Astronomy
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Browsing Astronomy by Subject "galaxies: formation"
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Item Average metallicity and star formation rate of Ly$\alpha$ emitters probed by a triple narrowband survey*(The American Astronomical Society, 2012) Nakajima, K.; Ouchi, M.; Shimasaku, K.; Ono, Y.; Lee, J.C.; Foucad, S.; Ly, C.; Dale, D.A.; Salim, S.; Finn, R.; Almaini, O.; Okamura, S.We present the average metallicity and star formation rate (SFR) of $Ly\alpha$ emitters (LAEs) measured from our large-area survey with three narrowband (NB) filters covering the $Ly\alpha, \big[O II\big]\lambda3727$, and $\text{H}\alpha+\big[\text{N} II\big]$ lines of LAEs at $z = 2.2$. We select 919 $z = 2.2$ LAEs from Subaru/Suprime-Cam NB data in conjunction with Magellan/IMACS spectroscopy. Of these LAEs, 561 and 105 are observed with KPNO/NEWFIRM near-infrared NB filters whose central wavelengths are matched to redshifted $\big[\text{O} II\big]$ and $\text{H}\alpha$ nebular lines, respectively. By stacking the near-infrared images of the LAEs, we successfully obtain average nebular-line fluxes of LAEs, the majority of which are too faint to be identified individually by NB imaging or deep spectroscopy. The stacked object has an Hα luminosity of $1.7 × 10^{42} erg s^{–1}$ corresponding to an SFR of $14 M_{\odot} yr6{–1}$. We place, for the first time, a firm lower limit to the average metallicity of LAEs of $Z \gtrsim 0.09 Z_{\odot} \big(2\sigma\big)$ based on the $\big[\text{O} II\big]/\big(\text{H}\alpha+\big[\text{N} II\big]\big)$ index together with photoionization models and empirical relations. This lower limit of metallicity rules out the hypothesis that LAEs, so far observed at $z \sim 2$, are extremely metal-poor $\big(Z < 2 × 10–2 Z_{\odot}\big)$ galaxies at the $4\sigma$ level. This limit is higher than a simple extrapolation of the observed mass-metallicity relation of $z \sim 2$ UV-selected galaxies toward lower masses $\big(5 × 108 M_{\odot}\big)$, but roughly consistent with a recently proposed fundamental mass-metallicity relation when the LAEs' relatively low SFR is taken into account. The $\text{H}\alpha$ and $\text{Ly}\alpha$ luminosities of our NB-selected LAEs indicate that the escape fraction of $\text{Ly}\alpha$ photons is $\sim12%-30%$, much higher than the values derived for other galaxy populations at $z \sim 2$.Item The dependence of quenching upon the inner structure of galaxies at 0.5 ≤ $z$ < 0.8 in the DEEP2/AEGIS survey(The American Astronomical Society, 2012) Cheung, E.; Faber, S.M.; Koo, D.C.; Dutton, A.A.; Simard, L.; McGrath, E.J.; Huang, J.-S.; Bell, E.F.; Dekel, A.; Fang, J.J.; Salim, S.; Barro, G.; Bundy, K.; Coil, A.L.; Cooper, M.C.; Conselice, C.J.; Davis, M.; Domínguez, A.; Kassin, S.A.; Kocevski, D.D.; Koekemoer, A.M.; Lin, L.; Lotz, J.M.; Newman, J.a.; Phillips, A.C.; Rosario, D.J.; Weiner, B.J.; Willmer, C.N.A.The shutdown of star formation in galaxies is generally termed "quenching." Quenching may occur through a variety of processes, e.g., active galactic nucleus (AGN) feedback, stellar feedback, or the shock heating of gas in the dark matter halo. However, which mechanism(s) is, in fact, responsible for quenching is still in question. This paper addresses quenching by searching for traces of possible quenching processes through their effects on galaxy structural parameters such as stellar mass ($M_{\ast }$), $M_{\ast }/r_{e}$, surface stellar mass density ($\sim M_{\ast }/r^{2}_{e}$), and Sérsic index ($n$). We analyze the rest-frame $U - B$ color correlations versus these structural parameters using a sample of galaxies in the redshift range $0.5 \lesssim z < 0.8$ from the DEEP2/AEGIS survey. In addition to global radii, stellar masses, and Sérsic parameters, we also use "bulge" and "disk" photometric measurements from GIM2D fits to $\textit{HST/ACS V}$ and $I$ images. We assess the tightness of the color relationships by measuring their "overlap regions," defined as the area in color-parameter space in which red and blue galaxies overlap; the parameter that minimizes these overlap regions is considered to be the most effective color discriminator. We find that Sérsic index ($n$) has the smallest overlap region among all tested parameters and resembles a step function with a threshold value of $n$ = 2.3. There exists, however, a significant population of outliers with blue colors yet high n values that seem to contradict this behavior; they make up $\approx$ 40% of $n$ > 2.3 