ABSTRACT The existence of a classical bulge in disk galaxies holds an important clue to the assembly history of galaxies. Finding observational evidence of very low mass classical bulges, ...particularly in barred galaxies, including our Milky Way, is a challenging task as the bar-driven secular evolution might bring significant dynamical change to these bulges alongside the stellar disk. Using high-resolution N-body simulation, we show that if a cool stellar disk is assembled around a non-rotating low-mass classical bulge, the disk rapidly grows a strong bar within a few rotation timescales. Later, the bar-driven secular process transforms the initial classical bulge into a flattened rotating stellar system whose central part also has grown a barlike component rotating in sync with the disk bar. During this time, a boxy/peanut (hereafter B/P) bulge is formed via the buckling instability of the disk bar, and the vertical extent of this B/P bulge being slightly higher than that of the classical bulge, it encompasses the whole classical bulge. The resulting composite bulge appears to be both photometrically and kinematically identical to a B/P bulge without any obvious signature of the classical component. Our analysis suggests that many barred galaxies in the local universe might be hiding such low-mass classical bulges. We suggest that stellar population and chemodynamical analysis might be required in establishing evidence for such low-mass classical bulges.
Abstract The relation between the observed UV continuum slope ( β ) and the infrared excess (IRX) is used as a powerful probe to understand the nature of dust attenuation law in high-redshift ...galaxies. We present a study of 83 UV-selected galaxies between a redshift of 0.5 and 0.7 from the AstroSat UV Deep Field north that encloses the GOODS-North field. Using empirical relation, we estimate the observed IRX of 52 galaxies that are detected in either one or both of the Herschel PACS 100 and 160 μ m bands. We further utilize the multiband photometric data in 14–18 filters from the Ultra-Violet Imaging Telescope, KPNO, Hubble Space Telescope, Spitzer, and Herschel telescopes to perform spectral energy distribution (SED) modeling. Both the observed and model-derived IRX– β values show a large scatter within the span of previously known relations, signifying diversity in dust attenuation. We found a distinct relation between the best-fit power-law slope of the modified Calzetti relation ( δ ) in the IRX– β plane, where the steeper SMC-like attenuation law prefers lower δ values. Our SED model-based IRX– β relation shows a preference for steeper SMC-like attenuation, which we further confirm from the agreement between extinction-corrected star formation rates derived using the H α emission line and the observed far-ultraviolet plus reprocessed far-IR fluxes. The current study reveals a strong positive correlation between IRX and the galaxy's stellar mass between 10 9.5 and 10 11.0 M ⊙ , signifying increased dust production in more massive star-forming galaxies.
Abstract
We estimate the UV continuum slope (
β
) of 465 galaxies (with luminosities of 0.028–3.3
L
z
=
0.5
*
) in the Great Observatories Origins Survey Northern field in the redshift range
z
= ...0.40–0.75. We use two AstroSat/UVIT (N242W, N245M) bands, two Hubble Space Telescope (F275W, F336W) bands, and a KPNO (
U
) band to sample the UV continuum slope of selected galaxies between 1215 and 2600 Å. The mean (median) and 1
σ
scatter in the observed
β
are found to be −1.33 ± 0.07 ( − 1.32) and 0.60 within the considered redshift range. We do not find any significant evolution in the mean
β
within our redshift window. Our measurements add new data points to the global
β
–
z
relation in the least-explored redshift regime, further reinforcing the gradual reddening of galaxy UV continuum with cosmic time. We notice no strong consistent trend between
β
and
M
1500
for the entire luminosity range −21 <
M
1500
< − 15 mag. However, the majority of the most-luminous galaxies (
M
1500
< − 19 mag) are found to have relatively redder slopes. Using UVIT, we detect galaxies as faint as
M
1500
= − 15.6 mag (i.e., 0.028
L
z
=
0.5
*
). The faintest galaxies (
M
1500
> − 16 mag) tend to be redder, which indicates they were less actively forming stars during this cosmic time interval. Our study highlights the unique capability of UVIT near-UV imaging to characterize the rest-frame far-UV properties of galaxies at redshift
z
∼ 0.5.
