Questions tagged [redshift]

Questions related to the phenomenon whereby electromagnetic radiation (such as visible light) generated by an object moving away from an observer will have increase in wavelength (i.e. shifted toward the red end of the spectrum) once it reaches the observer.

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1answer
316 views

How to shift an AGN X-ray spectrum to rest frame

I have limited information to shift a spectrum (in the X-ray 0.5-10 keV bandpass) from redshift $z=2$ to the rest frame. I have a plot of normalized (photon) counts s$^{−1}$ keV$^{−1}$ as a function ...
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1answer
40 views

Why are galactic nuclei yellow?

So we've seen millions of galaxies, and all of them have a bright, yellow nucleus. What is the cause of the yellowness? Is it because of aging stars, redshift from the SMBH, or something else?
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30 views

K-correction vs resizing photometric bands

Is there a difference between a K-correction (see Wiki https://en.wikipedia.org/wiki/K_correction) and simply "resizing" the bands, e.g. the 2-10 keV x-ray band, with the redshift? I've read ...
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1answer
52 views

How to find the distance between two galaxies from ra and dec and redshift [duplicate]

I have ra and dec and corresponding redshift of galaxies. I would like to calculate the distance between two galaxies. I did like this to find the nearest neighbour galaxies. Suppose I am trying to ...
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1answer
636 views

How to correct observed flux densities for redshift

Say I have a spectrum of a galaxy at a redshift $z$, in flux density units of erg/s/cm^2/Angstrom. I'd like to recover the spectrum (in the same flux density units) at z=0, i.e. at its rest ...
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1answer
56 views

Question about redshift and effective gravity at the rotating neutron star surface

For a rapidly rotating neutron star, if consider the star as a sphere, redshift at the equator surface will be larger than at the pole. But if consider the star as an obsolate sphereiod (the ellipsoid ...
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1answer
41 views

redshift in astrophysical plots [closed]

Why do astrophysicists often use redshift in plots? Why it is important to know how an astrophysical object evolves in function of redshift? I was seeing an "cosmological time per redshift ...
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8answers
7k views

Does matter accumulate just outside the event horizon of a black hole?

My understanding is that time slows and approaches stopping when approaching the event horizon of a black hole. I have seen this explained several places, including a brief explanation in the last ...
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0answers
32 views

Get the expression of probed volume between 2 redshifts

1) I can't manage to find/justify the relation (1) below, from the common relation (2) of a volume. 2) It seems the variable r is actually the comoving distance and not comoving coordinates (with ...
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0answers
43 views

Relation between photometric redshift in a given bin and conditional probability

I take over an old question that is not yet clear for me. Indeed, I would like to formulate the equation (1) below under the form of "conditional probability" but I can't manage to do that. ...
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4answers
409 views

Could a contracting Universe create the redshift effect observed by Hubble?

What if we just can’t see far enough and the redshift actually shows the galaxies speeding up towards a gigantic black hole to where everything is converging, repeating a cycle that took place ...
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1answer
128 views

Universe expansion

I'm reading the book "The theory of everything". In the second chapter it says the Universe is expanding, and we know this because the measurements made using the Doppler effect of other ...
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3answers
742 views

Cosmological redshift vs doppler redshift

I'm reading Harrison's "Cosmology: Science of the universe" because Harrison focuses on the distinction between cosmological redshift (he calls it expansion redshift) and the Doppler redshift. He ...
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2answers
33 views

is there a way to determine a distant galaxy’s speed relative to the Hubble Flow by measuring time dilation effects between there and this galaxy?

As I understand it there is a preferred frame of reference based on the velocity for the CMB and our galaxy is moving in relation to it (at about 600 km/s). I think this is how the Hubble Flow is ...
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1answer
43 views

Is there any way to distinguish the doppler redshift from a distant galaxy from the probably much greater cosmological redshift? [duplicate]

Is there any way to distinguish the doppler redshift from a distant galaxy, caused by its initial recessional velocity relative to this galaxy, from the probably much greater, cosmological redshift, ...
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0answers
24 views

What was the energy/wavelength/frequency of the CMB when first emitted/released?