galaxies. We hypothesize that their Sérsic values may be distorted by bursts of star formation, AGNs, and/or poor fits, leading us to consider central surface stellar mass density, $\sum^{\ast }_{1 \:\text{kpc}}$, as an alternative to Sérsic index. Not only does $\sum^{\ast }_{1 \:\text{kpc}}$ correct the outliers, but it also forms a tight relationship with color, suggesting that the $\textit{innermost structure of galaxies is most physically linked with quenching}$. Furthermore, at $z \sim 0.65$, the majority of the blue cloud galaxies cannot simply fade onto the red sequence since their GIM2D bulge masses are only half as large on average as the bulge masses of similar red sequence galaxies, thus demonstrating that stellar mass must absolutely increase at the centers of galaxies as they quench. We discuss a two-stage model for quenching in which galaxy star formation rates are controlled by their dark halos while they are still in the blue cloud and a second quenching process sets in later, associated with the central stellar mass buildup. The mass buildup is naturally explained by any non-axisymmetric features in the potential, such as those induced by mergers and/or disk instabilities. However, the identity of the second quenching agent is still unknown. We have placed our data catalog onlineItem Exploring the correlations between globular cluster populations and supermassive black holes in giant galaxies(The American Astronomical Society, 2012) Rhode, K.L.This paper presents an analysis of the correlation between the number of globular clusters ($N_{GC}$) in giant galaxies and the mass of the galaxies' central supermassive black hole ( $M_{SMBH}$). I construct a sample of 20 elliptical, spiral, and S0 galaxies with known SMBH masses and with accurately measured GC system properties derived from wide-field imaging studies. The coefficients of the best-fitting $N_{GC}-M_{SMBH}$ relation for the early-type galaxies are consistent with those from previous work but in some cases have smaller relative errors. I examine the correlation between $N_{GC}$ and $M_{SMBH}$ for various subsamples and find that elliptical galaxies show the strongest correlation, while S0 and pseudobulge galaxies exhibit increased scatter. I also compare the quality of the fit of the numbers of metal-poor GCs versus SMBH mass and the corresponding fit for metal-rich GCs. I supplement the 20 galaxy sample with 10 additional galaxies with reliable $N_{GC}$ determinations but without measured $M_{SMBH}$. I use this larger sample to investigate correlations between $N_{GC}$ and host galaxy properties like total galaxy luminosity and stellar mass, and bulge luminosity and mass. I find that the tightest correlation is between $N_{GC}$ and total galaxy stellar mass. This lends support to the notion that $N_{GC}$ and $M_{SMBH}$ are not directly linked but are correlated because both quantities depend on the host galaxy potential. Finally, I use the $N_{GC}-M_{SMBH}$ relation derived from the 20 galaxy sample to calculate predicted $M_{SMBH}$ values for the 10 galaxies with accurate $N_{GC}$ measurements but without measured SMBH masses.Item The globular cluster populations of giant galaxies: Mosaic imaging of five moderate-luminosity early-type galaxies(The American Astronomical Society, 2012) Hargis, J.R.; Rhode, K.L.This paper presents results from wide-field imaging of the globular cluster (GC) systems of five intermediate-luminosity ($M_{V}$ ~ –21 to –22) early-type galaxies. The aim is to accurately quantify the global properties of the GC systems by measuring them out to large radii. We obtained BVR imaging of four lenticular galaxies (NGC 5866, NGC 4762, NGC 4754, NGC 3384) and one elliptical galaxy (NGC 5813) using the KPNO 4 m telescope and Mosaic imager and traced the GC population to projected galactocentric radii ranging from ~20 kpc to 120 kpc. We combine our imaging with Hubble Space Telescope data to measure the GC surface density close to the galaxy center. We calculate the total number of GCs (N GC) from the integrated radial profile and find $N_{GC}$ = 340 ± 80 for NGC 5866, $N_{GC}$ = 2900 ± 400 for NGC 5813, $N_{GC}$ = 270 ± 30 for NGC 4762, $N_{GC}$ = 115 ± 15 for NGC 4754, and $N_{GC}$ = 120 ± 30 for NGC 3384. The measured GC specific frequencies are SN between 0.6 and 3.6 and T in the range 0.9-4.2. These values are consistent with the mean specific frequencies for the galaxies' morphological types found by our survey and other published data. Three galaxies (NGC 5866, NGC 5813, and