Browsing by Subject "galactic disks"

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Abundances Of Light Elements In Metal-Poor Stars IV. Fe/O And Fe/Mg Ratios And The History Of Star Formation In The Solar Neighborhood
(2000-06) Gratton, Raffaele Gratton; Carretta, Eugenio; Matteucci, Francesca; Sneden, Christopher; Sneden, Christopher
The accurate O, Mg and Fe abundances derived in previous papers of this series from a homogeneous reanalysis of high quality data for a large sample of stars are combined with stellar kinematics in order to discuss the history of star formation in the solar neighborhood. We found that the Fe/O and Fe/Mg abundance ratios are roughly constant in the (inner) halo and the thick disk; this means that the timescale of halo collapse was shorter than or of the same order of typical lifetime of progenitors of type Ia SNe (similar to 1 Gyr), this conclusion being somewhat relaxed (referring to star formation in the individual fragments) in an accretion model for the Galaxy formation. Both Fe/O and Fe/Mg ratios raised by similar to 0.2 dex while the O/H and Mg/H ratios hold constant during the transition from the thick to thin disk phases, indicating a sudden decrease in star formation in the solar neighbourhood at that epo;ch. These results are discussed in the framework of current views of Galaxy formation; they fit in a scenario where both dissipational collapse and accretions were active on a quite similar timescale.
Abundances Of Light Elements In Metal-Poor Stars IV. Fe/O And Fe/Mg Ratios And The History Of Star Formation In The Solar Neighborhood
(2000-06) Gratton, Raffaele Gratton; Carretta, Eugenio; Matteucci, Francesca; Sneden, Christopher; Sneden, Christopher
The accurate O, Mg and Fe abundances derived in previous papers of this series from a homogeneous reanalysis of high quality data for a large sample of stars are combined with stellar kinematics in order to discuss the history of star formation in the solar neighborhood. We found that the Fe/O and Fe/Mg abundance ratios are roughly constant in the (inner) halo and the thick disk; this means that the timescale of halo collapse was shorter than or of the same order of typical lifetime of progenitors of type Ia SNe (similar to 1 Gyr), this conclusion being somewhat relaxed (referring to star formation in the individual fragments) in an accretion model for the Galaxy formation. Both Fe/O and Fe/Mg ratios raised by similar to 0.2 dex while the O/H and Mg/H ratios hold constant during the transition from the thick to thin disk phases, indicating a sudden decrease in star formation in the solar neighbourhood at that epo;ch. These results are discussed in the framework of current views of Galaxy formation; they fit in a scenario where both dissipational collapse and accretions were active on a quite similar timescale.
Bulges Of Nearby Galaxies With Spitzer: The Growth Of Pseudobulges In Disk Galaxies And Its Connection To Outer Disks
(2009-05) Fisher, David B.; Drory, Niv; Fabricius, Maximilian H.; Fisher, David B.
We study star formation rates (SFRs) and stellar masses in bulges of nearby disk galaxies. For this we construct a new SFR indicator that linearly combines data from the Spitzer Space Telescope and the Galaxy Evolution Explorer. All bulges are found to be forming stars irrespective of bulge type (pseudobulge or classical bulge). At present-day SFR the median pseudobulge could have grown the present-day stellar mass in 8 Gyr. Classical bulges have the lowest specific SFR implying a growth times that are longer than a Hubble time, and thus the present-day SFR does not likely play a major role in the evolution of classical bulges. In almost all galaxies in our sample the specific SFR (SFR per unit stellar mass) of the bulge is higher than that of the outer disk. This suggests that almost all galaxies are increasing their B/T through internal star formation. The SFR in pseudobulges correlates with their structure. More massive pseudobulges have higher SFR density, this is consistent with that stellar mass being formed by moderate, extended star formation. Bulges in late-type galaxies have similar SFRs as pseudobulges in intermediate-type galaxies, and are similar in radial size. However, they are deficient in mass; thus, they have much shorter growth times, similar to 2 Gyr. We identify a class of bulges that have nuclear morphology similar to pseudobulges, significantly lower specific SFR than pseudobulges, and are closer to classical bulges in structural parameter correlations. These are possibly composite objects, evolved pseudobulges or classical bulges experiencing transient, enhanced nuclear star formation. Our results are consistent with a scenario in which bulge growth via internal star formation is a natural, and near ubiquitous phenomenon in disk galaxies. Those galaxies with large classical bulges are not affected by the in situ bulge growth, likely because the majority of their stellar mass comes from some other phenomenon. Yet, those galaxies without a classical bulge, over long periods of extended star formation are able to growth a pseudobulge. Though cold accretion is not ruled out, for pseudobulge galaxies an addition of stellar mass from mergers or accretion is not required to explain the bulge mass. In this sense, galaxies with pseudobulges may very well be bulgeless (or "quasi-bulgeless") galaxies, and galaxies with classical bulges are galaxies in which both internal evolution and hierarchical merging are responsible for the bulge mass by fractions that vary from galaxy to galaxy.