Given the z redshift of 1089, and the temperature difference between then and now of about 1089, I tried scaling the ev, freq. and wavelength by 1089.... Is that right? I get a freq. of about 240 ...
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1answer
21 views

Approximate formula to find velocity from cosmological redshift

From IOAA 2013 (Greece) Theory question no. 15, they stated that the approximate formula to find velocity from cosmological redshift is $$v = c*log_e (1+z)$$ and that it is often used by cosmologists. ...
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1answer
56 views

How to calculate the redshift of a line in a bunch of line spectra?

If in a spectra, there are many lines, mixed with gravitational redshift and non-redshift lines. For example, in this paper, they identify lines both redshifted and no-redshifted. Then how can i ...
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1answer
64 views

Were radio spectroscopic observations ever used to first measure a distance successfully?

Usually, especially for an extragalactic object, its redshift is determined by optical spectroscopic redshift. But the angular resolution of early radio observations is poor and an optical counterpart ...
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1answer
53 views

Intuitive explanation for why the Doppler effect (and red-shift) happens?

I was looking for an intuitive explanation as to why the Doppler effect happens. I haven't found any, but this is what I thought: -Waves emitted travel at a constant speed -The source emits a wave -If ...
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1answer
41 views

Which redshift is used to determine the Hubbleconstant?

I think they measure cosmological redshift to use in the Law of Hubble-Lemaître together with the distance to calculate $H_0$. Is this correct, or do they use Doppler shift (too)? $H_0$ indicates how ...
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2answers
48 views

Is light emitted from a heavier body more redshifted than light from a lighter body?

Is light emitted outwards from, say, a region very close to a black hole more shifted than light with the same wavelength that began next to a less massive object?
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0answers
51 views

Is cosmological redshift caused due to expansion of space or distance traveled by light?

If the universe wasn't expanding, would light still be redshifted after traveling some distance? Is there a way of being sure that it's the expansion that causes light to redshift and not just a ...
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1answer
682 views

Redshift quantization

A full disclosure to begin with: I'm a PhD student in mathematics and while I understand most of standard cosmological-astronomical terms and I've followed a one semester course on cosmology, I don't ...
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1answer
110 views

Interpretation and understanding of the relation for the photometric redshift in a given bin

In the context of photometric probe of surveys (like LSST), I need to understand the relation I have to use for photometric bins. Considering $p_{ph}(z_p|z)$ the probability to measure a ...
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0answers
55 views

Definition of space - to cope with “space expansion”

I've searched quite a lot but found no satisfying definition, so please: How do we actually define space when talking of "space expansion"? Isn't it just a metric, a dimension we measure, instead of ...
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1answer
112 views

How do we determine a redshift error?

I am struggling to calculate the redshift error of a galaxy. I was able to determine the redshifts of the absorption and emission lines from the galaxy's spectrum and then took the average of the ...
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2answers
277 views

Redshifting restframe SED

I have a template spectral energy distribution (SED) at z=0. I want to shift the template SED to an arbitrary redshift (z>0), how to do this in python?
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1answer
87 views

how to calculate distance between galaxies in the distant past

for example, assume there is a high redshift galaxy called "A" with z=6, so how to calculate the distance between us and another galaxy B when the light left the galaxy A? For example, say B has a ...
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1answer
87 views

Redshift Formulas

I was wondering where does the formula for redshift $$z=\frac vc$$ come from? Can it be derived from the definition of redshift $$z={\lambda_{observed} - \lambda_{emitted}\over \lambda_{emitted}}?$$
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2answers
129 views

Is there still a red shift when moving perpendicular to the direction of incidence?

This answer suggests that when there is zero radial velocity... there will be no Doppler shift but it's not likely to be intended as precise and absolute. I remember reading about something like this ...
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1answer
72 views

Redshift and Velocity relation

Let us say that we have an object so its total velocity is defined as \begin{equation} v_{tot} = v_{pec} + V_{rec} \end{equation} Where \begin{equation} V_{rec} = H_0r \end{equation} and \...
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0answers
38 views

Minimum apparent magnitude of galaxy

currently I'm working on astronomy problem which deals with finding redshift of a galaxy. But the first part asks to derermine apparent magnitude of a galaxy: Find what should be the minimum ...
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2answers
75 views

Photometric redshift - empirical methods (machine learning)

I am currently learning about estimation of photometric redshifts with machine learning methods (or empirical methods in general). These methods use the knowledge about the photometry and the ...
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0answers
21 views

How the depth[Mpc] is obtained from the redshift

I read this paper. In this paper, they show the three-dimensional distribution of the [OII] emitters. It is mentioned that The depth in z-axis is converted from the redshift, which is estimated from ...
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1answer
124 views

At what cosmological redshift $z$, does the recession speed equal the speed of light? How is it calculated?

