75 research outputs found
Local Analogs for High-redshift Galaxies: Resembling the Physical Conditions of the Interstellar Medium in High-redshift Galaxies
We present a sample of local analogs for high-redshift galaxies selected in
the Sloan Digital Sky Survey (SDSS). The physical conditions of the
interstellar medium (ISM) in these local analogs resemble those in
high-redshift galaxies. These galaxies are selected based on their positions in
the [OIII]/H versus [NII]/H nebular emission-line diagnostic
diagram. We show that these local analogs share similar physical properties
with high-redshift galaxies, including high specific star formation rates
(sSFRs), flat UV continuums and compact galaxy sizes. In particular, the
ionization parameters and electron densities in these analogs are comparable to
those in galaxies, but higher than those in normal SDSS galaxies
by 0.6~dex and 0.9~dex, respectively. The mass-metallicity
relation (MZR) in these local analogs shows ~dex offset from that in SDSS
star-forming galaxies at the low mass end, which is consistent with the MZR of
the galaxies. We compare the local analogs in this study with those
in other studies, including Lyman break analogs (LBA) and green pea (GP)
galaxies. The analogs in this study share a similar star formation surface
density with LBAs, but the ionization parameters and electron densities in our
analogs are higher than those in LBAs by factors of 1.5 and 3, respectively.
The analogs in this study have comparable ionization parameter and electron
density to the GP galaxies, but our method can select galaxies in a wider
redshift range. We find the high sSFR and SFR surface density can increase the
electron density and ionization parameters, but still cannot fully explain the
difference in ISM condition between nearby galaxies and the local
analogs/high-redshift galaxies.Comment: 13 pages, 11 figures, accepted by Ap
Testing the global star formation relation: An HCO+ (3-2) mapping study of Red MSX sources in the Bolocam Galactic Plane Survey
We present an analysis of the relation between the star formation rate (SFR)
and mass of dense gas in Galactic clumps and nearby galaxies. Using the
bolometric luminosity as a measure of SFR and the molecular line luminosity of
HCO+ (3-2) as a measure of dense gas mass, we find that the relation between
SFR and M_{dense} is approximately linear. This is similar to published results
derived using HCN (1-0) as a dense gas tracer. HCO+ (3-2) and HCN (1-0) have
similar conditions for excitation. Our work includes 16 Galactic clumps that
are in both the Bolocam Galactic Plane Survey and the Red MSX Survey, 27 water
maser sources from the literature, and the aforementioned HCN (1-0) data. Our
results agree qualitatively with predictions of recent theoretical models which
state that the nature of the relation should depend on how the critical density
of the tracer compares with the mean density of the gas.Comment: 38 pages (with full table), 3 figure
Testing Diagnostics of Nuclear Activity and Star Formation in Galaxies at z>1
We present some of the first science data with the new Keck/MOSFIRE
instrument to test the effectiveness of different AGN/SF diagnostics at z~1.5.
MOSFIRE spectra were obtained in three H-band multi-slit masks in the GOODS-S
field, resulting in two hour exposures of 36 emission-line galaxies. We compare
X-ray data with the traditional emission-line ratio diagnostics and the
alternative mass-excitation and color-excitation diagrams, combining new
MOSFIRE infrared data with previous HST/WFC3 infrared spectra (from the 3D-HST
survey) and multiwavelength photometry. We demonstrate that a high [OIII]/Hb
ratio is insufficient as an AGN indicator at z>1. For the four X-ray detected
galaxies, the classic diagnostics ([OIII]/Hb vs. [NII]/Ha and [SII]/Ha) remain
consistent with X-ray AGN/SF classification. The X-ray data also suggest that
"composite" galaxies (with intermediate AGN/SF classification) host bona-fide
AGNs. Nearly 2/3 of the z~1.5 emission-line galaxies have nuclear activity
detected by either X-rays or the classic diagnostics. Compared to the X-ray and
line ratio classifications, the mass-excitation method remains effective at
z>1, but we show that the color-excitation method requires a new calibration to
successfully identify AGNs at these redshifts.Comment: 7 pages, 4 figures. Accepted to ApJ Letter
ALMA CO Clouds and Young Star Complexes in the Interacting Galaxies IC 2163 and NGC 2207
Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Debra Meloy Elmegreen, et al, 'ALMA CO Clouds and Young Star Complexes in the Interacting Galaxies IC 2163 and NGC 2207', The Astrophysical Journal, 841:43 (10pp), 22 May 2017, doi: https://doi.org/10.3847/1538-4357/aa6ba5. © 2017. The American Astronomical Society. All rights reserved.ALMA observations of CO(1-0) emission in the interacting galaxies IC 2163 and NGC 2207 are used to determine the properties of molecular clouds and their association with star-forming regions observed with the Hubble Space Telescope. Half of the CO mass is in 249 clouds each more massive than 4.0x10^5Mo. The mass distribution functions for the CO clouds and star complexes in a galactic-scale shock front in IC 2163 both have a slope on a log-log plot of -0.7, similar to what is observed in Milky Way clouds. The molecular cloud mass function is steeper in NGC 2207. The CO distribution in NGC 2207 also includes a nuclear ring, a mini-bar, and a mini-starburst region that dominates the 24micron, radio, and Halpha emission in both galaxies. The ratio of the sum of the masses of star complexes younger than 30 Myr to the associated molecular cloud masses is ~4%. The maximum age of star complexes in the galactic-scale shock front in IC 2163 is about 200 Myr, the same as the interaction time of the two galaxies, suggesting the destruction of older complexes in the eyelids.Peer reviewe
Gemini Deep Deep Survey VI: Massive Hdelta-strong galaxies at z=1
We show that there has been a dramatic decline in the abundance of massive
galaxies with strong Hdelta stellar absorption lines from z=1.2 to the present.