NGC 4762) had sufficient numbers of GC candidates to investigate color bimodality and color gradients in the GC systems. NGC 5813 shows strong evidence (>3σ) for bimodality and a B – R color gradient resulting from a more centrally concentrated red (metal-rich) GC subpopulation. We find no evidence for statistically significant color gradients in the other two galaxies.Item Globular cluster systems of spiral and S0 galaxies: Results from WIYN imaging of NGC1023, NGC1055, NGC7332, and NGC7339(The American Astronomical Society, 2012) Young, M.D.; Dowell, J.L.; Rhode, K.L.We present results from a study of the globular cluster (GC) systems of four spiral and S0 galaxies imaged as part of an ongoing wide-field survey of the GC systems of giant galaxies. The target galaxies - the SB0 galaxy NGC1023, the SBb galaxy NGC1055, and an isolated pair comprised of the Sbc galaxy NGC7339 and the S0 galaxy NGC7332 - were observed in BVR filters with the WIYN 3.5m telescope and Minimosaic camera. For two of the galaxies, we combined the WIYN imaging with previously published data from the $\textit{Hubble Space Telescope}$ and the Keck Observatory to help characterize the GC distribution in the central few kiloparsecs. We determine the radial distribution (surface density of GCs versus projected radius) of each galaxy's GC system and use it to calculate the total number of GCs ($N_{GC}$). We find $N_{GC}$ = 490 ± 30, 210 ± 40, 175 ± 15, and 75 ± 10 for NGC1023, NGC1055, NGC7332, and NGC7339, respectively. We also calculate the GC specific frequency (N GC normalized by host galaxy luminosity or mass) and find values typical of those of the other spiral and E/S0 galaxies in the survey. The two lenticular galaxies have sufficient numbers of GC candidates for us to perform statistical tests for bimodality in the GC color distributions. We find evidence at a high confidence level (>95%) for two populations in the $B - R$ distribution of the GC system of NGC1023. We find weaker evidence for bimodality (>81% confidence) in the GC color distribution of NGC7332. Finally, we identify eight GC candidates that may be associated with the Magellanic dwarf galaxy NGC1023A, which is a satellite of NGC1023.Item Modeling the effects of star formation histories on H$\alpha$ and ultraviolet fluxes in nearby dwarf galaxies(The American Astronomical Society, 2012) Weisz, D.R.; Johnson, B.D.; Johnson, L.C.; Skillman, E.D.; Lee, J.C.; Kennicutt, R.C.; Calzetti, D.; Van Zee, L.; Bothwell, M.S.; Dalcanton, J.J.; Dale, D.A.; Williams, B.F.We consider the effects of non-constant star formation histories (SFHs) on $\text{H}\alpha$ and GALEX far-ultraviolet (FUV) star formation rate (SFR) indicators. Under the assumption of a fully populated Chabrier initial mass function (IMF), we compare the distribution of $\text{H}\alpha$-to-FUV flux ratios from ~1500 simple, periodic model SFHs with observations of 185 galaxies from the $\textit{Spitzer}$ Local Volume Legacy survey. We find a set of SFH models that are well matched to the data, such that more massive galaxies are best characterized by nearly constant SFHs, while low-mass systems experience burst amplitudes of ~30 (i.e., an increase in the SFR by a factor of 30 over the SFR during the inter-burst period), burst durations of tens of Myr, and periods of ~250 Myr; these SFHs are broadly consistent with the increased stochastic star formation expected in systems with lower SFRs. We analyze the predicted temporal evolution of galaxy stellar mass, R-band surface brightness, $\text{H}\alpha$-derived SFR, and blue luminosity, and find that they provide a reasonable match to observed flux distributions. We find that our model SFHs are generally able to reproduce both the observed systematic decline and increased scatter in $\text{H}\alpha$-to-FUV ratios toward low-mass systems, without invoking other physical mechanisms. We also compare our predictions with those from the Integrated Galactic IMF theory with a constant SFR. We find that while both predict a systematic decline in the observed ratios, only the time variable SFH models are capable of producing the observed population of low-mass galaxies $\big(M_{*} \lesssim 10^{7} M_{\odot}\big)$ with normal $\text{H}\alpha$-to-FUV ratios. These results demonstrate that a variable IMF alone has difficulty explaining the observed scatter in the $\text{H}\alpha$-to-FUV ratios. We conclude by considering the limitations of the model SFHs and discuss the use of additional empirical constraints to improve future SFH modeling efforts.