Abstract
We present deep UV imaging observations of the Great Observatories Origins Survey Northern (GOODS-N) field with AstroSat/UVIT (AstroSat UV Deep Field north—AUDFn), using one far-UV (FUV) ...(F154W, 34.0 ks) and two near-UV (NUV) filters (N242W, 19.2 ks; N245M, 15.5 ks). The nature of the UV sky background was explored across the UVIT field, and a global mean and rms were estimated for each filter. We reach 3
σ
detection limits of
m
AB
∼ 27.35, 27.28, and 27.02 mag for a point source in the F154W, N242W, and N245M bands respectively. The 50% completeness limits of the FUV and NUV images are
m
AB
= 26.40 and 27.05 mag respectively. We constructed point-spread functions for each band and estimated their FWHM, which were found to be almost the same: 1.″18 in F154W, 1.″11 in N242W, and 1.″24 in N245M. We used SExtractor to separately identify sources in the FUV and NUV filters and produce the UV source catalog of the entire AUDFn field. The source count slope estimated in FUV and NUV is 0.57 dex mag
−1
(between 19 and 25 mag) and 0.44 dex mag
−1
(between 18 and 25 mag), respectively. The catalog contains 6839 and 16,171 sources (brighter than the 50% completeness limit) in the FUV and NUV, respectively. Our FUV and NUV flux measurements of the identified sources complement existing multiband data in the GOODS-N field, and enable us to probe rest-frame FUV properties of galaxies at redshift
z
< 1 and search for candidate Lyman continuum leakers at redshift
z
> 0.97.
Abstract We report the direct detection of Lyman continuum (LyC) emission from nine galaxies and one active galactic nucleus at z ∼ 1.1–1.6 in the GOODS-North field using deep observations from the ...Ultraviolet Imaging Telescope (UVIT) on board AstroSat. The absolute escape fraction of the sources estimated from the far-ultraviolet and H α -line luminosities using Monte Carlo analysis of two intergalactic medium models span a range ∼10%–55%. The rest-frame UV wavelength of the sources falls in the extreme-ultraviolet regime ∼550–700 Å, the shortest LyC wavelength range probed so far. This redshift range remains devoid of direct detections of LyC emission due to the instrumental limitations of previously available facilities. With UVIT having very low detector noise, each of these sources is detected with an individual signal-to-noise ratio (S/N) > 3, while for the stack of six sources, we achieve an S/N ∼ 7.4. The LyC emission is seen to be offset from the optical centroids and extended beyond the UVIT point-spread function of 1.″6 in most of the sources. This sample fills an important niche between GALEX and Cosmic Origins Spectrograph at low z and Hubble Space Telescope's Wide Field Camera 3 at high z and is crucial in understanding the evolution of LyC leakers.
The central region of the enigmatic Malin 1 Saha, Kanak; Dhiwar, Suraj; Barway, Sudhanshu ...
Journal of astrophysics and astronomy,
10/2021, Volume:
42, Issue:
2
Journal Article
Peer reviewed
Open access
Malin 1, being a class of giant low surface galaxies, continues to surprise us even today. The HST/F814W observation has shown that the central region of Malin 1 is more like a normal SB0/a galaxy, ...while the rest of the disk has the characteristic of a low surface brightness system. The AstroSat/UVIT observations suggest scattered recent star formation activity all over the disk, especially along the spiral arms. The central
9
′
′
(
∼
14
kpc
) region, similar to the size of the Milky Way’s stellar disk, has a number of far-UV clumps—indicating recent star-formation activity. The high resolution UVIT/F154W image reveals far-UV emission within the bar region (
∼
4
kpc
)—suggesting the presence of hot, young stars in the bar. These young stars from the bar region are perhaps responsible for producing the strong emission lines such as H
α
, O
ii
seen in the SDSS spectra. Malin 1B, a dwarf early-type galaxy, is interacting with the central region and probably responsible for inducing the recent star-formation activity in this galaxy.
