58 research outputs found
High-Redshift Protoclusters Traced by Submillimeter Galaxies: Tracing Star Formation Activity out to z > 4
Clustering analysis indicate that at z ~ 2 submm-selected galaxies (SMGs) reside in very massive halos (M_(DM) > 5 Ă 10^(13)), suggesting that SMGs trace high-density environments that evolve into rich galaxy clusters. Conversely, recent work suggests that SMGs are tracers of a broader range of environments, including structures with more modest masses caught in highly active periods; since galaxies in these structures are likely caught during episodes of peak starbursts, SMGs may be tracers of a wider range of environments beyond the progenitors of todays very rich clusters, opening a window for a more complete exploration of the details underpinning the process of galaxy evolution in concert with the assembly of the large scale structure (LSS). We have undertaken a large observing program comprising deep narrow-band Ly-alpha imaging and multi-object spectroscopy using Palomar/Keck/Magellan/Gemini telescopes to probe for galaxy overdensities in SMG environments at z ~ 1 â 5. With ~200 spectroscopically-confirmed Ly-alpha emitters, we are in a position to gauge the level of galaxy overdensity in these regions
Quenching Star Formation at Intermediate Redshifts: Downsizing of the Mass Flux Density in the Green Valley
The bimodality in galaxy properties has been observed at low and high redshifts, with a clear distinction between star-forming galaxies in the blue cloud and passively evolving objects in the red sequence; the absence of galaxies with intermediate properties indicates that the quenching of star formation and subsequent transition between populations must happen rapidly. In this paper, we present a study of over 100 transiting galaxies in the so-called green valley at intermediate redshifts (z ~ 0.8). By using very deep spectroscopy with the DEIMOS instrument at the Keck telescope we are able to infer the star formation histories of these objects and measure the stellar mass flux density transiting from the blue cloud to the red sequence when the universe was half its current age. Our results indicate that the process happened more rapidly and for more massive galaxies in the past, suggesting a top-down scenario in which the massive end of the red sequence is forming first. This represents another aspect of downsizing, with the mass flux density moving toward smaller galaxies in recent times
Profile comparison of the 6â9 ÎŒm polycyclic aromatic hydrocarbon bands in starburst-dominated galaxies
Polycyclic aromatic hydrocarbons (PAHs) are of great astrochemical and astrobiological interest due to their potential to form prebiotic molecules. We analyse the 7.7 and 8.6 ÎŒm PAH bands in 126 pre-dominantly starburst-dominated galaxies extracted from the Spitzer/IRS ATLAS project. Based on the peak positions of these bands, we classify them into the different A, B, and C Peetersâ classes, which allows us to address the potential characteristics of the PAH-emitting population. We compare this analysis with previous work focused on the 6.2 ÎŒm PAH band for the same sample. For the first time in the literature, this statistical analysis is performed on a sample of galaxies. In our sample, the 7.7 ÎŒm complex is equally distributed in A and B objectâs class while the 8.6 ÎŒm band presents more class B sources. Moreover, 39 per cent of the galaxies were distributed into A class objects for both 6.2 and 7.7 ÎŒm bands and only 18 per cent received the same A classification for the three bands. The âA A Aâ galaxies presented higher temperatures and less dust in their interstellar medium. Considering the redshift range covered by our sample, the distribution of the three bands into the different Peetersâ classes reveals a potential cosmological evolution in the molecular nature of the PAHs that dominate the interstellar medium in these galaxies, where B class objects seem to be more frequent at higher redshifts and, therefore, further studies have to be addressed
Mid-IR Spectroscopy of High-z SMGs: First Results
We present mid-infrared spectra of 5 submmillimeter galaxies at redshifts z = 0.65 â 2.38 taken with the Infrared Spectrograph aboard the Spitzer Space Telescope. Four of these sources have strong PAH features and the strength of these features are consistent with these galaxies being dominated by star formation. The other source displays a Mrk 231-type broad emission feature at restframe ~8 ÎŒm that does not conform to the typical 7.7/8.6 ÎŒm PAH complex in starburst galaxies, suggesting a more substantial AGN contribution
Multiply charged naphthalene and its C10H8 isomers: bonding, spectroscopy and implications in AGN environments
Naphthalene (C10H8) is the simplest polycyclic aromatic hydrocarbon (PAH) and
an important component in a series of astrochemical reactions involving
hydrocarbons. Its molecular charge state affects the stability of its isomeric
structures, which is specially relevant in ionised astrophysical environments.