Halo Streams In The Seventh Sloan Digital Sky Survey Data Release
(2009-06) Klement, R.; Rix, Hans-Walter; Flynn, C.; Fuchs, B.; Beers, Timothy C.; Prieto, Carlos Allende; Bizyaev, Dmitry; Brewington, Howard; Lee, Y. S.; Malanushenko, Elena; Malanushenko, Viktor; Oravetz, Dan; Pan, K.; Fiorentin, P. R.; Simmons, Audrey; Snedden, Stephanie; Prieto, Carlos Allende
We have detected stellar halo streams in the solar neighborhood using data from the seventh public data release of the Sloan Digital Sky Survey (SDSS), which includes the directed stellar program Sloan Extension For Galactic Understanding and Exploration (SEGUE). In order to derive distances to each star, we used the metallicity-dependent photometric parallax relation from Ivezic et al. We examine and quantify the accuracy of this relation by applying it to a set of globular and open clusters observed by the SDSS/SEGUE and comparing the resulting sequence to the fiducial cluster sequences obtained by An et al. Our final sample consists of 22,321 nearby (d <= 2 kpc), metal-poor ([Fe/H] <= -0.5) main-sequence stars with six-dimensional estimates of position and space velocity ((r) over bar, (v) over bar). We characterize the orbits of these stars through suitable kinematic proxies for their "effective" integrals of motion, angular momentum, eccentricity, and orbital polar angle and compare the observed distribution to expectations from a smooth distribution in four [Fe/H] bins. The metallicities provide an additional dimension in parameter space that is well suited to distinguish tidal streams from those of dynamical origin. On this basis, we identify at least five significant "phase-space overdensities" of stars on very similar orbits in the solar neighborhood to which we can assign unambiguously peaked [Fe/H] distributions. Three of them have been identified previously, including the halo stream discovered by Helmi et al. at a significance level of sigma = 12.0. In addition, we find at least two new genuine halo streams, judged by their kinematics and [Fe/H], at sigma = 2.9 and 4.8, respectively. For one stream the stars even show coherence in the configuration space, matching a spatial overdensity of stars found by Juric et al. at (R, z) approximate to (9.5, 0.8) kpc. Our results demonstrate the practical power of our search method to detect substructure in the phase-space distribution of nearby stars without making a priori assumptions about the detailed form of the gravitational potential.
Integral Field Unit Spectroscopy of the Stellar Disk Truncation Region of NGC 6155
(2010-06) Yoachim, Peter; Roskar, Rok; Debattista, Victor P.; Roskar, Rok
Like the majority of spiral galaxies, NGC 6155 exhibits an exponential surface brightness profile that steepens significantly at large radii. Using the VIRUS-P integral field unit spectrograph, we have gathered spatially resolved spectra of the system. Modifying the GANDALF spectral fitting routine for use on the complex stellar populations found in spirals, we find that the average stellar ages increase significantly beyond the profile break radius. This result is in good agreement with recent simulations that predict the outskirts of disk galaxies are populated through stellar migration. With the ability to bin multiple fibers, we are able to measure stellar population ages down to mu(V) similar to 24 mag arcsec(-2).