At what cosmological redshift $z$, does the recession speed equal the speed of light? What equations are used to calculate this number (since at large redshifts, $z=v/c$ won't apply)? [The ...
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2answers
107 views

Why does the Supernova 2006cm give a very different value for the Hubble constant? Why doesn't it increase error bars for the Hubble constant?

The Supernova 2006cm has a redshift of 0.0153 which translates into a recession speed of 4600 km/s. It has a distance modulus of 34.71 which translates into a luminosity distance of 87 Mpc. This ...
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1answer
26 views

Does The ISW Effect Predict Net Redshift Over Distance?

The Integrated Sachs–Wolfe (ISW) effect is concerned primarily with the study of the Cosmic Background Microwave Radiation (CMBR), but seems to demonstrate that photons passing through voids suffer a ...
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1answer
73 views

Can a change in the density of interstellar and intergalactic quantum fields affect the Redshift of photons?

Using our solar system's heliosphere as an analagy to what is happening in our galaxy and even the universe as a whole, can light redshift with changes in quantum fields? The universe is constantly ...
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1answer
122 views

Difference between 3D real space and 3D redshift space?

With regards to cosmological simulations (e.g., N-body dark matter only simulations), I've seen people talk about the positions of objects (halos, galaxies) in the simulation in terms of 3D real space ...
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1answer
169 views

Cosmological redshift - How do we know it's not caused by the observer's time dilation?

Instead of cosmological redshift being caused by the metric expansion of space, why couldn't cosmological redshift be caused by our perspective moving faster in time? Said another way, if the ...
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1answer
46 views

Redshift, velocity, distance

How are redshift, (recessional) velocity and distance of a galaxy related? I understand what v=HD states. So I assume that Hubble measured a direct relation between the distance and recessional ...
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1answer
119 views

Dark energy and light red shifting

When light is red shifted from distant galaxies, the photons have lost energy. When dark energy pushes objects apart, those objects have gained energy from a larger gravitational potential. Is the ...
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1answer
120 views

SDSS Image Download: Swarp returns all zeros

In the CASJOBS Server, I created the following query for galaxies(DR12) ...
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2answers
4k views

Which star / galaxy is moving away from us the fastest?

I know that we have measured the rate a lot of stars and galaxies move away from us using the doppler shift, and I know that the further a star / galaxy is the faster they accelerate away from us due ...
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0answers
104 views

How to do logarithmic wavelength binning (in Python) in order to find redshift?

In Tonry and Davis (1979)(http://adsabs.harvard.edu/abs/1979AJ.....84.1511T), they formulated a cross-correlation method for extracting velocity redshifts. They bin the linear spectra into logarithmic ...
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1answer
707 views

Why do linear velocity redshifts correspond to linear pixel shifts when the spectra are binned in constant log wavelength?

In Tonry & Davis (1979), they describe spectroscopic redshift measurement via correlating with templates at known redshift. In Section IIIa, they say "Because the spectra are binned linearly with ...
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1answer
41 views

Where a database of different z values of redshift can be found?

I was working on reshift details for various galaxies but do not know any database to find the z values given to redshift galaxies. Can anyone help?
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1answer
47 views

How to identify component of velocity of a star from its red shift? [duplicate]

When a telescope observes a distant star, all it sees is the light being emitted by the star. Observing the wavelength of this light, we determine the velocity of the star. If the wavelength is red-...
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2answers
2k views

How Do we know about redshift?

Okay so scientists use the red shift to know that galaxies are moving away from us faster than light. How do we know that its not just red light from red stars. i mean most stars are in the infrared ...