We thus perform an extensive computational search for low-energy molecular
structures of neutral, singly, and multiply charged naphthalene and its isomers
with charge states +q = 0-4 and investigate their geometric properties and
bonding situations. We find that isomerisation reactions should be frequent for
higher charged states and that open chains dominate their low-energy
structures. We compute both the scaled-harmonic and anharmonic infrared spectra
of selected low-energy species and provide the calculated scaling factors for
the naphthalene neutral, cation, and dication global minima. All simulated
spectra reproduce satisfactorily the experimental data and, thus, are adequate
for aiding observations. Moreover, the potential presence of these species in
the emission spectra of the circumnuclear regions of active galactic nuclei
(AGNs), with high energetic X-ray photon fluxes, is explored using the
experimental value of the naphthalene photodissociation cross-section,
\sigma_{ph-d}, to determine its half-life, t_{1/2}, at a photon energy of 2.5
keV in a set of relevant sources. Finally, we show that the computed IR bands
of the triply and quadruply charged species are able to reproduce some features
of the selected AGN sources.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society. 15 pages, 7 pages, supplementary information available
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Formation of S0s in extreme environments I: clues from kinematics and stellar populations
Despite numerous efforts, it is still unclear whether lenticular galaxies (S0s) evolve from spirals whose star formation was suppressed, or formed trough mergers or disk instabilities. In this paper we present a pilot study of 21 S0 galaxies in extreme environments (field and cluster), and compare their spatially-resolved kinematics and global stellar populations. Our aim is to identify whether there are different mechanisms that form S0s in different environments. Our results show that the kinematics of S0 galaxies in field and cluster are, indeed, different. Lenticulars in the cluster are more rotationally supported, suggesting that they are formed through processes that involve the rapid consumption or removal of gas (e.g. starvation, ram pressure stripping). In contrast, S0s in the field are more pressure supported, suggesting that minor mergers served mostly to shape their kinematic properties. These results are independent of total mass, luminosity, or disk-to-bulge ratio. On the other hand, the mass-weighted age, metallicity, and star formation time-scale of the galaxies correlate more with mass than with environment, in agreement with known relations from previous work such as the one between mass and metallicity. Overall, our results re-enforce the idea that there are multiple mechanisms that produce S0s, and that both mass and environment play key roles. A larger sample is highly desirable to confirm or refute the results and the interpretation of this pilot study
The Thick Disk in the Galaxy NGC 4244 from S^4G Imaging
If thick disks are ubiquitous and a natural product of disk galaxy formation and/or evolution processes, all
undisturbed galaxies that have evolved during a significant fraction of a Hubble time should have a thick disk. The
late-type spiral galaxy NGC 4244 has been reported as the only nearby edge-on galaxy without a confirmed thick
disk. Using data from the Spitzer Survey of Stellar Structure in Galaxies (S^4G) we have identified signs of two disk
components in this galaxy. The asymmetries between the light profiles on both sides of the mid-plane of NGC 4244
can be explained by a combination of the galaxy not being perfectly edge-on and a certain degree of opacity of
the thin disk. We argue that the subtlety of the thick disk is a consequence of either a limited secular evolution in
NGC 4244, a small fraction of stellar material in the fragments which built the galaxy, or a high amount of gaseous
accretion after the formation of the galaxy
Bar properties as a function of wavelength: a local baseline with S4G for high-redshift studies