Full text
Available for:
DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Lenticular galaxies are generally thought to have descended from spirals via morphological transformation, although recent numerical simulations have shown that minor or even major mergers can also ...lead to an S0-like remnant. These mechanisms, however, are active in a dense environment such as a group or a cluster of galaxies-making it harder to explain the remarkable fraction of S0s found in the field. Here, we propose a new mechanism to form such lenticular galaxies. We show that an isolated cold disk settled into rotational equilibrium becomes violently unstable-leading to fragmentation and formation of stellar clumps that, in turn, not only cause the bulge to grow, but also increase the stellar disk velocity dispersion optimally in less than a billion years. Subsequently, the galaxy evolves passively without any conspicuous spiral structure. The final galaxy models resemble remarkably well the morphology and stellar kinematics of the present-day S0s observed by the Planetary Nebulae spectrograph. Our findings suggest a natural link between the high-redshift clumpy progenitors and the present-day S0 galaxies.
Stellar bars are the most common non-axisymmetric structures in galaxies and their impact on the evolution of disc galaxies at all cosmological times can be significant. Classical theory predicts ...that stellar discs are stabilized against bar formation if embedded in massive spheroidal dark matter haloes. However, dark matter haloes have been shown to facilitate the growth of bars through resonant gravitational interaction. Still, it remains unclear why some galaxies are barred and some are not. In this study, we demonstrate that corotating (i.e. in the same sense as the disc rotating) dark matter haloes with spin parameters in the range of 0 ≤ λdm ≤ 0.07 - which are a definite prediction of modern cosmological models - promote the formation of bars and boxy bulges and therefore can play an important role in the formation of pseudo-bulges in a kinematically hot dark-matter-dominated disc galaxies. We find continuous trends for models with higher halo spins: bars form more rapidly, the forming slow bars are stronger and the final bars are longer. After 2 Gyr of evolution, the amplitude of the bar mode in a model with λdm = 0.05 is a factor of ∼6 times higher, A
2/A
0 = 0.23, than in the non-rotating halo model. After 5 Gyr, the bar is ∼2.5 times longer. The origin of this trend is that more rapidly spinning (corotating) haloes provide a larger fraction of trailing dark matter particles that lag behind the disc bar and help growing the bar by taking away its angular momentum by resonant interactions. A counter-rotating halo suppresses the formation of a bar in our models. We discuss potential consequences for forming galaxies at high-redshift and present-day low-mass galaxies which have converted only a small fraction of their baryons into stars.
Why Are Some Galaxies Not Barred? Saha, Kanak; Elmegreen, Bruce
Astrophysical journal/The Astrophysical journal,
05/2018, Volume:
858, Issue:
1
Journal Article
Peer reviewed
Open access
Although more than two-thirds of star-forming disk galaxies in the local universe are barred, some galaxies remain unbarred, occupying the upper half of the Hubble tuning fork diagram. Numerical ...simulations almost always produce bars spontaneously, so it remains a challenge to understand how galaxies sometimes prevent bars from forming. Using a set of collisionless simulations, we first reproduce the common result that cold stellar disks surrounding a classical bulge become strongly unstable to non-axisymmetric perturbations, leading to the rapid formation of spiral structure and bars. However, our analyses show that galaxy models with compact classical bulges (whose average density is greater than or comparable to the disk density calculated within bulge half-mass radii) are able to prevent bar formation for at least 4 Gyr even when the stellar disk is maximal and having low Toomre Q. Such bar prevention is the result of several factors such as (a) a small inner Lindblad resonance with a high angular rate, which contaminates an incipient bar with x2 orbits, and (b) rapid loss of angular momentum accompanied by a rapid heating in the center from initially strong bar and spiral instabilities in a low-Q disk; in other words, a rapid initial rise to a value larger than ∼5 of the ratio of the random energy to the rotational energy in the central region of the galaxy.
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