These ``Hdelta-strong'', or HDS, galaxies have undergone a recent and rapid
break in their star-formation activity. Combining data from the Gemini Deep
Deep and the Sloan Digital Sky Surveys to make mass-matched samples
(M*>=10^10.2 Msun), with 25 and 50,255 galaxies, respectively), we find that
the fraction of galaxies in an HDS phase has decreased from about 50% at z=1.2
to a few percent today. This decrease in fraction is due to an actual decrease
in the number density of massive HDS systems by a factor of 2-4, coupled with
an increase in the number density of massive galaxies by about 30 percent. We
show that this result depends only weakly on the threshold chosen for the
Hdelta equivalent width to define HDS systems (if greater than 4 A) and
corresponds to a (1+z)^{2.5\pm 0.7} evolution. Spectral synthesis studies of
the high-redshift population using the PEGASE code, treating Hdelta_A, EW[OII],
Dn4000, and rest-frame colors, favor models in which the Balmer absorption
features in massive Hdelta-strong systems are the echoes of intense episodes of
star-formation that faded about 1 Gyr prior to the epoch of observation. The
z=1.4-2 epoch appears to correspond to a time at which massive galaxies are in
transition from a mode of sustained star formation to a relatively quiescent
mode with weak and rare star-formation episodes. We argue that the most likely
local descendants of the distant massive HDS galaxies are passively evolving
massive galaxies in the field and small groups.Comment: 16 pages, 12 figures, 3 tables, uses emulateapj.sty; updated to match
the version accepted by ApJ. One figure added, conclusions unchange
No More Active Galactic Nuclei in Clumpy Disks Than in Smooth Galaxies at z~2 in CANDELS / 3D-HST
We use CANDELS imaging, 3D-HST spectroscopy, and Chandra X-ray data to
investigate if active galactic nuclei (AGNs) are preferentially fueled by
violent disk instabilities funneling gas into galaxy centers at 1.3<z<2.4. We
select galaxies undergoing gravitational instabilities using the number of
clumps and degree of patchiness as proxies. The CANDELS visual classification
system is used to identify 44 clumpy disk galaxies, along with mass-matched
comparison samples of smooth and intermediate morphology galaxies. We note
that, despite being being mass-matched and having similar star formation rates,
the smoother galaxies tend to be smaller disks with more prominent bulges
compared to the clumpy galaxies. The lack of smooth extended disks is probably
a general feature of the z~2 galaxy population, and means we cannot directly
compare with the clumpy and smooth extended disks observed at lower redshift.
We find that z~2 clumpy galaxies have slightly enhanced AGN fractions selected
by integrated line ratios (in the mass-excitation method), but the spatially
resolved line ratios indicate this is likely due to extended phenomena rather
than nuclear AGNs. Meanwhile the X-ray data show that clumpy, smooth, and
intermediate galaxies have nearly indistinguishable AGN fractions derived from
both individual detections and stacked non-detections. The data demonstrate
that AGN fueling modes at z~1.85 - whether violent disk instabilities or
secular processes - are as efficient in smooth galaxies as they are in clumpy
galaxies.Comment: ApJ accepted. 17 pages, 17 figure
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