The Kinematics Of Thick Disks In Nine External Galaxies
(2008-08) Yoachim, Peter; Dalcanton, Julianne J.; Yoachim, Peter
We present kinematic measurements of thin- and thick-disk components in a sample of nine edge-on galaxies. We extract stellar and ionized gas rotation curves at and above the galaxies' midplanes using the Ca II triplet absorption features and H alpha emission lines measured with the GMOS spectrographs on Gemini-North and Gemini-South. For the higher mass galaxies in the sample, we fail to detect differences between the thin- and thick-disk kinematics. In the lower mass galaxies, there is a wide range of thick-disk behavior, including thick disks with substantial lag and one counterrotating thick disk. We compare our rotation curves with expectations from thick-disk formation models and conclude that the wide variety of thick-disk kinematics favors a formation scenario in which thick-disk stars are accreted or formed during merger events as opposed to models that form thick disks through gradual thin- disk heating.
Lick Indices In The Thin And Thick Disks Of Edge-On Disk Galaxies
(2008-08) Yoachim, Peter; Dalcanton, Julianne J.; Yoachim, Peter
We have measured Lick index equivalent widths to derive luminosity weighted stellar ages and metallicities for thin-and thick-disk-dominated regions of nine edge-on disk galaxies with the ARC 3.5 m telescope at Apache Point Observatory. In all cases, the thick disks are confirmed to be old stellar populations, with typical ages between 4 and 10 Gyr. The thin disks are uniformly younger than the thick disks, and show strong radial age gradients, with the outer regions of the disks being younger than 1 Gyr. We do not detect any significant metallicity differences or alpha-element enhancement in the thick-disk stars compared to the thin disk, due to the insensitivity of the Lick indices to these differences at low metallicity. We compare these results to thick disks measured in other systems and to predictions from thick-disk formation models.
A Petal Of The Sunflower: Photometry Of The Stellar Tidal Stream In The Halo Of Messier 63 (NGC 5055)
(2011-11) Chonis, Taylor S.; Martinez-Delgado, David; Gabany, R. Jay; Majewski, Steven R.; Hill, Gary J.; Gralak, Ray; Trujillo, Ignacio; Hill, Gary J.
We present deep surface photometry of a very faint, giant arc-loop feature in the halo of the nearby spiral galaxy NGC 5055 (M63) that is consistent with being a part of a stellar stream resulting from the disruption of a dwarf satellite galaxy. This faint feature was first detected in early photographic studies by van der Kruit; more recently, in the study of Martinez-Delgado and as presented in this work, from the loop has been realized to be the result of a recent minor merger through evidence obtained by wide-field, deep images taken with a telescope of only 0.16 m aperture. The stellar stream is clearly confirmed in additional deep images taken with the 0.5 m telescope of the BlackBird Remote Observatory and the 0.8 m telescope of the McDonald Observatory. This low surface brightness (mu(R) approximate to 26 mag arcsec(-2)) arc-like structure around the disk of the galaxy extends 14'0 (similar to 29 kpc projected) from its center, with a projected width of 1'6 (similar to 3.3 kpc). The stream's morphology is consistent with that of the visible part of a giant, >great-circle> type stellar stream originating from the recent accretion of a similar to 10(8) M-circle dot dwarf satellite in the last few Gyr. The progenitor satellite's current position and final fate are not conclusive from our data. The color of the stream's stars is consistent with dwarfs in the Local Group and is similar to the outer faint regions of M63's disk and stellar halo. From our photometric study, we detect other low surface brightness >plumes;> some of these may be extended spiral features related to the galaxy's complex spiral structure, and others may be tidal debris associated with the disruption of the galaxy's outer stellar disk as a result of the accretion event. We are able to differentiate between features related to the tidal stream and faint, blue extended features in the outskirts of the galaxy's disk previously detected by the Galaxy Evolution Explorer satellite. With its highly warped Hi gaseous disk (similar to 20 degrees), M63 represents one of the several examples of an isolated spiral galaxy with a warped disk showing recently discovered strong evidence of an ongoing minor merger.