The redshift evolution of bars is an important signpost of the dynamic maturity of disc galaxies. To characterize the intrinsic evolution safe from band-shifting effects, it is necessary to gauge how bar properties vary locally as a function of wavelength. We investigate bar properties in 16 nearby galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G) at ultraviolet, optical, and mid-infrared wavebands. Based on the ellipticity and position angle profiles from fitting elliptical isophotes to the two-dimensional light distribution, we find that both bar length and ellipticity â the latter often used as a proxy for bar strength â increase at bluer wavebands. Bars are 9 perâcent longer in the B band than at 3.6 ÎŒm. Their ellipticity increases typically by 8 perâcent in the B band, with a significant fraction (>40 perâcent) displaying an increase up to 35 perâcent. We attribute the increase in bar length to the presence of star-forming knots at the end of bars: these regions are brighter in bluer bands, stretching the bar signature further out. The increase in bar ellipticity could be driven by the apparent bulge size: the bulge is less prominent at bluer bands, allowing for thinner ellipses within the bar region. Alternatively, it could be due to younger stellar populations associated with the bar. The resulting effect is that bars appear longer and thinner at bluer wavebands. This indicates that band-shifting effects are significant and need to be corrected for high-redshift studies to reliably gauge any intrinsic evolution of the bar properties with redshift
The complex globular cluster system of the S0 galaxy NGC 4382 in the outskirts of the Virgo Cluster
NGC 4382 is a merger-remnant galaxy that has been classified as morphological type E2, S0, and even Sa. In this work, we performed a photometric and spectroscopic analysis of the globular cluster (GC) system of this peculiar galaxy in order to provide additional information about its history. We used a combination of photometric data in different filters, and multiobject and long-slit spectroscopic data obtained using the Gemini/GMOS instrument. The photometric analysis of the GC system, using the Gaussian Mixture Model algorithm in the colour plane, reveals a complex colour distribution within Rgal < 5 arcmin (26.1 kpc), showing four different groups: the typical blue and red subpopulations, a group with intermediate colours, and the fourth group towards even redder colours. From the spectroscopic analysis of 47 GCs, confirmed members of NGC 4382 based on radial velocities, we verified 3 of the 4 photometric groups from the analysis of their stellar populations using the ULySS code. NGC 4382 presents the classic blue (10.4 ± 2.8 Gyr, [Fe/H] = â1.48 ± 0.18 dex), and red (12.1 ± 2.3 Gyr, [Fe/H] = â0.64 ± 0.26 dex) GCs formed earlier in the lifetime of the galaxy, and a third group of young GCs (2.2 ± 0.9 Gyr; [Fe/H] = â0.05 ± 0.28 dex). Finally, analysis of long-slit data of the galaxy reveals a luminosity-weighted mean age for the stellar population of âŒ2.7 Gyr, and an increasing metallicity from [Fe/H] = â0.1 to +0.2 dex in Rgal < 10 arcsec (0.87 kpc). These values, and other morphological signatures in the galaxy, are in good agreement with the younger group of GCs, indicating a common origin as a result of a recent merger
The Spitzer Survey of Stellar Structure in Galaxies (S^4G)
The Spitzer Survey of Stellar Structure in Galaxies S^4G is an Exploration
Science Legacy Program approved for the Spitzer post-cryogenic mission. It is a
volume-, magnitude-, and size-limited (d < 40 Mpc, |b| > 30 degrees, m_(Bcorr)
< 15.5, D25>1') survey of 2,331 galaxies using IRAC at 3.6 and 4.5 microns.
Each galaxy is observed for 240 s and mapped to > 1.5 x D25. The final
mosaicked images have a typical 1 sigma rms noise level of 0.0072 and 0.0093
MJy / sr at 3.6 and 4.5 microns, respectively. Our azimuthally-averaged surface
brightness profile typically traces isophotes at mu_3.6 (AB) (1 sigma) ~ 27 mag
arcsec^-2, equivalent to a stellar mass surface density of ~ 1 Msun pc^-2. S^4G
thus provides an unprecedented data set for the study of the distribution of
mass and stellar structures in the local Universe. This paper introduces the
survey, the data analysis pipeline and measurements for a first set of
galaxies, observed in both the cryogenic and warm mission phase of Spitzer. For
every galaxy we tabulate the galaxy diameter, position angle, axial ratio,
inclination at mu_3.6 (AB) = 25.5 and 26.5 mag arcsec^-2 (equivalent to ~ mu_B
(AB) =27.2 and 28.2 mag arcsec^-2, respectively). These measurements will form
the initial S^4G catalog of galaxy properties. We also measure the total
magnitude and the azimuthally-averaged radial profiles of ellipticity, position
angle, surface brightness and color. Finally, we deconstruct each galaxy using
GALFIT into its main constituent stellar components: the bulge/spheroid, disk,
bar, and nuclear point source, where necessary. Together these data products
will provide a comprehensive and definitive catalog of stellar structures, mass
and properties of galaxies in the nearby Universe.Comment: Accepted for Publication in PASP, 14 pages, 13 figure
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