into account the upper limits, the mean and the dispersion of the distribution of the gas fraction in our Seyfert PSB sample are much smaller (mean = 0.025, std dev. = 0.018) than previous samples of Seyfert PSBs or PSBs in general (mean ~ 0.1 - 0.2, std dev. ~ 0.1 - 0.2).

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arXiv2017

No evidence for feedback: Unexceptional Low-ionization winds in Host galaxies of Low Luminosity Active Galactic Nuclei at Redshift z ~1

We study winds in 12 X-ray AGN host galaxies at z ~ 1. We find, using the low-ionization Fe II 2586 absorption in the stacked spectra, that the probability distribution function (PDF) of the centroid velocity shift in AGN has a median, 16th and 84th percentiles of (-87, -251, +86) km/s respectively. The PDF of the velocity dispersion in AGN has a median, 84th and 16th percentile of (139, 253, 52) km/s respectively. The centroid velocity and the velocity dispersions are obtained from a two component (ISM+wind) absorption line model. The equivalent width PDF of the outflow in AGN has median, 84th and 16th percentiles of (0.4, 0.8, 0.1) Angstrom. There is a strong ISM component in Fe II 2586 absorption with (1.2, 1.5, 0.8) Angstrom, implying presence of substantial amount cold gas in the host galaxies. For comparison, star-forming and X-ray undetected galaxies at a similar redshift, matched roughly in stellar mass and galaxy inclination, have a centroid velocity PDF with percentiles of (-74, -258, +90) km/s, and a velocity dispersion PDF percentiles of (150, 259, 57) km/s. Thus, winds in the AGN are similar to star-formation-driven winds, and are too weak to escape and expel substantial cool gas from galaxies. Our sample doubles the previous sample of AGN studied at z ~ 0.5 and extends the analysis to z ~ 1. A joint reanalysis of the z ~ 0.5 AGN sample and our sample yields consistent results to the measurements above.

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arXiv2016

The UV-optical Color Gradients in Star-Forming Galaxies at 0.5<z<1.5: Origins and Link to Galaxy Assembly

The rest-frame UV-optical (i.e., NUV-B) color index is sensitive to the low-level recent star formation and dust extinction, but it is insensitive to the metallicity. In this Letter, we have measured the rest-frame NUV-B color gradients in ~1400 large ($\rm r_e>0.18^{\prime\prime}$), nearly face-on (b/a>0.5) main-sequence star-forming galaxies (SFGs) between redshift 0.5 and 1.5 in the CANDELS/GOODS-S and UDS fields. With this sample, we study the origin of UV-optical color gradients in the SFGs at z~1 and discuss their link with the buildup of stellar mass. We find that the more massive, centrally compact, and more dust extinguished SFGs tend to have statistically more negative raw color gradients (redder centers) than the less massive, centrally diffuse, and less dusty SFGs. After correcting for dust reddening based on optical-SED fitting, the color gradients in the low-mass ($M_{\ast} <10^{10}M_{\odot}$) SFGs generally become quite flat, while most of the high-mass ($M_{\ast} > 10^{10.5}M_{\odot}$) SFGs still retain shallow negative color gradients. These findings imply that dust reddening is likely the principal cause of negative color gradients in the low-mass SFGs, while both increased central dust reddening and buildup of compact old bulges are likely the origins of negative color gradients in the high-mass SFGs. These findings also imply that at these redshifts the low-mass SFGs buildup their stellar masses in a self-similar way, while the high-mass SFGs grow inside out.

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arXiv2014

From Starburst to Quiescence: Testing AGN feedback in Rapidly Quenching Post-Starburst Galaxies

Post-starbursts are galaxies in transition from the blue cloud to the red sequence. Although they are rare today, integrated over time they may be an important pathway to the red sequence. This work uses SDSS, GALEX, and WISE observations to identify the evolutionary sequence from starbursts to fully quenched post-starbursts in the narrow mass range $\log M(M_\odot) = 10.3-10.7$, and identifies "transiting" post-starbursts which are intermediate between these two populations. In this mass range, $\sim 0.3\%$ of galaxies are starbursts, $\sim 0.1\%$ are quenched post-starbursts, and $\sim 0.5\%$ are the transiting types in between. The transiting post-starbursts have stellar properties that are predicted for fast-quenching starbursts and morphological characteristics that are already typical of early-type galaxies. The AGN fraction, as estimated from optical line ratios, of these post-starbursts is about 3 times higher ($\gtrsim 36 \pm 8 \%$) than that of normal star-forming galaxies of the same mass, but there is a significant delay between the starburst phase and the peak of nuclear optical AGN activity (median age difference of $\gtrsim 200 \pm 100$ Myr), in agreement with previous studies. The time delay is inferred by comparing the broad-band near NUV-to-optical photometry with stellar population synthesis models. We also find that starbursts and post-starbursts are significantly more dust-obscured than normal star-forming galaxies in the same mass range. About $20\%$ of the starbursts and $15\%$ of the transiting post-starbursts can be classified as the "Dust-Obscured Galaxies" (DOGs), while only $0.8\%$ of normal galaxies are DOGs.The time delay between the starburst phase and AGN activity suggests that AGN do not play a primary role in the original quenching of starbursts but may be responsible for quenching later low-level star formation during the post-starburst phase.

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